Pelagic Fish Biomass Research Articles

Seasonal Estimates of Fish Biomass and Length Distributions Using Acoustics and Traditional Nets to Identify Estuarine Habitat Preferences in Barataria Bay, Louisiana

AbstractWe conducted hydroacoustic, gill‐net, and push trawl surveys to quantify changes in habitat‐specific fish size and biomass in shallow (<2‐m) estuarine waters of Barataria Bay, Louisiana, in order to evaluate essential fish habitat. Surveys were conducted monthly between June 2003 and May 2004 among regions located along a north—south salinity gradient. The fish length distributions derived from the gill‐net and push trawl catches showed moderate concordance with the measured target strength distributions, indicating that our integrated approach more effectively characterized the fish community than using only a single gear type would have. Acoustic estimates showed that biomass was highest during fall (mean ± SE; 2.30 ± 0.27 g/m3) and next highest in spring (1.49 ± 0.20 g/m3), with relatively low biomass during summer (0.70 ± 0.14 g/m3) and winter (0.86 ± 0.14 g/m3); pelagic fish biomass from nets was low during winter (53.9 ± 14.9 grams per unit effort [gpue]) but relatively high in fall (846.1 ± 207.2 gpue), spring (774.3 ± 175.5 gpue), and summer (620.3 ± 140.7 gpue). Oyster habitat supported a greater biomass of pelagic fish (acoustic survey: 1.54 ± 0.15 g/m3; gill‐net survey: 467.3 ± 81.0 gpue) than soft‐bottom habitat (acoustic: 0.94 ± 0.11 g/m3; gill‐net: 315.2 ± 54.8 gpue). Among regions, the greatest biomass of pelagic fish was observed at polyhaline stations (acoustic: 1.78 ± 0.19 g/m3; gill‐net: 654.3 ± 136.5 gpue), followed by mesohaline (acoustic: 1.18 ± 0.15 g/m3; gill‐net: 378.5 ± 79.1 gpue) and oligohaline stations (acoustic: 0.82 ± 0.12 g/m3; gill‐net: 228.3 ± 50.2 gpue). Gill‐net biomass was linearly related to the acoustic biomass estimates of small pelagic fish. The complementary, multigear approach proved to be useful in evaluating habitat use and may be particularly helpful in identifying and monitoring ecosystem reference points to evaluate change and in standardizing ecosystem‐based assessment approaches.

The relationship between marine fronts and fish diversity in the Patagonian Shelf Large Marine Ecosystem

AbstractAim To evaluate the relationship between marine fronts and (1) fish diversity patterns, (2) fish biomass distribution, and (3) fish assemblage structure.Location Patagonian Shelf, Southwestern Atlantic Ocean.Methods Three main frontal systems and eight nearby zones (frontal vs. non‐frontal areas) were compared. An extensive fishery database, obtained during an a priori sampling scheme (spatial resolution 1 × 1) from 1978 to 1979, was employed. Analyses of 248 trawling stations were performed using a combination of diversity measures (species richness, Shannon diversity and evenness), biomass analyses and multivariate analyses [nonparametric analysis of similarity (ANOSIM), non‐metric multidimensional scaling ordination (NMDS) and similarity percentages (SIMPER)] to evaluate the effect of fronts on fish communities.Results In total, 46 fish taxa were identified. Demersal fish diversity increased at the tidal front of Península Valdés but decreased in the frontal zones of the Southern Shelf‐Break and Magellan frontal systems. The richness and biomass of pelagic fish were higher at the Península Valdés Tidal Front, while the biomass of demersal fish was higher at its non‐frontal counterpart. Fish diversity did not differ consistently along the Northern Shelf‐Break Front. However, a higher fish biomass was registered at the non‐frontal zone. Demersal and pelagic fish assemblages differed between frontal and non‐frontal zones.Main conclusions The Patagonian Shelf Large Marine Ecosystem has diverse frontal types that appear to have different effects on fish diversity and assemblage structure. Seasonal (tidal) or permanent fronts have different effects on the parameters analysed, which could be due to productivity–diversity relationships. The Península Valdés Tidal Front, a relatively short temporal scale system, demonstrates a stronger influence on fish diversity, biomass and assemblage structure than the other frontal areas studied. The seasonality of nutrient input owing to frontogenesis, which begins during the austral spring, may play an important role in determining local diversity. At permanent frontal features, diversity decreases when the nutrient availability is high. This is potentially because, when food resources increase, a few species become dominant.

