Seasonality in Diel Catch Rate of Small Fishes in a Shallow-water Fish Assemblage at Porto Pim Bay, Faial, Azores

An assemblage of shallow-water fishes was sampled with a beach seine, over 24 h periods, each month, between July 1989 and June 1990 in Porto Pim Bay, Faial, Azores. Each monthly set of samples was divided into daytime and night-time catches to examine the stability of day/night differences in assemblage structure over a 1 year period. A total of 24 species were caught over the sampling period of which four occurred only during day and three only during night. The dominant species,Chelon labrosusandSardina pilchardus, were caught in consistently higher numbers during day but onlyS. pilcharduswas caught at a consistently higher biomass during day.Echiichthys viperaconsistently had higher night-time catches in terms of both numbers of individuals and biomass. At the assemblage level, generally, more species were caught during night-time whereas a greater number of individuals were caught during daytime. Inconsistencies in day/night assemblage structure over seasonal cycles have numerous implications for studies on the seasonality of fish assemblages. Any study on seasonality must state when during the diel cycle samples were taken. In the case of Porto Pim Bay, and probably many other areas as well, there is a certain degree of consistency in the day/night fish assemblage structure over seasonal cycles. However, the major disruptions occur when the assemblage undergoes restructuring through periods of recruitment of new year classes and as the assemblage switches from an overwintering to a spring/summer structure. The tidal state (time of sampling relative to high or low water) may have some influence on the patterns observed. Therefore, to fully investigate patterns in day/night changes in community structure it may be necessary to design experiments which reduce the potential influence of tidal state.