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Baffling telemetry detections can be useful: an acoustic receiver design to monitor organisms along reserve boundaries and ecotones

Baffling telemetry detections can be useful: an acoustic receiver design to monitor organisms... Background: Many biotelemetry studies seek to detect movement of organisms across reserve boundaries or between adjacent habitat areas. Our objective was to enhance this capability in studies of aquatic organisms that are tagged with acoustic transmitters and tracked by passive data loggers. We installed an experimental shroud on a commercially available telemetry receiver. The shroud was designed to baffle incoming signals from transmitters along one hemisphere of the receiver and therefore more conclusively determine which side of a boundary line that a tagged organism occupies. Results: Range testing of shroud effectiveness was conducted along the border of a marine-protected area in a coral reef ecosystem. A transmitter of the type implanted into reef fish was deployed in various directions, distances, and landscape settings relative to 11 shrouded receivers. There was a significant difference in the detectability of trans- missions on the shrouded versus unshrouded side of the receivers. On the unshrouded hemisphere of the receiver, 75–100 % of transmissions within 100 m were detected and maximum effective detection range was ~180 m. On the shrouded hemisphere of the receiver, detections were rare at any distance with a maximum of 12 % of transmissions recorded even as close as 60 m away. Conclusions: The shroud modified the detection area of a standard omni-directional receiver into a hemi-directional receiver better able to detect transboundary presence. The approach is useful for applications that require detection of simple boundary-crossing events using a minimal number of receivers. Keywords: Array, Boundary crossing, Fish movement, Marine-protected area, Tracking suite of strengths and limitations. Active tracking of fish Background implanted with acoustic transmitters using a directional A key question in marine-protected area (MPA) design hydrophone offers precise positioning and avoids the need and performance is estimating the frequency and dura- for a costly array of many acoustic receivers, but of course tion of fish movement across borders [ 1]. The tendency requires presence of researchers and boats to acquire each of fishermen to “fish the line” just outside MPA bounda - position (e.g., [3]). Arrays of multiple acoustic receivers ries necessitates a detailed understanding of the spatial and data loggers deployed in grid or gate formations with movements of fish relative to MPA borders [ 2]. There are non-overlapping detection ranges offer relatively auto - a range of acoustic telemetry approaches now being uti- mated monitoring but yield only approximate positions lized to address this question, each with their particular based on receiver range [4]. Moderate densities of three or more acoustic receivers deployed with overlapping detec- *Correspondence: matt.kendall@noaa.gov tion ranges can be used to estimate approximate position NOAA/NOS/NCCOS/CCMA/Biogeography Branch, 1305 East West Highway, Silver Spring, MD 20910, USA or activity centers based on the detection rate of relatively Full list of author information is available at the end of the article © 2016 Kendall et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Kendall et al. Anim Biotelemetry (2016) 4:2 Page 2 of 7 stationary transmitters heard at multiple receivers using used the commercially available VR2W model receiver linear [5] and sigmoid [6] relationships between detection from VEMCO, Amirix Systems Inc. during these experi- rate and transmission distance. High densities of time- ments. This battery powered hydrophone and data logger synchronized receivers have also been used along with is an automated system that has been deployed in a wide time of arrival data from transmitters to calculate position diversity of telemetry studies worldwide. The ability of within 1–5  m [7–9]. However, all the techniques capable the standard VR2W omni-directional receivers to detect of discriminating fine-scale boundary crossing require fish movements across the boundary of the Monument close spacing of three or more receivers to ensure overlap was modified by attaching a directional acoustic shroud in the detection range, and in these systems, transmitter onto the hydrophone of the receivers. This modification position can only be calculated in the area of overlap. This was inspired by prior studies suggesting that materi- number of receivers increases hardware costs, deploy- als including mooring equipment near the hydrophone ment, maintenance and recovery logistics, as well as com- transducer can block acoustic signals [14, VEMCO pers. putational requirements. The estimated positions provide comm.]. The shroud was constructed from a 15.25-cm a wealth of habitat utilization information that may be diameter (6 inch), solid PVC foam float of the type com - extraneous to simply detect positions relative to linear monly used in boating and fishing applications. The air- boundaries associated with MPAs. filled foam of the shroud theoretically provided a barrier We sought to identify a technique that would sen- to acoustic pings from transmitters. The PVC float was sitively identify presence/absence along one side of a cut lengthwise in half to make two shrouds. Each half was boundary but eliminate the need for extensive arrays of partially hollowed out to fit snugly around the top of a closely spaced hydrophones and complex data processing VR2W such that the actual transducer on the top of the required by approaches presently available. Experiments VR2W would seat into the existing rope channel of the were conducted to convert standard omni-directional PVC float (Fig.  1). Shrouds had slots drilled through them acoustic receivers into hemi-directional receivers that could only detect fish presence in a specified hemisphere on one side of a receiver. We installed an experimental acoustic shroud over the hydrophone of a commercially available acoustic data logger to test whether it would enable more detailed quantification of boundary-crossing events