22.214.171.124 Impacts of decadal variability and extremes
Most of the large global marine-capture fisheries are affected by regional climate variability. Recruitment of the two tropical species of tuna (skipjack and yellowfin) and the sub-tropical albacore (Thunnus alalunga) in the Pacific is related to regimes in the major climate indices, ENSO and the Pacific Decadal Oscillation (Lehodey et al., 2003). Large-scale distribution of skipjack tuna in the western equatorial Pacific warm pool can also be predicted from a model that incorporates changes in ENSO (Lehodey, 2001). ENSO events, which are defined by the appearance and persistence of anomalously warm water in the coastal and equatorial ocean off Peru and Ecuador for periods of 6 to 18 months, have adverse effects on Peruvian anchovy production in the eastern Pacific (Jacobson et al., 2001). However, longer term, decadal anomalies appear to have greater long-term consequences for the food-web than the short periods of nutrient depletion during ENSO events (Barber, 2001). Models relating interannual variability, decadal (regional) variability and global climate change must be improved in order to make better use of information on climate change in planning management adaptations.
North Pacific ecosystems are characterised by ‘regime shifts’ (fairly abrupt changes in both physics and biology persisting for up to a decade). These changes have major consequences for the productivity and species composition of fisheries resources in the region (King, 2005).
Major changes in Atlantic ecosystems can also be related to regional climate indicators, in particular the NAO (Drinkwater et al., 2003; see also Chapter 1 on north-east Atlantic plankton, fish distribution and production). Production of fish stocks, such as cod in European waters, has been adversely affected since the 1960s by the positive trend in the NAO. Recruitment is more sensitive to climate variability when spawning biomass and population structure are reduced (Brander, 2005). In order to reduce sensitivity to climate, stocks may need to be maintained at higher levels.
Climate-related reductions in production cause fish stocks to decline at previously sustainable levels of fishing; therefore the effects of climate must be correctly attributed and taken into account in fisheries management.