Hypoxia-avoidance by planktivorous fish in Chesapeake Bay: Implications for food web interactions and fish recruitment

Chesapeake Bay has been experiencing severe eutrophication-driven bottom hypoxia for several decades, yet the effects of hypoxia on its food webs, especially its pelagic components, remain largely enigmatic. To better understand how hypoxia influences the interaction and spatial overlap between planktivorous fishes (e.g., bay anchovy Anchoa mitchilli) and their mesozooplankton prey (e.g., Acartia tonsa), we contrasted the spatial distributions of these food-web components along the Bay's entire north–south axis during spring, summer, and fall 1996, 1997, and 2000, and along several cross-Bay transects during summer of these same years. Pelagic fish biomass was estimated with a surface-towed split-beam echosounder. Dissolved oxygen and mesozooplankton biomass were simultaneously quantified using a towed, undulating ScanFish sensor package. Results indicate that hypoxia can disrupt the diel vertical migration behavior of planktivorous fishes in Chesapeake Bay during summer by reducing access to bottom waters and forcing fish to reside in well-oxygenated surface or nearshore waters. In turn, reduced access to bottom waters reduces spatial overlap with mesozooplankton prey, which appear to use the hypoxic zone as a refuge. Ultimately, we discuss how these hypoxia-driven changes in behavior and spatial overlap may influence food web interactions and bay anchovy recruitment in Chesapeake Bay.

A simulation model to explore the response of the Gulf of Maine food web to large-scale environmental and ecological changes

A dynamic simulation model was constructed using outputs from a balanced Gulf of Maine (GOM) energy budget model as the initial parameter set. The model was structured to provide a recipient control set of dynamics, largely based off of flows to and from different biological groups. The model was used to produce Monte Carlo simulations that were compared (percent change in biomass) with basecase simulations for a variety of scenarios. Changes in primary production, large increases in pelagic and demersal fish biomass, increases in fishing mortality, and large increases in top predators such as baleen whales and pinnepids were simulated. These scenarios roughly simulated the potential impacts of climate change, altered fishing pressure, additional protected species mitigations, and combinations thereof. Results suggest that the GOM system is primarily influenced by bottom-up processes involving phytoplankton, zooplankton, and bacterial biomass. Pelagic and demersal fish were important in determining trends in some of the scenarios. Marine mammals, large pelagic fish, and seabirds have a minor role in the GOM system in terms of biomass flows among the ecosystem components. The system is resilient to large-scale change due, in part to many predator–prey linkages. However, major alterations could occur from sustained climate change, high fishing rates, and by combinations of these types of external forcing mechanisms.

Decadal changes in a NW Mediterranean Sea food web in relation to fishing exploitation

