Showing papers in "Ices Journal of Marine Science in 2003"

An overview of seabed-mapping technologies in the context of marine habitat classification

Abstract: A wide range of seabed-mapping technologies is reviewed in respect to their effectiveness in discriminating benthic habitats at different spatial scales. Of the seabed attributes considered important in controlling the benthic community of marine sands and gravel, sediment grain size, porosity or shear strength, and sediment dynamics were highlighted as the most important. Whilst no one mapping system can quantify all these attributes at the same time, some may be estimated by skilful interpretation of the remotely sensed data. For example, seabed processes or features, such as bedform migration, scour, slope failure, and gas venting are readily detectable by many of the mapping systems, and these characteristics in turn can be used to assist a habitat classification (and monitoring) of the seabed. We tabulate the relationship between “rapid” continental shelf sedimentological processes, the seabed attributes affecting these processes, and the most suitable mapping system to employ for their detection at different spatial scales.

Phytoplankton trends in the Baltic Sea

Abstract: Wasmund, N., and Uhlig, S. 2003. Phytoplankton trends in the Baltic Sea. – ICES Journalof Marine Science, 60: 177–186.Monitoring data of phytoplankton abundance and biomass (1979–1999) and Chl. a (1979–2000) from surface samples (0–10m) of the Kattegat, Belt Sea and Baltic proper wereinvestigated for long-term trends. The Mann–Kendall test as well as the LOESS smootherwas applied for three taxonomic groups in spring, summer and autumn separately. Chl. atrends were analysed by linear regression. Downward trends were found for diatoms inspring and summer whereas dinoflagellates generally increased in the Baltic proper butdecreased in the Kattegat. In autumn, diatoms increased at some stations. For cyano-bacteria, downward trends were detected mainly in the Kattegat/Belt Sea area. Chl. aconcentrations showed a general decrease in the Kattegat/Belt Sea area but an increase inthe Baltic proper. Observed changes in trends during the two decades might indicate shiftsin the ecosystem.

Impact of a large-scale area closure on patterns of fishing disturbance and the consequences for benthic communities

Abstract: Seasonal area closures of fisheries are primarily used to reduce fishing mortality on target species. In the absence of effort controls, fishing vessels displaced from a closed area will impact fish populations and the environment elsewhere. Based on the observed response of the North Sea beam trawl fleet to the closure of the ‘‘cod box’’ and an existing size-based model of the impacts of beam trawling, we predict the effects of seasonal area closures on benthic communities in the central North Sea. We suggest that repeated seasonal area closures would lead to a slightly more homogeneous distribution of annual trawling activity, although the distribution would remain patchy rather than random. The increased homogeneity, coupled with the displacement of trawling activity to previously unfished areas, is predicted to have slightly greater cumulative impacts on total benthic invertebrate production and lead to localized reductions in benthic biomass for several years. To ensure the effective integration of fisheries and environmental management, the wider consequences of fishery management actions should be considered a priori. Thus, when seasonal closures increase the homogeneity of overall disturbance or lead to the redistribution of trawling activity to environmentally sensitive or previously unfished areas, then effort reductions or permanent area closures should be considered as a management option. The latter would lead to a single but permanent redistribution of fishing disturbance, with lower cumulative impacts on benthic communities in the long run. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights

Synthetic echograms generated from the relative frequency response

Abstract: Korneliussen, R. J., and Ona, E. 2003. Synthetic echograms generated from the relativefrequency response. – ICES Journal of Marine Science, 60:636–640.Calibrated, digitized data from multi-frequency echo sounders have been used to generatenew, synthetic echograms. The relative frequency response measured at four acousticfrequencies (18, 38, 120, and 200kHz) is the main acoustic feature used to characterize theacoustic targets. Synthetic echograms are used to enhance and colour-code sample volumeswith similar acoustic properties. The method is invaluable during scrutinizing, particularlyin areas with many types of target. Several broad acoustic categories can be clearlydistinguished, and detailed examples based on different fish species and zooplankton aredemonstrated. The limitations of the method are described and discussed.

An ecosystem model of food web and fisheries interactions in the Baltic Sea

Abstract: Because fisheries operate within a complex array of species interactions, scientists increasingly recommend multispecies approaches to fisheries management. We created a food web model for the Baltic Sea proper, using the Ecopath with Ecosim software, to evaluate interactions between fisheries and the food web from 1974 to 2000. The model was based largely on values generated by multispecies virtual population analysis (MSVPA). Ecosim outputs closely reproduced MSVPA biomass estimates and catch data for sprat (Sprattus sprattus), herring (Clupea harengus), and cod (Gadus morhua), but only after making adjustments to cod recruitment, to vulnerability to predation of specific species, and to foraging times. Among the necessary adjustments were divergent trophic relationships between cod and clupeids: cod exhibited top-down control on sprat biomass, but had little influence on herring. Fishing, the chief source of mortality for cod and herring, and cod reproduction, as driven by oceanographic conditions as well as unexplained variability, were also key structuring forces. The model generated many hypotheses about relationships between key biota in the Baltic Sea food web and may ultimately provide a basis for estimating community responses to management actions.