of tagged fish. Methods Study area The experimental system was tested in a natural ecosystem rather than in a highly controlled environment in order to evaluate its real-world potential. The St. John, US Virgin Islands (USVI) study area consists of many small, man- grove-lined bays with scattered patch reefs. At this latitude in the Caribbean (18.3  N, 64.7  W) the trade winds blow consistently from east to west. The Virgin Islands coral reef national Monument (VICRNM) is a no-take MPA that bisects portions of Coral Bay and Round Bay and was the focus of this evaluation [10]. The water column is well mixed. Wave energy and tidal or other currents are always minimal throughout the area. Biotic sounds experience small peaks around dawn and dusk but are uniform at other times [11]. These uniform water characteristics are not believed to be a major source of spatial or temporal variability on transmitter detections [4, 6, 12, 13]. Hemi‑directional receivers Fig. 1 Design of the acoustic shroud from: a whole PVC float, b float Eleven acoustic receivers were deployed along the cut in half and then hollowed out to fit over the top of the VR2W, c VICRNM boundary. All were between 17 and 24 m depth side view of shroud mounted on VR2W shown in black, and d facing view except the northwestern receiver which was at 6  m. We Kendall et al. Anim Biotelemetry (2016) 4:2 Page 3 of 7 to enable tight attachment onto the hydrophone using at each range-test site. This distance off the bottom simu - zip ties through the existing pry bar holes in the VR2W lated the near-benthic position typical of the reef fish in housing. In this arrangement, the shroud theoretically the area. This transmitter was deployed repeatedly in var - blocks incoming pings from one hemisphere around the ious directions, distances, and landscape settings relative receiver but allows normal detection of pings from the to each receiver (Fig. 3). open side. As a result an omni-directional receiver is Distance to range-test site and detection rate (the converted into a hemi-directional receiver. The mooring percentage of transmissions actually detected out of system therefore must maintain the orientation of the the transmissions emitted at each range-test site) were shroud along the axis of interest, in this case parallel to plotted for those range-test sites on the shrouded ver- the Monument boundary. sus unshrouded sides of the receiver for comparison. A The mooring system maintained receivers in a vertical detection rate of 50 % was recently suggested by the man- orientation and more importantly, prevented rotation ufacturer for identifying a reliable tag detection range of the shroud off the axis of the Monument boundary [15] although researchers typically customize acceptable to ensure monitoring of signals from the desired direc- cut-offs [16]. Detection range is defined here as the shape tion [14]. This was accomplished by installing two sand of the curve relating detection rate and distance between screws  ~  1.5  m apart along the Monument boundary, receiver and tag [16]. Maximum effective detection range and securing them to the sides of the shrouded receiver is defined as the distance where detection rate fell below such that it maintained its directional orientation (Fig. 2). 20  %. It was not possible, nor was it necessary to gener- Once installed, divers twisted and pulled on the assem- ate individual range-test curves for each receiver since bly to confirm that the floats would quickly return the all distances were not tested at all sites and these receiv- system to its desired orientation. It should be noted that ers were deployed in relatively consistent environmental this mooring was effective in our low-wave energy and settings. minimal current environment but a fixed or rigid design A compass plot was used to evaluate detection perfor- may be needed elsewhere. All shrouds were observed to mance relative to the shrouded versus unshrouded hem- be intact and in place when receivers were retrieved for ispheres of the receiver. For this, both the distance and download at the end of the study. Two unmodified, omni- angle of the range-test transmitter relative to the shroud directional receivers were deployed at two of the hemi- was depicted. Angles were standardized in the compass directional receiver sites as controls but unfortunately plot such that the shroud theoretically blocked detections both of these experienced component failures, did not from the south. Note that the actual angles of shroud record data, and will not be discussed further. deployment at each receiver varied to be aligned with Monument boundaries (Fig. 3). Testing detection range and direction Sensitivity of receivers to detect transmitter signals along Statistical analysis the VICRNM boundary was determined by deploying a Probability of detection was modeled with a quasibi- range-test transmitter in a diversity of locations around nomial generalized linear model with a logit link func- the modified receivers throughout the study area. In this tion and where the response data were the numbers analysis, a range-test transmitter (for VEMCO V7-4L of detected and undetected pings from each range- transmitters) with a  ~15  s transmission interval was test site. This model offers an advantage over a sim - deployed ~0.5 m off the bottom for a minimum of 10 min pler unweighted logistic regression in that it naturally accounts for any differences in deployment time at each range-test site. Samples of longer duration have more influence on model fit. Independent variables were dis - Float tance (continuous) and shrouded versus unshrouded side of the receiver (factor). The fitted model was used to Shroud predict the probability of detection within a circle whose radius equaled the maximum distance in the data. Statis- Receiver tical analyses were performed in R (R Development Core Cable Team, http://www.R-project.org). ~1.5 m MPA Boundary Results Sand screw All test sites (6/6) within  ~100  m of the receiver on the Fig. 2 Diagram of the directional mooring system to maintain unshrouded hemisphere experienced good or excellent receiver orientation along the Monument boundary detection (75–100  % of possible detections) and 9 out Kendall et al. Anim Biotelemetry (2016) 