| Research on the relationships of biotic and abiotic factors as they influence fish assemblage structure was conducted in the Tortuguero estuary on Costa Rica's Caribbean coast. The research had four major components: (1) Responses of the estuary's shallow-water fish assemblage to short-term environmental variability were investigated. Micro-habitat features (vegetation cover, substrate, bank formation), bottom salinity and season significantly influenced species distributions in the estuary, but surface salinity and rainfall did not. Species distributions tended to be skewed toward their respective supply ends of the estuary. (2) Salinity tolerance of four common species from the estuary (Centropomus pectinatus, Pomadasys crocro, Lutjanus jocu, and Amphilophus citrinellus) was tested by transferring them directly from freshwater into water at 28 ppt salinity or greater for 24 h. Mortality was 100% for A. citrinellus within 4 h of transfer. No mortality was experienced by P. crocro or L. jocu, and C. pectinatus suffered intermediate mortality. A second trial with C. pectinatus revealed that species could tolerate a transfer from freshwater to water at 20 ppt for 24 h, followed by a second transfer to water at 32 ppt for an additional 24 h, with no mortality. (3) Diets of five dominant piscivores that forage in the estuary (Centropomus pectinatus, Bairdiella ronchus, Micropogonias furnieri, Pomadasys crocro, Polydactylus virginicus) were examined. Fish accounted for 22 to 82% of the prey items consumed, with shrimp and crabs comprising most of the remaining volume. Prey selection among the species indicated that foraging space was segregated vertically. (4) The life history of Centropomus pectinatus, a dominant fish in the estuary, was investigated. Adults spawn throughout the year, probably at or near the mouth of the estuary. Juveniles <25 mm SL were captured only between July and October, and data suggested that habitats other than the lower estuary may be used as nursery grounds by this species. Strong sexual dimorphism with respect to size was observed in C. pectinatus , with protrandry being a potential causal mechanism. |
