ICES Journal of Marine Science: Journal du Conseil

Hammond, T. R.; Swartzman, G. L.

doi: 10.1006/jmsc.2001.1102pmid: N/A

An algorithm to identify classes of fish in acoustic backscatter images would improve the accuracy of acoustic biomass estimates over manually scrutinized images. A generalized Bayesian procedure for such identification called BASCET is presented, and two implementation strategies for the procedure are compared using simulated acoustic survey data. The procedure has several unusual characteristics: it evaluates schools not individually but in clusters; it makes use of human experience at cluster identification; it presents measures of uncertainty in all estimation results; and it constructs the training set required for supervised learning automatically using spatial and temporal assumptions. The simulation study comparison suggests that making use of temporal and spatial structure in the acoustic data leads to improved estimation performance. On the simulated data, the BASCET algorithm correctly identified the dominant fish class in 15 of 16 cases. However, the simulation model generates acoustic survey data based on the same assumptions used in BASCET, assumptions that may differ from a real acoustic survey. The study also assumed that the human experience incorporated in the Bayesian prior distributions was not misleading. Performance of BASCET on real acoustic data is presented in a companion paper.

Hammond, T. R.; Swartzman, G. L.; Richardson, T. S.

doi: 10.1006/jmsc.2001.1103pmid: N/A

This paper applies BASCET, a Bayesian Spatial Composition Estimation Tool for clusters of acoustically identified schools, to Bering Sea acoustic survey data collected during 1994. As the method employs prior information from an acoustic expert, procedures for eliciting such information are suggested and pitfalls of the process are indicated. Techniques for model checking using the posterior predictive distribution are employed, as is a multi-chain method for evaluating the convergence of the Markov-Chain Monte Carlo algorithm used in BASCET. Unlike methods based on neural networks, BASCET is able to provide confidence regions for its estimates of school cluster composition. In addition, it can indicate which school cluster attributes were most influential in determining a given estimate, a useful tool for model checking that is here demonstrated on a randomly selected cluster. Estimated abundance ratios of juvenile to adult pollock (Theragra chalcogramma) were compared, in two regions, to the values used by expert technicians. Ratios differed from expert values by less than 0.03 in both regions. The encouraging results reported here suggest that the BASCET method, originally tested on simulated data, may be usefully applied to real surveys.

Petitgas, P.; Reid, D.; Carrera, P.; Iglesias, M.; Georgakarakos, S.; Liorzou, B.; Massé, J.

doi: 10.1006/jmsc.2001.1130pmid: N/A

Small pelagic fish are known to aggregate into schools and clusters of schools. It is commonly assumed that the number of such schools and clusters, as well as their size and densities, will vary with the stock abundance. We have carried out a PCA based meta-analysis, using series of acoustic survey data from five different locations in Europe to examine this assumption. The study concluded that there was no discernible relationship between stock abundance and the number of schools seen, or on the clustering of those schools. The study also showed that the number and structure of the school clusters was strongly correlated with the number of schools seen. An increased number of schools in an area tended to be linked with denser clusters (more schools per kilometre) and a higher occupation of the survey area by those clusters. There was also a weaker tendency to find more clusters. It is not clear whether these relationships and the absence of a link to abundance are due to density independence in aggregation patterns or whether such density dependence is only functional at relatively low stock abundance levels.

Reid, D. G.; Maravelias, C. D.

doi: 10.1006/jmsc.2001.1111pmid: N/A

A series of three fine-spatial-scale acoustic studies was carried out to study the relationships between herring-school distribution and the seabed substrate. The study was carried out on three separate bank areas east of Shetland in July 1993. The study areas were characterised respectively as: mainly mud with some hard ground; mixed mud, sand and hard ground; and mostly hard ground. Herring schools were identified from the echo-sounder record. The substrate was mapped using the RoxAnn seabed classification system interfaced to the same echo-sounder. The relationship between herring distribution and substrate was examined at two different levels; map-based, using contour plots of the important variables, and track-based, using individual herring schools in relation to the specific substrate type found on the survey track under the schools. Data are presented which show a strong tendency for schools to be found preferentially over hard seabeds, particularly in the track-based analysis. There also appears to be a strong relationship between herring aggregations and particular topographic features. This is discussed in relation to specific features, a low ridge and two escarpments, identified in two of the study areas.

Orlowski, A.

doi: 10.1006/jmsc.2001.1117pmid: N/A

The diel cycle strongly affects fish behaviour and through its vertical migrations causes significant changes in the physical parameters of fish environment and physiology. There is evidence of the influence of these factors on acoustic fish-back-scattering cross-section values. This variability may be considered potentially as one of most important sources of bias in fisheries acoustics. The methods used and the results of analysing the acoustic, hydrographic, and biological data, collected continuously throughout the day at intervals of one nmi, by research vessels of the Polish Sea Fisheries Institute in the southern Baltic in autumns of the years 1989–1999 are described in this paper. Sa values corresponding to the whole water column (equivalent to column-scattering-strength) were calculated for each elementary sampling distance unit (ESDU) and applied for further analysis as a basic acoustic parameter. The mean depth of the fish biomass gravity centre and corresponding values of temperature and salinity at depths of acoustic “gravity centre” were estimated by computer interpolation of hydrological data for all ESDU. On the basis of numeric models of measurements interactions among environmental and behavioural factors and normalised fish acoustic response have been estimated. The significant influence of fish behaviour and physical factors on the effective value of acoustic back-scattering energy was found and has been described and discussed. The results allowed the identification of more characteristic periods of the fish diel cycle that need further research on fish target strength. Models of the basic components of the fish diel cycle can also be applied for wider ecological studies.