4:2 Page 4 of 7 Fig. 3 Coral Bay and Round Bay study area, St. John USVI of 13 test sites within  ~200  m of the unshrouded direc- manufacturer-recommended detection rate of 50  % tion had at least a few transmissions detected (Fig.  4). was never achieved and only 4 out of 17 test sites In rare cases, detection strength was good beyond  ~200 within ~200 m detected any transmissions in that direc- m away in this direction. Detection rate dropped below tion. Transmitters within  ~60  m of the receiver were 50 % at ~125 m, fell below 20 % at ~180 m, and reached detectable but never at more than a 12  % detection rate zero at all sites beyond ~230 m. These patterns were con - and detections beyond this distance were rare. sistent with the 150–300 m detection ranges observed for The plot of distance, detection rate, as well as angle omni-directional receivers that were deployed elsewhere relative to the acoustic shroud revealed the sensitivity of in our study region based on similar sized [pers. obs.] or shrouded receivers for detecting position near the Monu- slightly larger transmitters [17, 18]. The shape of this pat - ment boundary (Fig.  5a). The shroud appeared effective tern matches the typical sigmoid or logistic shape seen in at blocking signals along the hemispherical axis of the similar assessments [6, 12]. receivers (standardized along 90° and 270° in Fig.  5a, b). In contrast, detection rates were much lower in Transmissions emitted just 15° north of this axis were the hemisphere on the shrouded side of the receiver readily detectable, whereas those 15° south of it but at the (Fig.  4). Dramatically fewer transmissions were detected same distance away from the receiver were generally not in the direction blocked by the shroud such that the detectable. Kendall et al. Anim Biotelemetry (2016) 4:2 Page 5 of 7 unshrouded shrouded 0100 200 300 Distance (m) Fig. 4 Points represent the proportion of possible transmis- sions detected as a function of distance from the receiver to the unshrouded (n = 34) versus shrouded (n = 28) hemispheres of hemi-directional receivers. Curves denote the modeled probability of detection The modeled probability of detection decreased with distance from the receiver (p < 0.0001), and the shrouded and unshrouded hemispheres of the receiver exhib- ited significantly different probabilities of detection (p  <  0.001; Figs.  4 and 5b). The unshrouded side of the receiver showed a rapid drop in the modeled probability of detection from 80 to 20  % between  ~75 and  ~180  m. Based on model results, the 50 % detection rate occurred at 128 m and a hemispherical maximum detection range of  ~180  m (corresponding to 20  % probability of detec- tion) was estimated in the unshrouded direction along Fig. 5 a–b: Range-test results observed (a) and modeled (b) for the axis of the Monument boundary (Fig. 3). hemi-directional receivers. All receiver positions are at the center of these plots and all the compass bearings relative to the shroud Discussion [shown in orange in (a)] were standardized such that they were Tests here suggest that shrouded receivers can be an effec - theoretically blocking incoming transmissions from the south [hemi- tive tool for improved edge detection in acoustic telemetry sphere with gray slashing in (a)]. a Point locations represent range- test sites according to transmitter distance (m) and direction from the studies. Only transmitters on the unshrouded side of the receivers. Range-test sites were then gray scaled by the number of receiver could be detected reliably at a  >50  % detection possible transmissions actually detected with darker shades denoting rate [recommended by VEMCO, 15] or even a much more more detections. b Modeled probability of detection generous detection rate of 15 %. It is recognized that inter- pretation of detections and selection of appropriate cut-off values will depend on the particular setting, research ques- depend on the objectives of a particular study. It may tions, and behavior of the organism of interest and may be better to more conclusively detect when fish leave differ from the 20 % threshold estimated here [1 , 16]. This an MPA and are exposed to fishing than knowing that will require a customized set of decision rules to be estab- they remain safe in a given protected area. Additional lished to determine the probability that an organism is on omni-directional receivers could be deployed inside and one side of a boundary or the other (e.g., [1]). outside the MPA boundary to provide a more complete The direction of shroud placement, listening into ver - understanding of directional movements (Fig.  3). This sus out of an MPA, and other receiver placements will would also help with interpretation of low detection Proportion detected 0.00.2 0.40.6 0.8 1.0 Kendall et al. Anim Biotelemetry (2016) 4:2 Page 6 of 7 rates that could either be due to fish being present close (160 and 156  m, respectively) but did not record any by the shrouded side of the hemi-directional receiver or transmissions. This side of the shroud was of course far away but on the unshrouded side. Although it dou- identical to the other side that did have detections at bles the number of receivers needed, for some applica- similar angles and distances. We therefore looked tions it may also be useful to place two hemi-directional beyond the shroud, at habitats surrounding each test receivers on a single mooring but facing in opposite location for an explanation. One of those test sites lack- directions to determine on which side of a boundary ing detections was the shallowest and most complex line that an organism is located. The approach is not reef setting in our study and the other had patch reefs only useful for MPA boundary evaluation. For example, nearby but unfortunately no detailed habitat informa- shrouded receivers can be set along linear habitat bound- tion is available between the receiver and range-test site aries such as hardbottom/softbottom or reef/seagrass to further diagnose potential landscape interference interfaces to more conclusively detect which habitat a at the second site. Therefore, although we displayed tagged fish is utilizing. The approach may also be useful maximum detection range as a single, composite value in other