An expanded target-strength relationship for herring

Abstract: Target-strength (TS) experiments on captive, adult herring have been conducted at intervals over several years to investigate the acoustic effect of pressure and seasonal changes on fish physiology. Experiments in a large, net pen (4500 m 3 ) were done at 18, 38, and 120 kHz frequencies with calibrated, split-beam echosounders. The main ex situ results at 38 kHz, including a vertical-excursion experiment, were combined with in situ TS data collected at 38 kHz with a probing, split-beam transducer lowered into the dense herring layers, recorded during two surveys in the wintering area of the Norwegian spring-spawning stock. Multiple-linear regression analysis was used to investigate the functional relationship between TS and the measured parameters. The mean TS of herring was found to be significantly dependent on the depth (pressure) and the gonadosomatic index. These are the additional parameters included in the new TS relationship.

Morphometric variation among sardine (Sardina pilchardus) populations from the northeastern Atlantic and the western Mediterranean

Abstract: During the past decade, regional changes in the dynamics of the Atlanto-Iberian stock of sardine, and its exploitation by Portuguese and Spanish purse-seine fisheries, have increased the uncertainties in estimated trends of spawning biomass, stock abundance, and fishing mortality. Together with recent evidence for lack of discontinuities in the distribution of sardine eggs at the edges of the stock area, this casts doubts on the hypothesis that the stock is a panmictic, closed population. Sardine morphometric data (truss variables and landmark data) from 14 samples spanning the northeastern Atlantic and the western Mediterranean were analysed by multivariate and geometric methods. The analyses explored the homogeneity of sardine shape within the area studied, as well as its relation to that of adjacent and distant populations (Azores and northwestern Mediterranean). Principal components analysis on size-corrected truss variables and cluster analysis of mean fish shape using landmark data indicate that the shape of sardine off southern Iberia and Morocco is distinct from the shape of sardine in the rest of the area. The two groups of sardine are significantly separated by discriminant analysis, and their validity was confirmed by large percentages of correct classifications of test fish (87 and 86% of fish from the test sample were correctly classified into each group, respectively). There was also some evidence that fish from the western Mediterranean and the Azores form a separate morphometric group. These results question both the homogeneity within the AtlantoIberian sardine stock and the validity of its current boundaries.

A fisheries application of a dual-frequency identification sonar acoustic camera

Abstract: The uses of an acoustic camera in fish passage research at hydropower facilities are being explored by the U.S. Army Corps of Engineers. The Dual-Frequency Identification Sonar (DIDSON) is a high-resolution imaging sonar that obtains near video-quality images for the identification of objects underwater. Developed originally for the Navy by the University of Washington?s Applied Physics Laboratory, it bridges the gap between existing fisheries assessment sonar and optical systems. Traditional fisheries assessment sonars detect targets at long ranges but cannot record the shape of targets. The images within 12 m of this acoustic camera are so clear that one can see fish undulating as they swim and can tell the head from the tail in otherwise zero-visibility water. In the 1.8 MHz high-frequency mode, this system is composed of 96 beams over a 29-degree field of view. This high resolution and a fast frame rate allow the acoustic camera to produce near video-quality images of objects through time. This technology redefines many of the traditional limitations of sonar for fisheries and aquatic ecology. Images can be taken of fish in confined spaces, close to structural or surface boundaries, and in the presence of entrained air. The targets themselves can bemore » visualized in real time. The DIDSON can be used where conventional underwater cameras would be limited in sampling range to < 1 m by low light levels and high turbidity, and where traditional sonar would be limited by the confined sample volume. Results of recent testing at The Dalles Dam, on the lower Columbia River in Oregon, USA, are shown.« less

The pelagic foodweb in the upwelling ecosystem of Galicia (NW Spain) during spring: natural abundance of stable carbon and nitrogen isotopes

