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Groundfish survey data from the German Bight from 1902–08, 1919–23, and 1930–1932 and ICES International Bottom Trawl Survey (IBTS) quarter 3 data from 1991 to 2009 were analysed with respect to species frequencies, maximum length, trends in catch-per-unit-effort, species richness parameters (SNR) and presence of large fish (Φ 40 ), the latter defined as average presence of species per haul with specimens larger than 40 cm given. Four different periods are distinguished: (a) before 1914 with medium commercial CPUE and low landings, Φ 40 ≈ 2, high abundance in elasmobranchs and SNR conditions indicating highly diverse assemblages, (b) conditions immediately after 1918 with higher commercial CPUE, recovering landings, Φ 40 at > 4 in 1919, and SNR conditions indicating highly diverse assemblages, (c) conditions from 1920 to the early 1930's with decreasing commercial CPUE, increased landings, decreasing Φ 40 , SNR conditions similar to later years indicating less diverse assemblages, and a decrease in elasmobranchs. In the IBTS series (d), Φ 40 remains low indicating an increased rarity of large specimens, and SNR characteristics are similar to the third period. Dab, whiting and grey gurnard have increased considerably in the IBTS series as compared to the historic data. Φ 40 is suggested an alternative indicator reflecting community functional diversity when weight based indicators cannot be applied.

Lesser sandeels Ammodytes marinus are eaten by a range of predatory fishes including commercially fished species, but are also exploited at large scale by industrial fisheries. Is availability of sandeels, as key prey source, linked to the body condition of predatory fishes? In the North Sea, the largest sandeel biomass is concentrated in the Dogger Bank region. Here we studied predator–sandeel interactions at two sites differing widely in sandeel abundance and local sandeel fishing effort. Surveys took place in 2004, 2005, and 2006, years when local sandeel densities observed at these sites were low, intermediate, and high, respectively. Five predator species––whiting, lesser weever, grey gurnard, plaice, and haddock––showed better body condition indices in either the years or study area (or both) characterised by higher local sandeel densities, when compared to sandeel-poorer conditions. Moreover, whiting, weever, and gurnard condition was better for those individuals actually observed to have eaten sandeels (based on stomach contents) than for those that had not. As body condition relates to growth, reproduction, and survival, predators in sandeel-rich conditions may be inferred to have a higher fitness. These links between sandeel availability, sandeel consumption, and predator condition hint that, if large-scale localised depletions of sandeels were to occur, negative indirect effects on predatory fish might become apparent, underlining the importance of considering the sandeel fishery in an ecosystem context.

Uhlmann, S. S., Bierman, S. M., and van Helmond, A. T. M. 2011. A method of detecting patterns in mean lengths of samples of discarded fish, applied to the self-sampling programme of the Dutch bottom-trawl fishery. – ICES Journal of Marine Science, 68: 1712–1718. In 2009, a self-sampling programme was organized in the Netherlands, fishers sampling ca. 80 kg of discards from randomly selected bottom trawls in the North Sea. A statistical procedure is proposed to highlight samples, trips (with multiple samples), or vessels (which may have multiple trips within a year) where extreme mean lengths of discarded fish were observed. Randomization methods were used to test for evidence of non-randomness in patterns of highlighted discard samples, e.g. repeated observations of extreme mean lengths for consecutive discard samples across trips from the same vessel. European plaice (Pleuronectes platessa), common dab (Limanda limanda), grey gurnard (Eutrigla gurnardus), and whiting (Merlangius merlangus) were considered because these were the most abundant species in most of the discard samples. A linear mixed model was used to estimate random-sample effects on the estimated mean lengths by species. These random effects were incorporated into uni- and bivariate procedures to identify extreme samples that were summed for each vessel, and the probability of observing such numbers was estimated. Excluding these samples from the dataset had marginal effects on estimated size distributions of fish.