constrained settings where it is not desirable to of ~180 m due to the relatively homogeneous environ- detect fish presence throughout the entire circumfer - ment in which most receivers were deployed, it should ence of an omni-directional receivers’ detection range. be recognized that detection range on the unshrouded More detailed monitoring of arrival or departure from side of individual receivers will certainly vary depend- small landscape features such as artificial reefs, spawn - ing on their particular setting in the landscape [4, 13, ing sites, or boat channels may also be enhanced with this 15, 19]. technique. Our experiments were encouraging, that the shroud It is important that shrouds be consistently shaped and blocks most signal detection from one hemisphere, but snug against the transducer. Any deviations in shroud do not replace the need to conduct robust range tests geometry should be well under the wavelength of the at each receiver site [15, 16]. In addition, use of sentinel transmitters (69 kHz in this case, or ~21.7 cm). This will tags is also advisable to evaluate the long-term influence avoid irregularities in constructive or destructive inter- of variations in environmental noise on detection rate in ference with incoming signals on the unshrouded side most settings. Further tests are also needed to: (1) evalu- of the receiver (VEMCO pers. com.). Depending on the ate shroud performance on tagged fish now that con - material used to construct the shroud, an acoustically trolled field tests have yielded positive results, (2) test the absorbent coating or randomized scattering texture may approach on data loggers available from other manufac- also be useful to minimize any irregular lobes or nulls in turers, and (3) refine and evaluate other shroud materials, directional sensitivity. coatings, and designs. Of course the shroud is not suitable for all applica- Authors’ contributions tions. The approach doesn’t yield position estimates, it MK designed and built the receiver shrouds, analyzed the data, and drafted merely provides hemispheric presence/absence for a the manuscript. MM participated in the design and execution of the study and helped to draft the manuscript. AW performed statistical analysis. All authors better estimate of which side of a receiver an organism read and approved the final manuscript. may be positioned. For detailed location and habitat uti- lization information, other systems are required [7–9]. Author details NOAA/NOS/NCCOS/CCMA/Biogeography Branch, 1305 East West Highway, Those existing approaches enable fine-scale tracking of Silver Spring, MD 20910, USA. CSS-Dynamac Inc., 10301 Democracy Lane, fish position to within a few meters by deploying many Suite 300, Fairfax, VA 22030, USA. receivers in high density with overlapping detection Acknowledgements range. However, this reduces spatial coverage of a study Several individuals assisted with the deployment, range testing, retrieval, since so many receivers must be placed in a confined and download of the telemetry equipment including L Siceloff, C Cosgrove, area, yields much extraneous information (i.e., constant K Roberson, B Schwartz, A Glahn, and M Kent. We thank T Kelley and NPS for boat support. L Siceloff, J Christensen, and research and development position) for applications where only boundary-crossing staff at VEMCO evaluated an early version of this manuscript and provided data is of interest, and can be cost prohibitive due to suggestions for improvement. Two anonymous peer reviewers provided computational requirements and the large number and constructive suggestions which also greatly improved our manuscript. Funding for this project was from NPS and NOS/NCCOS/CCMA. Government density of receivers that are needed. contract labor (AW ) was provided by CSS-Dynamac, Inc. under NOAA contract Our analyses represent a composite value of perfor- EA-133C-14-NC-1384. mance for 11 receivers in a real landscape. The natu - Competing interests ral variability in landscape features present in the study The authors declare that they have no competing interests. area contributed to the variance in our results (Fig.  3). Two range-test sites in the unshrouded NW quadrant Received: 29 September 2015 Accepted: 21 December 2015 of Fig.  5a were within the detection range of receivers Kendall et al. Anim Biotelemetry (2016) 4:2 Page 7 of 7 References 11. Kaplan MB, Mooney TA, Partan J, Solow AR. Coral reef species assem- 1. Knip DM, Heupel MR, Simpfendorfer CA. Evaluating marine pro- blages are associated with ambient soundscapes. Mar Ecol Prog Ser. tected areas for the conservation of tropical coastal sharks. Biol Cons. 2015;533:93–107. 2012;148(1):200–9. 12. Hobday AJ, Pincock D. Estimating detection probabilities for linear acous- 2. Kellner JB, Tetreault I, Gaines SD, Nesbit RM. Fishing the line near marine tic monitoring arrays. In: Mckenzie JR, Parsons B, Seitz AC, Kopf RK, Mesa reserves in single and multispecies fisheries. Ecol Appl. 2007;17:1039–54. M, Phelps Q, editors. Advances in fish tagging and marking technology. 3. Hitt S, Pittman SJ, Nemeth RS. Diel movements of fishes linked to benthic Bethesda: American Fisheries Society Symposium 76; 2012. p. 325–46. seascape structure in a Caribbean coral reef ecosystem. Mar Ecol Prog 13. Mathies NH, Ogburn MB, McFall G, Fangman S. Environmental interfer- Ser. 2011;427:275–91. ence factors affecting detection range in acoustic telemetry studies 4. Heupel MR, Semmens JM, Hobday AJ. Automated acoustic tracking of using fixed receiver arrays. Mar Ecol Prog Ser. 2014;495:27–38. aquatic animals: scales, design and deployment of listening station arrays. 14. Clements S, Jepsen D, Karnowski M, Schreck CB. Optimization of an Mar Freshwater Res. 2006;57:1–13. acoustic telemetry array for detecting transmitter-implanted fish. N Am J 5. Simpfendorfer CA, Heupel MR, Hunter RE. Estimation of short-term cen- Fish Man. 2005;25:429–36. tres of activity from an array of omnidirectional hydrophones and its use 15. VEMCO. VEMCO range test software manual. DOC-5583-02. Nova Scotia; in studying animal movements. Can J Fish Aquat Sci. 2002;59:23–32. 2015. 