Trophic models of four benthic communities in Tongoy Bay (Chile): comparative analysis and preliminary assessment of management strategies

Steady-state trophic flow models of four benthic communities (seagrass, sand-gravel, sand and mud habitats) were constructed for a subtidal area in Tongoy Bay (Chile). Information of biomass, catches, food spectrum and dynamics of the commercial and non-commercial populations was used and the ECOPATH II software of Christensen and Pauly [Ecol. Modell. 61 (1992a) 169] was applied. The sea star Meyenaster gelatinosus and the crabs Cancer polyodon, C. porteri and Paraxanthus barbiger were found to be the most prominent predators in the benthic system. The scallop Argopecten purpuratus as well as other bivalves represented the principal secondary producers in the seagrass, sand-gravel and sand habitats, while the Infauna dominated the mud habitat. The highest total biomass and system throughput (33579.3 t/km²/year) was calculated for the sand-gravel habitat. The sand habitat had a negative net system production due to the amount of primary production imported from deeper waters to satisfy the food requirements of the large beach clam (Mulinia sp.) populations. The mean trophic level of the fishery varied between 2.06 (sand-gravel) and 3.92 (sand) reflecting the fact that the fishery concentrates on primary producers (i.e. algae and filter feeding), and on top predators (i.e. snails and crabs). Fishery is strongest in sand-gravel habitat, where annual catches amount to 122.05 g/m². Low values of the relative Ascendency (A/C) (from 27.4 to 32.7%) suggest that the systems analysed are immature and highly resistant to external perturbations. Manipulations of the input data for the exploited species suggest that seagrass and sand-gravel habitats have a potential for a ∼3 times higher than the present production of scallops and the red algae Chondrocanthus chamissoi. Preliminary results of Mixed Trophic Impacts (MTI) analysis suggest that any management policy aimed at a man-made increase in the standing stocks of A. purpuratus and Ch. chamissoi in seagrass and sand-gravel habitats, and a removal of the seastar M. gelatinosus in the seagrass habitat appears justified.