Transport of North Sea cod larvae into the Skagerrak coastal populations

The Atlantic cod (Gadus morhua) is economically one of the world's most important marine species--a species presently suffering from heavy overexploitation throughout its range of distribution. Although not fully understood, the Atlantic cod is believed to be structured into populations in a rather complex manner, whereby both highly migratory and more confined ocean-spawning stocks coexist with stationary coastal populations. Owing to the complex population structure, little is presently known about how overexploitation of offshore stocks may affect other segments of the species. Here, we use microsatellite DNA analyses of coastal and offshore cod in combination with oceanographic modelling to investigate the population structure of Atlantic cod in the North Sea-Skagerrak area and evaluate the potential for larval transport into coastal populations. Our results suggest an extensive but temporally variable drift of offshore cod larvae into coastal populations. In a year (2001) with high inflow of North Sea waters into the Skagerrak we find that juvenile cod caught along the Skagerrak coast are predominantly of North Sea origin, whereas in a year (2000) with low inflow juveniles appear to be of local origin. These findings indicate that offshore cod may influence coastal cod populations over large distances.     

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.     

Long-term stability and effective population size in North Sea and Baltic Sea cod (Gadus morhua)

DNA from archived otoliths was used to explore the temporal stability of the genetic composition of two cod populations, the Moray Firth (North Sea) sampled in 1965 and 2002, and the Bornholm Basin (Baltic Sea) sampled in 1928 and 1997. We found no significant changes in the allele frequencies for the Moray Firth population, while subtle but significant genetic changes over time were detected for the Bornholm Basin population. Estimates of the effective population size ( N _e ) generally exceeded 500 for both populations when employing a number of varieties of the temporal genetic method. However, confidence intervals were very wide and N _e 's most likely range in the thousands. There was no apparent loss of genetic variability and no evidence of a genetic bottleneck for either of the populations. Calculations of the expected levels of genetic variability under different scenarios of N _e showed that the number of alleles commonly reported at microsatellite loci in Atlantic cod is best explained by N _e 's exceeding thousand. Recent fishery-induced bottlenecks can, however, not be ruled out as an explanation for the apparent discrepancy between high levels of variability and recently reported estimates of N _e  << 1000. From life history traits and estimates of survival rates in the wild, we evaluate the compatibility of the species' biology and extremely low N _e / N ratios. Our data suggest that very small N _e 's are not likely to be of general concern for cod populations and, accordingly, most populations do not face any severe threat of losing evolutionary potential due to genetic drift.     

Mapping the spawning grounds of North Sea cod (Gadus morhua) by direct and indirect means

Despite recent evidence for sub-stock structuring, North Sea cod are assessed as a single unit. As a consequence, knowledge of sub-stock trends is poor. In particular, there are no recent evaluations of which spawning grounds are active. Here we report results from the first ichthyoplankton survey to cover the whole North Sea. Also, this survey, conducted in 2004, was the first to make extensive use of DNA-based molecular methods to unambiguously identify early developmental stage cod eggs. We compare the findings from the plankton survey with estimated egg production inferred from the distribution of mature cod in contemporaneous trawl surveys. Results from both approaches were in general agreement and showed hot spots of egg production around the southern and eastern edges of the Dogger Bank, in the German Bight, the Moray Firth and to the east of the Shetlands. These areas broadly coincide with known spawning locations from the period 1940 to 1970. We were, however, unable to directly detect significant numbers of cod eggs at the historic spawning ground off Flamborough (northeast coast of England). The results demonstrate that most of the major spawning grounds of cod in the North Sea are still active but that some localized populations may have been reduced to the point where it is now difficult to detect the presence of eggs in the plankton.     

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.     

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.     

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.     

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.     

Temporal analysis of archived samples indicates marked genetic changes in declining North Sea cod (Gadus morhua)

