Research Trawl Surveys Research Articles

An updated checklist of bony fishes from the Persian Gulf

This study provides an updated and comprehensive checklist of bony fishes occurring in the Persian Gulf based on a recent survey of the region. This study was conducted from 1995 to 2020 by research trawl survey for identification and biomass estimation of the demersal species each year from autumn to late winter, covering all waters of Iran in the Persian Gulf. In addition to the research cruises, routine field visits were conducted at major fishing ports to identify pelagic species captured through gillnet and trap fisheries. Among the over 700 species reported in previous studies conducted in the Persian Gulf, the occurrence of 486 species has been confirmed. Several species were omitted from the revised checklist due to insufficient evidence supporting their occurrence in the region or due to taxonomic confusion. Throughout the study, 486 species of bony fishes, representing 101 families, were identified, and a critical review of relevant literature was conducted. This checklist serves as a reliable reference to address existing contradictions in the scientific literature on bony fishes of the Persian Gulf and establishes a foundation for future stock assessment studies in the region.

Distributions of threatened skates and commercial fisheries inform conservation hotspots

Bycatch in commercial fisheries is a pressing conservation concern and has spurred global interest in adopting ecosystem-based management practices. To address such concerns, a thorough understanding of spatiotemporal relationships among bycatch species, their environment and fisheries is required. Here we used a generalized linear mixed model framework incorporating spatiotemporal random effects to model abundance patterns for 3 skate species caught as bycatch in commercial fisheries (thorny skate Amblyraja radiata, winter skate Leucoraja ocellata and smooth skate Malacoraja senta), as well as 10 target species on the Scotian Shelf, NW Atlantic. Spatiotemporal estimates of relative abundance for at-risk skates within the years 2005-2015 were modelled from research trawl survey data and overlaid with those for target species to identify hotspots of bycatch risk. In addition, abundance estimates for at-risk skates within the years 1975-1985, a period of higher stock abundance, were used to identify areas of previously important habitat. Historically, skate species densely occupied areas near Sable Island and Banquereau Banks, Georges Bank and the Bay of Fundy. Bycatch hotspots between at-risk skates and commercial targets were identified in regions across the Scotian Shelf. These hotspots were independently validated by predicting species presence from at-sea observer data that monitor skate bycatch directly. We discuss spatial relationships between target and bycatch species, highlighting limitations of at-sea observer programmes that this method helps to address. This framework can be applied more broadly to inform ecosystem management and priority areas for conservation or fisheries regulation.

Population structure and morphology of South African deep‐water parapagurid hermit crabs

AbstractThe parapagurid hermit crabs Sympagurus dimorphus (Studer, 1883) and Parapagurus bouvieri (Stebbing, 1910) form dominant components of South African deeper‐water benthic invertebrate communities and, as adults, forego the use of “traditional” gastropod shells in favour of symbiotically associated Epizoanthus carcinoecia. A total of 1,122 adult S. dimorphus and 549 adult P. bouvieri derived from demersal research trawl surveys were examined to investigate parameters of population structure, carcinoecia association and morphology. S. dimorphus had a female‐biased sex ratio of 1:1.5, and P. bouvieri an equal sex ratio of 1:1. The proportion of males of both species increased with cephalothoracic shield length (CL). The lengths of the major right chelipeds of both S. dimorphus males and females, and P. bouvieri males were positively allometric, and negatively allometric in P. bouvieri females. All P. bouvieri inhabited carcinoecia, while 3.39% (n = 38) of S. dimorphus specimens occupied gastropod shells. There was a positive relationship between S. dimorphus CL and rate of gastropod shell occupation (r2 = 0.96). In S. dimorphus, number of zoanthid polyps forming the carcinoecia pseudoshell increased with both total shell volume (R2 = 0.73) and colony height (R2 = 0.61), but no such relationships were found for P. bouvieri. Euspira napus was the most common original shell around which carcinoecia developed in both S. dimorphus (45.45%) and P. bouvieri (39.00%). We speculate that these hermit species exist along an evolutionary continuum of carcinoecia use. Sympagurus dimorphus retains the ability to occupy both gastropod shells and pseudoshells and, like most other hermit crabs, continue to use shells/carcinoecia as shelters to protect the entire body. In contrast, adult P. bouvieri appear to be totally reliant on carcinoecia, which they use more as a brood pouch covering only the abdomen, rather than as a shelter into which they can fully retract to escape predation.

