Complex interactions of ENSO and local conditions buffer the poleward shift of migratory fish in a subtropical seascape.

Ocean warming is associated with the tropicalization of fish towards higher latitudes. However, the influence of global climatic phenomena like the El Niño Southern Oscillation (ENSO) and its warm (El Niño) and cold (La Niña) phases on tropicalization has been overlooked. Understanding the combined effects of global climatic forces together with local variability on the distribution and abundance of tropical fish is essential for building more accurate predictive models of species on the move. This is particularly important in regions where ENSO-related impacts are known to be major drivers of ecosystem change, and is compounded by predictions that El Niño is becoming more frequent and intense under current ocean warming. In this study, we used long-term time series of monthly standardized sampling (August 1996 to February 2020) to investigate how ocean warming, ENSO and local environmental variability influence the abundance of an estuarine dependent tropical fish species (white mullet Mugil curema) at subtropical latitudes in the southwestern Atlantic Ocean. Our work revealed a significant increasing trend in surface water temperature in shallow waters (<1.5 m) at estuarine and marine sites. However, against our initial expectation, we did not observe an increasing trend in the abundance of this tropical mullet species. Generalized Additive Models revealed complex, non-linear relationships between species abundance and environmental factors operating at large (ENSO's warm and cold phases), regional (freshwater discharge in the coastal lagoon's drainage basin) and local (temperature and salinity) scales across the estuarine marine gradient. These results demonstrate that fish responses to global climate change can be complex and multifaceted. More specifically, our findings suggested that the interaction among global and local driving forces dampen the expected effect of tropicalization for this mullet species in a subtropical seascape.

Evidences of El Niño effects on the mullet fishery of the Patos Lagoon estuary

Based on biological and meteorological long-term database (1979-1983 and 1996-2000), we analyzed the variation in the juvenile recruitment and artisanal fishery landings of the mullet (Mugil platanus) in the Patos Lagoon estuary and its relationships with the regional rainfall and estuarine salinity anomalies during two strong El Niño events (1982-83 and 1997-98). Juvenile and adult mullets declined in abundance under the high rainfall and near-zero salinity that prevailed in the estuary during both El Niño events. We proposed two different hypotheses to explain the El Niño-induced effects on the juvenile and adult stages of the mullet in the estuary. First, high freshwater outflow during a very strong El Niño might render ineffective the mechanism of passive immigration of juvenile mullets into the estuary, which can lead to their decline in the estuary during El Niño events. Second, near-zero salinity at the estuarine area along several months during strong El Niño events could lead to higher spatial dispersion of the maturing mullet during their migration to the ocean , resulting in smaller shoals of individuals and, consequently, lower catches by artisanal fishermen.

Episódios El Niño ocorrem na região tropical do Oceano Pacífico e estão associados com o aumento da descarga continental de água na Lagoa dos Patos (10.360 km²) e sua região estuarina (971 km²). A partir de um banco de dados meteorológicos e biológicos de longo prazo (1979-1983 e 1996-2000), este trabalho investiga as variações no recrutamento de juvenis e no desembarque da pesca artesanal da tainha (Mugil platanus) no estuário da Lagoa dos Patos e suas relações com anomalias regionais de chuva e anomalias locais de salinidade durante dois fortes episódios El Niño (1982-1983 e 1997-1998). Tanto os juvenis quanto a captura dos adultos na pesca artesanal diminuíram em abundância durante as chuvas excessivas e salinidades próximas a zero que prevaleceram no estuário durante ambos eventos climáticos. Duas hipóteses são sugeridas para explicar os efeitos ocasionados pelo El Niño sobre as fases juvenis e adultas da tainha no estuário da Lagoa dos Patos. Primeiro, a elevada descarga continental durante forte eventos El Niño poderia afetar negativamente o transporte passivo de juvenis de tainha para o interior do estuário, o que levaria ao declínio de juvenis na região nesse período. Segundo, as salinidades próximas a zero durante vários meses no estuário durante forte eventos El Niño poderia acarretar maior dispersão dos cardumes de tainhas adultas durante a sua migração reprodutiva para o mar, resultando em menores capturas pelos pescadores artesanais da região.