Meiofauna as food source for small-sized demersal fish in the southern North Sea

Meiofauna play an essential role in the diet of small and juvenile fish. However, it is less well documented which meiofaunal prey groups in the sediment are eaten by fish. Trophic relationships between five demersal fish species (solenette, goby, scaldfish, dab <20 cm and plaice <20 cm) and meiofaunal prey were investigated by means of comparing sediment samples and fish stomach contents collected seasonally between January 2009 and January 2010 in the German Bight. In all seasons, meiofauna in the sediment was numerically dominated by nematodes, whereas harpacticoids dominated in terms of occurrence and biomass. Between autumn and spring, the harpacticoid community was characterized by Pseudobradya minor and Halectinosoma canaliculatum, and in summer by Longipedia coronata. Meiofaunal prey dominated the diets of solenette and gobies in all seasons, occurred only seasonally in the diet of scaldfish and dab, and was completely absent in the diet of plaice. For all fish species (excluding plaice) and in each season, harpacticoids were the most important meiofauna prey group in terms of occurrence, abundance and biomass. High values of Ivlev’s index of selectivity for Pseudobradya spp. in winter and Longipedia spp. in summer provided evidence that predation on harpacticoids was species-selective, even though both harpacticoids co-occurred in high densities in the sediments. Most surficial feeding strategies of the studied fish species and emergent behaviours of Pseudobradya spp. and Longipedia spp. might have caused this prey selection. With increasing fish sizes, harpacticoid prey densities decreased in the fish stomachs, indicating a diet change towards larger benthic prey during the ontogeny of all fish species investigated.     

Diet comparison between pelagic and demersal whiting in the North Sea

Pelagic North Sea whiting Merlangius merlangus preyed upon (pelagic) sprat and sandeel, while demersal whiting preyed upon (demersal) Norway pout and herring. Values of diet breadth were low for both feeding groups using Levin's index. Diet overlaps within layers were low ( D <0·25), while the between layer food overlap was moderate ( D =0·25–0·74) to high ( D >0·74) using Schoener's index. Selection of prey was density dependent. However, prey size also played an important role. The diet of whiting shifted from amphipods and mysids to fish with increasing predator length, and the length of prey consumed increased significantly with length of whiting. The fact that the stomach contents differed between the feeding groups demonstrates the need for a sampling design that includes both pelagic and demersal habitats when trying to quantify the diet of whiting.     

Marine molluscs and fish as biomarkers of pollution stress in littoral regions of the Red Sea, Mediterranean Sea and North Sea

The intensive development of industry and urban structures along the seashores of the world, as well as the immense increase in marine transportation and other activities, has resulted in the deposition of thousands of new chemicals and organic compounds, endangering the existence of organisms and ecosystems. The conventional single biomarker methods used in ecological assessment studies cannot provide an adequate base for environmental health assessment, management and sustainability planning. The present study uses a set of novel biochemical, physiological, cytogenetic and morphological methods to characterize the state of health of selected molluscs and fish along the shores of the German North Sea, as well as the Israeli Mediterranean and Red Sea. The methods include measurement of activity of multixenobiotic resistance-mediated transporter (MXRtr) and the system of active transport of organic anions (SATOA) as indicators of antixenobiotic defence; glutathione S-transferase (GST) activity as an indicator of biotransformation of xenobiotics; DNA unwinding as a marker of genotoxicity; micronucleus test for clastogenicity; levels of phagocytosis for immunotoxicity; cholinesterase (ChE) activity and level of catecholamines as indicators of neurotoxicity; permeability of external epithelia to anionic hydrophilic probe, intralysosomal accumulation of cationic amphiphilic probe and activity of non-specific esterases as indicators of cell/tissue viability. Complete histopathological examination was used for diagnostics of environmental pathology. The obtained data show that the activity of the defensive pumps, MXRtr and SATOA in the studied organisms was significantly higher in the surface epithelia of molluscs from a polluted site than that of the same species from control, unpolluted stations, providing clear evidence of response to stress. Enhanced frequency of DNA lesions (alkaline and acidic DNA unwinding) and micronucleus-containing cells was significantly higher in samples from polluted sites in comparison to those from the clean sites that exhibited genotoxic and clastogenic activity of the pollutants. In all the studied molluscs a negative correlation was found between the MXRtr levels of activity and the frequency of micronucleus-containing hemocytes. The expression of this was in accordance with the level of pollution. The complete histopathological examination demonstrates significantly higher frequencies of pathological alterations in organs of animals from polluted sites. A strong negative correlation was found between the frequency of these alterations and MXRtr activity in the same specimens. In addition to these parameters, a decrease in the viability was noted in molluscs from the polluted sites, but ChE activities remained similar at most sites. The methods applied in our study unmasked numerous early cryptic responses and negative alterations of health in populations of marine biota sampled from the polluted sites. This demonstrates that genotoxic, clastogenic and pathogenic xenobiotics are present and act in the studied sites and this knowledge can provide a reliable base for consideration for sustainable development. Received: 2 March 1999 / Received in revised form: 2 August 1999 / Accepted: 3 August 1999     

