Showing papers in "Ices Journal of Marine Science in 2002"
TL;DR: In this paper, the volume, area, and line coefficients relevant to multiple, distributed targets are defined, leading to practical formulas for the important application of remote biomass estimation from echo-integration.
Abstract: Long-standing problems with acoustical terminology in fisheries applications such as echo-integration indicate the need for a more consistent approach. Based where possible on existing terms, a scheme of explicitly named quantities is proposed, backed by clearly stated definitions and preferred symbols. The emphasis is on scattering phenomena because the terminology in this area presents the main source of difficulty. Starting with the scattering equations for a small target, the volume, area, and line coefficients relevant to multiple, distributed targets are defined, leading to practical formulas for the important application of remote biomass estimation from echo-integration. The aim is to incorporate, as far as possible, common practice in fisheries-acoustics terminology and related fields. The developed scheme has been commended by the ICES Fisheries Acoustics Science and Technology Working Group as a constructive approach to better communication standards in fisheries-acoustics publications.
647 citations
TL;DR: In this article, the authors used T-S analysis on the Greenland continental slope from the Fram Strait to south of the Denmark Strait, where the East Greenland Current encounters the northward-flowing branch of the Irminger Current.
Abstract: The East Greenland Current is the main conduit for the waters of the Arctic Ocean and the Nordic Seas to the North Atlantic. In addition to low salinity Polar Surface Water and sea ice, the East Greenland Current transports deep and intermediate waters exiting the Arctic Ocean and Atlantic Water re-circulating in the Fram Strait. These water masses are already in the Fram Strait and are dense enough to contribute to the Denmark Strait overflow and to the North Atlantic Deep Water. On its route along the Greenland slope the East Greenland Current exchanges waters with the Greenland and Iceland Seas and incorporates additional intermediate water masses. In 1998 RV "Polarstern" and RV "Valdivia" occupied hydrographic sections on the Greenland continental slope from the Fram Strait to south of the Denmark Strait, crossing the East Greenland Current at nine different locations. The Arctic Ocean waters and the re-circulating Atlantic Water could be followed to just north of Denmark Strait, where the East Greenland Current encounters the northward-flowing branch of the Irminger Current. There strong mixing occurs both within the East Greenland Current and between the waters of the two currents. No distinct contribution from the Iceland Sea was observed in the Denmark Strait but the temperature reduction of the warm core of the East Greenland Current just north of the strait could partly have been caused by mixing with the colder Iceland Sea Arctic Intermediate Water. The overflow plume south of the sill was stratified and covered by a low salinity lid. Less saline overflow water was also observed on the upper part of the slope. The less saline part of the overflow was identified as Polar Intermediate Water and its properties were similar to those of the thermocline present in the East Greenland Current already in the Fram Strait. It is thus conceivable that its source is the upper (<0) part of the Arctic Ocean thermocline. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.Author Keywords: Arctic Ocean, Denmark Strait overflow, East Greenland Current, Nordic Seas, T-S analysis, water masses.
222 citations
TL;DR: The study shows that effective large-scale sampling of benthic communities can be conducted during existing fisheries surveys, and indicates that concurrent bentho-fauna surveys should be conducted to monitor the environmental impacts of trawling disturbance, climate change, pollution and other natural and anthropogenic factors.
Abstract: The structure of North Sea benthic invertebrate and fish communities is an important indicator of anthropogenic and environmental impacts. Although North Sea fish stocks are monitored regularly, benthic fauna are not. Here, we report the results of a survey carried out in 2000, in which five nations sampled the epibenthic and fish fauna at 270 stations throughout the North Sea. The aim of the survey was to investigate the diversity and community structure of epibenthic and fish communities and to identify relationships with environmental factors, including the frequency of commercial otter and beam trawling disturbance. Epibenthic species diversity was lower in the southern North Sea than in central and northern areas. Fish, conversely, were more diverse in the south. The 50 m, 100 m and 200 m depth contours broadly defined the boundaries of benthic and fish communities. The abundance of epibenthos of the southern North Sea was dominated by free-living species, whilst north of the 50 m contour sessile species prevailed. A hybrid area, with sessile species typical of the north and free-living species characteristic of the south, was found off the Norfolk and Flamborough coast stretching towards the Dogger Bank. Large-scale hydrodynamic phenomena were most likely to be responsible for the main divisions between communities, especially the boundary between mixed and stratified water masses. However, bottom temperature, sediment parameters and beam trawling were closely correlated with species richness and diversity, as well as community patterns, and may modify regional species composition. Our study shows that effective large-scale sampling of benthic communities can be conducted during existing fisheries surveys. Since annual fisheries surveys are conducted throughout the northeast Atlantic shelf seas, concurrent benthic surveys would allow benthic sampling on unprecedented spatial and temporal scales. The samples would help to monitor the environmental impacts of trawling disturbance, climate change, pollution and other natural and anthropogenic factors.
174 citations
TL;DR: In this article, the authors concluded that whale displacement resulted from the deliberate introduction of noise into their environment, and that both mammal-eating and fish-eating killer whales were similarly impacted.