6. How JR, de Lestang S. Acoustic tracking: issues affecting the design, 16. Kessel ST, Cooke SJ, Heupel MR, Hussey NE, Simpfendorfer CA, Vagle S, analysis and interpolation of data from movement studies. Mar Freshwa- Fisk AT. A review of detection range testing in aquatic passive acoustic ter Res. 2012;63:312–24. telemetry studies. Rev Fish Biol Fisheries. 2014;24:199–218. 7. Andrews KS, Tolimieri N, Williams GD, Samhouri JF, Harvey CJ, Levin PS. 17. Friedlander AM, Monaco ME, Clark R, Pittman SJ. Fish movement patterns Comparison of fine-scale acoustic monitoring systems using home range in Virgin Islands national park, Virgin Islands coral reef national monu- size of a demersal fish. Mar Biol. 2011;158:2377–87. ment and adjacent waters. Silver Spring: NOAA Technical Memorandum 8. Espinoza M, Farrugia TJ, Webber DM, Smith F, Lowe CG. Testing a new NOS NCCOS; 2013. p. 172. acoustic telemetry technique to quantify long-term, fine-scale move - 18. Pittman SJ, Monaco ME, Friedlander AM, Legare B, Nemeth RS, Kendall ments of aquatic animals. Fish Res. 2011;108:364–71. MS, Poti M, Clark RD, Wedding LM, Caldow C. Fish with chips: tracking reef 9. Cooke SJ, Niezgoda GH, Hanson K, Suski CD, Phelan FJS, Tinline R, Philipp fish movements to evaluate size and connectivity of Caribbean marine DP. Use of CDMA acoustic technology to document 3-D positions of fish: protected areas. PlosOne. 2014;9(5):e96028 (11 pp). relevance to the design and monitoring of aquatic protected areas. Mar 19. Simpfendorfer CA, Heupel MR, Collins AB. Variation in the performance Tech Soc J. 2005;39:17–27. of acoustic receivers and its implication for positioning algorithms in a 10. Clinton WJ. Establishment of the Virgin Islands coral reef national monu- riverine setting. Can J Fish Aquat Sci. 2008;65:482–92. ment. 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Baffling telemetry detections can be useful: an acoustic receiver design to monitor organisms along reserve boundaries and ecotones

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Springer Journals
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Copyright © 2016 by Kendall et al.
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Life Sciences; Animal Systematics/Taxonomy/Biogeography
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Abstract

Background: Many biotelemetry studies seek to detect movement of organisms across reserve boundaries or between adjacent habitat areas. Our objective was to enhance this capability in studies of aquatic organisms that are tagged with acoustic transmitters and tracked by passive data loggers. We installed an experimental shroud on a commercially available telemetry receiver. The shroud was designed to baffle incoming signals from transmitters along one hemisphere of the receiver and therefore more conclusively determine which side of a boundary line that a tagged organism occupies. Results: Range testing of shroud effectiveness was conducted along the border of a marine-protected area in a coral reef ecosystem. A transmitter of the type implanted into reef fish was deployed in various directions, distances, and landscape settings relative to 11 shrouded receivers. There was a significant difference in the detectability of trans- missions on the shrouded versus unshrouded side of the receivers. On the unshrouded hemisphere of the receiver, 75–100 % of transmissions within 100 m were detected and maximum effective detection range was ~180 m. On the shrouded hemisphere of the receiver, detections were rare at any distance with a maximum of 12 % of transmissions recorded even as close as 60 m away. Conclusions: The shroud modified the detection area of a standard omni-directional receiver into a hemi-directional receiver better able to detect transboundary presence. The approach is useful for applications that require detection of simple boundary-crossing events using a minimal number of receivers. Keywords: Array, Boundary crossing, Fish movement, Marine-protected area, Tracking suite of strengths and limitations. Active tracking of fish Background implanted with acoustic transmitters using a directional A key question in marine-protected area (MPA) design hydrophone offers precise positioning and avoids the need and performance is estimating the frequency and dura- for a costly array of many acoustic receivers, but of course tion of fish movement across borders [ 1]. The tendency requires presence of researchers and boats to acquire each of fishermen to “fish the line” just outside MPA bounda - position (e.g., [3]). Arrays of multiple acoustic receivers ries necessitates a detailed understanding of the spatial and data loggers deployed in grid or gate formations with movements of fish relative to MPA borders [ 2]. There are non-overlapping detection ranges offer relatively auto - a range of acoustic telemetry approaches now being uti- mated monitoring but yield only approximate positions lized to address this question, each with their particular based on receiver range [4]. Moderate densities of three or more acoustic receivers deployed with overlapping detec- *Correspondence: matt.kendall@noaa.gov tion ranges can be used to estimate approximate position NOAA/NOS/NCCOS/CCMA/Biogeography Branch, 1305 East West Highway, Silver Spring, MD 20910, USA or activity centers based on the detection rate of relatively Full list of author information is available at the end of the article © 2016 Kendall et al. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/ publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Kendall et al. Anim Biotelemetry (2016) 4:2 Page 2 of 7 stationary transmitters heard at multiple receivers using used the commercially available VR2W model receiver linear [5] and sigmoid [6] relationships between detection from VEMCO, Amirix Systems Inc. during these experi- rate and transmission distance. High densities of time- ments. This battery powered hydrophone and data logger synchronized receivers have also been used along with is an automated system that has been deployed in a wide time of arrival data from transmitters to calculate position diversity of telemetry studies worldwide. The ability of within 1–5  m [7–9]. However, all the techniques capable the standard VR2W omni-directional receivers to detect of discriminating fine-scale boundary crossing require fish movements across the boundary of the Monument close spacing of three or more receivers to ensure overlap was modified by attaching a directional acoustic shroud in the detection range, and in these systems, transmitter onto the hydrophone of the receivers. This modification position can only be calculated in the area of overlap. This was inspired by prior studies suggesting that materi- number of receivers increases hardware costs, deploy- als including mooring equipment near the hydrophone ment, maintenance and recovery logistics, as well as com- transducer can block acoustic signals [14, VEMCO pers. putational requirements. The estimated positions provide comm.]