Aggregations and associations in deep-sea chondrichthyans

Abstract Group living has been recorded across a diverse range of chondrichthyans, but due to a variety of factors, any inferences of social associations in deep-sea cartilaginous fishes have yet to be described. Using a companion preference analysis, aggregating behaviour and associations in deep-sea chondrichthyans (four holocephalans, ten elasmobranchs) were evaluated across class and group size from long-term archived fisheries independent research trawl survey datasets. Results indicated that not all selected species engaged in aggregative behaviour, but those that did suggested patterns of sex- and size-specific associations, which varied with catch density. Adult females were caught most frequently in low densities, and were highly associated with other adult females. Adult males were consistently associated with each other. The nature of associations has important implications for selective mortality by spatial or temporally stratified, or aggregation-targeting, fisheries.

Population structure and distribution of deep-water cardinal fish Epigonus occidentalis (Epigonidae) and Epigonus pandionis (Epigonidae) in the Colombian Caribbean Sea

Cardinal fish are commercially valuable species. In the Colombian Caribbean Sea E. occidentalis and E. pandionis occurred in 37and 35% of tows during a research trawl survey (>200 m), respectively and the biology of these vulnerable species is unknown. The aim of this work is to describe the spatial distribution patterns of biomass, size structure and morphometric relationships of E. occidentalis and E. pandionis in deep waters of the Colombian Caribbean Sea. The samples were collected by trawling in depths between 200 and 550 m. No statistical differences were found in size by sexes in both species. In female and male E. occidentalis the growth was isometric, while in female and male E. pandionis it was positive allometric. E. occidentalis occurred mainly in the northern zone of Colombian Caribbean Sea, with highest aggregations off Santa Marta and Riohacha. E. pandionis was distributed in the northern area between Santa Marta and Riohacha and in the southern area between Cartagena and Morrosquillo Gulf. However, before the exploitation of these species is considered, further research is required to determine basic life history traits, such as growth, reproduction, recruitment, and mortality.

Reproductive biology of the two deep-sea chimaerids, longnose spookfish (Harriotta raleighana) and Pacific spookfish (Rhinochimaera pacifica)

The family Rhinochimaeridae, the long-nose chimaerids, consists of eight species across three genera, two of which occur in New Zealand waters. Very little is known about the biology of the rhinochimaerids. Longnose spookfish, (Harriotta raleighana, Goode and Bean, 1895), and Pacific spookfish, (Rhinochimaera pacifica,Mitsukuri, 1895), were collected from research trawl surveys and by commercial fishing vessels around New Zealand at depths between 400 and 1300m. A total of 300H. raleighana were caught which varied in length from 18.7 to 90.4cm chimaera length (CL), and 168R. pacifica at lengths of 20.9–139.9cm CL. External assessment of male claspers and female gonad mass and oviducal gland width were the best indicators for maturity. Both species matured at a large proportion of their maximum length. Length at maturity was estimated at 62.8cm CL and 75.8cm CL for male and female H. raleighana respectively, and 105.3cm CL and 125.0cm CL for male and female R. pacifica. Fecundity was low and measured up to 27 eggs for H. raleighana, and 31 eggs for R. pacifica. Sperm storage was confirmed in females of both species. Sexual dimorphism in snout length was found in H. raleighana, where male relative snout size increased at sexual maturity, suggesting the snout is a secondary sexual characteristic. This study contributes to a better understanding of the life histories of H. raleighana and R. pacifica and their vulnerability to exploitation as fisheries bycatch.

