Life‐history variation among local populations of Atlantic cod from the Norwegian Skagerrak coast

Small‐scale spatial variation in life history was found among genetically distinct local populations of Atlantic cod Gadus morhua from the Norwegian Skagerrak coast. Among populations, age at 50% maturity varied from 2·6 to 3·8 years, total ( L _T) length at 50% maturity from 35 to 60 cm, annual survival from 33 to 64%, mean L _T at age 4 years from 43 to 63 cm, and mean backcalculated L _T at age 1 year from 8 to 12 cm.     

Warm water occupancy by North Sea cod

The North Sea has warmed in recent years and there is an ongoing debate into how this is affecting the distribution of fishes and other marine organisms. Of particular interest is the commercially important Atlantic cod (Gadus morhua L.), which has declined sharply in abundance in the North Sea over the past 20 years. Observations of the temperature experienced by 129 individual cod throughout the North Sea were made during a large-scale electronic tagging programme conducted between 1999 and 2005. We asked whether individual cod fully occupied the thermal habitat available to them. To this end, we compared the temperature experience of cod with independently measured contemporaneous sea-bottom temperature data. The majority of cod experienced a warmer fraction of the sea than was potentially available to them. By summer, most of the individuals in the south experienced temperatures considered superoptimal for growth. Cooler waters were within the reach of the cod and a small number of individuals migrated to areas that allowed them to experience lower temperatures, indicating that the cod had the capacity to find cooler water. Most did not, however, suggesting that the changing thermal regime of the North Sea is not yet causing adult cod to move to cooler waters.     

Exotic flagellates of coastal North Sea waters

Flagellate species have been shown to survive transocean passage by ballast water and the large dinoflagellateGymnodinium catenatum was introduced from Japanese to Tasmanian waters in this way.Gymnodinium mikimotoi—better known asGyrodinium aureolum—andFibrocapsa japonica as well asAlexandrium leeii are good candidates to have been introduced recently. Species which seem to have been introduced recently into the North Sea but apparently are transported from adjacent seas by currents into the region areGymnodinium chlorophorum andAlexandrium minutum. Species reported as introduced due to misidentifications areGymnodinium catenatum andLepidodinium viride. Under other names the speciesProrocentrum minimum, Prorocentrum redfieldii, andHeterosigma akashiwo have been known for a long time in the North Sea. The recent reports of threeChattonella species may be either due to introduction or they have been overlooked. The reasons why the introduction of flagellates into coastal North Sea waters is difficult to prove will be discussed.     

Porewater chemistry of inorganic nitrogen compounds in the eastern Skagerrak (NE North Sea)

Porosity, Eh and interstitital nitrogen compounds profiles are given for four locations in the eastern Skagerrak. Sediments down to 30 cm depth, i.e. postindustrial in age, were analysed. The results together with a detailed age determination (²¹⁰Pb) suggest a high degree of both lateral and vertical variability as a result of local bottom topography and currents as well as event-controlled sedimentation processes.     

Ecological and genetic impact of Atlantic cod larval drift in the Skagerrak

We evaluate the hypothesis that Atlantic cod larvae are passively transported by sea currents from off-shore spawning areas to settle in coastal waters, a hypothesis which has recently gained support from genetic analysis of cod in the North Sea-Skagerrak area. Such larval transport has been suggested to be an important mechanism behind the commonly observed low spatial genetic differentiation in many marine organisms. Here, we apply an ARMAX(2,2) model for juvenile abundance and use long-term monitoring data from the Skagerrak coast, constituting 54 continuous annual series from 1945 to 1997. Analysing the model, we find that the product of the size of the North Sea breeding stock and the strength of the net inflow of North Sea waters had a significant, positive effect on the abundance of coastal juvenile cod. The peak effect occurs during the month of March, just after spawning, when eggs and larvae remain pelagic and sensitive to currents. In contrast, we find no evidence of any direct effect of the North Sea spawning stock alone. Our analyses indicate that 15-20,000 0-group larvae from the North Sea reach each fjord per year, on average. This corresponds to about 1-10% of the total 0-group population in each fjord on average. These findings clearly demonstrate a direct link between larval drift and gene flow in the marine environment.     