. The shroud was constructed from a 15.25-cm a wealth of habitat utilization information that may be diameter (6 inch), solid PVC foam float of the type com - extraneous to simply detect positions relative to linear monly used in boating and fishing applications. The air- boundaries associated with MPAs. filled foam of the shroud theoretically provided a barrier We sought to identify a technique that would sen- to acoustic pings from transmitters. The PVC float was sitively identify presence/absence along one side of a cut lengthwise in half to make two shrouds. Each half was boundary but eliminate the need for extensive arrays of partially hollowed out to fit snugly around the top of a closely spaced hydrophones and complex data processing VR2W such that the actual transducer on the top of the required by approaches presently available. Experiments VR2W would seat into the existing rope channel of the were conducted to convert standard omni-directional PVC float (Fig.  1). Shrouds had slots drilled through them acoustic receivers into hemi-directional receivers that could only detect fish presence in a specified hemisphere on one side of a receiver. We installed an experimental acoustic shroud over the hydrophone of a commercially available acoustic data logger to test whether it would enable more detailed quantification of boundary-crossing events of tagged fish. Methods Study area The experimental system was tested in a natural ecosystem rather than in a highly controlled environment in order to evaluate its real-world potential. The St. John, US Virgin Islands (USVI) study area consists of many small, man- grove-lined bays with scattered patch reefs. At this latitude in the Caribbean (18.3  N, 64.7  W) the trade winds blow consistently from east to west. The Virgin Islands coral reef national Monument (VICRNM) is a no-take MPA that bisects portions of Coral Bay and Round Bay and was the focus of this evaluation [10]. The water column is well mixed. Wave energy and tidal or other currents are always minimal throughout the area. Biotic sounds experience small peaks around dawn and dusk but are uniform at other times [11]. These uniform water characteristics are not believed to be a major source of spatial or temporal variability on transmitter detections [4, 6, 12, 13]. Hemi‑directional receivers Fig. 1 Design of the acoustic shroud from: a whole PVC float, b float Eleven acoustic receivers were deployed along the cut in half and then hollowed out to fit over the top of the VR2W, c VICRNM boundary. All were between 17 and 24 m depth side view of shroud mounted on VR2W shown in black, and d facing view except the northwestern receiver which was at 6  m. We Kendall et al. Anim Biotelemetry (2016) 4:2 Page 3 of 7 to enable tight attachment onto the hydrophone using at each range-test site. This distance off the bottom simu - zip ties through the existing pry bar holes in the VR2W lated the near-benthic position typical of the reef fish in housing. In this arrangement, the shroud theoretically the area. This transmitter was deployed repeatedly in var - blocks incoming pings from one hemisphere around the ious directions, distances, and landscape settings relative receiver but allows normal detection of pings from the to each receiver (Fig. 3). open side. As a result an omni-directional receiver is Distance to range-test site and detection rate (the converted into a hemi-directional receiver. The mooring percentage of transmissions actually detected out of system therefore must maintain the orientation of the the transmissions emitted at each range-test site) were shroud along the axis of interest, in this case parallel to plotted for those range-test sites on the shrouded ver- the Monument boundary. sus unshrouded sides of the receiver for comparison. A The mooring system maintained receivers in a vertical detection rate of 50 % was recently suggested by the man- orientation and more importantly, prevented rotation ufacturer for identifying a reliable tag detection range of the shroud off the axis of the Monument boundary [15] although researchers typically customize acceptable to ensure monitoring of signals from the desired direc- cut-offs [16]. Detection range is defined here as the shape tion [14]. This was accomplished by installing two sand of the curve relating detection rate and distance between screws  ~  1.5  m apart along the Monument boundary, receiver and tag [16]. Maximum effective detection range and securing them to the sides of the shrouded receiver is defined as the distance where detection rate fell below such that it maintained its directional orientation (Fig. 2). 20  %. It was not possible, nor was it necessary to gener- Once installed, divers twisted and pulled on the assem- ate individual range-test curves for each receiver since bly to confirm that the floats would quickly return the all distances were not tested at all sites and these receiv- system to its desired orientation. It should be noted that ers were deployed in relatively consistent environmental this mooring was effective in our low-wave energy and settings. minimal current environment but a fixed or rigid design A compass plot was used to evaluate detection perfor- may be needed elsewhere. All shrouds were observed to mance relative to the shrouded versus unshrouded hem- be intact and in place when receivers were retrieved for ispheres of the receiver. For this, both the distance and download at the end of the study. Two unmodified, omni- angle of the range-test transmitter relative to the shroud directional receivers were deployed at two of the hemi- was depicted. Angles were standardized in the compass