ICES Journal of Marine Science: Journal du Conseil

Maunder, Mark N.; Sibert, John R.; Fonteneau, Alain; Hampton, John; Kleiber, Pierre; Harley, Shelton J.

doi: 10.1016/j.icesjms.2006.05.008pmid: N/A

Despite being one of the most common pieces of information used in assessing the status of fish stocks, relative abundance indices based on catch per unit effort (cpue) data are notoriously problematic. Raw cpue is seldom proportional to abundance over a whole exploitation history and an entire geographic range, because numerous factors affect catch rates. One of the most commonly applied fisheries analyses is standardization of cpue data to remove the effect of factors that bias cpue as an index of abundance. Even if cpue is standardized appropriately, the resulting index of relative abundance, in isolation, provides limited information for management advice or about the effect of fishing. In addition, cpue data generally cannot provide information needed to assess and manage communities or ecosystems. We discuss some of the problems associated with the use of cpue data and some methods to assess and provide management advice about fish populations that can help overcome these problems, including integrated stock assessment models, management strategy evaluation, and adaptive management. We also discuss the inappropriateness of using cpue data to evaluate the status of communities. We use tuna stocks in the Pacific Ocean as examples.

Litzow, Michael A.

doi: 10.1016/j.icesjms.2006.06.003pmid: N/A

Climate regime shifts have recently occurred in the North Pacific (1998–1999) and the Arctic (2000), but the nature of biological reaction to these events is poorly understood. An index of local climate (1960–2005), and data from commercial fishery catches (1960–2004) and from small-mesh trawl surveys (1972–2005) are used to assess the impacts of these climate events in the Subarctic Gulf of Alaska. Non-linear regression showed that survey catch composition strongly responded to local climate at lags of 2 and 4 years, providing evidence of rapid ecological response to climate change in the system. A sequential regime shift detection method identified rapid change in local climate, and in survey and commercial catches following the well-documented regime shift to a positive state of the Pacific Decadal Oscillation (PDO) in 1976/1977. However, the analysis failed to detect the 1998/1999 regime shift in local climate, or in survey or commercial catches. This result is consistent with the view that the 1998/1999 climate regime shift did not represent a reversion to a negative PDO state. Local temperature increased and local sea level pressure decreased in the Gulf of Alaska during the years 2001–2005, consistent with anthropogenic warming and recent spatial reorganization in Arctic climate. There was no evidence of community reorganization following this climate event. Further observation will be required to evaluate the persistence of this new climate pattern, and the nature of community reaction to it.

Jaffe, Jules S.

doi: 10.1016/j.icesjms.2006.04.024pmid: N/A

Common current practice in fisheries acoustics is to use sound that has been backscattered at 180° in order to infer parameters of individual or aggregations of animals. This article proposes that there is interesting information that can be obtained by processing scatter from other observation angles. Using a simple one-dimensional model of scatter from a fish swimbladder, an expression is derived that predicts the location of the nulls of the scattered sound as a function of transmit angle, observation angle, and tilt. The model was used retrospectively to compute the size of a swimbladder from an existing data set (that of Foote, K. G. 1985. Rather-high-frequency sound scattering by swimbladdered fish. Journal of the Acoustical Society of America, 78: 688–700), with good agreement. In order to pursue the development of a pragmatic collection system, a method is suggested that uses a single transmitter with multiple receivers. The locations of the receivers can be determined using a design methodology that considers bandwidth, centre frequency, and the size of the intended object. The method ensures that a spatially unaliased backscattered waveform can be measured over a specified sampling interval. The technique is illustrated with a practical example that uses a small number of receivers, placed in the backscattered hemisphere.

