Length–weight relationships of 216 North Sea benthic invertebrates and fish
Leonie A. RobinsonS.P.R. GreenstreetHenning ReissRuth CallawayJohan CraeymeerschIngeborg de BooisS. DegraerS. EhrichH. M. FraserA. GoffinIngrid KrönckeL. Lindal JorgensonM. RobertsonJ. E. Lancaster
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Size-based analyses of marine animals are increasingly used to improve understanding of community structure and function. However, the resources required to record individual body weights for benthic animals, where the number of individuals can reach several thousand in a square metre, are often prohibitive. Here we present morphometric (length–weight) relationships for 216 benthic species from the North Sea to permit weight estimation from length measurements. These relationships were calculated using data collected over two years from 283 stations. For ten abundant and widely dispersed species we tested for significant spatial and temporal differences in morphometric relationships. Some were found, but the magnitude of differences was small in relation to the size-ranges of animals that are usually present and we recommend that the regression relationships given here, based on pooled data, are appropriate for most types of population and community analyses. Our hope is that the availability of these morphometric relationships will encourage the more frequent application of size-based analyses to benthic survey data, and so enhance understanding of the ecology of the benthic/demersal component of marine ecosystems and food webs.Keywords:
Demersal fish
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Benthos
The influence of pelagic larval duration (PLD) and egg type dispersal capabilities of 35 demersal and pelagic‐spawning tropical fish species is examined in relation to their abundance on the temperate coasts of Japan. The PLDs of pelagic spawners were significantly longer than those of demersal spawners, and a high occurrence of pelagic spawners on the temperate coasts suggests that these fishes are more easily transported to temperate coasts than demersal spawners. For demersal spawners, the common species on the temperate coasts had significantly longer PLDs than the rare species; this suggests that PLD is a major factor influencing the distribution patterns of tropical demersal spawners on temperate coasts. Moreover, a negative correlation between PLD and the abundance of some species of pelagic and demersal spawners suggests the presence of reproductively active fishes in northern subtropical and even in temperate waters.
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Nekton
Marine larval ecology
Ichthyoplankton
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Bottom-water hypoxia (≤2 mg O2 l-1) usually occurs on an annual basis on the Louisiana/Texas continental shelf from mid-May through mid-September over a large area (up to 20,000 km2 in mid-summer). The effects of hypoxia on the benthic infauna (potential prey) for demersal fish were examined, because changes in optimal diet can lead to negative impacts on growth and reproduction. Benthic samples were taken in three areas (inshore and offshore out of hypoxia and in the hypoxic area) during August 2003. Samples were also taken monthly from September 2003 to October 2004 at a fixed station (C6B) where summer hypoxia occurs consistently. The mean abundance of the benthic infauna in the three summer areas were not significantly different indicating similar prey abundances found in the study area. Diverse infaunal communities exist offshore of the hypoxic zone with similar species composition compared to the inshore but different compared to the hypoxic area. An abundance of benthos at the surface was not found at the summer 2003 hypoxic stations; therefore there was not an abundance of available prey at the surface. However, benthos migrated toward the surface at station C6B in June and July 2004 during hypoxia, providing an increase of prey at the surface compared to other months. During the spring months, the infaunal community was more diverse and abundant compared to the post hypoxic months (August, September, and October), which suggests fewer and less diverse potential prey in the fall for demersal predators. The most abundant prey items for demersal fish in the study area were polychaetes and secondarily molluscs. The benthic community abundances during the summer 2003 and 2004 were not expected and may be due to the storm events in summer 2003 and shorter duration of hypoxia in summer 2004.
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The Saya de Malha (SMB) and Nazareth Banks (NB) are the main offshore locations where fishing activities are carried out by the Republic of Mauritius, targeting mainly shallow water Lethrinids, deep-water snappers and groupers. A multi-disciplinary survey was carried out on the two banks in May 2018 on-board the R/V Dr. Fridtjof Nansen with the objective of studying the diversity of fish on both banks using pelagic trawls, bottom trawls, basket traps and video using a Remotely Operated Vehicle (ROV). Analysis of data showed that the main fish family recorded in the pelagic waters of SMB, using pelagic trawl, was Myctophidae while the bottom trawls on NB showed the presence of fishes from the families Gobiidae, Triglidae and Synodontidae, mainly at depths between 200 and 300 m. The ROV video analysis highlighted principally the presence of demersal fishes of the family Lethrinidae and other reef-associated and commercial families recorded between 20 and 50 m. The main catch from the basket traps set at the SMB was Lethrinus mahsena, caught at a depth of 21 m. While previous studies focussed mainly on commercial fishes, this study brings forth new information on other fish families, contributing to the knowledge of the fish community that exists at these two banks.