directional receiver sites as controls but unfortunately plot such that the shroud theoretically blocked detections both of these experienced component failures, did not from the south. Note that the actual angles of shroud record data, and will not be discussed further. deployment at each receiver varied to be aligned with Monument boundaries (Fig. 3). Testing detection range and direction Sensitivity of receivers to detect transmitter signals along Statistical analysis the VICRNM boundary was determined by deploying a Probability of detection was modeled with a quasibi- range-test transmitter in a diversity of locations around nomial generalized linear model with a logit link func- the modified receivers throughout the study area. In this tion and where the response data were the numbers analysis, a range-test transmitter (for VEMCO V7-4L of detected and undetected pings from each range- transmitters) with a  ~15  s transmission interval was test site. This model offers an advantage over a sim - deployed ~0.5 m off the bottom for a minimum of 10 min pler unweighted logistic regression in that it naturally accounts for any differences in deployment time at each range-test site. Samples of longer duration have more influence on model fit. Independent variables were dis - Float tance (continuous) and shrouded versus unshrouded side of the receiver (factor). The fitted model was used to Shroud predict the probability of detection within a circle whose radius equaled the maximum distance in the data. Statis- Receiver tical analyses were performed in R (R Development Core Cable Team, http://www.R-project.org). ~1.5 m MPA Boundary Results Sand screw All test sites (6/6) within  ~100  m of the receiver on the Fig. 2 Diagram of the directional mooring system to maintain unshrouded hemisphere experienced good or excellent receiver orientation along the Monument boundary detection (75–100  % of possible detections) and 9 out Kendall et al. Anim Biotelemetry (2016) 4:2 Page 4 of 7 Fig. 3 Coral Bay and Round Bay study area, St. John USVI of 13 test sites within  ~200  m of the unshrouded direc- manufacturer-recommended detection rate of 50  % tion had at least a few transmissions detected (Fig.  4). was never achieved and only 4 out of 17 test sites In rare cases, detection strength was good beyond  ~200 within ~200 m detected any transmissions in that direc- m away in this direction. Detection rate dropped below tion. Transmitters within  ~60  m of the receiver were 50 % at ~125 m, fell below 20 % at ~180 m, and reached detectable but never at more than a 12  % detection rate zero at all sites beyond ~230 m. These patterns were con - and detections beyond this distance were rare. sistent with the 150–300 m detection ranges observed for The plot of distance, detection rate, as well as angle omni-directional receivers that were deployed elsewhere relative to the acoustic shroud revealed the sensitivity of in our study region based on similar sized [pers. obs.] or shrouded receivers for detecting position near the Monu- slightly larger transmitters [17, 18]. The shape of this pat - ment boundary (Fig.  5a). The shroud appeared effective tern matches the typical sigmoid or logistic shape seen in at blocking signals along the hemispherical axis of the similar assessments [6, 12]. receivers (standardized along 90° and 270° in Fig.  5a, b). In contrast, detection rates were much lower in Transmissions emitted just 15° north of this axis were the hemisphere on the shrouded side of the receiver readily detectable, whereas those 15° south of it but at the (Fig.  4). Dramatically fewer transmissions were detected same distance away from the receiver were generally not in the direction blocked by the shroud such that the detectable. Kendall et al. Anim Biotelemetry (2016) 4:2 Page 5 of 7 unshrouded shrouded 0100 200 300 Distance (m) Fig. 4 Points represent the proportion of possible transmis- sions detected as a function of distance from the receiver to the unshrouded (n = 34) versus shrouded (n = 28) hemispheres of hemi-directional receivers. Curves denote the modeled probability of detection The modeled probability of detection decreased with distance from the receiver (p < 0.0001), and the shrouded and unshrouded hemispheres of the receiver exhib- ited significantly different probabilities of detection (p  <  0.001; Figs.  4 and 5b). The unshrouded side of the receiver showed a rapid drop in the modeled probability of detection from 80 to 20  % between  ~75 and  ~180  m. Based on model results, the 50 % detection rate occurred at 128 m and a hemispherical maximum detection range of  ~180  m (corresponding to 20  % probability of detec- tion) was estimated in the unshrouded direction along Fig. 5 a–b: Range-test results observed (a) and modeled (b) for the axis of the Monument boundary (Fig. 3). hemi-directional receivers. All receiver positions are at the center of these plots and all the compass bearings relative to the shroud Discussion [shown in orange in (a)] were standardized such that they were Tests here suggest that shrouded receivers can be an effec - theoretically blocking incoming transmissions from the south [hemi- tive tool for improved edge detection in acoustic telemetry sphere with gray slashing in (a)]. a Point locations represent range- test sites according to transmitter distance (m) and direction from the studies. Only transmitters on the unshrouded side of the receivers. Range-test sites were then gray scaled by the number of receiver could be detected reliably at a  >50  % detection possible transmissions actually detected with darker shades denoting rate [recommended by VEMCO, 15] or even a much more more detections. b Modeled probability of detection generous detection rate of 15 %. It is recognized that inter- pretation of detections and selection of appropriate cut-off values will depend on the particular setting, research ques- depend on the objectives of a particular study. It may tions, and behavior of the organism of interest and may be better to more conclusively detect when fish leave differ from the 20 % threshold estimated here [1 , 16]. This an MPA and are exposed to fishing than knowing that will require a customized set of decision rules to be estab- they remain safe in a given