Abstract
Forage fish play the crucial role of transferring energy from lower to upper trophic levels. These small pelagic fish feed on plankton and can compete with each other for food, leading to potentially complex interactions. Three forage fish species (Sprattus fuegensis, Strangomera bentincki, and Engraulis ringens) are commonly found in the inshore waters of Northern Chilean Patagonia, a region with large spatial variability in oceanographic conditions and in the zooplankton community. To better understand what factors may explain coexistence among these three forage fish species, we examined differences in resource availability (zooplankton community and abundances) in three locations (Chiloé, Cordillera, and Aysén regions) within Northern Patagonia. We compared the diet of the three fish species using both stable isotope and stomach content analyses, and we measured their gill rakers. S. fuegensis, the only species found in all three regions, had flexible diets which lent it greater success than the other two species at foraging in waters with fewer prey. We found little diet differentiation among the three species in the copepod-rich Cordillera region, but the diverging diets of S. bentincki and S. fuegensis in the copepod-poor Chiloé region suggest that diet partitioning in areas with fewer resources is due to competition. The small differences in diet facilitates coexistence among the three species, which may help explain the lack of correlation between the three species’ population abundances in the Cordillera region over the past 6 years. Finally, our results show that the high degree of oceanographic heterogeneity in Northern Patagonia may encourage species separation based on their specific environmental requirements.
Highlights
Forage fish are a key component of the marine ecosystem as they transfer energy from lower to upper trophic levels (Cury, 2000; Bakun et al, 2010; Pikitch et al, 2014)
Multi-decadal alternations in abundances between of anchovy and sardine in upwelling systems have long been observed (Chavez, 2003). This population dynamic is illustrated by biomass trends in the last two decades of Strangomera bentincki and Engraulis ringens in Central Chile to the north of our study area (Figure 1; data from acoustic surveys carried out by the Instituto de Fomento Pesquero provided by Jorge Castillo) where, from 1999 to 2019, the abundances of E. ringens and S. bentincki were negatively correlated with a lag of 2 years
Our results show that S. fuegensis were able to adapt their foraging strategy and diet to the varying prey availability of the three regions
Summary
Forage fish are a key component of the marine ecosystem as they transfer energy from lower to upper trophic levels (Cury, 2000; Bakun et al, 2010; Pikitch et al, 2014). This functional group is characterized by short-lived species with rapid reproductive turnovers (Peck et al, 2013) and highly fluctuating abundances (Cury, 2000) driven by large scale climactic changes to their environment (Tourre et al, 2007). While diet-mediated interactions between sardine and anchovy in upwelling systems are wellunderstood, it is less clear whether a similar mechanism facilitates coexistence in non-upwelling systems where several small pelagic fish species co-occur
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