Diet overlap among flatfish species in the southern North Sea

The diets of two non-commercial flatfish species (solenette Buglossidium luteum and scaldfish Arnoglossus laterna) and two commercial flatfish species (dab Limanda limanda and plaice Pleuronectes platessa) were compared in a study area in the German Bight (southern North Sea) to investigate prey-resource partitioning between these species. The diets of A. laterna and B. luteum mainly comprised crustaceans (harpacticoids, amphipods, cumaceans and decapods), whereas the diet of L. limanda and P. platessa consisted mainly of polychaetes. The Schoener index, calculated for different fish size classes between these flatfish species, showed a biologically significant diet overlap between small-sized L. limanda and P. platessa and B. luteum and A. laterna, using similar prey resources of smaller prey (e.g. amphipods, harpacticoids and juvenile bivalves). In contrast, with increasing body size, a change in the diet of L. limanda and P. platessa towards larger prey occurred (e.g. polychaetes and decapods), resulting in low diet overlap values with B. luteum and A. laterna. Due to these size-related differences in resource use, it is assumed that there is reduced interspecific competition for prey between larger L. limanda and P. platessa and both non-commercial flatfishes, probably facilitating resource partitioning within the same area. In contrast, smaller L. limanda and P. platessa may compete directly for the same prey resources with B. luteum and A. laterna. Furthermore, prey availability of most important prey items of the studied flatfishes was relatively low in the study area. Therefore, increasing abundances of B. luteum and A. laterna in the southern North Sea since the late 1980s, owing to fishing effects and climate change, might affect the population dynamics of L. limanda and P. platessa.     

Dinoflagellate resting cysts as factors in phytoplankton ecology of the North Sea

The occurrence and distribution of dinoflagellate resting cysts were investigated at 11 locations in the south-eastern part of the North Sea. Twenty-six known cyst species and 7 unknown cyst types, which may act as seed population for planktonic dinoflagellate blooms, have been recorded for the first time in the area. The most common cysts in recent sediments were those ofScrippsiella trochoidea, Zygabikodinium lenticulatum, Peridinium dalei, Scrippsiella lachrymosa, Protoceratium reticulatum, Protoperidinium denticulatum, andP. conicum. At all stations,S. trochoidea dominated the cyst assemblages with a maximal abundance of 1303 living cysts/cm³ in the uppermost half centimetre. Cysts of the potentially toxic dinoflagellatesAlexandrium cf.excavatum andA. cf.tamarense were scarce. In the upper 2-cm layer of sediment, dinoflagellate cysts were found in concentrations of 1.8 up to 682 living cysts/cm³. Empty cysts constituted 22–56% of total cyst abundance. The comparative distribution of the cysts showed a general increase in abundance from inshore sites to the offshore area, whereby sandy stations exhibited the lowest cyst abundance and diversity. The wide distribution of living and empty cysts ofScrippsiella lachrymosa suggests that its motile form, which has not been officially recorded in the area until now, is a common plankton organism in German coastal waters. The relatively high abundance of cysts in recent sediments demonstrates the potential importance of benthic resting stages for the initiation of dinoflagellate blooms in the study area.     