Distribution, habitat and movement of juvenile smooth hammerhead sharks (Sphyrna zygaena) in northern New Zealand

ABSTRACTThe smooth hammerhead shark, Sphyrna zygaena, occurs in warm temperate waters around the northern North Island of New Zealand. Commercial fishing records and research trawl survey data were used to determine their distribution. Highest catch rates were recorded in relatively sheltered bays and coastlines along the northeast coast of North Island. Neonate and juvenile sharks use shallow coastal waters and large harbours and estuaries as nursery areas up to an age of two years and total length of 150 cm. Five sharks were electronically tagged but two apparently died and three (137–160 cm total length) returned useful data. Two tagged sharks remained in or near the Bay of Islands for 6–55 days after tagging, moving extensively through the bay. A third shark moved about 155 km southeast in 250 days. That shark spent 70 days mostly shallower than 10 m (94% of time) with occasional dives to 40 m. Thereafter, it oscillated between the surface and depths of 60−80 m, with most time (55%) being spent at 40−60 m. Maximum recorded depth was 144 m.

Population dynamics of Japanese threadfin bream Nemipterus japonicus from Pakistani waters

Japanese threadfin bream Nemipterus japonicus (Bloch, 1791) is among the most abundant and commercially important species in Pakistan. From the coast of Pakistan, four demersal trawl surveys in October–November 2009 and May–June, August, October and November in 2010 were carried out. The purpose of this study is to estimate the population dynamics and status of the stock of the N. japonicus from Pakistani waters based on the research trawl surveys from the research area. The data consist of n=784 length-weight pairs and n=7 530 length frequency with the maximum length and weight of 29 cm and 358 g respectively. The length frequency data were analyzed using ELEFAN method in FiSAT computer package. The parameters of length and weight relationship were b=2.778, a=0.032 and R 2=0.973. The estimated von Bertalanffy growth function parameters were L ∞=30.45 cm, K=0.270 year−1. Based on length-converted catch curve analysis the total mortality (Z) during this study was estimated at 0.960 year−1. The natural mortality coefficient (M) was 0.74 year−1 using Pauly’s equation (the annual average sea surface temperature was 27°C), therefore, the fishing mortality coefficients (F) were 0.22 year−1. The yield per recruit analysis indicated that when t c was 2, F max was estimated at 1.2 and F 0.1 at 1.1. When t c was 1, F max was estimated at 0.95 and F 0.1 at 0.8. Because current age at first capture is about 1 year and F current was 0.22, F current is smaller than F 0.1 and F max, which indicated that the fishery is about in a safe condition. When using Gulland (1971) biological reference point, F opt was equals to M (0.74). The current fishing mortality rate of 0.22 was smaller than the target biological reference point.

Species composition and acoustic signatures of mesopelagic organisms in a subtropical convergence zone, the New Zealand Chatham Rise

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 503:23-40 (2014) - DOI: https://doi.org/10.3354/meps10731 Species composition and acoustic signatures of mesopelagic organisms in a subtropical convergence zone, the New Zealand Chatham Rise Stéphane Gauthier1,2,*, Johannes Oeffner1, Richard L. O’Driscoll1 1National Institute of Water and Atmospheric Research, Private Bag 14-901, Kilbirnie, Wellington, New Zealand 2Fisheries and Oceans Canada, Institute of Ocean Sciences, PO Box 6000, Sidney, British Columbia V8L 4B2, Canada *Corresponding author: stephane.gauthier@dfo-mpo.gc.ca ABSTRACT: The Chatham Rise is a submarine ridge east of New Zealand. The subtropical convergence occurs over this ridge and it is an area of high productivity, associated with important deepwater fisheries. Acoustic recordings suggest that there is a relatively high biomass of mesopelagic fish over the Chatham Rise, but little is known about the species composition, distribution, and spatio-temporal dynamics of the organisms responsible for the acoustic scattering layers. Two research trawl and acoustic surveys were carried out in May and June 2008 and November 2011 to assess the distribution and acoustic signatures of mesopelagic animals on the Chatham Rise. A total of 171 species or species groups were caught in 86 midwater trawls to a maximum depth of 1000 m. Seven species assemblages were described corresponding to different acoustic mark types. These 7 assemblages were dominated by euphausiids, Maurolicus australis (Stomiiformes), Lampanyctodes hectoris, Symbolophorus boops, Electrona carlsbergi, Diaphus danae and Gymnoscopelus piabilis (all Myctophiformes). Euphausiids could be separated from mesopelagic fish species based on their multi-frequency acoustic response, but the 6 fish groups could not be separated based on their frequency response alone. These different fish assemblages were distinguished based on their unique acoustic mark characteristics (amplitude and dimension features of the volume backscatter), vertical distribution and ancillary information. Classification results were synthesised in a decision model. M. australis and L. hectoris were associated with shallower water on the central and western Chatham Rise, D. danae occurred mainly in deeper water on the north side of the Rise, and E. carlsbergi was associated with cooler water on the south Rise. KEY WORDS: Mesopelagic fish · Acoustics · Myctophids · Deep scattering layer · Spatial distribution Full text in pdf format Supplementary material PreviousNextCite this article as: Gauthier S, Oeffner J, O’Driscoll RL (2014) Species composition and acoustic signatures of mesopelagic organisms in a subtropical convergence zone, the New Zealand Chatham Rise. Mar Ecol Prog Ser 503:23-40. https://doi.org/10.3354/meps10731 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 503. Online publication date: April 29, 2014 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2014 Inter-Research.