We analysed changes in the ecological roles of species, trophic structure and ecosystem functioning using four standardized mass-balance models of the South Catalan Sea (North-western Mediterranean). Models represented the ecosystem during the late 1970s, mid 1990s, early 2000s, and a simulated no-fishing scenario. The underlying hypothesis was that ecosystem models should quantitatively capture the increasing exploitation in the ecosystem from the 1970s to 2000s, as well as differences between the exploited and non-exploited scenarios. Biomass showed a general decrease, while there was an increase in biomass at lower trophic levels (TL) from the 1970s to 2000s. The efficiency of energy transfer (TE) from lower to higher TLs significantly increased with time. The ecosystem during the 1990s showed higher biomass and flows than during the 1970s and 2000s due to an increase in small pelagic fish biomass (especially sardines). Exploited food webs also showed similarities in terms of general structure and functioning due to high intensity of fishing already in the 1970s. This intensity was highlighted with low trophic levels in the catch, high consumption of production by fisheries, medium to high primary production required to sustain the catches and high losses in secondary production due to fishing. Significant differences on ecosystem structure and functioning were highlighted between the exploited and no-fishing scenarios. Biomass of higher TLs increased under the no-fishing scenario and the mean trophic level of the community and the fish/invertebrate biomass ratios were substantially lower in exploited food webs. The efficiency of energy transfer (TE) from lower to higher TLs was lower under the no-fishing scenario, and it showed a continuous decrease with increasing TL. Marine mammals, large hake, anglerfish and large pelagic fish were identified as keystone species of the ecosystem when there was no fishing, while their ecological importance notably decreased under the exploited periods. On the contrary, the importance of small-sized organisms such as benthic invertebrates and small pelagic fish was higher in exploited food webs.

Measuring herring densities with one real and several phantom research vessels

Abstract Patel, R., and Ona, E. 2009. Measuring herring densities with one real and several phantom research vessels. – ICES Journal of Marine Science 66: 1264–1269. Vessel-induced avoidance can potentially cause a large bias in acoustic estimates of schooling, pelagic-fish biomass. This paper presents a method for quantifying this uncertainty. Volume-backscattering strength (Sv) from a horizontally projecting, multibeam sonar (Simrad MS70) is resampled to form synthetic, vertical, echosounder beams to the side of the survey vessel. These data are analysed as if they were collected from phantom vessels surveying parallel transects at fixed ranges from the real vessel. The nautical-area-backscattering coefficients (sA) from the synthetic echograms are compared with those measured by conventional 70 and 120 kHz echosounders (Simrad EK60) on the real vessel. Data collected in 2006 from schools of Norwegian spring-spawning herring are used to illustrate the method and explore its limitations. Potential effects of vessel-induced avoidance are evaluated by comparing the mean sA values observed from the phantom vessels with those observed from the real vessel. The technique also allows direct estimates of the mean lateral-aspect target strength of in situ herring.

The Lake Huron pelagic fish community: persistent spatial pattern along biomass and species composition gradients

Spatial patterns in the biomass of pelagic fish in Lake Huron have persisted over 10 years even though biomass decreased 86% and the fish community shifted from dominance by non-native species (rainbow smelt, Osmerus mordax ) to dominance by native species (bloater, Coregonus hoyi ). Based on multivariate analyses of acoustic biomass data and abiotic variables from the years 1997, 2004, 2005, and 2007, the strength of relationships between abiotic variables (primarily bottom depth) and fish community composition gradients decreased with fish biomass, suggesting that at high biomass, the influence of the measured abiotic variables is minimal. We observed consistently higher biomass in the North Channel and Georgian Bay than in the Main Basin, and as a result, we conclude that these smaller basins are likely important contributors to lakewide fish biomass, production, and dynamics. These results suggest that at current biomass levels, efforts to understand ecology, population dynamics, and lakewide abundance need to incorporate the effects of depth and geographic variation on fish distributions and ecology.

Exploring the effect of Marine Protected Areas on the dynamics of fish communities in the southern Benguela: an individual-based modelling approach

AbstractYemane, D., Shin, Y-J., and Field, J. G. 2009. Exploring the effect of Marine Protected Areas on the dynamics of fish communities in the southern Benguela: an individual-based modelling approach. – ICES Journal of Marine Science, 66: 378–387. Marine Protected Areas (MPAs) have been suggested as a tool that can achieve some of the goals of an Ecosystem Approach to Fisheries (EAF), e.g. prevention of overexploitation, biodiversity conservation, recovery of overexploited population, but the consequences of their establishment on the dynamics of protected components are often unclear. Spatial and multispecies models can be used to investigate the effects of their introduction. An individual-based, spatially explicit, size-structured, multispecies model (known as OSMOSE) is used to investigate the likely consequences of the introduction of three MPAs off the coast of South Africa, individually or in combination. The simultaneous introduction of the MPAs affected varying proportions of the distribution of the modelled species (5–17%) and 12% of the distribution of the whole community. In general, the introduction of the MPAs in the different scenarios resulted in a relative increase in the biomass of large predatory fish and a decrease in the biomass of small pelagic fish. The simulation demonstrates that consideration of trophic interactions is necessary when introducing MPAs, with indirect effects that may be detrimental to some (mainly smaller prey) species.