The dominant fish fauna in the North Sea and its determination

The 224 species of fish reported from the North Sea are regarded as being composed of elements of three faunas—Boreal, Lusitanian and Atlantic, and they can be grouped accordingly on both a number of species and a biomass basis. The estimation of biomass of more than 10 important species is based on stock estimates obtained from population data. For 65 non-standard species the estimate is based on a comparison of catch rates for 'standard'and non-standard species from groundfish survey data. This involves assumption about the relative catchability of different sets of species. For the other species the biomass are computed in different ways. An analysis of the dominant fish fauna in the North Sea is attempted. The fish fauna in the area is analysed in two ways: by the conventional total number of species from each zoogeographic area, and by the biomass of the representatives of each fauna. It is concluded that for establishing the dominant faunal element of fish, biomass is a better index than is the number of species. The dominant faunal element of fish in the North Sea is Boreal.     

A decline in primary production in the North Sea over 25 years,associated with reductions in zooplankton abundance and fish stock recruitment

Phytoplankton primary production is at the base of the marine food web; changes in primary production have direct or indirect effects on higher trophic levels, from zooplankton organisms to marine mammals and seabirds. Here, we present a new time‐series on gross primary production in the North Sea, from 1988 to 2013, estimated using in situ measurements of chlorophyll and underwater light. This shows that recent decades have seen a significant decline in primary production in the North Sea. Moreover, primary production differs in magnitude between six hydrodynamic regions within the North Sea. Sea surface warming and reduced riverine nutrient inputs are found to be likely contributors to the declining levels of primary production. In turn, significant correlations are found between observed changes in primary production and the dynamics of higher trophic levels including (small) copepods and a standardized index of fish recruitment, averaged over seven stocks of high commercial significance in the North Sea. Given positive (bottom‐up) associations between primary production, zooplankton abundance and fish stock recruitment, this study provides strong evidence that if the decline in primary production continues, knock‐on effects upon the productivity of fisheries are to be expected unless these fisheries are managed effectively and cautiously.     

Year-to-year variation in larval fish assemblages of the Southern North Sea

In order to test the temporal stability within and the reproducibility of larval fish assemblages between years, the larval fish assemblage at Helgoland Roads, North Sea (NE Atlantic) was quantitatively sampled almost daily from January 2003 to December 2005. The survey resulted in a total of 462 samples containing 50,632 larval fish of at least 42 taxa. In winter the larval fish assemblage was mainly dominated by larvae emerging from demersal eggs. This changed gradually to larvae hatching from pelagic eggs. Larvae from pelagic eggs dominated the ichthyoplankton assemblage in summer. A remarkably stable seasonality in terms of dominance patterns with recurring, season-specific fish assemblages was observed over the 3 years, despite substantial variation in environmental conditions such as a temperature difference of almost 20°C between summer and winter. The lesser sandeel (Ammodytes marinus), was the only species which showed significant fluctuations in abundance between the years. After removal of this species from the analysis, the dominance patterns of the remaining fish species were almost identical between years.     

Spawning stock and recruitment in North Sea cod shaped by food and climate

In order to provide better fisheries management and conservation decisions, there is a need to discern the underlying relationship between the spawning stock and recruitment of marine fishes, a relationship which is influenced by the environmental conditions. Here, we demonstrate how the environmental conditions (temperature and the food availability for fish larvae) influence the stock–recruitment relationship and indeed what kind of stock–recruitment relationship we might see under different environmental conditions. Using unique zooplankton data from the Continuous Plankton Recorder, we find that food availability (i.e. zooplankton) in essence determines which model applies for the once large North Sea cod (Gadus morhua) stock. Further, we show that recruitment is strengthened during cold years and weakened during warm years. Our combined model explained 45 per cent of the total variance in cod recruitment, while the traditional Ricker and Beverton–Holt models only explained about 10 per cent. Specifically, our approach predicts that a full recovery of the North Sea cod stock might not be expected until the environment becomes more favourable.     

Mortality of fish from the by-catch of shrimp vessels in the North Sea

The results from 3 different rogrammes concerning by-catch mortality in shrimp fisheries are reported. Mortalities increased consigrably after the catch passed the sorting sieve. 100 % mortality was detected for whiting ( Merlangius merlangus ). Mortalities were about 10% for sculpin ( Myxocephalus scorpius ), hooknose ( Agonus catapbractus ), and eelpout ( Zoarces viviparus ) in the discard fraction. Survival of flatfish depended strongly on the species, the size of the secimens as well as the catch and catch processing conditions, and ranged from 17 to 100%. No difirences could be detected in the survival after sorting on different machines. However, due to better sorting efficiency, the rotary sieve may reduce mortality of fish in the by-catch.

