Biological and environmental factors influencing recruitment success of North Sea demersal and pelagic fish stocks Alan Sinclair Fisheries and Oceans Canada Pacific Biological Station, Nanaimo, BC Laurence Kell and Georgi Daskalov CEFAS Lowestoft Laboratory, UK

Motivation North Sea stocks are assessed on a single stock basis However fishing fleets exploit a range of species –For example cod are taken by many gears and as a bycatch in various non-target fisheries. It is important therefore to look at whether stocks vary together and how environmental factors influence the main commercial fish stocks Since this has important implications both for yields to the various fishery sectors and for the management of the North Sea fisheries

The Main Question Clearly there must be spawners (S) to have recruits (R) –However, inter-annual variability in recruitment far outweighs variability in spawners How Do Environmental Conditions Affect Recruitment? Do environmental conditions determine –the number of recruits, regardless of spawning stock size? –or the juvenile survival rate (R/S)? Is recruitment affected by biological processes such as predation, competition or spawning condition? Is recruitment affected by physical processes such as temperature, salinity …?

Preliminary analysis of commercial North Sea Stocks We set out to Test alternative biological and physical hypotheses within a common framework Data sets Commercial Fish Stocks –ICES stock assessment WGs Environment –Sea surface temperature COADS –North Atlantic Oscillation (NAO) Other data series could be included in future analyses –plankton, salinity, advection …

Main North Sea Commercial Stocks Plaice Sole Cod Haddock Saithe Whiting Sandeel Herring

Stepwise Analysis of Environmental Effects Using Likelihood Ratio Test R = recruitment S = spawning stock biomass E = environmental covariate α = maximum recruitment β = biomass at ½ max recruitment κ = environmental parameter σ = residual standard deviation

Biotic Hypotheses Competition / Predation –Recruitment of one species is negatively affected by R or S of another species in the year of spawning. Juvenile feeding –Recruitment of a pisciverous juvenile (cod, plaice, saithe, whiting) is positively affected by R of a suitable prey species (herring, sandeel) in the year of spawning. Feeding and spawning fitness –Recruitment of cod, haddock, plaice, saithe, sole or whiting is positively affected by R of any species in the year prior to spawning. –Recruitment of cod, saithe or whiting is positively affected by S of herring or sandeel in the year prior to spawning.

Abiotic Hypotheses North Atlantic Oscillation (NAO) –A single annual mean NAO index was used –Positive or negative effect of NAO on recruitment Temperature –May act on different part of life history, therefore temperature variables were created from monthly time series and from North Sea sea surface temperature (SST) –Effect of temperature variables may be positive or negative

Temperature Variables SSTY –Sea surface temperature annual mean anomaly Q1Y, Q2Y, Q3Y, Q4Y –Quarterly mean anomalies in year of spawning Q1Y-1, Q2Y-1, Q3Y-1, Q4Y-1 –Quarterly mean anomaly in the year prior to spawning SSTDJF: –Mean winter anomaly, Dec Y-1, Jan Y, Feb Y; SSTFJ –Mean anomaly Feb – July PC1, PC2, PC3 –First 3 principle components

PCA of Monthly Sea Surface Temperature PC1 ~ 54% of Variance –annual signal PC2 ~ 16% of Variance –contrast between first and second half of year PC3 ~ 10% of Variance –contrast between summer and winter temperature

Results A preliminary first cut at an analysis of this type A broad look at how the North Sea commercial fish species vary together and the possible mechanisms

Cod Recruitment density dependent Negative effect of SSTemp (PC1) on recruitment –this has been noted by Planque and Frédou 1999 among others α = maximum recruitment β = biomass at ½ max recruitment

Cod Positive effect of Sandeel Recruitment (R1San) and negative effect of SST (PC1) The PC1 term not significant in model with both terms The stock/recruitment parameters are very sensitive to the environmental effect Resolving which environmental effect is operating is important for interpreting stock/recruitment dynamics α = maximum recruitment β = biomass at ½ max recruitment

Haddock Cannot reject density independent recruitment hypothesis (i.e. no evidence of a significant stock recruitment relationship) Negative effect of sole recruitment (R1Sol) and herring spawning biomass (S1Her) The S1Her term not significant in model with both terms High residual standard deviation (Sigma) regardless of model

Sole Cannot reject density independent recruitment hypothesis Positive effect of contrast in SST between winter and summer (PC2) Positive effect of plaice recruitment (R1Pla) Both effects significant in 2-parameter model

Herring Recruitment density dependent Negative effect of Saithe spawning biomass (S1Sai) on recruitment

Plaice Cannot reject density independent recruitment hypothesis Negative effect of SSTemp Feb-Jun (SSTFJ) for and periods Positive effect of Sandeel recruitment (R1San)for period Sandeel recruitment not significant (barely) in model with both effects

Summary of Biological Effects A very large number of plausible biological hypotheses were tested involving competition, predation, feeding of juveniles and feeding of spawners. It was surprising how little evidence was found to support any of these.

Summary of Temperature and NAO Effects Temperature effects may be important for cod, plaice and sole recruitment. For cod and plaice the effects are negative and related to annual or seasonal temperature values. For sole, recruitment was best in years with high seasonal contrast in temperatures. The NAO did not enter in any of the ‘best’ models. However, there is a strong correlation between the NAO and SST, especially SST in the first quarter. Thus, colinearity may be masking important relationships with NAO.

Summary and Additional Questions Why do there appear to be so few significant relationships Is there ancillary information to support the findings through diets, laboratory study, earlier publications, etc.? Are some of the ‘significant’ relationships obviously spurious or at odds with accepted conditions in the North Sea? Are there other mechanisms that should be investigated? –For example, is it temperature or Sandeel that affects cod recruitment? What are the implications of specific ‘environmental’ relationships for management targets and limits