Abstract: Whale displacement by acoustic “pollution” has been difficult to document, even in cases where it is strongly suspected, because noise effects can rarely be separated from other stimuli. Two independent studies on the natural history of killer whales (Orcinus orca) monitored frequency of whale occurrence from January 1985 through December 2000 in two adjacent areas: Johnstone Strait and the Broughton Archipelago. Four high-amplitude, acoustic harassment devices (AHDs) were installed throughout 1993 on already existing salmon farms in the Broughton Archipelago, in attempts to deter predation on fish pens by harbour seals (Phoca vitulina Linnaeus). While whale occurrence was relatively stable in both areas until 1993, it then increased slightly in the Johnstone Strait area and declined significantly in the Broughton Archipelago while AHDs were in use. Both mammal-eating and fish-eating killer whales were similarly impacted. Acoustic harassment ended in the Broughton Archipelago in May 1999 and whale occurrence re-established to baseline levels. This study concludes that whale displacement resulted from the deliberate introduction of noise into their environment.
173 citations
TL;DR: The results confirm the prominent role of habitat complexity in relation to artificial reef design on diversity and abundance of fish assemblages and show higher values of all community metrics investigated after treatment.
Abstract: Large artificial reef units (LARUs; 158 m 3 ) comprise 38% of 40 000 m 3 of artificial reefs deployed in France since 1985. Habitat complexity of one LARU was increased in situ in 1991 when 37 m 3 of small-sized building materials was placed randomly inside the empty chambers. The fish fauna, before and after added complexity, was compared with the fauna of an unmodified control LARU through 1987–1989 and 1997–1998, respectively, by visual censuses. The experimental LARU showed higher values of all community metrics investigated after treatment: total species richness had become twice as high, mean number of species per census 3 times higher, density 10 times, and biomass 40 times higher. Exceptionally large increases were observed among commercially important species such as sparids. In contrast, few changes were observed at the control unit: species richness had not changed significantly, while density and biomass showed moderate increases by a factor of 2–3, which might be explained by reef maturation. The results confirm the prominent role of habitat complexity in relation to artificial reef design on diversity and abundance of fish assemblages.
164 citations
TL;DR: Overall, there appears to have been a reduction in mortality during the last 30 years, which implies that seabird and seal food supply in terms of sandeel may be strongly dependent on decisions regarding management of stocks of mackerel and gadoids.
Abstract: Furness, R. W. 2002. Management implications of interactions between fisheries and sandeel-dependent seabirds and seals in the North Sea. – ICES Journal of Marine Science, 59: 261–269. The lesser sandeel, Ammodytes marinus, is a key food for many seabirds and seals, and is also the target of the largest single-species fishery in the North Sea. Despite claims that sandeel fishing has harmed dependent predator populations, census data show that most seabirds and grey seals increased in numbers as the fishery grew and reached peak harvest. Generally high breeding success of black-legged kittiwakes at North Sea colonies also suggests that sandeel abundance has remained good for breeding seabirds at the broad scale, though local and small-scale effects of sandeel fishing should not be overlooked. VPA and CPUE data suggest that abundance increased as the fishery grew. A negative correlation between sandeel recruitment and total stock size preceding spawning suggests that there is now resource competition (bottom-up control). Bioenergetics modelling indicates that predatory fish take far more sandeel than taken by the industrial fishery or wildlife. Effects of decreases in predatory fish stocks have been greater than increases in the take by seabirds and seals and by the fishery. Thus, overall, there appears to have been a reduction in mortality during the last 30 years. Changes in predatory fish abundances, especially mackerel and whiting, may influence sandeel stocks more than changes in industrial fishery, at least at the scale of the North Sea as a whole. These interactions imply that seabird and seal food supply in terms of sandeel may be strongly dependent on decisions regarding management of stocks of mackerel and gadoids. The overwhelming influence of predation on ‘‘food-fish’’ by predatory fish may be a feature of many marine food webs worldwide, where ‘‘fishing down the food web’’ has occurred, and this has clear management implications if wildlife and fisheries are to coexist. 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
159 citations
TL;DR: The potential for enhancing fish abundance, species richness, and biomass on artificial reefs was examined by attaching floating attractants and manipulating structural complexity of small concrete reefs each approximately 1.3 m in diameter, 1 m high.
Abstract: The potential for enhancing fish abundance, species richness, and biomass on artificial reefs was examined by attaching floating attractants and manipulating structural complexity of small concrete reefs each approximately 1.3 m in diameter, 1 m high. Experimental design consisted of a comparison of fish assemblages among three treatments (10 replicate, hemisphere-shaped reefs each): 10-m floating line attached (Streamer); concrete block in the central void space (Block); and no floating line or concrete block (Control). Reefs were deployed on sandy substrate at 20-m depth off Fort Lauderdale, Florida, USA. Divers recorded fish census data on slates 18 times over 24 months. Species composition, numbers of individuals per species, and estimated total length (TL; by size class: 20 cm) for all fishes within 1 m of each reef were recorded. Size classes were used to calculate fish biomass. There was a significant difference among treatments. Block reefs had higher numbers of individuals, species, and biomass than Streamer or Control reefs (p 0.05). These results highlight the importance of structural complexity in artificial reefs designed to enhance fish recruitment, aggregation, and diversity. Copyright 2002 Published by Elsevier Science Ltd on behalf of the International Council for the Exploration of the Sea.
154 citations
TL;DR: In this paper, a special version of the Simrad EK500 multi-frequency, split-beam echosounder and the Bergen Echo Integrator (BEI) post-processing system were used.
