Fish communities and environmental variables during dry season in Pampanga estuary (Philippines)

Abstract

Baseline assessment of fish communities and selected environmental variables (salinity, water temperature, water transparency, current speed, total suspended solid, chlorophyll a) was conducted in Pampanga estuary (Philippines) during a specific dry season (April 2010). The aims were: (a) to examine the variations of fish diversity and community structure, in relation to neap and spring tides and estuary zones and; (b) to examine the influence of environmental variables on fish communities. Reconnaissance showed that salinity was measurable at 6.8 kilometers from the rivermouth, for the surface and mid-water layers, and at 15.9 kilometers for the bottom layers, indicating the occurrence of a salt wedge. Bottom salinity was used to establish the sampling stations, grouped into 3 zones (lower 1–3, middle 4–6, upper 7–9). Sampling of fish, using otter trawl, and measurement of environmental variables were done under four tidal conditions at each station (neap low and high tide; spring low and high tide). Neap tide showed low and homogenous horizontal salinity, higher temperature, slower current speed, high TSS and chlorophyll a, whereas, spring tide showed higher salinity, horizontal saline stratification, lower temperature, faster current speed, low TSS and chlorophyll a. A total of 330 fish were captured from 32 trawl samples, encompassing 20 families and 27 species. Number of species generally did not vary between neap (20) and spring tide (18), but showed a decreasing pattern from lower to the upper estuary. Only 36% of total species were shared between neap and spring tides. Fish abundance was significantly affected by neap and spring tide, and zonation, whereas fish biomass was solely affected by zonation. Arius manillensis was the most typifying species, suggesting high probability of encounter. Chlorophyll a, TSS, salinity and current speed have a significant influence on the variations of abundance, biomass and frequency of occurrence of particular fish species.