ENSO EVENTS IN THE NORTHERN GULF-OF-ALASKA, AND EFFECTS ON SELECTED MARINE FISHERIES

The 1991-93 El Nino-Southern Oscillation (ENSO) event first appeared i n the northern Gulf of Alaska in autumn 1991 with warm sea-surface tem peratures. In winter 1992, there were pulses of increased sea level an d anomalous circulation. El Nino conditions persisted at least through summer 1993. The effects of this ENSO event on major groundfish speci es and Pacific herring in the northern Gulf of Alaska were examined an d compared with the effects of previous ENSO events. There is little e vidence that the 1991-93 or 1982-83 ENSO events affected landings of w alleye pollock, Pacific cod, Pacific halibut, or arrowtooth flounder. Some changes in distribution of groundfish species were observed in 19 93, but the effect was similar to changes observed in non-ENSO warm ye ars. In general, warm ocean conditions have a positive effect on recru itment of northern stocks, but ENSO events appear to have an inconsist ent effect on year-class strength within species and among different s pecies. For example, strong year classes of halibut and arrowtooth flo under sometimes, but not always, coincide with ENSO events; ENSO event s are associated with moderate to weak year classes of cod and pollock . However, post-ENSO warm years often are associated with strong recru itment of many groundfish species. Major changes have occurred in the Gulf of Alaska ecosystem since 1977. The influence of the 1976 ENSO ev ent in precipitating these changes and the role of the frequency or st rength of subsequent El Nino events is presently unknown. Herring and other stocks of small pelagic fishes may be more affected by ENSO even ts. In particular, decreased catches, recruitment, and weight-at-age o f herring are sometimes associated with ENSO events. Furthermore, a va riety of seabirds which feed mostly on pelagic forage fishes or the pe lagic juvenile stages of groundfish suffered widespread mortalities an d breeding failures in the Gulf of Alaska during the ENSO years of 198 3 and 1993. These effects on seabirds were also observed over a wider geographic range, from California to the western Bering Sea.