The biodiversity value of marine protected areas for multi‐species fishery management in the Gulf of Maine

ABSTRACTThis work examined the potential for marine protected areas based on the efficient design of essential fish habitat (EFH) for juvenile life stages of economically important demersal fish species to represent non‐exploited demersal fish species in the Gulf of Maine.Distribution and abundance data from research trawl surveys conducted from 1963–2000 were pooled into a 10‐min lat/long square sampling grid and density per square of each species was calculated as the mean of counts from all surveys.A simulated annealing algorithm (Marxan) was used to identify minimum sets of 10‐min squares that represented at least 10–30% of the regional abundance of each species under constraints for degree of spatial aggregation among sites. Numbers of non‐exploited species in these sites, and species represented at abundance levels equal to or greater than the targets set for juveniles of economically important species were identified, and both were compared with mean totals from 100 sets of randomly drawn squares of equivalent number to simulation results.Percentages of non‐exploited species among all scenarios varied from 81–97%. All species absent from simulation results were rare regionally. Species richness was lower in each case compared with randomly drawn squares, but only marginally. However, more species were represented in complementary sites at abundance levels equal to or greater than the targets set for economically important species.These results were interpreted with respect to a definition of biodiversity that includes species as well as functional and structural ecosystem components, and it was concluded that the dual objectives of fishery management and biodiversity conservation can be achieved, at least in some cases, within a single network of marine protected areas of moderate size. Copyright © 2013 John Wiley & Sons, Ltd.

Hypotheses of spatial stock structure in orange roughy Hoplostethus atlanticus inferred from diet, feeding, condition, and reproductive activity.

We evaluate hypotheses for meso-scale spatial structure in an orange roughy (Hoplostethus atlanticus) stock using samples collected during research trawl surveys off the east coast of New Zealand. Distance-based linear models and generalised additive models were used to identify the most significant biological, environmental, and temporal predictors of variability in diet, proportion of stomachs containing prey, standardised weight of prey, fish somatic weight, fish total weight, and reproductive activity. The diet was similar to that observed elsewhere, and varied with ontogeny, depth, and surface water temperature. Smaller sized and female orange roughy in warmer bottom water were most likely to contain food. Fish condition and reproductive activity were highest at distances more than 20 km from the summit of the hills. Trawl survey catches indicated greater orange roughy densities in hill strata, suggesting hill habitat was favoured. However, analyses of feeding, condition, and reproductive activity indicated hill fish were not superior, despite fish densities on hills being reduced by fishing which, in principle, should have reduced intra-specific competition for food and other resources. Hypotheses for this result include: (1) fish in relatively poor condition visit hills to feed and regain condition and then leave, or (2) commercial fishing has disturbed feeding aggregations and/or caused habitat damage, making fished hills less productive. Mature orange roughy were observed on both flat and hill habitat during periods outside of spawning, and if this spatial structure was persistent then a proportion of the total spawning stock biomass would remain unavailable to fisheries targeting hills. Orange roughy stock assessments informed only by data from hills may well be misleading.

