A trophic model for Tongoy Bay —a system exposed to suspended scallop culture (Northern Chile)

Abstract

A steady-state model of 17 compartments was constructed for the Tongoy Bay ecosystem using the ECOPATH II software of Christensen & Pauly (1992). The system is driven by planktonic production which is governed by periodical intrusions of upwelling water from a nearby upwelling centre. Of the total system biomass (236.3 g/m 2), 47% is comprised of benthic invertebrates whose food intake exceeds that of pelagic fish and birds. Scallops in hanging cultures account for 71% of the biomass in the water column. The model suggests that benthic invertbrate predators are more important in the cycling of biomass than demersal fish. A significant part of the production of the groups Macrophytes, Zooplankton, Phytoplankton, Suspended Scallops and a minor portion of other groups enters the detritus pool, from which an important amount of biomass re-enters the filter feeder compartments through bacteria. Global system properties such as total system through put (T), development capacity (C), ascendency (A) (sensu Ulanowicz & Mann, 1981; Ulanowicz, 1986; Baird & Ulanowicz, 1993) were also calculated by the means of net work analysis implemented in the ECOPATH II software. Results indicate that Tongoy Bay is an intermediate sized system (in terms of the sum of flows, T) of low maturity and high capacity to withstand ecological perturbations. The mean transfer efficiency between trophic levels is 14.7%, the fishery's gross efficiency (primary production/catch and harvest) is 0.89%. In terms of global system properties these results are similar to those reported by Jarre et al. (1991) for the “open” Peruvian upwelling system during the peak period of anchovy catches (1963–1969). However, the mean trophic level of the fishery (3.6 for Tongoy Bay compared to 2.2 for the open upwelling system) and the biomass pathways differ significantly between these systems. Manipulation of the input data suggest that the bay has a potential for the production of biomass of filter feeders that is 10 to 15 times higher than the present production.