Weak 'top-down' control of dinoflagellate growth in the coastal Skagerrak

Effects of planktivorous fish (Sprattus sprartus), copepods(mainly Oithona sp.) and benthic filter feeders (Mytilus edulis)on the biomass and species composition of a natural marine planktoncommunity, dominated by dinoflagellates, was studied experimentallyin September 1988. Plastic 300 I cylinders were filled withSkagerrak surface water, filtered through a 90 µm nylonnet in order to remove mesozooplankton. Copepods were addedin three different concentrations (0, 1x and 6x the ambientdensity) with and without nutrient additions (nitrate, silicateand phosphate). In two nutrient-enriched cylinders with addedcopepods (1x and 6x), 5 cm sprats were also added (10 per cylinder).In two nutrient-enriched cylinders without added copepods. bluemussels (20 per cylinder. 3 cm length) were introduced. Chlorophylla. inorganic nutrients and densities of phytoplankton, ciliatesand copepods were measured throughout the experimental periodof 10 days. Gyrodinium aureolum increased in cell numbers inall but the mussel cylinders, while the other dominating dinoflagellate,Ceratium furca, decreased. This decrease was not due to grazing,but probably a container artefact. Diatoms, Rhizosolenia fragilissinia,Leptocylindrus danicus and Chaetoceros spp., increased slowlyin treatments without fish. In the fish treatments, diatomshad appreciably higher growth rates and cell numbers, probablydue to predation by fish on the copepods. Gyrodiniun aureolunalso had a somewhat higher growth rate in the presence of fish.although this was probably not due to reduction of copepod grazing.The results show that planktivorous fish can affect the biomassand species composition of natural phytoplankton communitiesin marine waters, but that the effect on the late summer-autumndinoflagellate dominated community is weak. Blue mussels areefficient grazers of the whole phytoplankton community and arenot particularly selective with respect to either size or species.     

Evidence for metapopulation structuring in cod from the west of Scotland and North Sea

The scale of population structuring in cod Gadus morhua from the west of Scotland (ICES stock area VIa) and northern North Sea was investigated using a combination of non-genetic methods. Site fidelity of spawning aggregations was examined using historic tag-recapture experiments and individual geolocation estimates from data storage tagged fish. The extent of movements from spawning areas indicated by these two tagging approaches was broadly similar. Between 67 and 97% of adult cod remained within 100 km of spawning areas throughout the year, suggesting resident spawning groups. A small proportion of cod, however, did appear to stray between spawning areas. A comparison of the elemental signature of the otoliths of 0 year-group and the 0 year-group component from the same year-class at age 2 years indicated that most adults originate from local nursery areas. Moreover, there did not appear to be detectable exchange between spawning areas >200 km apart, such as the Clyde and Minch or Shetland and the Inner Hebrides. As such, population processes may operate at a smaller spatial scale than the stock level, with spawning aggregations functioning as local populations within a metapopulation. The relevance of this scale of dynamics is discussed in relation to recently imposed closed areas in the region.     

Dynamics of coastal cod populations: intra- and intercohort density dependence and stochastic processes

Skagerrak populations of Atlantic cod (Gadus morhua L.) have been surveyed at several fixed stations since 1919. These coastal populations consist of local stocks with a low age of maturity and a short life span. We investigated 60 time-series of 0-group juveniles (i.e. young of the year) sampled annually from 1945 to 1994. An age-structured model was developed which incorporates asymmetrical interactions between the juvenile cohorts (0-group and 1-group; i.e. one-year-old juveniles) and stochastic reproduction. The model was expressed in delay coordinates in order to estimate model parameters directly from the time-series and thereby test the model predictions. The autocovariance structure of the time-series was consistent with the delay coordinates model superimposed upon a long-term trend. The model illustrates how both regulatory (density-dependent) and disruptive (stochastic) forces are crucial in shaping the dynamics of the coastal cod populations. The age-structured life cycle acts to resonance the stochasticity inherent in the recruitment process.     