Abstract: Calibrated and digitized data from two or more discrete echosounder frequencies can be combined for the purpose of separating and extracting the acoustic scattering from zooplankton and fish in mixed recordings. This method is also useful for quantifying the relative contribution of each frequency to the total acoustic-backscattering when scrutinizing records in large-scale, acoustic surveys. Echosounder hardware requirements are defined which would permit the ideal extraction of such information. These include calibration, transducer specification, pulse resolution and digital representation of the signals. During this initial study a special version of the Simrad EK500 multi-frequency, split-beam echosounder and the Bergen Echo Integrator (BEI) post-processing system were used. The echosounder transmitted pulses simultaneously at four frequencies, 18, 38, 120 and 200 kHz and transferred the received signals to the post-processing system in calibrated, raw, digitized format. Methods are described for echogram manipulation and for the construction of new, synthetic, combined-frequency [c(f)] echograms. Examples of extracted scattering information from mixed layers of fish and small scattering-organisms, such as copepods and euphausiids, are shown, and the potential of the method is discussed.
146 citations
TL;DR: Artificial reefs have been placed in European waters for around 30 years as discussed by the authors, and the majority now play a role in protecting valuable Mediterranean seagrass beds from trawl damage, and most aspire to a fisheries function.
Abstract: Artificial reefs have been placed in European waters for around 30 years. The majority now play a role in protecting valuable Mediterranean seagrass beds from trawl damage, and most aspire to a fisheries function. Until relatively recently, reef-building has been carried out locally, in some cases without national collaboration or international cooperation. This is changing; in 1991, Italian artificial reef scientists formed a national reef group to encourage liaison between research groups, and the Spanish created one in 1998. There is now also an association of Mediterranean artificial reef scientists. Research in Europe has reached a stage where scientific priorities for the future need to be developed in the light of previous research and experience. This is the aim, and the reason for the creation in 1995, of the European Artificial Reef Research Network (EARRN) funded by the European Commission "AIR" programme. Reefs have now been formally licensed and deployed in Finland, France, Greece, Israel, Italy, Malta, Poland, Portugal, Romania, Spain, The Netherlands, Turkey, Ukraine, and the United Kingdom, and Denmark, Ireland, Russia, and Sweden have an interest, although no specific reef structures have, as yet, been placed. Norway has deployed experimental concrete units and has an interest in the "rigs-to-reefs" concept.
141 citations
TL;DR: In this article, the authors examined growth, condition, and reproductive potential with respect to capelin diet in 18, 000 cod sampled primarily in January and June of the years 1996-2000.
Abstract: The stock of northern cod (Gadus morhua) off Newfoundland and Labrador is at its lowest level in recorded history, with no rebuilding of northern spawning aggregations since the fishing moratorium in 1992. Cod diet was historically dominated by capelin (Mallotus villosus), which have been scarce in the northern areas since 1990. Using the study areas Hawke Channel and Trinity Bay within the historical northern cod range, and Placentia Bay (south coast), we examine growth, condition, and reproductive potential with respect to capelin diet in 18 000 cod sampled primarily in January and June of the years 1996-2000. Overall diet weights differed among areas and seasons (Placentia=Hawke>Trinity in January; Placentia>Trinity=Hawke in June). However, just 7 of 3383 cod stomachs (0.2%) from Hawke contained capelin (PFI=0). In contrast, 10% of cod of ages 3-7 in Placentia and Trinity Bays preyed on capelin and overall capelin partial fullness indices (PFI) were 0.2-0.4. Capelin PFIs were correlated with an index of availability (potential contact of capelin within 40 km of cod) calculated from acoustic surveys. Seasonally adjusted cod liver condition was strongly associated with capelin availability (p Trinity≫Hawke).
131 citations
TL;DR: Kang et al. as mentioned in this paper derived the common observation range of at least up to 150 m for a quantitative echosounder operating at 38 and 120 kHz with the same beam widths of 8.5.
Abstract: Kang, M., Furusawa, M., and Miyashita, K. 2002. Effective and accurate use ofdifference in mean volume backscattering strength to identify fish and plankton. –ICES Journal of Marine Science, 59: 794–804.Acoustic species identification is very important for fisheries’ operations and surveys.One of the most promising methods for identification is to utilize the difference ofmean volume backscattering strengths ( MVBS) among frequencies. Improvement ofthis technique is the aim of this study. The MVBS must be obtained for a commonobservation range among frequencies so that the difference can be attributed solely tofrequency characteristics of the sound scattering of targets organisms. We derived thecommon observation range of at least up to 150 m for our quantitative echosounderoperating at 38 and 120 kHz with the same beam widths of 8.5 . We related MVBSdata obtained off northeastern Japan to specific marine organisms combined with theswimming depth and water temperature information. The echoes with small MVBS( 1dB< MVBS<4 dB) were attributed to the walleye pollock (
TL;DR: Application of the reconstruction method to a set of measurements on Northeast Arctic cod (Gadus morhua) collected between 1933 and 1944 illustrates how the approach allows new information to be extracted from real data.
