Stephen B. Reilly

Physical and biological effects of Los Niños in the eastern tropical Pacific, 1986–1989

Abstract The eastern tropical Pacific Ocean was surveyed in August–November 1986, 1987, 1988 and 1989 as part of a long-term program to monitor dolphin stocks. Temperature, salinity, chlorophyll and nutrients were monitored to help interpret variability in dolphin stock estimates. The four surveys reveal major environmental changes during the moderate 1987 El Nino and the cold La Nina episode that followed in 1988. During the “onset” phase of El Nino in fall 1986, surface temperatures were up to 1.5°C above normal in equatorial water, but near normal in tropical water north of the equator. The equatorial thermocline ridge was deepened by 10–30 m. During the “mature” phase of El Nino in fall 1987, surface temperatures anomalies were up to +2.5°C in equatorial water and about +1°C in tropical water. Thermocline topography was anomalously flat. Surface chlorophyll and nutrient concentrations declined by 11–48% compared to 1986, with the greatest declines occurring in coastal and equatorial upwelling systems and along the countercurrent thermocline ridge. During La Nina 1988, equatorial surface temperatures were up to 2.5°C below normal and the equatorial thermocline ridge was 10–50 m more shallow than normal. Chlorophyll and nitrate concentrations increased by 58–65% compared to 1987. In 1989, surface temperature was within ±1°C of normal throughout the study area and chlorophyll concentrations were similar to those observed in 1986. Changes in nutrient availability and biological productivity during the El Nino/La Nina cycle were caused by variations in both the rate of wind-driven upwelling and in the nutrient content of subsurface water entrained by upwelling.

Ecological Metrics of Biomass Removed by Three Methods of Purse-Seine Fishing for Tunas in the Eastern Tropical Pacific Ocean

An ecosystem approach to fisheries management is a widely recognized goal, but describing and measuring the effects of a fishery on an ecosystem is difficult. Ecological information on the entire catch (all animals removed, whether retained or discarded) of both species targeted by the fishery and nontarget species (i.e., bycatch) is required. We used data from the well-documented purse-seine fishery for tunas (Thunnus albacares, T. obesus, and Katsuwonus pelamis) in the eastern tropical Pacific Ocean to examine the fisherys ecological effects. Purse-seine fishing in the eastern tropical Pacific is conducted in 3 ways that differ in the amount and composition of target species and bycatch. The choice of method depends on whether the tunas are swimming alone (unassociated sets), associated with dolphins (dolphin sets), or associated with floating objects (floating-object sets). Among the fishing methods, we compared catch on the basis of weight, number of individuals, trophic level, replacement time, and diversity. Floating-object sets removed 2-3 times as much biomass as the other 2 methods, depending on how removal was measured. Results of previous studies suggest the ecological effects of floating-object sets are thousands of times greater than the effects of other methods, but these results were derived from only numbers of discarded animals. Management of the fishery has been driven to a substantial extent by a focus on reducing bycatch, although discards are currently 4.8% of total catch by weight, compared with global averages of 7.5% for tuna longline fishing and 30.0% for midwater trawling. An ecosystem approach to fisheries management requires that ecological effects of fishing on all animals removed by a fishery, not just bycatch or discarded catch, be measured with a variety of metrics.