Refined estimates of South African pelagic fish biomass from hydro-acoustic surveys: quantifying the effects of target strength, signal attenuation and receiver saturation

The biomass of small pelagic fish species off the coast of South Africa has been monitored since 1984 using hydro-acoustic survey techniques. These time-series of spawner biomass and recruitment estimates form the basis for management of both the South African sardine Sardinops sagax and anchovy Engraulis encrasicolus resources and are central to the setting of annual total allowable catch levels. However, these survey estimates have, for the most part, been treated as relative indices as there are several biases inherent in acoustic survey methodology that remain difficult to quantify. Advances in acoustic technology together with an improved understanding of the major sources of survey errors have enabled estimation of and correction for biases such as receiver saturation, acoustic signal attenuation and target strength. Incorporation of these corrections over the entire time-series has resulted in an improved accuracy of acoustic survey estimates and substantial changes to the biomass estimates of both species, without jeopardising the requirement that the time-series remains comparable throughout its duration. Furthermore, the resultant decrease in the level of uncertainty associated with these abundance estimates has rendered improved utilisation of these resources possible.

Ecosystem simulations supporting ecosystem-based fisheries management in the Coral Triangle, Indonesia

A comprehensive field study ongoing in Eastern Indonesia has provided data for a trophodynamic Ecosim ecosystem model of the Raja Ampat archipelago on the west coast of New Guinea. Model dynamics have been tuned to agree with local catch and relative biomass time series data developed for this project, and validated by experts. The model is used in this article to investigate five high priority research questions related to ecosystem-based fisheries management (EBFM) in the region. Regency fisheries managers and scientific partners working in Indonesia posed the questions. Here, we analyze the ecosystem impacts of blast fishing, including trophic effects and removal of refuge space. Removal of refuge space is as harmful to juvenile reef fish populations as direct mortality from the fishery itself; reef damage is cumulative due to the slow re-growth rate of corals. We quantify the likely ecological and economic impacts of limiting commercial fisheries for groupers. Artisanal fisheries benefit slightly and system biodiversity is improved, but the improvement is lost if artisanal fisheries increase effort to compensate for missing catch. We forecast the effects of limiting commercial net fisheries for reef fish. There is a marginal increase in reef fish biomass and an unexpected benefit to large pelagic species due to reduced interception of their anchovy prey. We evaluate the exploitation status of anchovy and tuna and report on the ecosystem effects of these fisheries. All fisheries appear fully exploited in Raja Ampat or nearly so. There is an indication that anchovies provide an ecosystem service: a large population may act to buffer fluctuations in large pelagic fish biomass under climate variation.

Seasonal changes in pelagic fish biomass around the Chiswell Island Steller sea lion rookery in 2003

Fisheries acoustics surveys were conducted around the Chiswell Island rookery in the northern Gulf of Alaska at night in April and August 2003 to assess seasonal changes in prey available to Steller sea lions ( Eumetopias jubatus) foraging around the rookery. Adult walleye pollock ( Theragra chalcogramma) ≥28 cm fork length was the dominant biomass in the upper 50 m of the water column in both months, increasing from 122.8 kg/nmi 2 in April to 457.9 kg/nmi 2 in August. A similar pattern was observed for Pacific herring ( Clupea pallasii), which averaged 2.8 and 65.6 kg/nmi 2 in April and August, respectively. Incidental trawl catch suggested the appearance of age-0 pollock and juvenile salmonids ( Oncorhynchus spp.) around the rookery in August as well. The increased biomass of these key prey species is linked to increased foraging trip durations by lactating sea lions from Chiswell Island, and supports the general view that sea lions in the northern Gulf of Alaska are not food limited during summer months.

