Sleeping in separate rooms: Habitat partitioning between a small epipelagic and a mesopelagic fish in a semi-enclosed deep Gulf

Spatial aggregation patterns, feeding ecology, and potential dietary overlap of a small epipelagic fish (Engraulis anchoita) and a mesopelagic fish (Maurolicus spp.) were examined in the semi-enclosed San Matías Gulf (Southwest Atlantic Ocean; 40°47′ S – 42°13′ S) during austral spring, focusing on two sectors of the tidal front (NW: northwest; SE: southeast). Acoustic data, stomach content analysis (prey relative importance; Schoener index), CTD profiles (in situ temperature and salinity), satellite chlorophyll-a, zooplankton sampling (200  μm mesh), and multi-model inference were combined to assess competition and/or resource partitioning between species. Engraulis anchoita occurred mostly nearshore, while Maurolicus spp. was associated with deeper waters, both co-occurring at 100–150  m and more abundant in NW, influenced by bottom temperature and salinity. The dominant prey for both species was the copepod Calanus australis (more consumed in SE). E. anchoita also consumed Decapoda larvae, Copepoda (1–2  mm), and Chaetognatha (preferred in NW), while Maurolicus spp. fed on Euphausiacea (more in SE) and conspecific eggs (in NW). These patterns aligned with zooplankton availability, shaped by southern shelf water intrusion. A dietary shift was observed in both species, from meso- to macrozooplankton with increasing size. No dietary overlap was detected, suggesting habitat partitioning and coexistence. These results underscore the ecological role of both species in linking zooplankton to higher trophic levels in a unique deep Gulf environment, providing an opportunity to test coexistence and resource use by this pelagic species.