Ecology of survival in some gadoid larvae of the northern North Sea

Synopsis The development of the larvae of five gadoid species in the northern North Sea was examined in May 1978 and May 1979. Mortality varied with larval size and period of sampling about high average rates. The limits on survival were due primarily to prey density and prey size. Competition for food did not appear to be a critical survival factor. The growth rates of larvae in the northern North Sea were higher than the growth rates of larvae reared on high densities of prey in the laboratory. Without involving the hypothesis of patchiness, it is suggested that sufficient quantities of prey were available in the field to sustain the high growth rates observed. On the other hand, competitive grazing in the laboratory reduced the amount of prey which fell to the share of each larva. Scarcity of large prey during the first half of May did not favour the survival of late larvae. The significance of spawning in relation to the characteristics of the prey production cycle is discussed.     

Environmental influences on Vibrio populations in northern temperate and boreal coastal waters (Baltic and Skagerrak Seas)

Even if many Vibrio spp. are endemic to coastal waters, their distribution in northern temperate and boreal waters is poorly studied. To identify environmental factors regulating Vibrio populations in a salinity gradient along the Swedish coastline, we combined Vibrio-specific quantitative competitive PCR with denaturant gradient gel electrophoresis-based genotyping. The total Vibrio abundance ranged from 4 x 10(3) to 9.6 x 10(4) cells liter(-1), with the highest abundances in the more saline waters of the Skagerrak Sea. Several Vibrio populations were present throughout the salinity gradient, with abundances of single populations ranging from 5 x 10(2) to 7 x 10(4) cells liter(-1). Clear differences were observed along the salinity gradient, where three populations dominated the more saline waters of the Skagerrak Sea and two populations containing mainly representatives of V. anguillarum and V. aestuarianus genotypes were abundant in the brackish waters of the Baltic Sea. Our results suggest that this apparent niche separation within the genus Vibrio may also be influenced by alternate factors such as nutrient levels and high abundances of dinoflagellates. A V. cholerae/V. mimicus population was detected in more than 50% of the samples, with abundances exceeding 10(3) cells liter(-1), even in the cold (annual average water temperature of around 5 degrees C) and low-salinity (2 to 4 per thousand) samples from the Bothnian Bay (latitude, 65 degrees N). The unsuspected and widespread occurrence of this population in temperate and boreal coastal waters suggests that potential Vibrio pathogens may also be endemic to cold and brackish waters and hence may represent a previously overlooked health hazard.     

Histology of pseudobranchial tumours in Atlantic cod(Gadus morhua) from the North Sea and the Baltic Sea

Atlantic cod bearing pseudobranchial tumours were investigated from both North Sea and Baltic Sea areas. Light and electron microscopic studies revealed a tumorous structure previously described in cod from Atlantic and Pacific waters. Additionally, cysts, multinucleated cells and early stages of mitosis are described. These findings support the hypothesis that pseudobranchial tumours are instigated by a protozoan infection causing xenotumours associated with the pseudobranch organ. The cells contained in these lesions showed similarities to amoebae.     

Epidemiology of pseudobranchial tumours in Atlantic cod(Gadus morhua) from the North Sea and the Baltic Sea

Investigations during several cruises (1979–1981) showed that Atlantic cod with pseudobranchial tumours occurred in all areas investigated. Statistical analysis of data revealed that there was a marked prevalence of the condition in the centre of the German Bight, whereas a lower incidence of the condition was observed on the Dogger Bank, off the Dutch coast and in the western Baltic Sea. The higher prevalence of pseudobranchial tumours in the German Bight coincides with a dense population of cod and impacts of pollution.     