Abstract: The structure of the pelagic foodweb in the upwelling ecosystem of Galicia (NW Spain) was studied by means of the enrichment in 13 Ca nd 15 N isotopes in plankton (four size classes from 20 to >1000 m), sardine (primary consumer) and dolphin (top consumer). Samples were collected in March 1998, during the spawning period of sardines at the beginning of the upwelling season. Isotopic enrichment across plankton size classes was lower than either enrichment between plankton and sardine or enrichment between sardine and dolphin. This difference may be attributed to the presence of chain-forming phytoplankton in some plankton size classes but also to a large degree of omnivory among zooplankton. Also, there was a size-dependent effect on 15 N enrichment of sardine. Sardines <18 cm had a variable isotopic enrichment but generally higher than those of larger sardines, revealing omnivorous but mostly zoophagous diets. In contrast, sardines 18 cm showed a linear decrease of 15 N enrichment with length, as a consequence of the increase in the efficiency of filter-feeding and in the amount of phytoplankton ingested. According to 15 N enrichment, we concluded that probably there are no more than four trophic levels in the studied ecosystem. Nevertheless, our results revealed that the pelagic food web in this upwelling area is more complex than expected due to the generalized omnivory in all organisms. Furthermore, the obtained relationships between 15 N and the size of organisms provide the basis for a quantitative analysis of changes in the trophic structure of this ecosystem.

A simulation study of the implications of age-reading errors for stock assessment and management advice

Abstract: Catch-at-age data are a crucial input to many stock assessments, so errors in age determination could have an adverse effect on the quality of the stock assessment and the scientific advice based on that assessment. The results of simulation studies presented in this study are intended to quantify the effects of age-reading errors on the perception of stock trends and short-term management advice. The study is based on Eastern Baltic cod, in which problems with consistent interpretation of otolith structures result in the catch-at-age data being particularly problematic. The results indicate a clear distinction between the performance of the assessment, and the performance of catch forecasts and advice based on that assessment. The ageing error affected the absolute level of estimates of fishing mortality and spawning stock biomass from stock assessments, although overall trends are similar, and general conclusions about the state of the stock are likely to be broadly correct. Greater problems arose in catch forecasts and advice, for which ageing error led to discrepancies between the required and the effective fishing mortality, and a general tendency for ageing error to lead to advice on Total Allowable Catch that would be too optimistic and, therefore, less effective for stock conservation.

Age estimation, growth and maturity of the European hake (Merluccius merluccius (Linnaeus, 1758)) from Iberian Atlantic waters

Abstract: Difficulties in age estimation for hake (Merluccius merluccius) have hampered the assessment of stocks. Here, we describe new, agreed ageing criteria based on the interpretation of the pattern of otolith growth. Improved estimates of von Bertalanffy growth parameters, and new estimates of maturity ogive parameters and length–weight relationships for European hake from Iberian Atlantic waters are presented. The results came from a study carried out during 1996–1997 and provide the first published account of the main life history traits of Southern stock hake. von Bertalanffy growth parameters of males were L1 ¼ 70 cm, K ¼ 0.18 year � 1 , and t0 ¼� 0.97 year, and those of females were L1 ¼ 89 cm, K ¼ 0.13 year � 1 , and t0 ¼� 1.15 year. Growth of sexes differed from age 3 onwards, with females being on average larger and heavier than males. The estimated total length (L, cm)–total weight (W, g) relationships were W ¼ 0:0132135L 2:8134246 for males and W ¼ 0:0086471L 2:942563 for females. Spawning took place from December to May with a peak in February. The mean length and age at first maturity were 32.8 cm at 2.5 years for males and 45 cm at 4.4 years for females. Application of new ageing criteria showed that otolith sections may be used to determine ages up to 5 years in a consistent manner. These results indicate that hake of the Southern stock grow at higher rates and mature earlier than previously considered. Summaries of hake’s life history parameters from other marine regions are also presented in order to make information that belongs largely to the grey literature available.

Sustained exponential population growth of grey seals at Sable Island, Nova Scotia

Abstract: Grey seal pup production on Sable Island, Nova Scotia, has been monitored since the early 1960s. We estimated pup production on Sable Island in 1997 using aerial photography with a correction for detection of pups on the imagery and a statistical model to account for the proportion of pups not born at the time of the survey. A total of 20 863 pups were counted on the colour positives. When corrected for the proportion of pups seen on the imagery (0.96) and the proportion of pups born before the survey (east colony 0.94, west colony 0.66), estimated total pup production was 25 400 with approximate 95% confidence limits of 23 500 and 26 900. The 1997 estimate indicates that pup production in this population, now the largest grey seal colony in the world, has been increasing exponentially at an annual rate of 12.8% for four decades in the face of considerable environmental variability.