Zusammenfassung

Sterblichkeit von Fischen aus dem Beifang von Garnelenkuttern in tier Nordsee
Es werden die Ergebnisse von 3 verschiedenen Untersuchungsprogramrnen zur Beifansterblichkeit in der Garnelenfischerei mitgeteilt. Die Sterblichkeit nahm beträchtlich zu, wenn der Beikng das Sortiersieb passiert hatte. 100% Sterblichkeit wurde für Wittlinge ( Merlangius merlangus ) ermittelt. Für Seeskorpione ( Myoxocephalus scorpius ), Steinpicker ( Agonus cataphractus ) und Aalmuttern ( Zoarces viviparus ) lag die Sterblichkeit in der Discard-Fraktion iei 10%. Die Überlebensraten bei Plattfischen waren stark von der jeweilien Art, der Größe derTiere, wie auch vom Fang und der Fangbearbeitung abhängig und bewegten sici zwischen 17 und 100%. Es konnten keine Unterschiede in den Überlebensraten nach der Sortierung mit verschiedenen Maschinen festgestellt werden. Allerdins kann der Trommelsonierer aufgrund seiner besseren Sortiereigenschaften die Sterblichkeit im Beifang herabsetzen.     

An annotated checklist of North Sea cartilaginous fish species

A checklist is compiled of all cartilaginous fishes (sharks, skates, rays and rat-fishes) which were so far recorded from the North Sea, along with their English common names. The objective of the study was to provide background information in light of the rapidly changing species diversity with overfishing and climate change as the main cause of changes in their status, thereby providing a reference tool for comparative purposes. This was also undertaken because usually the literature provides general information on the biology and covers the entire geographic range of a species but is not concerned with the status in a particular area. Presently the list contains 57 species, including 37 sharks, 19 species of skates and rays and 1 rat-fish species (also frequently called rabbit-fish or chimaera). Annotations were made for a number of selected species, as to their historical distribution and the present status.     

The Body Size-Dependent Diet Composition of North American Sea Ducks in Winter

Daily food requirements scale with body mass and activity in animals. While small species of birds have higher mass-specific field metabolic rates than larger species, larger species have higher absolute energy costs. Under energy balance, we thus expect the small species to have a higher energy value diet. Also the weight and time constraints for flighted and diurnal foragers should set a maximum to the amount of prey items taken in one meal and to the daily number of meals, respectively. Further, avoidance of competition causes the species to reduce the amount of shared prey in their diet. Some diet segregation is therefore to be expected between species. We tested these hypotheses and investigated the role of body mass in the diet composition of 12 sea duck species (Somateria mollissima, Somateria spectabilis, Somateria fischeri, Polysticta stelleri, Bucephala clangula, Bucephala islandica, Bucephala albeola, Melanitta nigra, Melanitta perspicillata, Melanitta deglandi, Histrionicus histrionicus and Clangula hyemalis) wintering in North America. This study was based on a literature survey with special emphasis given to the diet data from the former US Bureau of Biological Survey. The data supported our hypothesis that the energy value of winter diet of sea ducks scales negatively with body mass. Diet diversity also scaled negatively with body mass. Our results suggest the existence of a minimum for the energy value of avian diets.     

Trace element levels in fish from clean and polluted coastal marine sites in the Mediterranean Sea, Red Sea and North Sea