Identification of DNA-synthesizing bacterial cells in coastal North Sea plankton

We describe a method for microscopic identification of DNA-synthesizing cells in bacterioplankton samples. After incubation with the halogenated thymidine analogue bromodeoxyuridine (BrdU), environmental bacteria were identified by fluorescence in situ hybridization (FISH) with horseradish peroxidase (HRP)-linked oligonucleotide probes. Tyramide signal amplification was used to preserve the FISH staining during the subsequent immunocytochemical detection of BrdU incorporation. DNA-synthesizing cells were visualized by means of an HRP-labeled antibody Fab fragment and a second tyramide signal amplification step. We applied our protocol to samples of prefiltered (pore size, 1.2 micro m) North Sea surface water collected during early autumn. After 4 h of incubation, BrdU incorporation was detected in 3% of all bacterial cells. Within 20 h the detectable DNA-synthesizing fraction increased to >14%. During this period, the cell numbers of members of the Roseobacter lineage remained constant, but the fraction of BrdU-incorporating Roseobacter sp. cells doubled, from 24 to 42%. In Alteromonas sp. high BrdU labeling rates after 4 to 8 h were followed by a 10-fold increase in abundance. Rapid BrdU incorporation was also observed in members of the SAR86 lineage. After 4 h of incubation, cells affiliated with this clade constituted 8% of the total bacteria but almost 50% of the visibly DNA-synthesizing bacterial fraction. Thus, this clade might be an important contributor to total bacterioplankton activity in coastal North Sea water during periods of low phytoplankton primary production. The small size and low ribosome content of SAR86 cells are probably not indications of inactivity or dormancy.     

Diazotrophic microbial community of coastal microbial mats of the southern North Sea

The diazotrophic community in microbial mats growing along the shore of the North Sea barrier island Schiermonnikoog (The Netherlands) was studied using microscopy, lipid biomarkers, stable carbon (δ(13) C(TOC) ) and nitrogen (δ(15) N) isotopes as well as by constructing and analyzing 16S rRNA gene libraries. Depending on their position on the littoral gradient, two types of mats were identified, which showed distinct differences regarding the structure, development and composition of the microbial community. Intertidal microbial mats showed a low species diversity with filamentous non-heterocystous Cyanobacteria providing the main mat structure. In contrast, supratidal microbial mats showed a distinct vertical zonation and a high degree of species diversity. Morphotypes of non-heterocystous Cyanobacteria were recognized as the main structural component in these mats. In addition, unicellular Cyanobacteria were frequently observed, whereas filamentous heterocystous Cyanobacteria occurred only in low numbers. Besides the apparent visual dominance of cyanobacterial morphotpyes, 16S rRNA gene libraries indicated that both microbial mat types also included members of the Proteobacteria and the Cytophaga-Flavobacterium-Bacteroides group as well as diatoms. Bulk δ(15) N isotopes of the microbial mats ranged from +6.1‰ in the lower intertidal to -1.2‰ in the supratidal zone, indicating a shift from predominantly nitrate utilization to nitrogen fixation along the littoral gradient. This conclusion was supported by the presence of heterocyst glycolipids, representing lipid biomarkers for nitrogen-fixing heterocystous Cyanobacteria, in supratidal but not in intertidal microbial mats. The availability of combined nitrogen species might thus be a key factor in controlling and regulating the distribution of the diazotrophic microbial community of Schiermonnikoog.     