Abstract: Reaction norms for age and size at maturation describe the probability of immature fish maturing at a certain age and size. Knowledge of such reaction norms is increasingly important, both for observing and understanding changes in maturation and for calibrating size- and age-structured population models. Estimating the reaction norms for age and size at maturation by logistic regression requires data on the size and age distribution of immature and maturing fish. To permit such estimation when measurements of the size and age distribution of immature fish are not available, the information can be reconstructed by means of a back-projection procedure. For the reconstruction, only the size and age distribution of maturing fish, or first-time spawners, together with the age-dependent proportion of mature fish, given in the form of maturity ogives, have to be known. The method of Gulland (1964) is used to generate maturity ogives in the absence of data on immature fish. The robustness of the estimation method is demonstrated by analysing artificial data generated with a known reaction norm. Application of the reconstruction method to a set of measurements on Northeast Arctic cod (Gadus morhua) collected between 1933 and 1944 illustrates how the approach allows new information to be extracted from real data. 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
TL;DR: Salinity requirements for successful egg development and opportunities for the eggs to obtain neutral buoyancy suggest that successful reproduction may occur regularly in the Sound, the Arkona, and Bornholm basins as well as in the Gdansk and Gotland basins, and that there are three stocks of flounder with pelagic eggs.
Abstract: The reproductive success and thus abundance and distribution of dab, plaice, and flounder in the Baltic Sea, a large brackish water area, is restricted by salinity. By measuring spermatozoa mobility and fertilisation rates at different salinities and determining the salinity at which eggs are neutrally buoyant, the salinity requirements for successful egg development were assessed. The results were used for the evaluation of potential spawning areas and for stock discrimination by analyses of differences in the salinity requirements of fish from different areas (ICES Subdivisions (SD) 23-28). The results suggest that there are two stocks of dab and successful reproduction may occur in the Sound (SD 23) and, occasionally, in the Arkona and Bornholm basins (SD 24 and SD 25). Opportunities for successful reproduction of plaice exist regularly in the Arkona and Bornholm basins and occasionally in the Gdansk and Gotland basins (SD 26 and SD 28). No differences in salinity requirements for fish from SD 24-28 suggest one stock of plaice in the Baltic proper. There are two different types of flounder, one with demersal eggs and the other with pelagic eggs. The former, constituting one distinct stock, may reproduce successfully as far north as the Bothnian Sea and the Gulf of Finland (SD 30 and SD 32), up to the 6 psu isohaline. For flounder with pelagic eggs, opportunities for the eggs to obtain neutral buoyancy suggest that successful reproduction may occur regularly in the Sound, the Arkona, and Bornholm basins as well as in the Gdansk and Gotland basins, and that there are three stocks of flounder with pelagic eggs.
TL;DR: In this article, the main oceanographic features of Icelandic waters and the Iceland Sea are described and capelin migrations related to the distribution of water masses and the ocean current systems in the area.
Abstract: The stock of capelin, Mallotus villosus (Muller), that inhabits the area between Iceland, East Greenland, and the island of Jan Mayen spawns in shallow coastal water south and west of Iceland. Juveniles grow up over the continental shelf off north Iceland and off East Greenland west of the Denmark Strait. The main feeding area of adults is the Iceland Sea, the oceanic area from about 68 to 72°N, between the Jan Mayen Ridge and the East Greenland continental shelf. After the feeding season, the adult stock assembles over the outer shelf off north Iceland and migrates to the spawning grounds along the south and west coasts from December to March. The main oceanographic features of Icelandic waters and the Iceland Sea are described and capelin migrations related to the distribution of water masses and the ocean current systems in the area. In the past two decades there have been large variations in capelin migrations and catchability, especially during the feeding season. However, these variations can only be explained in part by the available environmental data. Year-class abundance appears to be determined by survival during the first winter, in tune with the greater environmental variability off north Iceland than south and west of Iceland, where these capelin spawn and the larvae start drifting. Adult growth is positively related to the flow of Atlantic water into the area north of Iceland, indicating improved feeding conditions in the Iceland Sea when the Irminger Current is strong. There can be large interannual variations in number and weight-at-age in the adult stock. The main predators are whales, seabirds, and fish, especially cod. The combined annual removal by predators is estimated to have been 2.1-3.4 million tonnes in the early 1990s. The mean weight-at-age of cod aged 5-8 years dropped by up to 25-30% when capelin abundance was low in the early 1980s and 1990s. The relatively low mean weight of cod in the past 3 years may well be due to changed distribution and migration of capelin, resulting in reduced access of cod to this most important item in their diet.
TL;DR: The finding that long-lived, slower growing, late maturing species are highly vulnerable to exploitation is supported, and conservation measures should focus on juvenile and adult stages of these species.
Abstract: Over the past thirty years catches of skates have increased in the western Atlantic as a result of targeted fisheries and as by-catch. Presently, sustainable harvest levels for skates in the western Atlantic are unknown. Available life history information was used to model three western Atlantic skate species, little skate Leucoraja erinacea, winter skate Leucoraja ocellata, and barndooor skate Dipturus laevis, to determine their population growth rate and susceptibility to population decline under exploitation. Population characteristics were estimated using age-based matrix analyses for little and winter skate and a stage-based matrix approach for barndoor skate. The intrinsic rate of population increase (r) for little, winter and barndoor skates were 0.21, 0.13 and 0.20, respectively. Fishing mortality resulting in equilibrium conditions, where the intrinsic rate of increase is zero, were 0.35 for little skate, 0.16 for winter skate and 0.20 for barndoor skate. Elasticity analyses indicated that juvenile survival contributed most to population growth in little skate and winter skate, and adult survival contributed the most in barndoor skate. Thus, conservation measures should focus on juvenile and adult stages of these species. In the absence of fishing mortality, elasticity of fecundity was low for all species, however; at high exploitation levels the contribution of fecundity to population growth rate increased. Stochastic analyses were performed by varying first year mortality (egg stage to the completion of one year of life), and model parameters for each species to generate estimates of the distributions of biological reference points. Our study supports the finding that long-lived, slower growing, late maturing species are highly vulnerable to exploitation. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
TL;DR: For scavenger species that live in areas where discarding is continuous, such additional inputs could be a major trophic resource and for this reason, continuous anthropogenic inputs may bring about environmental perturbations that can alter benthic ecology and community stability.
