Clive Fox

Climate variability and north sea cod

The stock of North Sea cod is under pressure because of overfishing, and we show here that it is also threatened by a decline in the production of young cod that has paralleled warming of the North Sea over the past ten years. The combination of a diminished stock and the possible persistence of adverse warm conditions is endangering the long-term sustainability of cod in the North Sea. To decrease the risk of collapse, fishing pressure must be reduced.

Fisheries: Climate variability and North Sea cod

The stock of North Sea cod is under pressure because of overfishing, and we show here that it is also threatened by a decline in the production of young cod that has paralleled warming of the North Sea over the past ten years. The combination of a diminished stock and the possible persistence of adverse warm conditions is endangering the long-term sustainability of cod in the North Sea. To decrease the risk of collapse, fishing pressure must be reduced.

Synchrony in the recruitment time-series of plaice (Pleuronectes platessa L) around the United Kingdom and the influence of sea temperature

Abstract Historical time-series of recruitment to plaice stocks around the United Kingdom were examined for evidence of synchrony. Correlations were significant for most pair-wise comparisons and a meta-analysis test over all the stocks was highly significant. Previous studies on plaice recruitment in the North Sea have suggested that a negative relationship exists between sea temperature, during the first few months of the year, and subsequent year-class strength. When we examined the correlations between sea surface temperature and plaice recruitment, we found statistically significant negative relationships for most areas. These were strongest for the period February–June. Our results are consistent with the hypothesis that the temperature effect on recruitment of North Sea plaice is due to changes in predation pressure on the planktonic stages. However, western (North Sea and Channel) and eastern (Irish Sea and Celtic Sea) stocks appeared to respond to different time-scales of temperature variability which may imply that different mechanisms could be operating in these stocks. Management of plaice is based upon projections of future stock dynamics. Compared with fisheries on some other species, those on plaice are less dependent on the in-coming year-class. Therefore, the incorporation of sea temperature data into plaice stock dynamic models will probably not have a large impact on short-term projections. However, incorporation of a temperature relationship will allow medium to long-term forecasts to be made under varying environmental scenarios. These may provide a more realistic range of projections of future stock trends than is currently achieved.

TaqMan DNA technology confirms likely overestimation of cod (Gadus morhua L.) egg abundance in the Irish Sea: implications for the assessment of the cod stock and mapping of spawning areas using egg‐based methods

Recent substantial declines in northeastern Atlantic cod stocks necessitate improved biological knowledge and the development of techniques to complement standard stock assessment methods (which largely depend on accurate commercial catch data). In 2003, an ichthyoplankton survey was undertaken in the Irish Sea and subsamples of ‘cod‐like’ eggs were analysed using a TaqMan multiplex, PCR (polymerase chain reaction) assay (with specific probes for cod, haddock and whiting). The TaqMan method was readily applied to the large number of samples (n = 2770) generated during the survey and when combined with a manual DNA extraction protocol had a low failure rate of 6%. Of the early stage ‘cod‐like’ eggs (1.2–1.75 mm diameter) positively identified: 34% were cod, 8% haddock and 58% whiting. As previous stock estimates based on egg surveys for Irish Sea cod assumed that the majority of ‘cod‐like’ eggs were from cod, the TaqMan results confirm that there was probably substantial contamination by eggs of whiting and haddock that would have inflated estimates of the stock biomass.

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.

Effectiveness of TaqMan probes for detection of fish eggs and larvae in the stomach contents of a teleost predator

Experiments were conducted on the ability of TaqMan molecular probes to detect plaice Pleuronectes platessa DNA from eggs, and cod Gadus morhua DNA from eggs and larvae following ingestion by a teleost predator, whiting Merlangius merlangus. Estimated half-life detection rate (T50) for eggs was 31 h, and 26 h for larvae, with some positive detections occurring even after visual inspection indicated complete gut clearance. Because TaqMan probes are taxon specific, the results presented demonstrate that this technique can provide a means of rapid and unambiguous detection of predation by teleosts on fish eggs and larvae.

Plaice egg mortality : can we determine survivorship?

The daily mortality rate of cohorts of plaice eggs in the Irish Sea is estimated throughout the spawning season in 1995 and 2000, using general additive models of egg production. Daily mortality (z) was found to vary between 0.15 and 0.29. Mortality rates declined through the season in 1995 but not in 2000. There were significant differences in the mortality rates of individual cohorts of eggs. The suitability of an exponential decay model to describe daily mortality is discussed. The mortality rates conform to previously described relationships between fish eggs and temperature. The estimates of mortality rate were found to be robust to imprecision in egg ageing, but ageing imprecision led to a bias in the estimation of survivorship of eggs from spawning to hatch. The bias will vary with mortality rate.

Noise can affect acoustic communication and subsequent spawning success in fish

There are substantial concerns that increasing levels of anthropogenic noise in the oceans may impact aquatic animals. Noise can affect animals physically, physiologically and behaviourally, but one of the most obvious effects is interference with acoustic communication. Acoustic communication often plays a crucial role in reproductive interactions and over 800 species of fish have been found to communicate acoustically. There is very little data on whether noise affects reproduction in aquatic animals, and none in relation to acoustic communication. In this study we tested the effect of continuous noise on courtship behaviour in two closely-related marine fishes: the two-spotted goby (Gobiusculus flavescens) and the painted goby (Pomatoschistus pictus) in aquarium experiments. Both species use visual and acoustic signals during courtship. In the two-spotted goby we used a repeated-measures design testing the same individuals in the noise and the control treatment, in alternating order. For the painted goby we allowed females to spawn, precluding a repeated-measures design, but permitting a test of the effect of noise on female spawning decisions. Males of both species reduced acoustic courtship, but only painted gobies also showed less visual courtship in the noise treatment compared to the control. Female painted gobies were less likely to spawn in the noise treatment. Thus, our results provide experimental evidence for negative effects of noise on acoustic communication and spawning success. Spawning is a crucial component of reproduction. Therefore, even though laboratory results should not be extrapolated directly to field populations, our results suggest that reproductive success may be sensitive to noise pollution, potentially reducing fitness.

Climate variability and North Sea cod: Fisheries

The stock of North Sea cod is under pressure because of overfishing, and we show here that it is also threatened by a decline in the production of young cod that has paralleled warming of the North Sea over the past ten years. The combination of a diminished stock and the possible persistence of adverse warm conditions is endangering the long-term sustainability of cod in the North Sea. To decrease the risk of collapse, fishing pressure must be reduced.