Plankton changes and cod recruitment in the North Sea

1. ECOREG Early life cycle ECOREG EcosystemCOmplexityREsearchGroup Plankton changes and cod recruitment in the North Sea Grégory Beaugrand1,3*, Keith M. Brander2, Sami Souissi1 (2)ICES, Palaegade 2-4, 1261 Copenhagen K, Denmark (3) SAHFOS, The Laboratory, Citadel Hill, Plymouth PL1 2PB, England (1) EcosystemCOmplexity REsearchGroup,Station Marine de Wimereux, Université des Sciences et Technologies de Lille CNRS - UMR 8013 ELICO, 28 avenue Foch, BP 80 - F-62930 Wimereux, France. (*) Gregory.Beaugrand@univ-lille1.fr Introduction A number of studies have reported that temperature influences cod recruitment although the relationships found were often weak for the North Sea. Food quantity and quality are also essential as they influence growth of fish larvae and attempts have been made to establish relationships between changes in plankton (i.e. availability of prey) and cod. The match/mismatch hypothesis relates survival to the match between the time of larval occurrence and that of the production of their food. The critical period for this match probably extends through the late larval/early juvenile stages and the prey must be of a suitable size. Figure 4 suggests that larval cod survival depends on three key biological parameters of their prey: the mean size, the seasonal timing and abundance of preys. A plankton indicator of cod larval survival in the North Sea A new plankton index has been recently proposed by Beaugrand et al. (2003). It integrates biological parameters important for larval cod survival (Fig. 1). This index includes information on calanoid copepod biomass, mean size of calanoid copepod preys and on the abundance of the key prey species Calanus finmarchicus, C. helgolandicus and Pseudocalanus spp. The annual index is calculated by using a principal component analysis performed for all months between March and September (current period: 1958-2002) while the monthly index uses information from January to December. Fig. 1. Simplified summary of preys eaten by larval cod from March to September The index revealed a clear distinction between the periods 1963-1983 and both the periods 1984-1999 and 1958-1962 (Fig. 2). The period 1963-1983 (Gadoid Outburst) was characterized by high abundance of prey for larval cod (positive anomalies in the biomass of calanoid copepods, in the abundance of C. finmarchicus, euphausiids and Pseudocalanus spp.) and a high mean size of calanoid copepods. Cod recruitment decreased from the mid-1980s, coincident with unfavorable changes in the plankton ecosystem, compared to the earlier period 1963-1983. Fig. 4. Plankton changes (1958-1999) and their consequences for larval / juvenile cod survival. b. Long-term changes in the ratio size of calanoid copepod:length of cod larvae. Larval cod (from 15 to 45 mm long) feed on adult C. finmarchicus in July and according to Munk's model, the ratio should be close to 0.05, becoming highly unfavorable below 0.03 and above 0.08 These results provide evidence that changes in the plankton ecosystem are the probable cause of the increased recruitment during the period 1963-1983, which resulted in the “Gadoid Outburst” and suggest that unfavorable changes in the plankton ecosystem have exacerbated the impact of overfishing in reducing recruitment of North Sea cod since the mid 1980s. Figure 5 shows the relationships between recruitment, spawning stock biomass and the plankton index. Three local dynamic equilibriums are identified. P3 was characterized by the highest SSB but recruitment was lower than P2/P4, probably as a result of a plankton environment less favorable for larval cod survival. Fig. 2. Long-term monthly changes (1958-1999) in the plankton index of larval cod survival in relation to changes in cod recruitment (at age 1, one-year lag). Positive anomalies in the index indicate high quantity and quality of preys and a match between prey and larval cod occurrence. Negative anomalies mean an opposite biological environment Figure 3 shows that the plankton index is highly correlated positively with cod total biomass with a two-year lag until the mid-1960s and a one-year lag (r=0.88, pACF<0.01) after the mid-1960s onwards. This increase in the speed of cod response to plankton change may be related to the increasing impact of fishing at that time on the age structure of the stock. Fig. 5. Long-term changes in plankton, cod recruitment (age 1, one-year lag) and spawning stock biomass. Three dynamic equilibriums are identified Conclusions This research suggests that plankton changes should be taken into account in the management of cod in the North Sea. References Beaugrand G., Brander K.M., Lindley J.A., Souissi S & P.C. Reid. (2003) Plankton effect on cod recruitment in the North Sea. Nature, 426: 661-664. Munk P. (1997) Prey size spectra and prey availability of larval and small juvenile cod. Journal of Fish Biology, 51 (Supplement A): 340-351. Fig. 3. Long-term changes in plankton and total cod biomass (with a one-year lag) Paris 31 March – 3 April 2004 International symposium on quantitative ecosystem indicators for fisheries management