Differential modes of selection on the rhodopsin gene in coastal Baltic and North Sea populations of the sand goby,Pomatoschistus minutus

An excellent model to elucidate the mechanisms and importance of evolution in the marine environment is the spectral tuning mechanism of the visual pigment in vertebrates. In the sand goby Pomatoschistus minutus (Teleostei; Gobiidae), a distribution‐wide study showed that spatial variation at the rhodopsin gene (RH1) matches the characteristics of specific light environments. This match suggests that populations are locally adapted to selective light regimes targeting the RH1 gene. If so, then the direction of selection should depend on the regional spatial and temporal stability of the light conditions. We tested this prediction by comparing goby populations from two regions: the Baltic Sea, characterized by divergent, but temporally stable light conditions, and the North Sea, characterized by locally heterogeneous and temporally variable light conditions. RH1 sequences of 491 Pomatoschistus minutus individuals from 15 locations were analysed. We found that variation at the RH1 gene in the Baltic populations showed signatures of diversifying selection, whereas the RH1 gene in the North Sea showed signatures of stabilizing selection. These different modes of selection are consistent with the regional light conditions and hence support our predictions, but may also be influenced by migration between the open sea and more turbid estuarine environments. An interesting observation is that within one gene, synonymous and non‐synonymous SNPs show a totally different pattern between populations. Population differentiation based on non‐synonymous SNPs of the RH1 gene correlated with spectral variation of the local environment of the sand goby populations. In contrast, the differentiation based on synonymous SNPs of RH1 reflects more the neutral historical pattern of the species.     

Influences of human activities on seabird populations in the North Sea

This paper describes the North Sea's seabird resource, which is of great international importance. It reviews the significance of human activities on seabirds; these are persecution, fishing, eutrophication, chemical pollution, introduction of alien predators, rising sea level, and oil pollution. The paper speculates on future trends. Some suggestions to reduce human pressures, to use seabirds as monitors of the marine environment and to conserve their populations more effectively are made.     

Multi-torsking: simultaneous measurements of cod behaviour show differences between North Sea and Irish Sea stocks*

Fish behaviour can vary considerably in response to environmental and biological conditions. We used electronic Data Storage Tags (DSTs) to investigate the behaviour of cod in different environments; 74 North Sea cod and 20 Irish Sea cod were tagged and released in late-March 1999. So far, 32 tags (>2000 days of data) have been returned from the North Sea and four (>700 days) from the Irish Sea. The maximum recorded depth of a North Sea cod was 84 m, while Irish Sea cod utilised a depth range of 154 m. Active depth changes were of greater magnitude and rate in Irish Sea cod than North Sea cod. In addition, Irish Sea cod were continually active and made proportionally more active depth changes than North Sea cod, which were very active during migration (March–June) but exhibited little vertical movement during the summer months. To our knowledge, this is the first time that the behaviour patterns of cod from different stocks have been measured simultaneously over extended periods at such a fine temporal scale. In addition to describing considerable inter-stock differences in behaviour, our results suggest that while cod in the Irish Sea remain active and highly mobile throughout the year, North Sea cod reduce their movements during the summer feeding period. Our findings will contribute to the development of more biologically realistic management models used for evaluating the efficacy of fisheries technical measures, such as closed areas.     

New data on the longevity of coastal cod Gadus morhua Linnaeus, 1758 in the White Sea

The cod, Gadus morhua, is a common and abundant demersal fish in the White Sea coastal zone. The published data on the maximum age of White Sea cod still requires additional documentation, because some authors have noted difficulties in age determination by otoliths of large specimens. To obtain accurate data on the longevity of this species in the White Sea, an image analysis of thin‐sections of otoliths was made for age estimation. Research surveys in the Chupa Inlet and adjacent waters of the Kandalaksha Bay were conducted in June to August from 2007 to 2013. The five largest specimens of a total of 3564 captured fish were selected for age determination. Age of these largest individuals ranged between 7 and 12 years, total length and weight varied from 60.2 to 77.0 cm and from 2.4 to 6.1 kg, respectively. According to the data, maximal White Sea cod longevity is greater than in previously published data, and demonstrates similar longevity to conspecifics from the Baltic and North seas, whereas its life cycle is much shorter than cod from the Northeast Arctic, Iceland, Greenland, Newfoundland and Labrador stocks.