Gerlotto, François; Bertrand, Sophie; Bez, Nicolas; Gutierrez, Mariano

doi: 10.1016/j.icesjms.2006.04.023pmid: N/A

Most pelagic fish live in schools. To allow fast reactions, for instance to predator attacks, these collective structures require behavioural mechanisms authorizing fast, coordinated movements. Considering the large number of individuals constituting a school of small pelagic fish, a crucial premise to coordinated movements and school reorganization is an ability to transfer quickly and efficiently information across the whole collective structure. We observed anchovy school movements and reactions to sea-lion attacks while the ship was drifting in Peruvian waters. The main process of information transfer we could observe was that of waves of agitation crossing large anchovy schools. The average speed of these waves (7.45 m s−1) was much greater than the average 0.3 m s−1 school speeds measured during this experiment. The internal organization of each school modified dramatically after the waves of agitation had crossed them. Changes in school external morphology and internal structure were described and measured using geostatistics. Our results show that information transfer is a crucial process for the cohesion and plasticity of schools. As such, it allows efficient reactions of schools of pelagic fish to variations in their immediate environment in general, and to predation in particular.

Chandrasekar, Subhashini; Sorial, George A.; Weaver, James W.

doi: 10.1016/j.icesjms.2006.04.019pmid: N/A

When a dispersant is applied to an oil slick, its effectiveness in dispersing the spilled oil depends on factors such as oil properties, wave-mixing energy, temperature, and salinity of the water. Estuaries represent water with varying salinity, so in this study, three salinity values in the range 10–34 psu were investigated, representing potential salinity concentrations found in typical estuaries. Three oils were chosen to represent light refined oil, light crude oil, and medium crude oil. Each was tested at three weathering levels to represent maximum, medium, and zero weathering. Two dispersants were chosen for evaluation. A modified trypsinizing flask termed a baffled flask was used to conduct the experimental runs. A full factorial experiment was conducted for each oil. The interactions between the effects of salinity and three environmental factors, temperature, oil weathering, and mixing energy, on dispersion effectiveness were investigated. Each experiment was replicated four times in order to evaluate the accuracy of the test. Statistical analyses of the experimental data were performed for each of the three oils independently for each dispersant treatment (two dispersants and oil controls). A linear regression model representing the main factors (salinity, temperature, oil weathering, flask speed) and second-order interactions among the factors was fitted to the experimental data. Salinity played an important role in determining the significance of temperature and mixing energy on dispersant effectiveness for almost all the oil–dispersant combinations. The impact of salinity at different weathering was only significant for light crude oil with dispersant A.

Røstad, Anders; Kaartvedt, Stein; Klevjer, Thor A.; Melle, Webjørn

doi: 10.1016/j.icesjms.2006.03.026pmid: N/A

Fish rapidly accumulated below research vessels (RVs) at anchor, kept stationary by dynamic satellite positioning, or freely drifting. This happened by day and night, beneath vessels with different noise levels, in fjords and the open ocean, comprised different species assemblages of fish, and spanned depths of several hundred metres. Acoustic backscatter (fish abundance) increased by more than an order of magnitude in less than an hour. One of the study sites was characterized by much ship traffic, and intermittent, strong decreases in the local fish aggregation beneath the RV were caused by fish swimming towards passing commercial vessels, before returning to the stationary RV. The study suggests more complex relationships between fish, vessels, and noise than previously anticipated. If fish are commonly attracted to vessels, this has implications for fish abundance estimates and basic ecological research.

Aksland, Magnar

doi: 10.1016/j.icesjms.2006.03.027pmid: N/A

Estimators of mean Echo Value Constant (the ratio between echo abundance and the number of fish) in an alternative echo-integrating method were tried with the SIMRAD EK 60 split-beam echosounder. The mean fish-density estimates of NE Arctic cod were compared with corresponding estimates by the classical echo-integration method; the two methods gave similar results. The alternative method uses integrated single-fish echoes, and a new algorithm to extract and integrate single-target echoes is introduced and used. This algorithm uses echo shape and angle stability, not echo amplitude, to test for the presence of single-target echoes. Apparent single-target echoes with a dynamic range of 60 dB in integrated echo intensity were extracted.