Development of wild Atlantic salmon stocks in the rivers of the northern Baltic Sea in response to management measures

Abstract: Since 1980, the abundance of wild Atlantic salmon has been monitored by means of catch records, adult counts, electrofishing and smolt trapping in six rivers flowing into the northern Baltic Sea. River abundance (spawners, parr and smolts) was compared with implemented large-scale and river-specific management measures and with natural factors potentially affecting abundance. Since the 1980s, the wild stocks have recovered in a synchronous cyclical pattern. The recovery occurred mainly in two jumps, first a sudden increase dating back to around 1990 and a second sharp rise in the late 1990s. River abundance of young salmon commonly rose about 10-fold and approached the previously estimated production capacity in some of the rivers. This positive development may be explained by a decline in fishing pressure together with covarying natural factors influencing survival and growth. The offshore fishery started to decline at the time of the first increase, while the reduction in the total allowable catches together with seasonal restrictions on the coastal fishery strengthened the second increase. Improved natural conditions seem to have increased both survival and escapement during the first rise. Spawners producing the second rise were the offspring of the spawners of the first rise. The outbreak of the M74 mortality syndrome among alevins reduced the abundance of several year-classes that hatched during the first half of the 1990s. In most rivers, the fraction of older and female fish in the spawning run has increased over the period, thereby increasing the reproductive capacity of the populations. No distinct effects of variations in river-specific management regimes were observed. Instead, the results emphasize the role of fisheries management in the open sea as well as in coastal waters, and also of non-human factors in controlling overall abundance of wild salmon in northern Baltic rivers.

Demersal assemblages and depth distribution of elasmobranchs from the continental shelf and slope off the Balearic Islands (western Mediterranean)

Abstract: The analysis of 131 hauls from four bottom trawl fishing surveys carried out between depths of 46 and 1713 m in two different areas off the Balearic Islands yielded a total of 23 elasmobranch species belonging to eight families. Cluster analysis and multidimensional scaling (MDS) ordination were applied to detect zonation patterns and some ecological parameters (e.g. species richness, abundance and biomass, mean weight, diversity and evenness) were calculated for each assemblage. For each area, analysis of similitude (ANOSIM) and similarity percentage analysis (SIMPER) were also applied to detect differences between seasons and depths. For the most important species (Galeus melastomus, Scyliorhinus canicula, Centroscymnus coelolepis, Etmopterus spinax, Squalus blainvillei, Raja naevus, Raja asterias, Raja clavata, Raja miraletus and Raja oxyrhinchus), abundance and size distributions were analysed by depth.

Acoustic characterization of gelatinous plankton aggregations: four case studies from the Argentine continental shelf

Abstract: During routine acoustic surveys for the assessment of fish abundance in the Argentine Sea, large-scale, plankton-like scattering layers covering thousands of square nautical miles are commonly observed. Net sampling revealed that many of these scattering layers comprised gelatinous zooplankters aggregated in dense concentrations in the main. Even though echoes from gelatinous zooplankton are expected to be weak, because of the low reflectivity of their bodies, dense aggregations are capable of producing sound-scattering levels high enough to mask even the overlapping echoes from swimbladdered fish. The objective of this study is to relate the aggregations of four gelatinous species identified by means of nets to the presence of sound-scattering layers. Selected sections of echo recordings from aggregations of Lychnorhiza lucerna (Scyphozoa), Iasis zonaria (Salpidae), Mnemiopsis leidyi (Ctenophora), and Aequorea sp. (Hydrozoa) were obtained at 38 or 120 kHz, at different locations along the Argentine shelf. Some features of the spatial distributions of the aggregations are described. The feasibility of remote detection for different gelatinous groups is of great importance considering the impact that blooms of these organisms could have on some particularly sensitive ecosystems (e.g. fish spawning and nursery grounds). The characterization of specific aggregation and behavioural patterns will allow the mapping ofjellyfish distributions by the analysis of previous cruise databases. This methodology will provide a baseline for the study of spatial and temporal changes and trends in their abundance.

Spring–summer climatological circulation in the upper layer in the region of Cape St. Vincent, Southwest Portugal