Trophic modeling of Eastern Boundary Current Systems: a review and prospectus for solving the “Peruvian Puzzle”

Eastern Boundary Current systems (EBCSs) are among the most productive fishing areas in the world. High primary and secondary productivity supports a large biomass of small planktivorous pelagic fish, “small pelagics”, which are important drivers of production to the entire system whereby they can influence both higher and lower trophic levels. Environmental variability causes changes in plankton (food) quality and quantity, which can affect population sizes, distribution and domi-nance among small pelagics. This variability combined with impacts from the fishery complicate the development of management strategies. Consequently, much recent work has been in the development of multispecies trophic models to better understand interdependencies and system dynamics. Despite similarities in extent, structure and primary productivity between EBCSs, the Peruvian system greatly differs from the others in the magnitude of fish catches, due mainly to the incredible production of the anchovy Engraulis ringens. This paper reviews literature concerning EBCSs dynamics and the state-of-the-art in the trophic modeling of EBCSs. The objective is to critically analyze the potential of this approach for system understanding and management and to adapt existing steady-state models of the Peruvian system for use in (future) dynamic simulations. A guideline for the construction of trophodynamic models is presented taking into account the important trophic and environmental interactions. In consideration of the importance of small pelagics for the system dynamics, emphasis is placed on developing appropriate model compartmentalization and spatial delineation that facilitates dynamic simulations. Methods of model validation to historical changes are presented to support hypotheses concerning EBCS dynamics and as a critical step to the development of predictive models. Finally, the identification of direct model links to easily obtainable abiotic parameters is emphasized to add practicality to the model as a predictive tool.

Estimating abundance of pelagic fishes using gillnet catch data in data-limited fisheries: a Bayesian approach

We describe a Bayesian modelling approach to estimate abundance and biomass of pelagic fishes from gillnet catches in data-limited situations. By making a number of simple assumptions, we use fish sustained swimming speed to calculate the effective area fished by a gillnet in a specified soak time to estimate abundance (fish·km–2) from the number of fish caught. We used catch data from various sampling methods in northern Australia and elicited anecdotal information from experts to build a size distribution of the true population to compensate for size classes that were unlikely to be represented in the catch because of size selectivity of the gear. Our final abundance estimates for various-sized scombrids (0.04–4.17 fish·km–2) and istiophorids (0.004–0.005 fish·km–2) were similar to what has been estimated for similar species in more data-rich situations in tropical regions of the Pacific Ocean. The model is particularly useful in data-limited situations in which abundance or biomass estimates are required for pelagic fish species of low economic importance. These data are often required for ecosystem models (e.g., Ecopath) that are increasingly being considered as potential tools for ecosystem-based fisheries management.

Evidence of a variable “unsampled” pelagic fish biomass in shallow water (<20 m): the case of the Gulf of Lion

Abstract Studies of small pelagic fish biomass are limited by the fact that research vessels and fishing boats are usually restricted to working areas with a bottom depth >20 m. Consequently, “unsampled” areas can represent a large proportion of the continental shelf, and the biomass in those areas can be important and must be taken into account in assessment methods in order to avoid misleading interpretations in population dynamics. A time-series ten years long has been compiled from acoustic-assessment surveys of small pelagic fish stocks, and the results show an overall increase in the acoustic fish density towards the coast, where values were the highest. Additional experiments on transects covering shallow-water areas (5–20 m) were conducted from 2001 to 2003 with small boats and a research vessel to evaluate the acoustic fish density in those areas. The results confirmed that the fish biomass in shallow water is significant, sometimes very large, and should be evaluated to avoid underestimation. Therefore, surveys should be conducted in shallow water, if at all possible, as well as at greater depths when carrying out surveys destined to support assessment exercises.

