A trophic model for exploring possible ecosystem impacts of fishing in the Gulf of Paria, between Venezuela and Trinidad

Abstract

The Gulf of Paria is a semi-enclosed estuarine area between Trinidad and Venezuela. Fisheries for demersal and pelagic species are important, and shared by nationals from both countries. In this study, a trophic model is constructed, and several whole system statistics and network flow indices determined for this ecosystem. Possible impacts of trawling on the biomass of model components, through simulation of the effects of varying fishing mortality rate, were also explored. The model components consist of 15 fish groups, 5 invertebrate groups, seabirds, phytoplankton, detritus and discards. Results indicate that the food web is dominated by the detrital pathway, and that the export of detritus out of the system is significant. Mixed trophic impacts show that detritus and lower trophic levels have a significant positive impact on other groups, i.e. they exert bottom-up control of the food web. Mean transfer efficiency is 12.2%, Finn cycling index 7.2%, path length 6.2, omnivory index 0.2, and system ascendency and overhead 42 and 58%, respectively. Results suggest that, although the Gulf appears to be relatively mature, it may experience some degree of instability due to exploitation and the large seasonal variation in salinity, among other factors. Simulations of different fishing mortality rates show a strong impact on the biomass of system components. Increasing fishing mortality by 50% for 5 years resulted in a marked decrease in biomass of fish groups accompanied by an increase in that of invertebrates, notably penaeids and crabs. Fish biomass recovers after fishing pressure is relaxed, while that of crabs declined further. On the other hand, a reduced fishing mortality rate elicited the opposite response in system components, except for crab of which the biomass again declined significantly when that of fish increased. Biomass of groups at higher trophic levels obtained during trawl surveys conducted in 1945, before trawl fishing was introduced in the Gulf, were significantly higher than current biomass, except for carangids, penaeids, and possibly clupeoids. This may indicate a possible shift towards a system dominated by lower trophic levels.