Understanding the links between large scale spatial structuring of fish assemblages and shaping factors is essential to develop comprehensive ecosystem-based fisheries management. In this study, we investigated spatial patterns of bottom fish assemblages in the North Sea in relation to prevailing water masses in the region. We based our analysis on catch data from the German Small-Scale Bottom Trawl Survey conducted between 1987 and 2005 and used both ordination techniques and Mantel tests. Spatial variability of bottom fish assemblages was larger than inter-annual variability. Five significantly different bottom fish assemblages were associated with the following prevailing hydrographical regimes: i) the English Channel, ii) Continental Coastal, iii) central North Sea, iv) northern North Sea, and v) northern Atlantic water masses. Associations were generated by gradients in relative proportions of abundant species such as grey gurnard (Eutrigla gurnardus), dab (Limanda limanda), whiting (Merlangius merlangus), haddock (Melanogrammus aeglefinus) and Norway pout (Trisopterus esmarki). Taking into account large scale spatial structuring of catch data Mantel tests confirmed significant correlation between the fish assemblages and hydrographical variables. In summary, our results strongly support the hypotheses that hydrographical features such as water masses, fronts, and residual currents could shape bottom fish associations in the North Sea. Spatial demarcations of bottom fish assemblages indicated by this study can be used to support ecosystem-based fisheries management strategies.

This paper presents the first estimation of discarding levels by the demersal fishery in the waters around Ireland. The analysis is based on the Irish discard programme, involving observer's onboard commercial vessels that participate on a voluntary basis. The programme started in 1993 and, until 2002, 225 trips were sampled, corresponding to 2189 sampled tows. Approximately one-quarter of the catch of otter trawlers and “Scottish” seiners is discarded, while two-thirds of the catch of beam trawls is discarded. Furthermore, otter trawl fleet components (defined by the area visited, targeted species and gear used) discard between 20 and 60% of their catch. Discards usually comprise 11 fish species per haul in the 3 gears types studied. Whiting (Merlangius merlangus), haddock (Melanogrammus aeglefinus), bluemouth (Helicolenus dactylopterus), megrim (Lepidorhombus whiffiagonis) and dogfish (Scyliorhinus spp.) are the main species discarded by otter trawlers. “Scottish” seiners discard mostly whiting, haddock and grey gurnard (Eutrigla gurnardus). Dab (Limanda limanda) and plaice (Pleuronectes platessa) are the most species discarded by beam trawls. The majority of these discarded species consist of immature fish. Discarding appears to be increasing in recent years, especially in traditionally bycaught species such as haddock and whiting.

Grey gurnard (Eutrigla gurnadus) is a widely distributed demersal species in the North Sea that has been ranked frequently among the 10 dominant species. Since the late 1980s, grey gurnard catch rates in the international bottom trawl surveys showed a pronounced increase and it was included as an "other predator" in the North Sea multispecies virtual population analysis (MSVPA) in 1997. The MSVPA results estimated grey gurnard to be responsible for approximately 60% of the total predation mortality on age-0 Atlantic cod (Gadus morhua). Long-term MSVPA predictions led to the extinction of North Sea cod. As a possible technical reason, the Holling type II functional response implemented in the model was discussed. In the current analysis, it was demonstrated that the Holling type II functional response was not responsible for the extinction of cod in the model, which was rather a true effect of high grey gurnard predation. Further, it was shown that grey gurnard predation had a significant top-down effect on whiting (Merlangius merlangus) and potentially also on cod recruitment, which was linked to the spatial distribution of the three species. Eventually, the implications of the results for North Sea cod stock recovery plans were discussed.

Size preference for prey fish of North Sea whiting, saithe, and grey gurnard was analysed. The analysis combined size-specific prey abundance estimates derived from bottom-trawl surveys with size frequencies of prey in predator stomachs from the International North Sea Stomach Database. To estimate the abundance of all potential prey fish in the sea, predator-specific length-based number spectra were calculated. Prey spectra were weighted by local predator abundance to take the spatial–temporal overlap between predator and their prey into consideration. Species-specific prey size preference models are presented. Contrary to former results, the preferred predator–prey weight ratio of whiting and grey gurnard is an exponentially increasing function of predator size and an exponentially decreasing function of the slope of the number spectrum. When predators grow, they prefer larger prey in absolute units. However, from a species-specific body size onwards they increasingly shift their prey preference towards relatively smaller prey sizes. From a bioenergetic point of view, this behaviour most likely maximizes the predator's foraging efficiency by reducing the expenditure of costly, anaerobically generated energy expended during burst swimming.