Habitat-forming cold-water corals show affinity for seamounts in the New Zealand region

Determining the distribution of habitat- forming scleractinian corals in the New Zealand region is necessary in order to understand the ecological significance of these taxa and the likely impact of an- thropogenic activities on their persistence. Historical records from early publications, research trawl surveys, commercial fishing bycatch and recent biodiversity sur- veys were compiled for the habitat-forming coral spe- cies Madrepora oculata, Solenosmilia variabilis, Gonio- corella dumosa, Enallopsammia rostrata and Oculina virgosa. These data were used to describe the observed depth, geographic distribution and geomorphic habitat associations of the study corals in the region. A boosted regression trees analysis was also used to identify which of 11 environmental variables best describe the distribu- tion of the 5 species across the New Zealand region, and to predict their spatial distribution. The contribution of the environmental variables differed greatly between species, but consistently identified depth and seamount occurrence as important factors de scribing observed coral species distribution. The models identified that M. oculata, S. variabilis and E. rostrata occurred in deep waters (>1000 m) where seabed slopes were steep, tidal current and orbital velocities were slow, sea surface pri- mary productivity was low, and where seamounts were generally present. By contrast, G. dumosa and O. vir- gosa were found in relatively shallower waters, where sea surface primary productivity was high and tidal current speeds were generally fast. Spatial predictions were consistent with the recorded observations and identified that all species, apart from O. virgosa, were distributed throughout the region a nd were found primarily between ~200 and 2000 m.

Impacts of Interannual Environmental Forcing and Climate Change on the Distribution of Atlantic Mackerel on the U.S. Northeast Continental Shelf

AbstractThe Northwest Atlantic stock of Atlantic mackerel Scomber scombrus is distributed from Cape Hatteras to Newfoundland and migrates great distances on a seasonal basis. Atlantic mackerel are sensitive to changes in temperature, preferring water greater than 5°C. Annual changes in the winter and early‐spring distributions of Atlantic mackerel were investigated using satellite imagery, research trawl surveys, geographical information systems, and spatial and standard statistical analyses. During the past 40‐plus years (1968–2008), the distribution of the stock has shifted about 250 km to the north and east. Also, a change has occurred in the depth distribution of the stock, from deeper off‐shelf locations to shallower on‐shelf ones. Areal and bathymetric changes in distribution are correlated with interannual temperature variability and gradual warming. These results have implications for U.S. commercial and recreational mackerel fisheries because, despite the current high abundance of the stock, the changes in distribution will probably make it more difficult to find and catch Atlantic mackerel in certain areas in the future.

Diet and dietary variation of New Zealand hoki Macruronus novaezelandiae

The diet of hoki was determined from examination of stomach contents of 1992 fish of 26–112 cm total length (TL) sampled at depths of 209–904m on Chatham Rise, New Zealand, from summer research trawl surveys and seasonal commercial fishing trawls, during 2004–2008. Prey was predominantly euphausiids, mesopelagic fishes and natant decapods. Multivariate analyses using distance-based linear models, non-parametric multi-dimensional scaling and similarity percentages indicated that the best predictors of diet variability were the position of the fish in relation to the subtropical front (STF), fish size and longitude. Pasiphaeids were more important to the north of the STF, and sternoptychid fishes and euphausiids more important in the STF convergence area. Euphausiids and sternoptychid fishes were important for smaller hoki (26–55 cm TL), myctophid fishes and natant decapods for larger hoki, and macrourids for the largest hoki (>84 cm TL). The longitudinal effect was characterised by pasiphaeids, euphausiids and sternoptychids to the west, and myctophids in the centre of Chatham Rise. Feeding activity was analysed using generalised additive models, and was found to vary with time of day, sample source (research or commercial), longitude, hoki size and depth. The variability in diet suggested hoki forage opportunistically within their preferred habitat and biological limits.

