Trophic modelling of the Gulf of Nicoya, Costa Rica

Anne van Dam, Farid Tabash and Mauricio Vargas

The Gulf of Nicoya is an estuary on the Pacific coast of Costa Rica that is an important fish producing area. Since the 1970's, several research projects have been carried out in the gulf. In 1997, the Marine Biology Station of the Universidad Nacional in Costa Rica was opened. In this station, various research projects on fisheries management are conducted. There is thus a large amount of published information about the gulf. To summarize and integrate available information and to get more insight into the trophic relationships in the Gulf of Nicoya, an ecotrophic model was constructed using the Ecopath model. Ecopath is a program for the construction of mass balance models of aquatic ecosystems.

The Ecopath model

The basic equation for an Ecopath model is, for each functional group (i):

Production(i) = all predation on (i) + nonpredation losses of(i) + export of (i), for all (i),  or:
B_i · (P/B)_i =S B_j · (Q/B)_j · DC_ji + (P/B)_i · B_i · (1-EE_i) + EX_i

Where B_i is the biomass, (P/B)_i is the production/biomass ratio of group (i), (Q/B)_i is the consumption/ biomass ratio of group (i), DC_ji is the fraction of prey (i) in the diet of predator (j) and EX_i is the export of group (i). With an equation for each group in the system, a set of linear equations results that can be solved for the unknowns. This means that for each functional group we had to get estimates of P/B, Q/B, B, EE and the diet composition.
Fourteen functional groups were defined for the Gulf of Nicoya: birds, sharks, large pelagics, small pelagics, juvenile fish, demersal fish, other shrimp, white shrimp, crabs, molluscs, benthos, zooplankton, phytoplankton and detritus. A diet composition matrix was defined based on studies of stomach content. With this information, an Ecopath model was balanced with quantified flows between all groups (including flows to detritus, respiration and fishing) and some summary statistics for the whole ecosystem calculated.

Results

Of the primary production, only 14.7% is directly consumed by molluscs, zooplankton and small pelagics. The remaining primary production flows into detritus and is the main input (88%) into total detritus production. Because of this inflow, a large accumulation of detritus occurs. Only 1.8% of detritus production is consumed directly by shrimps and benthos. All benthic groups with the exception of "other shrimp" show high ecotrophic efficiencies. Most of these groups are important preys for other groups or are exploited by the fishery.
The summary statistics suggest that the Gulf of Nicoya is an immature system, with high net system production and total primary production/total respiration, and low total biomass/total throughput ratios. The system omnivory index suggests that the system does not display web-like features. This is a result of the relatively simple diet-matrix we used, based on basic ideas about the trophic structure of the gulf.

Further work

• Split the current model in separate models for the inner and outer gulf
• Split the "demersal fish" group into its major components
• Include benthic algae
• Estimate the import of detritus through river-runoff into the gulf.
• Account for the role of bacteria by including export of detritus.
• Introduce variability in time and space using the new Ecosim and Ecospace software

Acknowledgement

This work was presented at the "Taller Internacional Pesca '97, Evaluación y Manejo de Recursos Pesqueros", 17-21 November 1997, Havana, Cuba.

Ecopath workshop, Puntarenas, April 1998

For a report of the workshop "Placing fisheries in their ecosystem context" at the Marine Biology Station of the Universidad Nacional, see the Ecopath homepage (link to www.ecopath.org).

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