Abstract: Geostrophic transport and hydrographic measurements derived from a historical database (1900–1998) were used to study the spring–summer mean circulation in the upper layer south and west of Cape St. Vincent, Southwest (SW) Portugal. The larger-scale circulation scheme is forced by equatorward winds from May to September, when the Iberian coastal transition zone (CTZ) is dominated by a generalized upwelling of cold, low-salinity water. A partially separated surface jet intensified at the shelf break conveys � 1 Sv of upwelled water equatorward parallel to the bathymetry, while offshore a poleward flow transports � 0.4–0.6 Sv of upwelled water. Although alongshore transports dominate the circulation pattern of the upper layers, cross-shore transports are significant at the climatological scale. Anticyclonic circulation with an exchange of � 0.5 Sv from the equatorward jet to the offshore poleward flow and the partial re-circulation further north, back into the equatorward flow are discussed. A coherent, cyclonic re-circulation pattern inshore of the upwelling jet is also speculated. From these results the shelf break is considered a climatological border at both sides of which two major re-circulation cells occur. The climatological equatorward flow has offshore protrusions, interpreted as recurrent episodes of major contortions of the upwelling flow. These features bring about considerable ‘‘crossshelf flow’’ re-circulation reaching up to 50% of the main flow. The most significant exchanges are found to be associated with major changes of orientation of the coastline. Off Cape St. Vincent the upwelling front stretches to both west and south and contributes to the cross-shelf re-circulations. Additionally, convergence of the upwelling flow and a branch of the Azores current, with associated re-circulation is found diagonally from the cape. On the southern coast the upwelling jet is seen to meander offshore in the vicinity of Cape St. Maria. Individual synoptic cruise data showed agreement with the climatological circulation features. We conclude that these oceanographic features leave an imprint on the climatic circulation in spite of the ‘‘smoothing out’’ of recurrent events over the spring– summer period of the years of 1900–1998.

Spatial and temporal patterns of cuttlefish (Sepia officinalis) abundance and environmental influences – a case study using trawl fishery data in French Atlantic coastal, English Channel, and adjacent waters

Abstract: The spatial and temporal distribution patterns of cuttlefish abundance and the relationships between cuttlefish abundance and environmental variables in the French Atlantic coast, the English Channel, and adjacent waters were studied using both geographical information system and statistical methods. Cuttlefish have a clear general annual migration pattern, consistently occurring in broadly the same areas in different years. The strength of the Atlantic currents into the west part of the English Channel and the south part of the Celtic Sea may be the dominant influence on the timing of cuttlefish migration to these areas. Local abundance shows a positive correlation with SST, although it is difficult to determine if this reflects any causal link. Cuttlefish expand their distribution further north in the spawning season in warm years and shift south in cool years. The centre of high abundance in offshore deep water shifts north in warm winters and south in cool winters.

Spatial and temporal aspects of the decline in cod (Gadus morhua L.) abundance in the Kattegat and eastern Skagerrak

Abstract: Svedang, H., and Bardon, G. 2003. Spatial and temporal aspects of the decline in cod (Gadus morhua L.) abundance in the Kattegat and eastern Skagerrak. - ICES Journal of Marine Science, 60: 32-37. We explored the temporal and spatial evolution of cod abundance along the Swedish west coast during the past 20-40 years. Analyses of cpue data from the Swedish Nephrops fishery revealed a persistent decline in offshore abundance since the beginning of the 1980s in both the Skagerrak and Kattegat, culminating in extremely low densities in the late 1990s. Owing to a lack of reliable logbook and survey data for the area inside the trawling limit, informal statistics such as records of sport fishing clubs and notes on catches made by a coastal fishermen were used to sketch the development of cod abundance in the coastal zone. These data indicate that the decline in catches of inshore cod preceded the one in offshore areas at the end of the 1970s. The information presented indicates that the depletion of the cod stock in these areas is much worse than suggested by routine annual assessments. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.

Autonomous underwater vehicles: future platforms for fisheries acoustics

Abstract: Autonomous underwater vehicles (AUVs) are unmanned submersibles that can be pre-programmed to navigate in three dimensions under water. The technological advances required for reliable deployment, mission control, performance, and recovery of AUVs have developed considerably over the past 10 years. Currently, there are several vehicles operating successfully in the offshore industries as well as in the applied and academic oceanographic sciences. This article reviews the application of AUVs to fisheries- and plankton-acoustics research. Specifications of the main AUVs currently in operation are given. Compared to traditional platforms for acoustic instruments, AUVs can sample previously impenetrable environments such as the sea surface, the deep sea, and under-sea ice. Furthermore, AUVs are typically small, quiet, and have the potential to operate at low cost and be unconstrained by the vagaries of weather. Examples of how these traits may be utilized in fisheries-acoustics science are given with reference to previous work in the North Sea and Southern Ocean and to potential future applications. Concurrent advances in multi-beam sonar technology and species identification, using multi-frequency and broadband sonars, will further enhance the utility of AUVs for fisheries acoustics. However, before many of the more prospective applications can be accomplished, advances in power-source technology are required to increase the range of operation. The paper ends by considering developments that may turn AUVs from objects sometimes perceived as science fiction into instruments used routinely to gather scientific facts