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Pelagic and demersal cod (Gadus Morhua) specimens were collected respectively during a pelagic and a demersal survey in the Eastern Baltic sea in the fourth quarter of the years 2015-2017. Stomach contents were analysed and compared for the purpose of evaluating differences in the diet among pelagic and demersal specimens. Furthermore, generalized additive models (GAMs) were employed to investigate the daily fluctuations of stomach content weights of this predator. My results showed significant differences in the diet composition of demersal and pelagic cod mainly attributable to the higher weight share of sprat in the pelagic stomachs. Moreover, a remarkable diel variation in the stomach contents weights was present, indicating morning and evening peaks. The present study furnished novel insights into cod feeding pattern in the South-Western Baltic Sea. The implications of these findings for stock assessment multispecies models are also briefly discussed.
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Variations in assemblages of demersal fishes occurring in different depths of water (30, 60 and 100 m) on the inner continental shelf off Sydney are described. Assemblages comprised a diverse and abundant ichthyofauna; many species were of economic importance. Classification analyses showed that assemblages of demersal fishes at 30 and 60 m depth were most similar to each other and that they consistently differed from those at 100 m depth. This difference may reflect a change in the demersal ichthyofauna from a nearshore to an offshore assemblage. The depth-delineated differences between assemblages agree with those found in similar studies on demersal fishes in coastal waters elsewhere. The distributions and relative abundances of many species differed markedly among depths, but such differences were not always consistent between localities or throughout time. Despite this, some species showed some temporal affinity with a particular site and/or depth. The study confirmed spatial and temporal variabilities in the demersal fish fauna in coastal waters off Sydney; such heterogeneity must be considered when future studies are designed.
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This study proposed the newly-designed Pelagic and demersal trawls for the fishing vessels operating in Cameroonian waters in pelagic and demersal fishing grounds.The engineering performances of both trawls were investigated using physical modelling method and analytical method based on the predicted equations.In a flume tank, a series of physical model tests based on Tauti's law were performed to investigate the hydrodynamic and geometrical performances of both trawls and to assess the applicability of the analytical methods based on predicted equations.The results showed that in model scale, the working towing speed and door spread for the pelagic trawl were 3.5 knots and 1.85 m, respectively, and for the bottom trawl net they were 4.0 knots and 1.8 m.At that speed and door spread, the drag force, net opening height, and wing-end spread of the pelagic model trawl were 36.73N, 0.89 m, and 0.86 m, respectively, and the swept area was 0.76 m 2 .Bottom trawl speed and door spread were 30.43 N, 0.38 m, and 0.45 m, respectively, and the swept area was 0.25 m 2 .The maximum difference between the experimental and analytical results of hydrodynamic performances was less than 56.22% and 41.45%, respectively, for pelagic and bottom trawls, the results of the geometrical performances obtained using predicted equations were close to the experimental results in the flume tank with a maximum relative error less than 12.85%.The newly developed pelagic and bottom trawls had advanced
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Fish eggs can be divided into three main groups: marine pelagic eggs which float of their own accord, marine demersal, and freshwater demersal eggs. The embryological development of freshwater teleosts follows an accepted pattern, demonstrated by morphologists during the nineteenth century. Marine embryos, both pelagic and demersal, do not conform in all respects to this pattern, and misunderstandings have frequently arisen by lack of attention to known structural differences between the three groups. Mclntosh & Prince (1890), in their comprehensive review offish development, emphasized some outstanding anomalies in pelagic structure, but gave no adequate explanation of their occurrence.
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A description of the small-scale fisheries of the coastal pelagic and demersal resources off Kribi, Cameroon, is presented. The major fishing grounds are within the estuarine zone, an area of high productivity. Catch estimates of 19.5 t year super(-1) and 6.5 t year super(-1) were obtained for the pelagic motorized and nonmotorized canoes, respectively, while estimates of 11 t year super(-1) were obtained for the demersal motorized canoes. The social benefits from these fisheries are also discussed.
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Fish stomachs from 18 demersal and pelagic fi shes from the coast of Terengganu in Malaysia were examined. The components of the fi shes’ diets varied in number, weight, and their frequency of occurrence. The major food items in the stomachs of each species were determined using an Index of Relative Importance. A “conceptual” food web structure indicates that fi sh species in the study area can be classifi ed into three predatory groups: (1) predators on largely planktivorous or pelagic species; (2) predators on largely benthophagous or demersal species; and (3) mixed feeders that consume both pelagic and demersal species.
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