Influence of detached macroalgae on fish size and condition in nearshore habitats

Abstract

Nearshore habitats are important for reproduction, feeding and growth of many fish species and are usually subject to seasonal influxes of adults from other habitats. Those areas consist mostly of small-scale sites whose environments are affected by meso- or large-scale and continental events, making it difficult to establish seasonality of habitat use. Understanding which drivers make these habitats suitable for fish species and, in particular, why this might vary seasonally is relevant to resource management. Detached macrophytes are likely to improve refuge and food for fishes and invertebrates, increasing abundance (especially juveniles) and diversity. On the southeastern Brazilian coast, detached macroalgae are carried by surface currents and swell and deposited along beaches in the winter (Dry period), but are largely absent in the summer, nutrient-enriched Rain period. This project aimed to investigate the potential importance of macroalgae to fish populations in the southeastern Brazilian coastal region. Four areas were surveyed on the southern Espírito Santo coast. The main diet of Larimus breviceps, Stellifer rastrifer and S. stellifer (Sciaenidae) was determined and some key food items were then sampled from macrophytes. Samples of fishes, food items, algae and higher plants were subject to carbon and nitrogen stable isotope analysis in order to define food chains from algae and higher plants to fish. Fish sizes were compared among Dry and Rain periods. Fish condition was addressed through linear regression models fitted with Log-transformed eviscerated weight as the response variable, and Log-transformed Standard Length, Period, Site, Species and Reproductive Class (and their first-order interactions) as the explanatory variables. Trophic positions of fish and their food items were estimated using stable isotopes signatures and macrophytes as the baseline. For most combinations of species and site, fish were shorter and lighter in the dry period than the rain period. However, fish of all three species were almost always predicted to be heavier for a given length in the Dry period, when macroalgae were present. The data suggest that detached macroalgae in the Dry period are a regional environmental driver of fish and crustacean distributions and that management of macroalgae may be important for biodiversity and fisheries conservation.