protected area. Additional lished to determine the probability that an organism is on omni-directional receivers could be deployed inside and one side of a boundary or the other (e.g., [1]). outside the MPA boundary to provide a more complete The direction of shroud placement, listening into ver - understanding of directional movements (Fig.  3). This sus out of an MPA, and other receiver placements will would also help with interpretation of low detection Proportion detected 0.00.2 0.40.6 0.8 1.0 Kendall et al. Anim Biotelemetry (2016) 4:2 Page 6 of 7 rates that could either be due to fish being present close (160 and 156  m, respectively) but did not record any by the shrouded side of the hemi-directional receiver or transmissions. This side of the shroud was of course far away but on the unshrouded side. Although it dou- identical to the other side that did have detections at bles the number of receivers needed, for some applica- similar angles and distances. We therefore looked tions it may also be useful to place two hemi-directional beyond the shroud, at habitats surrounding each test receivers on a single mooring but facing in opposite location for an explanation. One of those test sites lack- directions to determine on which side of a boundary ing detections was the shallowest and most complex line that an organism is located. The approach is not reef setting in our study and the other had patch reefs only useful for MPA boundary evaluation. For example, nearby but unfortunately no detailed habitat informa- shrouded receivers can be set along linear habitat bound- tion is available between the receiver and range-test site aries such as hardbottom/softbottom or reef/seagrass to further diagnose potential landscape interference interfaces to more conclusively detect which habitat a at the second site. Therefore, although we displayed tagged fish is utilizing. The approach may also be useful maximum detection range as a single, composite value in other constrained settings where it is not desirable to of ~180 m due to the relatively homogeneous environ- detect fish presence throughout the entire circumfer - ment in which most receivers were deployed, it should ence of an omni-directional receivers’ detection range. be recognized that detection range on the unshrouded More detailed monitoring of arrival or departure from side of individual receivers will certainly vary depend- small landscape features such as artificial reefs, spawn - ing on their particular setting in the landscape [4, 13, ing sites, or boat channels may also be enhanced with this 15, 19]. technique. Our experiments were encouraging, that the shroud It is important that shrouds be consistently shaped and blocks most signal detection from one hemisphere, but snug against the transducer. Any deviations in shroud do not replace the need to conduct robust range tests geometry should be well under the wavelength of the at each receiver site [15, 16]. In addition, use of sentinel transmitters (69 kHz in this case, or ~21.7 cm). This will tags is also advisable to evaluate the long-term influence avoid irregularities in constructive or destructive inter- of variations in environmental noise on detection rate in ference with incoming signals on the unshrouded side most settings. Further tests are also needed to: (1) evalu- of the receiver (VEMCO pers. com.). Depending on the ate shroud performance on tagged fish now that con - material used to construct the shroud, an acoustically trolled field tests have yielded positive results, (2) test the absorbent coating or randomized scattering texture may approach on data loggers available from other manufac- also be useful to minimize any irregular lobes or nulls in turers, and (3) refine and evaluate other shroud materials, directional sensitivity. coatings, and designs. Of course the shroud is not suitable for all applica- Authors’ contributions tions. The approach doesn’t yield position estimates, it MK designed and built the receiver shrouds, analyzed the data, and drafted merely provides hemispheric presence/absence for a the manuscript. MM participated in the design and execution of the study and helped to draft the manuscript. AW performed statistical analysis. All authors better estimate of which side of a receiver an organism read and approved the final manuscript. may be positioned. For detailed location and habitat uti- lization information, other systems are required [7–9]. Author details NOAA/NOS/NCCOS/CCMA/Biogeography Branch, 1305 East West Highway, Those existing approaches enable fine-scale tracking of Silver Spring, MD 20910, USA. CSS-Dynamac Inc., 10301 Democracy Lane, fish position to within a few meters by deploying many Suite 300, Fairfax, VA 22030, USA. receivers in high density with overlapping detection Acknowledgements range. However, this reduces spatial coverage of a study Several individuals assisted with the deployment, range testing, retrieval, since so many receivers must be placed in a confined and download of the telemetry equipment including L Siceloff, C Cosgrove, area, yields much extraneous information (i.e., constant K Roberson, B Schwartz, A Glahn, and M Kent. We thank T Kelley and NPS for boat support. L Siceloff, J Christensen, and research and development position) for applications where only boundary-crossing staff at VEMCO evaluated an early version of this manuscript and provided data is of interest, and can be cost prohibitive due to suggestions for improvement. Two anonymous peer reviewers provided computational requirements and the large number and constructive suggestions which also greatly improved our manuscript. Funding for this project was from NPS and NOS/NCCOS/CCMA. Government density of receivers that are needed. contract labor (AW ) was provided by CSS-Dynamac, Inc. under NOAA contract Our analyses represent a composite value of perfor- EA-133C-14-NC-1384. mance for 11 receivers in a real landscape. The natu - Competing interests ral variability in landscape features present in the study The authors declare that they have no competing interests. area contributed to the variance in our results (Fig.  3). 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Journal

Animal BiotelemetrySpringer Journals

Published: Jan 11, 2016

References