Dietary shifts and niche partitioning throughout ontogeny reduce intraspecific competition in a pelagic generalist predator

Abstract

Patterns of feeding strategy change throughout ontogeny according to size-specific abilities and requirements. Characterizing the extent and potential repeated occurrence of dietary differences within the population can improve understanding of the intraspecific predation regime and population dynamics. Here, we investigated size-related feeding habits and trophic niche partitioning of an iconic pelagic generalist predator, the jumbo squid Dosidicus gigas, by a combined analysis of morphologic indicators of feeding capability (fin and feeding apparatuses), trophic biochemical tracers (stable isotopes and fatty acids), and stomach contents. Results showed significant variation in prey composition, isotopic values, and fatty acid profiles with size. D. gigas exhibited a shift in diet at approximately 250 mm mantle length (ML). The trophic niche of small (ML ≤ 250 mm) and large (ML > 250 mm) squid revealed low potential for resource overlap, suggesting segregation in terms of spatial and food resources of different size groups. Large individuals tend to feed deeper and on smaller prey to optimize food capture costs and energy benefits. This feeding strategy is likely related to variability in the development of feeding capacity, increasing swimming ability, and metabolic demand as squid grow and may reduce intraspecific competition to improve survival. These results highlight the dietary flexibility of D. gigas and demonstrate that niche differentiation acts as a major factor in a cohort, which may have important implications for their population dynamics and management. In addition, this study demonstrates that using multiple diet tracers can highlight subtle differentiations in diet correlated to growth in a pelagic generalist predator.