Power-law scaling in dimension-to-biomass relationship of fish schools

Motivated by the finding that there is some biological universality in the relationship between school geometry and school biomass of various pelagic fishes in various conditions, I here establish a scaling law for school dimensions: the school diameter increases as a power-law function of school biomass. The power-law exponent is extracted through the data collapse, and is close to 3 5 . This value of the exponent implies that the mean packing density decreases as the school biomass increases, and the packing structure displays a mass-fractal dimension of 5 3 . By exploiting an analogy between school geometry and polymer chain statistics, I examine the behavioral algorithm governing the swollen conformation of large-sized schools of pelagics, and I explain the value of the exponent.

Density and biomass of smelt (Osmerus eperlanus) in five Finnish lakes

Abstract Hydro-acoustic stock assessment and exploratory sampling with small mesh-sized trawls and seines have sometimes suggested that the importance of smelt (Osmerus eperlanus) in pelagic fish communities might be greater than sampling from commercial fishery and gillnetting indicate. We studied the proportion of smelt in the total fish density (fish ha−1) and biomass (kg ha−1) of pelagic fish with echo sounding and trawling in five southern boreal lakes. Fish density varied between 460 and 2000 fish ha−1 in the study lakes. Smelt and vendace (Coregonus albula) accounted for more than 95% of the exploratory trawl-catches. The total fish biomass in the study areas varied between 3 and 13 kg ha−1. In four lakes the proportion of smelt was more than 60%. In one lake the proportion of vendace was over 85%. These estimates indicate the importance of smelt in the study lakes. Studies of the co-occurrence of smelt with other fish species in the pelagic area of boreal lakes are needed to get a less biased picture of the fish community. This underlines the need for relevant sampling methods for the species involved.

Fat content in Black Sea sprat as an indicator of fish food supply and ecosystem condition

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 293:201-212 (2005) - doi:10.3354/meps293201 Fat content in Black Sea sprat as an indicator of fish food supply and ecosystem condition Georgiy E. Shulman1,*, Victor N. Nikolsky1, Tatyana V. Yuneva1, Galina S. Minyuk1,Vladimir Ya. Shchepkin1, Alla M. Shchepkina1, Elena V. Ivleva1, Oleg A. Yunev1,Ivan S. Dobrovolov2, Ferit Bingel3, Ahmet E. Kideys3 1Institute of Biology of the Southern Seas, National Academy of Sciences of Ukraine, Nakhimov Av. 2,Sevastopol 99011, Crimea, Ukraine2Institute of Fishery Resources, Primorsky Bulv. 4, Varna 9000, Bulgaria3Middle East Technical University, Institute of Marine Sciences, PO Box 28, 33731 Erdemli, Mersin, Turkey *Email: shulman@ibss.iuf.net ABSTRACT: Fat content (as % of total body wet wt) of the sprat Sprattus sprattus phalericus in the Black Sea was determined annually during the peak feeding period (i.e. summer) between 1960 and 2001. Interannual variations of this characteristic were quite high (from 8.0 to 16.0%). Sprat fat content can be used as an indicator of food supply (or recent feeding history). In the Black Sea, different periods could be distinguished with respect to food supply of this species: Period I (1960 to 1964) with stable sprat fat content values of 11.2 to 13.7%; Period II (1965 to 1972) with decreased values of 8.5 to 10.3%; Period III (1974 to 1982) with higher values of 11.4 to 15.7%; Period IV (1983 to 1990) when there were the highest interannual fluctuations and a decrease to 9.9 to 10.7%; Period V (1991 to 1997) again with high fat content values of 12.2 to 14.7%; Period VI (1998 to 2001) with stabilized fat content values of 12.0 to 12.3%. These periods are closely related to the general situation in the Black Sea ecosystem caused by climatic changes, increased trophic competition, eutrophication and pollution. Relationships between interannual fluctuations in sprat fat content, on the one hand, and sprat biomass and phytoplankton concentration, on the other, are revealed. The features of the interannual dynamics of the Black Sea sprat fat content are similar to those (biomass and catches) of small pelagic fishes in the Mediterranean and in some other parts of the World Ocean. KEY WORDS: Sprat · Sprattus sprattus phalericus · Black Sea · Fat content · Food supply · Interannual periods · Stock biomass · Phytoplankton · Mesozooplankton Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 293. Online publication date: June 02, 2005 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2005 Inter-Research.

