Long-term trends in the abundance of an estuarine fish and relationships with El Niño climatic impacts and seagrass meadows reduction

Abstract

El Niño events and their climatic effects vary markedly in intensity due to several factors related to oceanographic and atmospheric conditions in the Pacific equatorial ocean. As a result of fluctuations in the climate system, rainfall and temperature deviate from average conditions which causes severe biological, economic and social impacts worldwide. In this study, long-term (23 years) standardized monthly sampling of Brazilian silverside, Atherinella brasiliensis, resident fish that inhabits the shallow waters of a subtropical estuary, was performed to investigate the temporal variability in its abundance and relationship with local and regional environmental factors associated with El Niño events and temporal changes in seagrass meadows biomass. We investigated the following hypotheses: (i) A. brasiliensis abundance has presented a long-term decreasing trend, (ii) El Niño events with higher intensities are related to a higher decrease in the species abundance in the estuary and (iii) the decline in A. brasiliensis abundance is influenced by reduction of seagrass meadows, which serve as attachment sites for developing embryos of this species. Time series analysis revealed a statistically significant decrease in the abundance of A. brasiliensis, which was characterized by marked decline between 1996 and 2004 followed by a relative stability between 2005 and 2019. Generalized linear mixed models (GLMM) showed that its abundance decreased during high rainfall and lower salinity conditions associated with moderate and very strong El Niño events, but not during neutral and weak events. An additional GLMM also revealed significant effects of seagrass biomass on the fish abundance. In fact, years with marked decline in the fish abundance coincided with lower or absence of seagrass biomass in the estuary, especially between 1997 and 2005. Hence, negative climatic impacts on A. brasiliensis abundance may have been intensified by synergistic effects caused by periods of reduction in seagrass meadows. These findings raise concerns for conservation, especially when we consider that the studied species is ubiquitous in southwestern Atlantic estuaries and that climate disturbances such as El Niño are expected to become more frequent in the future due to rising ocean temperatures driven by global warming.