Comparative analysis of trophic structure of commercial fishery species off Central Chile in 1992 and 1998

Abstract

Trophic interactions and community structure of commercial fishery species off Central Chile (33°–39°S) were analyzed and compared for 1992 and 1998 by ecotrophic modelling, using the Ecopath modelling software. The model encompasses the fishery, pinnipeds (sea lions), small pelagic fish (anchovy, pilchard), medium-sized pelagic fish (horse mackerel), demersal fish (e.g. Chilean hake, black conger), benthic invertebrates (carrot prawn, yellow prawn), and other groups such as zooplankton, phytoplankton, and detritus. Input information for the model was gathered from published and unpublished reports and our own estimates. Also, the effects of fishing and predation on fishery resources and on the most important components of the system were investigated, within an ecotrophic framework.

Predators consumed the greater part of the production of the most important fishery resources, particularly juvenile stages, and the fishery removed a large fraction of adult production. Mortality by predation is an important component of natural mortality, especially in recruit and prerecruit groups. Analysis of direct and indirect trophic impact shows that adult Chilean hake have a negative impact on juvenile Chilean hake through cannibalism, and on pilchard, anchovy, and carrot prawn through predation. Also, fishing has a strong impact on fishery resources, such as Chilean hake, pilchard, and anchovy. Total biomass in 1998 was 1.5 times higher than in 1992. However, total catches in 1998 were about 80% of those in 1992. Changes in biomass and total yields of the system between 1992 and 1998 can be observed in such properties as total flows, consumption, respiration, and production. It is concluded that ecotrophic modelling is an useful tool for fishery management, since it can improve our understanding of the predator–prey interactions among the exploited (fishery resources) and unexploited but potential fishery resources of the system.

Introduction

Although short-term fishery management objectives may be partially fulfilled in the absence of ecosystem information, long-term strategies necessarily require placing fisheries in their ecosystem context, that is by incorporating knowledge of interspecific interactions of both exploited and unexploited populations, their physical environment and their habitat (Christensen et al., 1996, Sinclair et al., 1997).

The Humboldt Current system of Central Chile is a typical upwelling ecosystem that sustains some of the most productive fisheries of the world (FAO, 1995) due to coastal upwelling (Vergara, 1992).

Arancibia, 1989, Arancibia, 1992 and Quiñones et al. (1997), analyzed interspecific relationships between fishery resources and their prey in Central Chile, and concluded that most species that are fishery resources play important ecological roles in the marine system. Nonetheless, research on the trophic relationships between the main components, such as Chilean hake (Merluccius gayi) and horse mackerel (Trachurus symmetricus), have been both sporadic and short-term (Arancibia, 1987, Arancibia, 1991, Miranda et al., 1998). Additionally, some species that are not fishery resources could have important roles as either predators or preys (Neira and Arancibia, 2000).

Since the early 1980’s, the development and later westward expansion of an important purse-seine horse mackerel fishery has influenced total landing in Central Chile. During the 1990’s horse mackerel landings ranged from 2.4 million tonnes to a historical maximum of more than 4 million tonnes in 1995 (Fig. 1a). During 1997–1998, remarkable changes occurred in the length structure of jack mackerel catches, as juveniles dominated the fishing grounds. This has been attributed to both the entrance of one or two strong year classes and changes in oceanographic conditions associated with the ENSO event of 1997–1998 (Cubillos et al., 1999, Arcos et al., 2001). The dramatic drop in horse mackerel landings in the later years was accompanied by an increasing trend in landings of other important fishery resources, such as Chilean hake, pilchard (Strangomera bentincki), and anchovy (Engraulis ringens) (Fig 1b). The ecosystem effects of these changes in landings of the most important fishery resources—especially those of horse mackerel—are still poorly understood in Central Chile.

Consequently, the objective of this paper is to investigate changes in the trophic interactions and the community structure of the Central Chile marine ecosystem (33°–39°S) between early 1990’s—when the horse mackerel fishery was plenty—and late 1990’s—when the horse mackerel fishery had collapsed. To accomplish this objective, we constructed two mass-balanced ecotrophic models summarizing biomass, catches and production of the main trophic groups, with emphasis on fishery resources in the Central Chile marine ecosystem in 1992 and 1998.