Abstract: The consumption rate of fishery discards is evaluated and the organisms involved in recycling discarded fish analysed. For this purpose, a fixed baited camera was deployed on the continental shelf and upper slope of the Catalan Sea (northwestern Mediterranean), where trawlers discard large quantities of non-target species. The results showed considerable bait consumption after 12 h of immersion. Between 48 and 64% of the bait was consumed within this period on the shelf, at a mean rate of 23.8±5.7 g h −1 , and >90% of the bait on the slope, at a mean rate of 30.8±10.5 g h −1 . After 24 h, generally just the bones of the bait were left. Amphipods, isopods, cephalopods, ophiuroids, nine species of fish, and seven decapods were identified feeding on the bait. Typical scavenger species stayed longer in the vicinity of the bait, but pelagic species exhibited only occasional scavenging behaviour, were slower to arrive at the bait, and remained for a shorter time. The snake eel, Ophichthus rufus, was the most abundant fish attracted to the bait, appearing in 75% of the photographic series. Isopods and amphipods reached the bait 80 minutes after camera deployment and were also involved in the recycling and dispersal of organic matter in the system. In fact, these organisms congregated at the bait and consumed it entirely in just a few hours. For scavenger species that live in areas where discarding is continuous, such additional inputs could be a major trophic resource. For this reason, continuous anthropogenic inputs may bring about environmental perturbations that can alter benthic ecology and community stability.
TL;DR: No reliable auxiliary variable could be found when various factors were investigated to explain the quantities of commercial species discarded and the only explanatory factor valid for any species was the smallest size of that species in the catch.
Abstract: Discards of the French trawler fleet operating in the Celtic Sea in 1997 were studied. Twenty-six trips, representing 0.8% of the total fishing effort, were sampled. This fleet consists of three metiers, benthic trawlers, Gadoid trawlers and Nephrops trawlers. The fleet discarded an estimated 30 000 tons of animals in 1997, while landing 63 000 tons. The total quantity discarded did not differ among metiers, but the species composition of discards did. Benthic trawlers discarded mainly by-catch species, whereas Gadoid and Nephrops trawlers discarded primarily their target species. Whiting, megrim, Nephrops and hake were discarded in larger numbers than landed. Hence discards should be taken into account in catch-based assessments. However, discards were found to be highly variable between trips and between years. In addition, no reliable auxiliary variable could be found when various factors were investigated to explain the quantities of commercial species discarded. The only explanatory factor valid for any species was the smallest size of that species in the catch. Hence there is a need for regular sampling of discards.
TL;DR: The porbeagle is primarily an opportunistic piscivore with a diet characterized by a wide range of species and diet composition changed seasonally following a migration from deep to shallow water.
Abstract: Stomachs of 1022 porbeagle sharks (Lamna nasus) ranging in size from 85-264 cm were examined from the Canadian porbeagle shark fishery and a scientific cruise in the northwest Atlantic between February 1999 and January 2001. Teleosts occurred in the majority of stomachs and constituted 91% of the diet by weight. Cephalopods occurred in 12% and were the second most important food category consumed. Pelagic fish and cephalopods comprised the largest portion of the diet in the spring while groundfish dominated the diet in the fall. Diet did not differ significantly between the sexes. Stomach fullness differed slightly but significantly across months and declined slightly with fork length. The porbeagle is primarily an opportunistic piscivore with a diet characterized by a wide range of species. Diet composition changed seasonally following a migration from deep to shallow water.
TL;DR: In this article, generalized linear models (GLMs) were applied to assess the influence of depth, latitude, length, year, sex and reproductive status on condition of ten exploited fish species.
Abstract: We examine the variation in condition of ten exploited fish species as an indicator of large-scale habitat quality in the north-western Mediterranean. To determine fish condition, we used morphometric (Fulton's K) and physiological (hepatosomatic, HSI; digestivesomatic, DSI; and gonadosomatic, GSI) indices. Generalized linear models (GLMs) were applied to assess the influence of depth, latitude, length, year, sex and reproductive status on condition. The GLMs incorporating all independent variables accounted for between 25 and 74% of the variance in Fulton's K condition factor, 15-77% of the variance in HSI, 14-35% of the variance in DSI, and 77% of the variance in GSI. The mean condition factor of the species studied showed considerable interannual fluctuations along with an increasing trend over the time series (1994-2001). Length is an important explanatory variable in explaining condition of fish species. Condition typically increased with fish length and explained 72% of the deviance in GSI of M. barbatus. For some species, there were differences in condition due to sex and reproductive status, with adults being usually better in condition than juveniles and, among adults, females better than males. The influence of latitude and depth on condition of a given species varied according to the index used to assess condition. When there was a significant effect, fish with the highest condition factor, HSI and/or DSI inhabited the shallowest and northernmost parts of the area of study. Considering that condition influences growth, reproduction and survival of fish, our results support the importance of shallow habitats to productivity of demersal stocks in the Mediterranean Sea.