Despite increasing evidence that current exploitation rates can contribute to shifts in life-history traits and the collapse of marine fish stocks, few empirical studies have investigated the likely evolutionary impacts. Here, we used DNA recovered from a temporal series of archived North Sea cod (Gadus morhua) otoliths, to investigate genetic diversity within the Flamborough Head population between 1954 and 1998, during which time the population underwent two successive declines. Microsatellite data indicated a significant reduction in genetic diversity between 1954 and 1970 (total number of alleles: 1954, 46; 1960, 42; 1970, 37), and a subsequent recovery between 1970 and 1998 (total number of alleles: 1970, 37; 1981, 42; 1998, 45). Furthermore, estimates of genetic differentiation (F(ST) and R(ST)) showed a significant divergence between 1998 and earlier samples. Data are consistent with a period of prolonged genetic drift, accompanied by a replacement of the Flamborough Head population through an increased effective migration rate that occurred during a period of high exploitation and appreciable demographic and phenotypic change. Other studies indicate that diversity at neutral microsatellite loci may be correlated with variability at selected genes, thus compromising a population's subsequent recovery and adaptive potential. Such effects are especially pertinent to North Sea cod, which are threatened by continuing exploitation and rising sea temperatures.     

Changes of the tidal water levels at the German North Sea coast

For 10 selected tide gauges at the German North Sea Coast, the time series of the mean high water (MHW) and mean low water (MLW) were analysed. Since about 1950, both levels deviate from earlier trends; MHW shows an accelerated rise, while MLW even fell after an earlier rise. Presented at the VI International Wadden Sea Symposium (Biologische Anstalt Helgoland, Wattenmeerstation Sylt, D-2282 List, FRG, 1–4 November 1988).     

The dispersal patterns and behaviour of North Sea cod (Gadus morhua) studied using electronic data storage tags*

The migratory behaviour of adult cod was studied using electronic data storage tags (DSTs). Fifty-eight cod were externally tagged with LOTEK LTD-100 DSTs and released off Lowestoft in March and April 1999. Twenty-two tags were returned by March 2000. Individuals demonstrated significant shifts in behaviour patterns that were associated with migration and residence on feeding grounds. During migration, individuals were very active and utilised a large vertical range. In contrast, individuals demonstrated low activity and a much reduced vertical range whilst on summer feeding grounds. Use of the Tidal Location Method showed that individuals dispersed to feeding grounds in the central and southern North Sea. Our results demonstrate that individual-based studies of fish behaviour can provide information relevant to fisheries management.     

Recruitment variability in North Atlantic cod and match-mismatch dynamics

Background

Fisheries exploitation, habitat destruction, and climate are important drivers of variability in recruitment success. Understanding variability in recruitment can reveal mechanisms behind widespread decline in the abundance of key species in marine and terrestrial ecosystems. For fish populations, the match-mismatch theory hypothesizes that successful recruitment is a function of the timing and duration of larval fish abundance and prey availability. However, the underlying mechanisms of match-mismatch dynamics and the factors driving spatial differences between high and low recruitment remain poorly understood.

Methodology/Principal Findings

We used empirical observations of larval fish abundance, a mechanistic individual-based model, and a reanalysis of ocean temperature data from 1960 to 2002 to estimate the survival of larval cod (Gadus morhua). From the model, we quantified how survival rates changed during the warmest and coldest years at four important cod spawning sites in the North Atlantic. The modeled difference in survival probability was not large for any given month between cold or warm years. However, the cumulative effect of higher growth rates and survival through the entire spawning season in warm years was substantial with 308%, 385%, 154%, and 175% increases in survival for Georges Bank, Iceland, North Sea, and Lofoten cod stocks, respectively. We also found that the importance of match-mismatch dynamics generally increased with latitude.

Conclusions/Significance

Our analyses indicate that a key factor for enhancing survival is the duration of the overlap between larval and prey abundance and not the actual timing of the peak abundance. During warm years, the duration of the overlap between larval fish and their prey is prolonged due to an early onset of the spring bloom. This prolonged season enhances cumulative growth and survival, leading to a greater number of large individuals with enhanced potential for survival to recruitment.     

Microbial community structure analysis of produced water from a high-temperature North Sea oil-field