Hydroacoustic Estimates of Abundance and Spatial Distribution of Pelagic Prey Fishes in Western Lake Superior

Lake herring ( Coregonus artedi ) and rainbow smelt ( Osmerus mordax ) are a valuable prey resource for the recovering lake trout ( Salvelinus namaycush ) in Lake Superior. However, prey biomass may be insufficient to support the current predator demand. In August 1997, we assessed the abundance and spatial distribution of pelagic coregonines and rainbow smelt in western Lake Superior by combining a 120 kHz split beam acoustics system with midwater trawls. Coregonines comprised the majority of the midwater trawl catches and the length distributions for trawl caught fish coincided with estimated sizes of acoustic targets. Overall mean pelagic prey fish biomass was 15.56 kg ha −1 with the greatest fish biomass occurring in the Apostle Islands region (27.98 kg ha −1 ), followed by the Duluth Minnesota region (20.22 kg ha −1 ), and with the lowest biomass occurring in the open waters of western Lake Superior (9.46 kg ha −1 ). Biomass estimates from hydroacoustics were typically 2–134 times greater than estimates derived from spring bottom trawl surveys. Prey fish biomass for Lake Superior is about order of magnitude less than acoustic estimates for Lakes Michigan and Ontario. Discrepancies observed between bioenergetics-based estimates of predator consumption of coregonines and earlier coregonine biomass estimates may be accounted for by our hydroacoustic estimates.

Spatial distribution and biomass of pelagic fish in the East China Sea in summer, based on acoustic surveys from 1997 to 2001

: Annual acoustic surveys were conducted using a quantitative echo sounder (frequency: 38 kHz) in the East China Sea from July to September of 1997–2001. Three groups of fish were identified from the echograms, based on school shape and swimming depth characteristics: (i) anchovy Engraulis japonicus and round herring Etrumeus teres; (ii) jack mackerel Trachurus japonicus and chub mackerel Scomber japonicus; and (iii) lantern fish Diaphus spp. and pearlside Maurolicus japonicus. The area backscattering strength (Sa) of each group was divided by the total weight of fish species caught in midwater trawls. Anchovy, round herring, jack mackerel, lantern fish and pearlside were the most abundant. The estimated biomass indices of anchovy and round herring were positively correlated with the commercial landings of these species, indicating the usefulness of acoustic surveys.

Transition to an alternate state in a continental shelf ecosystem

The collapse of Atlantic cod (Gadus morhua) stocks in the northwest Atlantic Ocean ranks among the most dramatic and widely known ecological changes of the 20th century. Less widely known are the systemic changes that occurred within these areas prior to and coincident with the cod collapse. Our analysis of a fishery-independent, long-term, standardized database collected on the eastern Scotian Shelf off Nova Scotia revealed that during the past four decades, coherent, community-level reductions in body size, biomass, and physiological condition have occurred in the resident demersal fish species. The changes occurred over large spatial (>104 km2) and short temporal (<10 years) scales, suggesting a progressive decline in the nature and extent of the energy flow through the benthic system. The unexpected and persistent poor condition of a variety of morphologically and functionally dissimilar demersal fish species, living in an environment of reduced intra- and inter-specific competition, suggests a decoupling of the benthic– pelagic systems. This decoupling appears to be attributable to a cascading series of processes involving the cumulative removal of biomass resulting from commercial fishing, compensatory and self-stabilizing increases in pelagic fish biomass, and a decline in groundfish productivity exacerbated by decadal scale variability in water temperature and stratification.