Effort in trawl surveys is standardized by using a common gear and fixing haul duration and vessel speed. Such standardization should result in a fairly constant distance trawled and area or volume swept. Protocols for the North Sea International Bottom Trawl Survey (IBTS) establish hauls of 30 min at a target speed of 4 knots over ground with a standard GOV (Grande Ouverture Verticale) trawl. Primarily to evaluate the effect of departures from the target speed and of trawl speed through water, a fishing experiment was performed under IBTS protocol conditions. The experiment consisted of 30 hauls made by RV “Walther Herwig III” in a small area in the northern North Sea during 5 days in November 1997. Speed over ground was calculated from the distance between GPS shooting and hauling positions. Current speed and direction were continuously recorded by a current metre set a few metres above the sea bottom in the centre of the area to allow trawl speed through water to be calculated. We used generalized additive and linear models to analyse variation in catch rates of Norway pout (Trisopterus esmarki), haddock (Melanogrammus aeglefinus), whiting (Merlangius merlangus), dab (Limanda limanda) and grey gurnard (Eutrigla gurnardus) with speed over ground and through water, and also with area and volume swept by the gear, together with time of day to account for diel fluctuations. Catch rates of small haddock and whiting, grey gurnard and dab increased significantly with speed over ground while rates of Norway pout and large whiting increased with speed through water. We propose that this difference is indicative of the vertical distribution of the fish. Most affected by speed were small haddock and whiting: catches in numbers doubled within the range of 3.9 to 5.2 knot ground speed observed during the experiment. Catches of large haddock were stable. Area swept affected small haddock and whiting and volume swept affected small haddock only. With the exception of large whiting, catch rates of all species and sizes varied with time of day typically within a factor of 2 between day and night. Copyright 2002 International Council for the Exploration of the Sea. Published by Elsevier Science Ltd. All rights reserved.

Catches of demersal fish from research vessel surveys in three areas around the British Isles from 1901 to 1907 were compared with survey catches in the same areas from 1989 to 1997. Mesh size and other characteristics of the trawls used suggested that some of these data were comparable, and could be used to describe changes in demersal fish assemblages over the last 80 years. In Start Bay (NW English Channel) and the Irish Sea, species diversity was the same in both periods, although the most abundant species in each period were not the same. In English coastal regions of the southern North Sea, fish populations became more diverse, as plaice Pleuronectes platessa and whiting Merlangius merlangus became less abundant, and the relative abundance of several non-target species such as dragonet Callionymus spp., bib Trisopterus luscus, and bull-rout Myoxocephalus scorpius increased. The proportion in the catch of small fish species (maximum body length 30 cm) in catches decreased in all regions during the time period, except in the Irish Sea where plaice replaced grey gurnard Eutrigla gurnardus as a dominant species. There was a decline in abundance of large sharks, skates and rays, including the common skate, Raja batis, white skate R. alba and the angel shark Squatina squatina. During historic surveys, 60% of the elasmobranch fauna consisted of thornback ray Raja clavata, whereas in contemporary surveys the lesser spotted dogfish Scyliorhinus canicula was the most abundant elasmobranch. Changes in length-frequency distribution of fish in both target and non-target categories, and other observed changes, were thought to be a response to commercial exploitation, and corresponded to similar observations recorded elsewhere. (C) 2000 British Crown Copyright.

Fish predation on 0-group flatfish (plaice Pleuronectes platessa, dab Limanda limanda and flounder Pleuronectesflesus) populations on Tralee Beach. Scotland (56" 31' N, 5" 29' W) was studied by stomach content analysis during the summers of 1992 and 1993. I-group grey gurnard Eutrigla gurnardus, poor-cod Trisopterus minutus and whiting Merlanyius inerlangus and 0-group cod Gadus rnorhua were the major fish predators of the 0-group flatfishes. Flatfishes >45 mm total length appeared to have a refuge in size from predation by the fishes sampled. Predation by fishes was shown to be negatively size-selective, concentrated on the smaller size-classes of flatfishes. Smaller individuals were eaten by a greater taxonomic variety and a larger size range of predators than larger flatfishes. Smaller flatfishes'were therefore vulnerable to a larger number of predators. The size distributions of predators affected the pattern of predation because only the larger individuals ate flatfishes. It is therefore proposed that the degree of overlap in the size distributions of predators and prey determine the magnitude of predation. Growth rates and the timing of settlement in any one year will therefore have a considerable influence on predation, with the potential to result in a 'match or mismatch' of predator and prey size distributions.

