Ecology of fish communities from the soft bottoms of Bahía Concepción, México

Fish assemblages from soft bottoms were studied to produce an ecological index of abundance, biomass, species richness, and species dominance in a Mexican coastal lagoon. Sampling was done by trawling between February and September of 1989. In total 1336 fish from 25 families, 44 genera, and 55 species were caught. The majority of species were small (5 to 20 cm SL) and medium sized (21 to 27 cm SL), which may be a result of the use of a small otter trawl net, a fishing method that cannot catch larger and more elusive individuals. The spatial and temporal distribution of the fish composition showed a close correlation with water temperature. In February and March, when temperatures are low, the bay is suitable for species of temperate affinity. In July to September, when the average temperature is high, there are many tropical species present. The high species richness in March and September coincided with low abundances and the largest biomass. In May and July, we found lower values of species richness, with greater abundance and lesser biomass, because of the large numbers of juveniles and smaller species. Fifteen species were permanent found, whereas the other 40 species had patterns of seasonal change, resulting from migration and environmental changes. The dominance of the permanent species indicate their ecological importance in Bahía Concepción. Most of the abundant and dominant fish were the species best adapted to the area such as Paralabrax maculatofasciatus, Urobatis halleri, Etropus crossotus, Sphoeroides sp., Balistes polylepis, and Citharichthys gilberti. A principal component analysis was done to compare the effect between abundance and biomass of the dominant species with external variables as water temperature, salinity and transparency. Also a presence-absence analysis of species allowed comments to be made on the changes between the different locations, based on depth, type of bottom, and typical fauna in each location.