ECOLOGY OF FISH COMMUNITIES FROM THE SOFT BOTTOMS OF BAHIA-CONCEPCION, MEXICO

Fish assemblages from soft bottoms were studied to produce an ecologic al index of abundance, biomass, species richness, and species dominanc e in a Mexican coastal lagoon. Sampling was done by trawling between F ebruary and September of 1989. In total 1336 fish from 25 families, 44 genera, and 55 species were caught. The majority of species were smal l (5 co 20 cm SL) and medium sized (21 to 27 cm SL), which may be a re sult of the use of a small otter trawl net, a fishing method that cann ot catch larger and more elusive individuals. The spatial and temporal distribution of the fish composition showed a dose correlation with w ater temperature. In February and March, when temperatures are low, th e bay is suitable for species of temperate affinity. In July to Septem ber, when the average temperature is high, there are many tropical spe cies present. The high species richness in March and September coincid ed with low abundances and the largest biomass. In May and July, we fo und lower values of species richness, with greater abundance and lesse r biomass, because of the large numbers of juveniles and smaller speci es. Fifteen species were permanent found, whereas the other 40 species had patterns of seasonal change, resulting from migration and environ mental changes. The dominance of the permanent species indicate their ecological importance in Bahia Concepcion. Most of the abundant and do minant fish were the species best adapted to the area such as Paralabr ax maculatofasciatus, Urabatis halleri, Etropus crossotus, Sphaeroides sp., Balistes polylepis, and Citharichthys gilberti. A principal comp onent analysis was done to compare the effect between abundance and bi omass of the dominant species with external variables as water tempera ture, salinity and transparency. Also a presence-absence analysis of s pecies allowed comments to be made on the changes between the differen t locations, based on depth, type of bottom, and typical fauna in each location.