Kalikhman, I.

doi: 10.1006/jmsc.2001.1106pmid: N/A

A mathematical model (Kriging) was used to examine the effects of choosing various units of sampling distance on the adequacy of reconstructing patchy distribution fields. The model simulates fish or plankton patches (or gaps) of different shapes and spatial orientations, and an acoustic survey by parallel transects along which a unit of sampling distance is set. Conformity of the reconstructed fields to those originally generated was evaluated by calculating their correlations. If a priori information on patch orientation is available, the optimal ratios of the distance between transects and the sampling distance unit to the corresponding autocorrelation radius for the field have been determined, ensuring the required match between the reconstructed field and its original. The experiments with a certain unit of sampling distance confirm that a posteriori determination of patch orientation allows reconstruction of the best field attainable on the basis of the data from the survey conducted. In cases of field movement, the criterion for choosing a survey direction is based on the relationship between the dimension of moving patches in the direction of movement and that of the surveyed area; the criterion remains valid when a unit of sampling distance is set along transects.

Griffiths, Gwyn; Enoch, Peter; Millard, Nicholas W.

doi: 10.1006/jmsc.2001.1120pmid: N/A

Autonomous Underwater Vehicles (AUVs) can be very quiet platforms from which to make observations of marine living resources. With few moving parts and very low propulsion power they should have a very low underwater radiated noise signature. The purpose of this paper is to describe measurements on Autosub to test this assumption. The measured noise level and spectrum are compared to the ICES proposal for the radiated noise of fisheries research vessels. Integrated over the band 100 Hz to 5000 Hz the AUV source level, at 124 dB relative to 1 μPa at 1 m, was about 39 dB below of the ICES specification and about 30 dB quieter than the noise-reduced vessel “Scotia”. Autosub is in fact exceedingly quiet. The difficulties of making measurements of the underwater radiated noise of a quiet submerged vehicle are discussed and suggestions are made for improvements in techniques.

Nishibori, Naoyoshi; Nishii, Akinori; Takayama, Haruyoshi

doi: 10.1006/jmsc.2001.1115pmid: N/A

Polyamines are widely distributed in living organisms and are known to be essential elements for normal growth and development. Although they may affect phytoplankton growth their existence and concentration in seawater is unclear. In this paper the polyamine concentrations in coastal seawater are measured by high performance liquid chromatography (HPLC) using cation exchange resin and OPA reagent. This method seems sufficient for their detection in seawater. Putrescine and spermidine were found to be the major polyamines in the coastal seawater, though their respective concentrations varied widely. In addition to them cadaverine, norspermidine, norspermine, and spermine were also detected.

Bulgakova, Tatiana; Vasilyev, Dmitri; Daan, Niels

doi: 10.1006/jmsc.2001.1107pmid: N/A

Multispecies Virtual Population Analysis (MSVPA) is based on parameterization of the average relative food compositions for all possible prey-age predator-age combinations in the model by year and quarter. This sets high demands on stomach sampling programmes in terms of spatial coverage of the predator population and of sampling intensity for individual cohorts. In practice, there are many sources of error in the input data and large variances, which call for a smoothing procedure to avoid outliers in the MSVPA output. We investigate the potential of geostatistics (specifically kriging) in improving (1) estimates of total and partial stomach content weights from spatially non-uniformly distributed samples and (2) smoothing of the average stomach content weights over the two-dimensional input array of predator age and years. The examples shown indicate that kriging provides an efficient method to deal with geographical variability in food composition and predator abundance as well as to fill gaps and make extrapolations within a two-dimensional array in temporal space characterized by many empty cells or cells not sufficiently well sampled.

Rueda, Mario; Defeo, Omar

doi: 10.1006/jmsc.2001.1122pmid: N/A

We estimated the spatial population structure and biomass of Eugerres plumieri, Mugil incilis, and Cathorops spixii in a tropical estuarine lagoon in Colombia, based on survey data carried out seasonally in 1993–1994 and 1997. Geostatistical techniques and the swept area method were used to map and estimate fish biomass, whereas uncertainty in spawning biomass was estimated by Monte Carlo analysis to assess the status of the fishery. Biomass tended to be spatially autocorrelated and populations were distributed in high-biomass patches of 2–15 km in diameter. The spatial dependence was variable among species and seasons. High-biomass patches did not overlap in space between species, which could be viewed as a way of reducing or avoiding potential interactions. However, the occurrence of highest biomass for E. plumieri and M. incilis during the same season might be directed to overcome the effect of a widely fluctuating environment such as occurs in estuarine lagoons. Selectivity experiments were performed to account for the effect of fishing gear vulnerability in biomass estimates. Application of the encounter probability model showed increasing probabilities of capture with individual size and differential avoidance behaviour among species. Risk analysis, used for testing a biological reference point defined in terms of harvesting and spawning biomass, suggested the need to take immediate management actions for E. plumieri.