Interaction Between Anchovy and Squat Lobster in Northern Patagonia: Effects on Trophic Relationships and Ecological Implications

In the last decades, the population of the squat lobster Munida gregaria inhabiting the water column in the Patagonian Sea, Argentina, has expanded. This crustacean shares its habitat with the Argentine anchovy Engraulis anchoita, a key species in the marine food web. Existing information suggests that M. gregaria may compete with anchovy causing spatial segregation or interfere with the formation of fish schools. This study examines anchovy schools and squat lobster swarms sharing the water column in San Matías and Nuevo gulfs, in northern Patagonia. A total of four acoustic zigzag surveys were performed in San Matías gulf and five in Nuevo gulf, covering all the seasons. The echoes of both species were characterized and discriminated, measuring a set of shape, location, and density properties. A linear model was built to determine which factors affect anchovy school morphology. Anchovy schools presented well-defined edges, were amoeboid in shape, and occupied a wide range of depths in the water column. M. gregaria formed large and irregular aggregations, mainly in the upper half water column. Anchovy schools that interacted with M. gregaria tended to be larger and less dense, especially in spring and summer. This may result from either physical interference between individuals, leading to greater laxity in anchovy schools, or the partial dispersal of anchovies feeding on juvenile M. gregaria.