This paper examines long-term changes in the structure and composition of the groundfish species assemblage in three regions of the north-western North Sea. Scottish fisheries research vessel data collected during the months July-September over the period 1929-53 are compared with more recent August groundfish survey data covering the period 1980-93. Trends in the whole groundfish assemblage, and in a subset of the assemblage that is not targeted by commercial fisheries, are described. Long-term differences in species assemblage were subtle, and were most apparent in the dominance structure. Species diversity in the whole groundfish assemblage was marginally greater in the period 1929-53, but no difference was apparent within the non-target species assemblage. For the whole groundfish assemblage, diversity was greatest in the inshore region and least in the offshore area, but there was no obvious spatial gradient for the non-target species assemblage. Multivariate analyses indicated long-term changes and between-area differences in the species composition for both the whole groundfish assemblage and the non-target species subset. More detailed examination of the data revealed that the long-term changes resulted from relatively small and subtle differences in the relative abundance of rarer species, such as grey gurnard Eutrigla gurnardus (L.) and spur-dog Squalus acanthius L.. In contrast, changes in the relative abundance of the more common species, such as Norway pout Trisopterus esmarkii (Nilsson) and whiting Merlangius merlangus (L.), explained much of the between-area variation. Examination of species aggregated length-frequency distributions suggested that by the 1980s there had been a shift towards assemblages in which smaller fish were more highly represented. This was only apparent, however, in the whole groundfish species assemblage ; the length-frequency distributions of non-targeted species were almost identical in the two time periods. Overall, the results suggest that, although differences in the structure of the whole fish assemblage can be detected, the non-target groundfish assemblage appears to have remained relatively unchanged, despite a century of intensive fishing activity.

A main feature of the data here presented is the number of new records. For the sake of brevity these are tabulated below, under their respective headings.Another feature is the wide geographical distribution of a large percentage of the parasites herein recorded, particularly of such forms as Zoogonoides viviparus (Olsson), Derogenes varicus (Mueller) and Hemiurus communis Odhn. The former has been recorded for St Andrews, Millport, Aberdeen, Plymouth, Northumberland, Liverpool, Galway, and also for foreign seas. Derogenes varicus has been recorded for St Andrews, Millport, Plymouth, Aberdeen, Northumberland, Galway, and for foreign seas. Hemiurus communis has been recorded for St Andrews, Millport, Plymouth, Northumberland, Liverpool, Aberdeen, Galway, and for foreign seas.This suggests that the intermediate host of the larval stages of these parasites must be some common marine form, in all probability an invertebrate, which itself enjoys a very wide geographical distribution.A further feature is the tendency of certain parasites to infect a variety of fish, the more notorious being Derogenes varicus (Mueller) and Hemiurus communis Odhn. Of the twenty-six species of marine fishes examined at Galway, eleven (42 per cent.) were found to harbour Derogenes various. The infected fishes were the bib (G. luscus), cod (G. morrhua), pollack (G. pollachius), dab (Pleuronectes limanda), plaice (P. platessa), tub (Trigla hirundo), grey gurnard (T. gurnardus), brill (Rhombus laevis), hake (Merluccius vulgaris), common sole (Solea vulgaris), common sea bream (Sparus centrodontus).Hemiurus communis was found parasitising eight species of fish, i.e. 31 per cent, of the total number of species examined. The hosts affected were the bib (G. luscus), whiting (G. merlangus), cod (G. morrhua), pollack (G. pollachius), flounder (P. flesus), tub (T. hirundo), plaice (P. platessa), and the common sea bream (S. centrodontus).At the moment it is a little premature to prophesy to what extent the discovery of encysted specimens of Prosorhynchus crucibulum (Rud.) in the gill tissues of the cod (Gadus morrhua) will assist in elucidating the life-history of this parasite. Mature specimens of Prosorhynchus crucibulum have previously been recorded as occurring in the alimentary canal of the conger eel (Conger niger). The writer discovered partly digested specimens of small cod to form a considerable bulk of the stomach contents of the conger eel.