Origin and migration of wild and escaped farmed Atlantic salmon, Salmo salar L., in oceanic areas north of the Faroe Islands

Abstract: We examined the distribution, migration and origin of wild and escaped farmed Atlantic salmon, Salmo salar L., in the northeast Atlantic ocean north of the Faroe Islands based on individual tagging of salmon in this area. Recoveries of wild salmon were reported from homewaters in nine north Atlantic countries, and in a number of different rivers throughout the distribution range of Atlantic salmon. Most tags were recovered in Norway, but relatively large numbers of returns were observed in Scotland and Ireland as well. No fish were recaptured at Faroes. Fish tagged in the autumn tended to return to areas closer to the tagging site than fish tagged in the winter. This strongly suggests that salmon originating from most areas of the distribution range are at some life stage present in this area, but in variable proportions at different times. Most of the salmon returned home to spawn the next autumn, and the fish that stayed for another year originated from northern areas of Europe. All recoveries of farmed salmon were in Norway except one on the west coast of Sweden, suggesting that they could have escaped mainly from Norwegian fish farms. Assessment of the proportion of wild salmon from different countries present north of the Faroe Islands revealed that 40% of the fish were of Norwegian origin, and Scotland and Russia accounted for about 20% each. Four tags of wild fish were reported from Canada, all in the same year they were tagged. This demonstrates that adult Atlantic salmon can cross the north Atlantic ocean in less than 6 months. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.

Age determination, validation and growth of Grand Bank yellowtail flounder (Limanda ferruginea)

Abstract: Yellowtail flounder (Limanda ferruginea) (Storer, 1839) on the Grand Bank off Newfoundland were traditionally aged using surface-read whole otoliths. Age determination of otoliths from recaptures of fish tagged in the early 1990s indicated that the traditional ageing technique was underestimating the ages of yellowtail flounder when compared with the time at liberty. Age comparisons between whole and thin-sectioned otoliths showed agreement in age readings up to 7 years; thereafter whole otoliths tended to give much lower ages than those estimated by thin sections. Length-frequency analysis of pelagic and demersal juveniles, captive rearing of juveniles and marginal increment analysis all corroborated age determination based on thin sections. Tag-recaptures and bomb radiocarbon assays validated age interpretations based on thin sections in young and old yellowtail flounder, respectively. Ages were validated up to 25 years for females and 21 years for males. However, because of increased narrowing of annuli in thin-sectioned otoliths from old fish, even thin sections may underestimate the true age of the fish. von Bertalanffy growth curve parameters (combined sexes) were L ∞ = 55.6 cm total length, K=0.16 and t 0 =−0.003. These results challenge the conventional view that yellowtail flounder on the Grand Banks are a relatively fast growing, short-lived species.

Cod in fjords and coastal waters of North Norway: distribution and variation in length and maturity at age

Abstract: The distribution of cod along the Norwegian coast and in fjords from 62 � N north to the Russian border was examined using data from annual trawl surveys carried out between 1995 and 2001. Based on differences in growth zones of the otoliths, cod are traditionally classified into two types: Northeast Arctic cod and coastal cod. Both types were found throughout the area investigated. The catch rate of both increased northwards and from offshore to inshore. In a statistical model of length at age, abiotic factors such as area and year of capture explained more of the variance than biotic factors such as sex, stage of maturity, and type of cod. Length at age increased in a southward direction and was higher for cod captured offshore than for those captured inshore. In a statistical model of the proportion mature at age, area, type, and year of capture explained more of the variance than sex and depth of capture. On average, coastal cod attained 50% maturity (M50) more than a year younger than Northeast Arctic cod. Although there were relatively large differences in age at maturity between neighbouring areas, age at maturity was lowest in the south and inshore, and in general, lower inshore than offshore. As genetic analysis clearly indicates that cod in the study area consist of at least two genetically separated stocks, it is likely that the differences found here in age at M50 might have a genetic component.

Ocean thermal conditions in the post-smolt nursery of North American Atlantic salmon

Abstract: The effect of climate on the post-smolt survival of North American Atlantic salmon is obscure owing to the difficulty in interpreting the only relationships thus far observed between the abundance of these stocks and climate, which focuses on winter conditions. Placing significance on winter post-smolt survival is contrary to conventional thinking that the spring period is more important, because that is when the post-smolts migrate to sea and transition to ocean life takes place. The pre-fishery abundance for North American stocks was compared to thermal conditions in potential post-smolt nursery areas during the period 1982-1999. Pre-fishery abundance was modeled as a reconstruction of one-sea-winter (1SW) and two-sea-winter (2SW) age salmon populations. Cohort abundance was compared to mean temperature and thermal habitat (sea surface area within a given temperature range) in five index areas. Stock size was negatively correlated with mean sea surface temperature during June. Correlations were comparatively stronger between stock abundance and thermal habitat, further asserting that June conditions - the first month at sea for most stocks in the region - may be pivotal to survival. These correlations suggest that post-smolt survival is negatively affected by the early arrival of warm ocean conditions in the nursery area. Hypotheses related to post-smolt migration, predation, and the availability of suitable prey are discussed.