TL;DR: It is suggested that the female reproductive cycle is biennial, similar to those from other populations, although size-at-maturity of males was slightly different from South African and Australian populations.
Abstract: This study analyses the sexual activity and segregation of sand tiger sharks, Carcharias taurus, from Anegada Bay (Argentina). Reproduction-linked movements along the South American Atlantic coast were inferred from data from several SW Atlantic localities. Male sand tigers (n=162) matured at 193 cm total length (L T ). Females (n=77) matured between 218 and 235 cm L T . These figures are similar to those from other populations, although size-at-maturity of males was slightly different from South African and Australian populations. In females, the size of ovarian follicles was positively correlated with gonadosomatic index and negatively correlated with hepatosomatic index, while the liver was significantly larger than in males. Sand tiger sharks were present in Anegada Bay from December to April. Males were significantly more abundant than females (2:1). Significant differences in reproductive condition through time were observed in males. During January and February males had seminal vesicles full of spermatozeugmata but by March and April the vesicles were empty. As the proportion of males with a lighter colouration peaked from January to March, it is very likely that mating takes place during January and February. The skewed sex-ratio during the mating season indicates a possible strong competition for mates among males, as observed in captivity. Males, females and some juveniles occur in Argentinean and Uruguayan waters, where mating takes place. Pregnant females occur in subtropical waters of southern Brazil, where they give birth. Given that pregnant and non-pregnant females occur at the same time in different zones, we suggest that the female reproductive cycle is biennial. Striking differences among migratory patterns of sand tiger sharks from the SW and NW Atlantic and South Africa were observed. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.
TL;DR: Increasing populations of seabirds are related in part to their lessened mortality from gillnets since the closures of the eastern Canadian groundfishery in 1992, whereas declining populations of gulls can be related to reduced food availability from fishery discards and offal.
Abstract: Trophic interactions among seabirds and capelin (Mallotus villosus), a key forage species off Newfoundland and Labrador, are examined. During the 1990s, estimates of relative year-class strength of capelin were similar to estimates in the 1980s, capelin spawned later and matured younger, mean fish size was smaller, and there were large-scale distributional shifts of capelin. Most of these changes were linked to below-normal sea temperatures during the early 1990s, but the changes have persisted even though temperatures have returned to normal. Seabirds bred later in the 1990s and changed diets and foraging strategies. Off eastern Newfoundland, the breeding success of black-legged kittiwakes declined during the early 1990s owing to a suite of factors, including their inability to dive to capture capelin, the late arrival of capelin inshore and predation by gulls. Common murres and puffins did not suffer breeding failures because they could dive and catch capelin at depth. Off Labrador, black-legged kittiwakes experienced breeding failures because of the lack of capelin while common murres were able to find alternate prey and bred successfully. The diets of gannets in the 1990s contained a higher proportion of cold-water prey than in earlier periods when warm-water prey predominated. The population sizes of all seabird species remained stable or increased, with the exception of gulls. Increasing populations of seabirds are related in part to their lessened mortality from gillnets since the closures of the eastern Canadian groundfishery in 1992, whereas declining populations of gulls can be related to reduced food availability from fishery discards and offal. This food limitation has led gulls to switch much of their foraging effort from scavenging to predation on seabird adults and chicks before capelin arrive inshore.
TL;DR: In this article, an experimental scrap tyre artificial reef was constructed in Poole Bay to provide data on the environmental impact of the material, which was monitored for comparison with that on concrete control modules deployed at the same time.
Abstract: Scrap tyres have been widely used around the world to construct artificial reefs. They are a popular construction material, being readily available at no cost, durable, and with large void spaces. However, published information about the environmental impact of tyres in the marine environment is limited. When used, successful colonization by epibiota and mobile species seems to be taken as empirical proof of their suitability. In 1998, an experimental scrap tyre artificial reef was constructed in Poole Bay to provide data on the environmental impact of the material. Epibiotic colonization is being monitored for comparison with that on concrete control modules deployed at the same time. Samples of the epibiota have been analysed for heavy metals and organic compounds. Results from the first year of deployment are presented.
TL;DR: In this article, the authors examined variation at seven microsatellite loci and PanI in some 5230 Atlantic cod (Gadus morhua) from 19 inshore and offshore locations around Newfoundland and Labrador.
Abstract: We examined variation at seven microsatellite loci (Gmo3, Gmo8, Gmo19, Gmo34, Gmo35, Gmo36, and Gmo37) and PanI in some 5230 Atlantic cod (Gadus morhua) from 19 inshore and offshore locations around Newfoundland and Labrador. The mean F ST estimate over all loci was 0.0080. Overall, the cod populations surveyed conformed to an isolation-by-distance structure, cod from more distant locations tending to be more genetically distinct. Among offshore sites, the Flemish Cap population (NAFO Division 3M) was the most distinctive, and among inshore sites, the Gilbert Bay population in southern Labrador (2J) was the most distinctive. In NAFO Divisions 3KL, no significant genetic differentiation was observed among inshore northern cod sampled in four bays (Notre Dame, Bonavista, Trinity, and Conception) along the northeast coast of Newfoundland, and the data do not support the hypothesis of separate “bay stocks”. Annual variation within sampling sites was as large, on average, as the differentiation among sampling sites. The inshore northern cod were distinct from the population in Gilbert Bay and from most offshore northern cod populations. On average, over all populations, regional differences in allele frequencies were seven times larger than annual variation. The offshore samples were more heterogeneous, and there may be at least three distinct offshore spawning populations of northern cod. In Subdivision 3Ps, no consistent differentiation was observed among sampling sites, two in inshore bays (Placentia, Fortune), and two offshore (Burgeo Bank, Halibut Channel). The southern Grand Bank (Divisions 3NO) may have a separate spawning population from those in other offshore sites, but additional sampling is required to confirm population distinctiveness.
