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 fishery's 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. Resumen: Un enfoque ecosistemico en el manejo de pesquerias es una meta ampliamente reconocida, pero describir y medir los efectos de una pesqueria sobre un ecosistema es dificil. Se requiere informacion ecologica de toda la captura (todos los animales extraidos, ya sea retenidos o descartados) tanto de especies objetivo de la pesqueria como de especies no objetivo (captura incidental). Utilizamos datos de la pesqueria de atunes (Thunnus albacares, T. obesus y Katsuwonus pelamis) con red de cerco en el Oceano Pacefico oriental tropical, bien documentada, para examinar los efectos ecologicos de la pesqueria. La pesca con red de cerco en el Pacifico oriental tropical se lleva a cabo de tres formas, con distintas cantidades y composiciones de las especies objetivo y de la captura incidental. La seleccion del metodo depende de si los atunes se encuentran nadando solos (lances no asociados), asociados con delfines (lances sobre delfines) o asociados con objetos flotantes (lances sobre objetos flotantes). Entre los metodos de pesca, comparamos la captura con base en el peso, numero de individuos, nivel trofico, tiempo de reemplazo, y diversidad. Los lances sobre objetos flotantes extrajeron dos a tres veces mas biomasa que los otros metodos, dependiendo de como se midio la extraccion. Los resultados de estudios previos sugieren que los efectos ecologicos de los lances sobre objetos flotantes son miles de veces mayores que los efectos de otros metodos, pero estos resultados fueron derivados del numero de animales descartados solamente. El manejo de la pesqueria ha sido impulsado en grado sustancial por una concentracion en la reduccion de la captura incidental, aunque los descartes actualmente forman el 4.8% de la captura total en peso, comparado con los promedios globales de 7.5% para la pesca atunera con palangre y 30.0% para la captura con red de arrastre a media agua. Un enfoque ecosistemico al manejo de las pesquerias requiere que los efectos ecoligicos de la pesca sobre todos los animales extraidos por una pesqueria, no solo la captura incidental o captura descartada, sean cuantificados con una variedad de metricas.