Macrobenthic resources of the shallow soft-bottom sediments in the eastern English Channel and southern North Sea

Abstract: To obtain a baseline for future comparisons and to assist in the conservation of marine biodiversity, the distribution patterns and faunal composition of shallow (0–15 m depth) macrobenthic assemblages were studied along the French coast of the eastern English Channel and southern North Sea from two surveys conducted in 1998 and 2000. A total of 227 sites were sampled from Cap d’Ailly to the Belgian border, from which a total of 167 species were collected. Species richness, abundance and biomass were all increased in the vicinity of outflows from harbours, major bays and estuaries. Three principal macrobenthic assemblages were defined: (i) an Ophelia borealis medium to fine sand assemblage; (ii) a muddy heterogeneous sediment assemblage; and (iii) an Abra alba muddy fine sand assemblage. The Abra alba assemblage covered approximately 80% of the seabed in the survey area. Sediment characteristics and a latitudinal gradient accounted for a significant proportion of the observed variability in assemblage distribution patterns. In the eastern English Channel, the distribution patterns of species diversity, abundance and biomass values were most continuous, whereas in the southern North Sea a more heterogenous distribution was observed. 2003 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.

Managing marine genetic diversity: time for action?

Abstract: The 1992 Convention on Biological Diversity has established an international framework for broader conservation objectives for the management of ocean use activities (cf. Sainsbury and Sumaila, 2003). The Convention calls for preservation of biological diversity, including genetic, species and ecosystem diversity, thereby creating a demand for developing management forms that can cope with this issue. A number of initiatives have been developed to address this and other related international agreements. Notably, Ecosystem-Based Fishery Management (EBFM; cf. Brodziak and Link, 2002; Sainsbury and Sumaila, 2003) has emerged as a holistic approach to maintaining ecosystems and sustainable fisheries. EBFM should incorporate all levels of diversity, but in practice has focused on species and ecosystem diversity (e.g., Brodziak and Link, 2002). The application of EBFM must be broadened to include conservation of genetic diversity, including intraspecific diversity, which are not necessarily protected by maintaining diversity at higher levels (cf. Kenchington, 2003). Scientific justification for conserving genetic diversity stems from several sources including: (1) maintaining adaptability of natural populations; (2) the future utility of genetic resources for medical and other purposes; and (3) changes in life history traits and behaviour that influence the dynamics of fish populations, energy flows in the ecosystem, and ultimately, sustainable yield. The challenge is to formulate appropriate management actions for the preservation of genetic diversity (e.g., Sainsbury and Sumaila, 2003). This will require consensus on what it is we are trying to preserve (e.g., alleles, traits, population structure) and some means of assessing genetic status. This paper endeavours to outline a process to develop management advice for marine genetic diversity. We challenge fish conservation geneticists to consider their work in a more applied context so that management actions can be developed to preserve genetic resources.

Standardization of bluefin tuna, Thunnus thynnus, catch per unit effort in the baitboat fishery of the Bay of Biscay (Eastern Atlantic)

Abstract: Relative indices of abundance by age class for bluefin tuna from 1975 to 2000 were estimated using generalized linear mixed models. Age class was included as a fixed factor within the model specifications, with the Year×Age interaction as a fixed factor component in order to obtain annual indices by age. Catch and effort data on bluefin tuna were available from two sources, catches by trip, and daily catches from logbooks. Catches were modelled using the delta-lognormal model. The model finally selected included the following explanatory factors: Year, Age, Year×Age, Month, number of Crew, number of Bait Tanks, and with the Year×Month interaction as a random component. Overall, year trends and coefficients of variation were similar from both datasets by age class. In general, the standardization procedure showed that vessel characteristics and technological advances related to fishing have a relatively minor explanatory effect on the observed catch rates of bluefin tuna in the fishery. Neither geographical distribution nor type of bait had an explanatory effect on the observed catch rates. The 1994 cohort stands out as an exceptionally large year class; it can be followed through the standardized series. This standardized catch rate index at age of bluefin tuna is the most complete yet available and is the only one currently used for juvenile fish in calibrating population models to evaluate the Eastern Atlantic and Mediterranean stock. It is particularly valuable because, apart from improvements in electronic technology, there are no changes in the method of exploitation during the entire series.