TL;DR: Net and acoustic data from two cruises around South Georgia in 1996 are compared to assess the reliability of acoustic-based, target-identification techniques and use of ΔMVBS to identify Antarctic krill is advantageous because it is more objective than using echo-chart appearance.
Abstract: Acoustic surveys to estimate krill biomass require that the sound backscattered by krill can be identified and distinguished from all other types of backscatter. Sampling acoustic targets with nets to verify their identity have achieved this traditionally. More recently backscattered sound has been partitioned into krill or non-krill groups using differences in mean volume backscattering strength (MVBS) at two acoustic frequencies (ΔMVBS=MVBS120−MVBS38; where ΔMVBS between 2 and 12 dB indicated krill). Here we compare net and acoustic data from two cruises around South Georgia in 1996 to assess the reliability of acoustic-based, target-identification techniques. MVBS data at 120 and 38 kHz were collected with a Simrad EK500 echosounder and net samples were collected with an RMT8. Around 80% of the echo integration cells from targets believed to be krill on the basis of their appearance on echo-charts, were also identified as krill from their difference in backscatter at 38 and 120 kHz. Krill biomass estimated from acoustic targets identified using echo-chart appearance or ΔMVBS were broadly similar (regression: ΔMVBS=0.94 visual classification, r2=0.99). Krill size was calculated from scattering models using the two frequency data and compared with that obtained in net hauls. This comparison revealed that a simplified bent-cylinder model was a better predictor of krill length than a fluid-filled, sphere model. We conclude that use of ΔMVBS to identify Antarctic krill is advantageous because it is more objective than using echo-chart appearance.
TL;DR: Bjorn et al. as mentioned in this paper examined the abundance of salmon lice in two stocks of sympatric anadromous Arctic char and sea trout in sub-Arctic regions in northern Norway in June, July, and August 1992 and 1993.
Abstract: Bjorn, P. A., and Finstad, B. 2002. Salmon lice, Lepeophtheirus salmonis (Kroyer), infestation in sympatric populations of Arctic char, Salvelinus alpinus (L.), and sea trout, Salmo trutta (L.), in areas near and distant from salmon farms. – ICES Journal of Marine Science, 59: 131–139. The abundance of salmon lice was examined in two stocks of sympatric anadromous Arctic char and sea trout in sub-Arctic regions in northern Norway in June, July, and August 1992 and 1993. One stock feeds in a coastal area exposed to moderate salmon farming activity (exposed area), while the other feed in a region without salmon farms (unexposed area). The salmon lice infestation on both species differed significantly between the exposed and unexposed area as well as between years and also between weeks within the same year. We did not detect, however, any clear significant differences in salmon lice abundance between sympatric populations of Arctic char and sea trout, or between different size groups of the species. The 1992 and 1993 infestation pattern in the exposed area showed an epidemic tendency in both Arctic char and sea trout, characterised by a sudden increase in both prevalence and abundance of lice larvae in July 1992 (23.625.7 lice/fish) and August 1993 (19.920.8 lice/fish). We therefore suggest that salmon lice epidemics, previously only observed on sea trout, may also occur in populations of Arctic char, and that fish farming contributes to the elevated lice level in wild fish. The fish in the unexposed area were also infested, although at significantly lower levels than fish from the exposed area. The infestation peaked in August 1992 at 13.018.1 lice/fish and August 1993 at 3.94.5 lice/fish, suggested that lice originating on ascending wild Atlantic salmon, or lice larvae drifting from farming areas, may infest Arctic char and sea trout also in unexposed localities in Subarctic areas. 2002 International Council for the Exploration of the Sea
TL;DR: In this paper, a digital acoustic seabed classification system, QTC View (Series IV), was used in the coastal waters of Newfoundland to characterize and classify marine benthic habitats.
Abstract: A digital acoustic seabed classification system, QTC View (Series IV) was used in the coastal waters of Newfoundland to characterize and classify marine benthic habitats. The QTC View system was calibrated in Placentia Bay at sites identified independently during a submersible research program. Four different habitats were used for calibration of the QTC View system: mud, gravel, rock, and macroalgae on rock. These different habitats were used as a ‘‘training’’ catalogue for real-time classification of marine habitats carried out in Bonavista Bay. The classification data were based on over 2000 km of survey tracks ranging in depth from approximately 10-m to 220-m depth. Post classification analyses were carried out using data visualization techniques, simultaneously comparing the classification data in mathematical and geographic settings. Following post classification, eight different marine habitats were identified using the acoustic system: mud, loose gravel, gravel, rock, sparse algae/cobble, macroalgae, high relief/deep cobble, and wood chips. Throughout the surveyed area, rock habitat dominated, followed by sparse algae/cobble and high relief/cobble habitat types. The wood chip habitat type was identified within a small area that historically had been associated with logging in coastal Newfoundland.