Molecular and culture-based methods were used to investigate the microbial diversity in produced water obtained from the high-temperature Troll oil formation in the North Sea. 16S rRNA gene libraries were generated from total community DNA, using universal archaeal or bacterial oligonucleotide primer sets. Sequence analysis of 88 clones in the bacterial library indicated that they originated from members of Firmicutes (48 sequences), Bacteroidetes (17 sequences), δ-Proteobacteria (15 sequences), Spirochaetes (5 sequences), Thermotogales (2 sequences) and γ-Proteobacteria (1 sequence). Twenty-two sequences in the archaeal library were close relatives to members of the genera Methanococcus (18 sequences), Methanolobus (3 sequences) and Thermococcus (1 sequence). Most of the bacterial sequences shared less than 95% identity with their closest match in GenBank, indicating that the produced water harbours a unique community of novel bacterial species or genera. Members of the thermophilic genera Thermosipho, Thermotoga, Anaerophaga and Thermovirga were isolated. The Troll formations are not injected with sea water. Thus, dramatic changes of the in situ conditions have been avoided, and a common source of continuous contamination from injection water can be excluded. However, the majority of the organisms detected in the gene libraries were most closely related to cultivated organisms with optimum temperatures for growth well below the in situ reservoir temperature (70°C), indicating that produced water from the Troll platform harbours a substantial amount of non-indigenous organisms. This was confirmed by the isolation of a number of mesophilic and moderately thermophilic organisms that were unable to grow at reservoir temperature.     

Microbial analysis of backflowed injection water from a nitrate-treated North Sea oil reservoir

Reservoir souring in offshore oil fields is caused by hydrogen sulphide (H₂S) produced by sulphate-reducing bacteria (SRB), most often as a consequence of sea water injection. Biocide treatment is commonly used to inhibit SRB, but has now been replaced by nitrate treatment on several North Sea oil fields. At the Statfjord field, injection wells from one nitrate-treated reservoir and one biocide-treated reservoir were reversed (backflowed) and sampled for microbial analysis. The two reservoirs have similar properties and share the same pre-nitrate treatment history. A 16S rRNA gene-based community analysis (PCR-DGGE) combined with enrichment culture studies showed that, after 6 months of nitrate injection (0.25 mM NO₃ ⁻), heterotrophic and chemolithotrophic nitrate-reducing bacteria (NRB) formed major populations in the nitrate-treated reservoir. The NRB community was able to utilize the same substrates as the SRB community. Compared to the biocide-treated reservoir, the microbial community in the nitrate-treated reservoir was more phylogenetically diverse and able to grow on a wider range of substrates. Enrichment culture studies showed that SRB were present in both reservoirs, but the nitrate-treated reservoir had the least diverse SRB community. Isolation and characterisation of one of the dominant populations observed during nitrate treatment (strain STF-07) showed that heterotrophic denitrifying bacteria affiliated to Terasakiella probably contributed significantly to the inhibition of SRB. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Diseases in North Sea fishes

Prior to the studies reviewed here, only lymphocystis and skeletal deformities of a variety of fish species and certain diseases of eel were known to occur in the German Bight (North Sea). From 1977 until now, 9 externally visible lesions on North Sea fishes were observed; in addition to those mentioned before, they comprise: fin rot, ulcerations, epidermal papilloma, hyperplasia, pseudobranchial tumour, eye diseases and gill swellings. With the exception of information on changes in frequencies of vertebral deformities of herring from the 1950's to the 1970's, there are no long-term data characterizing changes in frequencies of the diseases under study. For pseudobranchial tumours of cod and epidermal papilloma of dab, information is provided on occurrence and abundance. The distribution pattern of cod afflicted with pseudobranchial tumours is strongly influenced by the migratory behaviour of the fish. Epidermal papillomas of dab were more frequently found at stations within the inner German Bight than in neighbouring areas. The Bight is used for dumping of wastes from titaniumdioxide production. Further disease hot spots are areas off the Humber estuary and the British coast. Analysis of chromium in dab from the German Bight revealed elevated concentrations in epidermal tissues of specimens from the dumping area compared with that found in dab from neighbouring localities. Particulate iron was demonstrated to occur in mucous cells of dab from the dumping area. From increased levels of heavy metals with cancerogenic potential in sensitive target tissues and from increased prevalences of diseased fish in the dumping area it is concluded that these phenomena are possibly causally linked. In the vicinity of the Humber estuary high disease rates were encountered and areas with high prevalences of dab afflicted with epidermal papilloma extended over regions shown to be transport routes for persistent pollutants such as radioactive materials. It is therefore suggested that the long-range distribution of fish diseases in the southern North Sea might reflect the long-range transport of persistent pollutants.     

Euthanasia across the North Sea

The United Kingdom and The Netherlands are separated by a narrow stretch of sea but in terms of an understanding of euthanasia they seem to be light years apart. An attempt to bridge the information gap seems in order.The position paper on euthanasia published by the Royal Dutch Medical Association contains 16 typed pages.¹ In contrast, the report on euthanasia issued by a working party of the British Medical Association is 80 pages in small print.²