A pilot tagging experiment on European hake (Merluccius merluccius): methodology and preliminary results

Abstract: European hake (Merluccius merluccius) were caught alive in the northern Bay of Biscay in June/July 2002, using a codend specially designed to avoid crushing fish and also to retain water while hauling the trawl. In all, 1307 fish were tagged with anchor T-bar tags, injected with tetracycline, then released. The length range of the tagged and marked fish varied from 13 to 58 cm, and the modal size was 28 cm, at which length they were assumed to be 2-3 years old. A mean survival rate of 68.2% was estimated. Mortality was mainly caused by stress of capture and physical damage, and depended on the size of the catch and the depth of trawling. By the end of March 2003, 32 fish and three tags had been returned to the laboratory (a 2.7% return rate), the recapture data indicating that the probability of survival after release does not depend on catch depth or depth at location of release. For combined sexes, the somatic growth rate was estimated at 0.033 ± 0.019 cm day −1 (n=15). Males and females did not differ significantly in somatic growth rate, which were, respectively, 0.028±0.018 (n=6) and 0.033 ± 0.012 cm day −1 (n=6). This pilot experiment represents the first recorded mass tagging of European hake, which is known to be a very fragile species. The preliminary results indicate that it would be possible to carry out a large-scale tagging experiment of the European stocks in order to improve assessment and subsequent management decisions.

A method for evaluating the effects of biological factors on fish target strength

Abstract: Hazen, E L, and Horne, J K 2003 A method for evaluating the effects of biologicalfactors on fish target strength – ICES Journal of Marine Science, 60:555–562Understanding the relationship between fish biology and target strength potentially im-proves the accuracy of acoustic assessments The effects of individual biological factors(eg, length, tilt, and depth) on backscatter amplitude have been examined, but the relativecontribution of each factor has not been quantified Dimensionless ratios, whichfacilitate comparison of disparate quantities, were used to evaluate the effects of individualbiological factors on echo intensities Ratios from 25 adult walleye pollock (Theragrachalcogramma) were calculated using a Kirchhoff-ray-mode, backscatter model parameter-ized for each fish This comparative approach can be used to identify the influence ofbiological factors on backscatter intensity and is potentially a tool for improving accuracywhen converting acoustic size to fish length

Application of in situ target-strength estimations in lakes: examples from rainbow-smelt surveys in Lakes Erie and Champlain

Abstract: Acoustic abundance of fish depends directly on the target strength (TS) of the fish surveyed. We analyzed 70 and 120 kHz acoustic data from two lakes with abundant rainbow-smelt (Osmerus mordax) populations. Using repeated surveys through the summer growing season, we derived a relationship between TS (dB) and fish length L (cm) at 120 kHz (TS = 19.9 log 10 L − 67.8). Values for 70 kHz were similar. In situ TS increased with fish density, indicating a bias from accepting multiple targets at high fish densities. Correcting for this bias increased estimates of smelt abundance by up to 18% in Lake Erie and up to 100% in Lake Champlain. Multiple modes in the TS distributions observed for older fish do not reflect different size groups, as the same modes can be observed from measurements from a single fish. Smelt released gas bubbles during the evening ascent, and these bubbles had TS (−60 to −58 dB) within the range of TS observed from the fish. Gas-bubble release occurred mostly during the migration. Conducting surveys after the ascent is completed will decrease bias associated with counting bubbles as fish.

Inference of biological and physical parameters in an internal wave using multiple-frequency, acoustic-scattering data

Abstract: High-frequency sound (>10 kHz) is scattered in the ocean by many different processes. In the water column, marine organisms are often assumed to be the primary source of acoustic backscatter. Recent field experiments and theoretical work suggest that the temperature and salinity microstructure in some oceanic regions could cause acoustic scattering at levels comparable to that caused by marine life. Theoretical acoustic-scattering models predict that the scattering spectra for microstructure and organisms are distinguishable from each other over certain frequency ranges. A method that uses multiple-frequency acoustic data to exploit these differences has been developed, making it possible to discriminate between biological and physical sources of scattering under some conditions. This method has been applied to data collected in an internal wave in the Gulf of Maine. For regions of the internal wave in which the dominant source of scattering is either biological or physical in origin, it is possible to combine the acoustic-scattering data and temperature and salinity profiles with acoustic-scattering models to perform a least-squares inversion. Using this approach, it is possible to estimate the dissipation rate of turbulent kinetic energy for some regions of the internal wave, and the length and numerical abundance of the dominant biological scatterer, euphausiids, in others.