Feeding habits and niche separation among the deep-sea chimaeroid fishes Harriotta raleighana, Hydrolagus bemisi and Hydrolagus novaezealandiae

The diets of Harriotta raleighana, Hydrolagus bemisi and Hydrolagus novaezealandiae were determined from examination of digestive tract contents from specimens sampled at depths of 219 to 876 m on the submarine ridge, Chatham Rise, New Zealand. All 3 species fed predominantly on benthic epifauna and infauna. Similarity percentages indicated the diet of H. raleighana was characterised by polychaetes and molluscs, H. bemisi by crabs and polychaetes, and H. novaezealandiae by crabs and sea urchins. Multivariate analyses using distance-based linear models found the most important predictors of diet variability were a categorical predictor for the subtropical front (STF) and bottom temperature. Cluster analysis of species subgroups indicated there was a greater similarity in diet between H. raleighana and H. bemisi, and between H. bemisi and H. novaezealandiae, in similar locations, than between subgroups of H. bemisi in different locations. Generalised additive models indicated that depth was the most important spatial predictor of research trawl survey catch rates, and depth separation was most pronounced between H. raleighana and H. novaezealandiae. Other significant predictors of diet variability included longitude, and fish length or weight. In all 3 species the diet of smaller fish included more polychaetes, small crustaceans and gastropods, and the diet of larger fish contained more decapod crustaceans. The primary factor determining niche separation among the 3 species appears to be depth and distribution rather than dietary specialisation.

The diet of deepwater sharks and the benefits of using DNA identification of prey

Stomachs from the sharks Dalatias licha, Centrophorus squamosus, Centroscymnus owstoni, Centroselachus crepidater, Proscymnodon plunketi, and Galeorhinus galeus were sampled from three research trawl surveys on Chatham Rise, east of New Zealand. Between 14 and 50 stomachs were examined for each species, of which 8–62% were empty. Prey were visually identified in 80 stomachs, and by DNA barcoding in a further 28 stomachs. The use of DNA methods allowed the identification of chunks of flesh found in the stomachs of D. licha and P. plunketi, and nearly doubled the rate of data accumulation for D. licha, C. squamosus, and C. owstoni. Between 84 and 223 stomachs were estimated to be needed to measure 90% of the extrapolated total prey richness. The prey of D. licha, C. squamosus, and P. plunketi were predominantly benthic or demersal fishes and cephalopods. The prey of C. owstoni and C. crepidater were predominantly mesopelagic fishes and squids. G. galeus foraged throughout the water column. Scavenging of discards from commercial fishing vessels was likely in C. squamosus, P. plunketi, and G. galeus. The diet of all species except C. crepidater was dominated by the commercially important benthopelagic species hoki Macruronus novaezelandiae.

Top-down control of lobster in the Gulf of Maine: insights from local ecological knowledge and research surveys

American lobster Homarus americanus landings in the Gulf of Maine have been steadily increasing since the 1980s. As a result, lobsters now support one of the most important fisheries in the USA and Canada. One hypothesis for this pattern is that lobsters have been released from predation as groundfish stocks declined, expanding both in abundance and habitat. Lobster habitat is typically rocky substrate of the inshore region, which is difficult to sample. Some long-term fisheries-indepen- dent abundance indices for lobsters and their predators are available for the Gulf of Maine in the USA, but not in Canada. To try and fill those research gaps, we designed a local ecological knowl- edge (LEK) survey. Semi-structured interviews of 42 fishermen in southwest Nova Scotia, Canada, revealed consistent trends of the depletion of large groundfish, particularly Atlantic cod Gadus morhua. Eighty-three percent of fishermen concluded this depletion was the main reason for an observed increase in lobster abundance. They also reported the expansion of lobsters to new habitats and depths. We further show that the proposed top-down control mechanism is independently sup- ported by USA research trawl surveys, which revealed a negative correlation (p < 0.05) between the summed abundance indices of 5 groundfish predators of lobster and lobster abundance (kg tow -1 ) at time lags ranging from 0 to 9 yr. Survey-based diet data also corroborated direct observations by fish- ermen on lobster predation by groundfish. These results suggest that LEK may be a useful supple- mentary tool to investigate the ecosystem effects of fishing, particularly in data-poor situations.