The bioaccumulation of Hg, Cd, Zn, Cu, Mn and Fe was evaluated in the muscle and liver tissue of four fish species (Siganus rivulatus, Diplodus sargus, Lithognatus mormyrus and Plathychtis flesus) from clean and polluted marine coastal sites in the Red Sea, Mediterranean Sea and North Sea within the framework of the MARS 1 program. Representative liver samples were screened for organic contaminants (DDE, PCBs and PAHs) which exhibited very low concentrations. The levels of Cd, Cu, Zn, Fe and Mn found in the muscle tissue in this study were similar among the four species and within the naturally occurring metal ranges. However, differences were found among the sites. In the Red Sea, Cu was higher in the muscle of S. rivulatus at Ardag and Zn at the Observatory (OBS). Cu, Zn and Mn were higher in the Red Sea than in the specimens from the Mediterranean. The differences were attributed to different diets derived from distinctively different natural environments. D. sargus from Haifa Bay (HB) had higher Cd, Cu and Mn values than specimens from Jaffa (JFA), and L. mormyrus higher Cd, Fe and Mn in HB, corresponding to the polluted environmental status of the Bay. No differences in metal levels were found among the North Sea sites, except for Fe that was lower at the Eider station. Hg was low in all the specimens, but the values varied with species and sites. The lowest Hg values were found in S. rivulatus, the herbivorous species, as expected from its trophic level. Hg in P. flesus was higher than in S. rivulatus but still low. Higher Hg values were found in the muscle tissue of L. mormyrus,with the highest values in D. sargus, both carnivorous species from the same family. Hg in D. sargus was higher in HB than in JFA, as expected, but in the larger specimens of L. mormyrus from JFA values were higher, while in the small specimens there were no differences in Hg values. The levels of all metals were higher in the liver than in the muscle, with enrichment factors ranging from 3 to 104, depending on species and sites. The lowest enrichment values were found for Hg. Based on liver values, the specimens of S. rivulatus from the OBS had the highest levels, as well as D. sargus and L. mormyrus from JFA, contrary to the known relative environmental status of the sites. Received: 25 February 1999 / Received in revised form: 5 June 1999 / Accepted: 7 June 1999     

Oxygen availability as a possible limiting factor in LDL oxidation

Kinetic studies of copper-induced LDL peroxidation commonly assume that the availability of molecular oxygen in the reaction media is not a limiting factor. The present study reveals that this assumption is valid only at low LDL concentrations. At high LDL concentrations, accumulation of oxidation products, as monitored spectroscopically under conditions of various oxygen concentrations in the medium, comes to a halt when the oxygen concentration in the solution, as measured by an oxygen electrode, decreases to near zero levels. Bubbling of the oxygen into the solution results in resumption of peroxidation. These results are important with respect to the ex vivo assaying of lipoprotein peroxidation because many previous studies have been conducted with LDL concentrations that corresponded to polyunsaturated fatty acid concentrations in access of the concentration of molecular oxygen. The possible pathophysiological significance of the results of this study has yet to be evaluated.     

Species assemblages of pelagic fish embryos in the southern North Sea between 1984 and 2000

The occurrence and abundance of the pelagic eggs of southern North Sea spring-spawning fish were analysed between 1984 and 2000. Species number varied between six (1986) and 14 (1999) and was positively correlated with sea surface temperature. With one exception, dab eggs were always the most abundant and usually highly dominant. Ranking of species depended on temperature, but no significant differences in ranking between years was discernible. Although with the increase in temperature in the 1990s a change in species assemblage was evident [species belonging to the boreal-Mediterranean (Lusitanian) group became more apparent in the species assemblage] this did not lead to an increased species diversity (Shannon Index) or a change in other community parameters. It appears that the recent developments regarding spawning stock biomass of commercial North Sea fish is reflected in the declining egg abundance of the respective commercial and larger species (i.e. cod, flounder, plaice) and an increase in abundance of the eggs of small species (i.e. long rough dab, rockling) over the years. Received in revised form: 5 November 2001 Electronic Publication     

Review on the effects of pollution on marine fish life and fisheries in the North Sea