Reiss, Henning; Kröncke, Ingrid; Ehrich, Siegfried

doi: 10.1016/j.icesjms.2006.06.001pmid: N/A

The catching efficiency of a standard 2-m beam trawl for sampling epifauna was estimated by removal experiments at two sites in the southern North Sea. In order to allow repeated sampling of the same area, a rig of three beam trawls one behind the other was constructed, the three being tied together by steel ropes. Catching efficiency was estimated on the basis of the sum of the catches of all three trawls relative to the numbers caught in the first trawl. The catching efficiency of the beam trawl for epifauna ranged from 36% to 44% of total abundance between sites. Efficiency was least for partly buried species (Liocarcinus holsatus, 9% and 18%; Buglossidum luteum, 27%; Arnoglossus laterna, 35%), and slightly better for species living on the surface of the sediment (Asterias rubens, 42% and 46%; Pomatoschistus minutus, 58% and 46%; Pagurus bernhardus, 51%). The abundance of epifauna will be underestimated by a factor of 1.4–11 relative to the abundance in the three trawls. Also, sediment type seems to influence the catching efficiency of the beam trawl, efficiency being less at the sandy study site than at the muddy site for most species.

Kang, Myounghee; Honda, Satoshi; Oshima, Tatsuki

doi: 10.1016/j.icesjms.2006.06.007pmid: N/A

The purpose of this study was to investigate the possibility of identifying the age of walleye pollock (Theragra chalcogramma) using acoustic information. Acoustic data targeting walleye pollock were collected at 38 and 120 kHz from 16 June to 12 July 2000 in the Pacific, off Hokkaido, Japan. To complement these data 33 trawl hauls were made and the species and age of the sample fish were accurately examined. The echoes of walleye pollock schools according to age were used to determine the morphological and bathymetric characteristics such as mean height, maximum length, centre depth, seabed depth, and distance from the seabed, as well as the frequency characteristics, this latter being the difference of mean volume backscattering strengths at 38 and 120 kHz, respectively (ΔMVBS). The ΔMVBS method is elaborated using MVBS (mean volume backscattering strength) from an integration cell of optimal size, the cell being examined by means of various integration periods to highlight the characteristics of the walleye pollock schools resulting in 20 pings (120 m), and by applying this method only in a common observation range for two frequencies. The ages of the schools are identified by a combination of morphological and bathymetric characteristics, and ΔMVBS characteristics. Age-0 groups are easy to distinguish from other age groups because they exist in distinct, small schools, are close to the coast, and have a narrow range of ΔMVBS regardless of time of day. Age-1 schools are low in height and very long, are distributed close to the sea floor, and have an ΔMVBS range of −1 to 8 dB, with most between 3 and 5 dB. These characteristics of age-1 schools are distinct from other age groups. As age-2 and age-5 schools have similar maximum length and distribution depth, it is almost impossible to identify these two by just morphological and bathymetric characteristics. However, the ΔMVBS of age-2 and age-5 schools show characteristic patterns that can be used as a means of identification. The pattern of ΔMVBS, which reflects an internal structure (swimming angles) of a school, is different for each age class, and is essential in the identification of the age of a walleye pollock school.

Siira, Antti; Suuronen, Petri; Kreivi, Petri; Erkinaro, Jaakko

doi: 10.1016/j.icesjms.2006.04.020pmid: N/A

We estimated the total size of the Atlantic salmon (Salmo salar) population complex (wild and cultured) in the Gulf of Bothnia, northern Baltic Sea, using a stratified mark-recapture method. In 2001 and 2002, 1970 salmon were captured by the commercial trapnet fishery and tagged with external arrow tags. A total of 349 tagged fish was later recaptured among 65 180 salmon screened for tags. Recoveries were gathered by the commercial trapnets and by screening fish entering counting facilities in rivers and broodstock fisheries at the river mouths. In addition, tags were recovered from recreational river catches. Our estimates suggest that the total size of the migrating population in both years was about 230 000 fish. Proportions of wild and cultured salmon differed between the two years. In 2001, the proportion of wild salmon was 37%; the corresponding figure for 2002 was 62%. Based on estimates of wild salmon smolt production and the number of released smolts, the estimated proportion of cultured smolts that survived the feeding migration and returned to the Gulf of Bothnia (2–4%) was approximately 2.5–4.5 times lower than that of wild smolts (9–10%).