Seasonal and interannual variability in grey seal diets on Sable Island, eastern Scotian Shelf

We studied seasonal and interannual variability in the diet of grey seals (Halichoerus grypus) using faecal samples collected from Sable Island, Nova Scotia between 1991 and 1998. More than 28,000 prey from at least 28 taxa were identified from 1,245 faecal samples collect mainly in spring, fall and winter. Sand lance (Ammodytes dubius) dominated the diet in all seasons and years, but the importance of this and other species varied over time. There was also evidence of seasonal and interannual variation in the size of prey consumed both within and among species. We compared diet composition with estimates of prey numbers and biomass from annual researchtrawl surveys conducted in March and July. Species-specific numerical corrections were applied to otolith counts to account for the complete digestion of otoliths, and fish catchability correction factors applied to trawl survey catches to account for trawl selectivity. Based on an odds ratio index of prey selectivity, grey seals positively selected sand lance in both seasons. Other species were either relatively avoided or eaten roughly in proportion to their estimated abundance.

Intercalibration of Research Survey Vessels on Lake Erie

AbstractFish abundance indices obtained from annual research trawl surveys are an integral part of fisheries stock assessment and management in the Great Lakes. It is difficult, however, to administer trawl surveys using a single vessel−gear combination owing to the large size of these systems, the jurisdictional boundaries that bisect the Great Lakes, and changes in vessels as a result of fleet replacement. When trawl surveys are administered by multiple vessel−gear combinations, systematic error may be introduced in combining catch‐per‐unit‐effort (CPUE) data across vessels. This bias is associated with relative differences in catchability among vessel−gear combinations. In Lake Erie, five different research vessels conduct seasonal trawl surveys in the western half of the lake. To eliminate this systematic bias, the Lake Erie agencies conducted a side‐by‐side trawling experiment in 2003 to develop correction factors for CPUE data associated with different vessel−gear combinations. Correcting for systematic bias in CPUE data should lead to more accurate and comparable estimates of species density and biomass. We estimated correction factors for the 10 most commonly collected species age‐groups for each vessel during the experiment. Most of the correction factors (70%) ranged from 0.5 to 2.0, indicating that the systematic bias associated with different vessel−gear combinations was not large. Differences in CPUE were most evident for vessels using different sampling gears, although significant differences also existed for vessels using the same gears. These results suggest that standardizing gear is important for multiple‐vessel surveys, but there will still be significant differences in catchability stemming from the vessel effects and agencies must correct for this. With standardized estimates of CPUE, the Lake Erie agencies will have the ability to directly compare and combine time series for species abundance.

Geographic, seasonal and bathymetric distribution of demersal fish species in the eastern Mediterranean

Summary The distributional abundance of three demersal fish species, Merluccius merluccius, Mullus barbatus and Lophius budegassa, was studied as a function of sampling season, bathymetry and geographic area. Data were collected during research trawl surveys in the Aegean and Ionian seas under the same sampling scheme, thereby allowing comparisons to be made on the mean regional densities in numbers and biomass of these three commercially important species in Greek waters. Results indicated that European hake, M. merluccius, demonstrated a wide bathymetric and geographic distribution, with specimens encountered in all regions between 20 and 500 m depth, although density was found to be highest usually between 101 and 200 m. The mean regional density of hake was observed to vary seasonally, being higher mainly in winter. Red mullet, M. barbatus, was distributed in shallow water depths (<100 m) throughout the Greek seas, particularly in the northern part of the Aegean Sea. Abundance of this species steadily decreased with increasing depth. The geographic distribution of anglerfish, L. budegassa, was found to be restricted to the central and northern part of the Aegean Sea, with the highest mean densities encountered in the Thermaikos Gulf and in intermediate water depths between 101 and 200 m. Spatial patterns of observed density are assumed to be attributed mainly to prevailing topographic and hydrographic conditions and related biological productivity levels.