TL;DR: In this article, the transition from single-beam to multibeam echo sounders is discussed, and the types of equipment investigated, the limitations of several acoustic systems examined, and how geomatics and 3D visualization can be used to enhance our knowledge of pelagic fish schools.
Abstract: Traditional acoustic approaches to the estimation of fish biomass have relied upon single-beam echo sounders that sample a relatively small volume of the water column within the survey area. Mean transect values, after bottom removal, are extrapolated to provide an estimate of number or biomass within a survey area. Over the past 20 years, many multibeam sonars have been developed but these systems which are commonly employed to collect detailed bathymetric and seafloor-type data, have been designed to remove mid-water returns. Only recently has a multibeam sonar been developed that allows for continuous digital recording of mid-water returns. For fisheries acoustics, the movement from single-beam to multibeam surveys provides a mechanism to greatly enhance the area and volume of coverage. The large volume of data generated by these systems, however, presents serious challenges for analysis and interpretation. This paper describes initial studies related to the transition from single to multibeam applications including the types of equipment investigated, the limitations of several acoustic systems examined, and how geomatics and 3D visualization can be used to enhance our knowledge of pelagic fish schools. Early results indicate that multibeam sonars, in conjunction with 3D visualization software can be powerful tools for assessing fish stocks, investigating fish school behavior, for exploring habitat preferences and for addressing questions related to vessel avoidance. As the technology improves so will the capability to investigate and to incorporate additional multi-parameter data such as water column properties and bottom type and as calibration techniques are developed for multibeam sonars estimates of biomass may also be possible.
TL;DR: The radius of the lapillus at hatching was poorly correlated with larval length at day 56 for both stocks, suggesting that the potential for fast growth may not necessarily be reflected in traits present at hatch, according to the assumption of constant proportionality between otolith growth and fish growth.
Abstract: The otolith (lapillus) size-fish size relationship was examined for offspring of two Atlantic cod stocks, reared at different temperatures. Larvae and early juveniles reared at high temperatures (fast growing), had larger otoliths at a given length than fish reared at low temperatures (slow growing). Within a given temperature group, however, faster growing cod tended to have proportionally smaller otoliths, although the difference was not always significant. Moreover, the otolith radius of Norwegian coastal cod was larger, at given fish lengths, compared to that of the northeast Arctic cod. An ontogenetic shift in the allometric otolith size-fish size relationship occurred at onset of metamorphosis (12 mm). Mean daily otolith growth of the lapillus radius increased with increasing temperature from 4 to 14°C and was size dependent and peaked at a larval length of about 25 mm. The radius of the lapillus at hatching was poorly correlated with larval length at day 56 for both stocks, suggesting that the potential for fast growth may not necessarily be reflected in traits present at hatching. The effects of temperature, stock and ontogeny are discussed with regard to the assumption of constant proportionality between otolith growth and fish growth.
TL;DR: In this article, the authors developed a model that incorporates the simultaneous effects of habitat augmentation, competition among reefs for larval settlers, and post-settlement density-dependence, and proposed two experimental approaches for evaluating the effects of artificial reefs on local production of natural reefs.
Abstract: Artificial reefs have been proposed as a tool to mitigate impacts on natural reefs, augment natural fish production, and divert detrimental harvesting activities from sensitive natural habitats. The efficacy of this strategy depends on the extent to which artificial reefs contribute to new production or simply redistribute fish during or after settlement. Small coral reef fishes are ideal study subjects because redistribution is most likely during the larval stage, given limited dispersal of benthic stages. We develop a model that incorporates the simultaneous effects of habitat augmentation, competition among reefs for larval settlers, and post-settlement density-dependence, and propose two experimental approaches for evaluating the effects of artificial reefs on local production of natural reefs. One is based on small-scale studies using replicated patch reefs, the other on unreplicated studies using larger reefs and the Before-After-Control-Impact Paired Series (BACIPS) design. Using field data for six fish species, we estimated spatial and temporal variance and thus statistical power of both designs. Power varied among species, but in most cases was sufficiently high to detect local reductions in density of 10–40% with modest levels of replication (6–20 patch reefs or sampling dates). 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd.
TL;DR: In this article, the authors investigated the benthic communities of the soft seabed inside and outside the Cesano-Senigallia artificial reef (central Adriatic Sea) were investigated seasonally for 2 years (spring 1997 to winter 1999) to evaluate the effects of the reef on the surrounding bottom.
Abstract: The benthic communities of the soft seabed inside and outside the Cesano-Senigallia artificial reef (central Adriatic Sea) were investigated seasonally for 2 years (spring 1997 to winter 1999) to evaluate the effects of the reef on the surrounding bottom. The community living close to the reef is typical for the coastal area. Mean species richness and densities over the entire sampling period were comparable inside and outside the reef, while higher values were recorded at the control site. However, the Shannon-Wiener diversity index was higher close to the reef than at the control. Physical factors associated with the presence of the artificial structures appear to affect composition and abundance of the infauna community more than biological factors do (predation). The area inside the reef was characterized by fine mud favouring the settlement of deposit and suspension feeders, mainly polychaetes. Outside the reef, molluscs were numerically dominant, with an increasing proportion of sandy-bottom species with increasing distance from the structures. These differences were more marked in spring and autumn, while in the other seasons the reef effect appeared to be reduced to a narrow area close to the structures. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.