Information on possible effects of pollution on fish life is reviewed with special emphasis to the North Sea. Concentrations of heavy metals are high in onshore waters, sediments and organisms. Offshore regions are considered to be not endangered. In estuaries concentrations of certain heavy metals are often surpassing thresholds, which have been experimentally demonstrated to produce sublethal effects on marine organisms. Also mussel culture areas had to be closed due to excess of cadmium contamination. Mercury in fillets of cod in the southern North Sea sometimes reaches 0.3 ppm representing 1/3 of the legal limit. Reductions of inputs are necessary due to the bioaccumulative and toxic potential of heavy metals. Organochlorine contamination of water, sediments and organisms is higher in the southern than in the northern North Sea. In offshore areas clearly elevated levels of PCB's, which are the dominating organochlorine compound in marine organisms, have been analyzed. At present only a minor fraction of total organochlorine residues accumulated in marine biota is analyzed. Margins between experimentally derived lethal levels and those occurring in marine biota are narrow suggesting that sublethal effects might occur. One of the special concerns in the context of organochlorine contamination has to be seen in the potential of these substances affecting the reproductive capacity of marine organisms. Reduction measures are urgently necessary and should include substances with bioaccumulative and toxicological potential, which are not presently included in Marine Monitoring Programmes. Impact of oil pollution is considered to be a regional problem, although especially in estuaries concentrations in the water column might be high. Input through oil platforms has led to local reductions in benthic communities. Measures have to be taken, to further reduce these inputs also in view to avoid contamination of fish in the vicinity of oil fields. Oxygen depletion in German and Danish coastal waters has been shown to be a severe degradation in coastal water quality resulting in significant impact on fish populations. Indications for large-scale avoidance reactions of low DO-areas and mortalities of fish and benthos organisms have been demonstrated. It is suspected that the low DO-situations are related to increased nutrients input and high phytoplankton production. Improvement of the situation can only be expected, if input of nutrients is reduced. Reasons for the occurrence of toxic plankton blooms are not understood. An intensified research is necessary, to avoid damage to fisheries and mariculture. Although activity of radionucleids in North Sea waters is increased due to input of radioactive material from La Hague and Sellafield reprocessing units, the present levels of activity are not considered to be a threat for marine life. The same holds for thermal pollution, which is, depending on the recipient area, expected to have only local effects. Sewage sludge dumping and discharge of untreated sewage introduce significant amounts of accompanying heavy metals, which can be expected to have a negative influence on the total metal budget of recipient waters. Nutrient input through sewage will exhibit only local effects. Dumping of wastes from titanium dioxide industry are suspected to be causally linked to the occurrence of increased prevalences of certain diseases of dab in the German Bight. Present management strategies should take account of the fact that through dumping of waste from titanium dioxide production significant quantities of heavy metal are introduced into the southern North Sea. Also the incineration of chemical wastes at sea should be reduced due to uncertainties concerning quantities and toxicity of incineration products and their potential impact on marine life. It is concluded that clear evidence exists for pollution impact in estuarine regions. Most of the major estuaries at the North Sea coast show benthos reductions, which can be related to water quality and it has to be assumed that areas of a sublethal effect are located in the vicinity of those areas of well demonstrated effects. They are of unknown size and attempts have to be undertaken to quantify areas of sublethal impact. The occurrence of fish diseases is interpreted as an expression of degraded water quality. Some of the diseases detected can produce mortality, thereby having an as yet unquantified impact on fish populations. The presence of fish diseases also reduces the marketability of catches. It cannot be excluded that changes in species composition of fish populations in German Wadden Sea areas are related to pollution. Concentrations of organochlorine substances in fishes of the southern North Sea give rise to the concern that their reproductive capability might be impaired. Future work has to be directed to this problem. Impact of pollution on populations of marine organisms has so far not been demonstrated.     

The dangers of ignoring stock complexity in fishery management: the case of the North Sea cod

The plight of the marine fisheries is attracting increasing attention as unsustainably high exploitation levels, exacerbated by more extreme climatic conditions, are driving stocks to the point of collapse. The North Atlantic cod (Gadus morhua), a species which until recently formed a major component of the demersal fisheries, has undergone significant declines across its range. The North Sea stock is typical of many, with a spawning stock biomass that has remained below the safe biological limit since 2000 and recruitment levels near the lowest on record. Cod within the North Sea are currently managed as a single stock, and yet mounting empirical evidence supports the existence of a metapopulation of regionally variable, genetically distinct, sub-stocks. Applying the same management strategies to multiple stocks that differ in their resilience to exploitation inevitably results in the overfishing and likely collapse of the weaker components. Indeed, recent studies have identified two North Sea spawning stocks that have undergone disproportionally large collapses with very substantial reductions in egg production. Similarly affected cod stocks in the northwest Atlantic have shown little evidence of recovery, despite fishery closures. The possible implications of ignoring sub-structuring within management units for biocomplexity, local adaptation and ecosystem stability are considered.     

Preliminary observations on the near-surface fish fauna of the northern North Sea in late autumn

In the course of cruises by FRV Scotia to study the ecology of O-group gadoids in the northern North Sea in October 1977 and November 1978, the fish fauna of the upper 1 m of the water column were sampled with an improvised gear. The most abundant species were O-group rocklings and clupeids and three spined sticklebacks. The frequency of occurrence of the last named and their relatively high abundance (up to 99 individuals in a 30 min haul) suggests that sticklebacks are more widely distributed throughout the open North Sea than is presently supposed. Young rocklings may amount to a considerable biomass in the near-surface waters.