Contribution of prey to Humboldt squid Dosidicus gigas in the northern California Current, revealed by stable isotope analyses

Abstract

Diet studies have shown Humboldt squid Dosidicus gigas to be aggressive oppor- tunistic predators, yet this approach has provided only a limited and potentially biased view of their trophic feeding behavior. As an alternative, we measured the δ 13 C and δ 15 N of D. gigas and their prey from the northern California Current ecosystem (NCC) and applied stable isotope Bayesian mixing models (Stable Isotope Analysis in R (SIAR)) to assess if D. gigas isotopically matched NCC or southern California Current (SCC) migratory end-members and to examine the proportional trophic contributions of prey groups from the NCC to their diet. For the trophic SIAR model, cluster analysis of prey taxa by their respective δ 13 C and δ 15 N values was first applied to consolidate prey into groups, which were then incorporated into the model as source groups to the diet mixture. Model results from examination of NCC and SCC migratory end-members indicated greatest contributions from the NCC system, indicating D. gigas was more integrated with the regional NCC isotopic signature. From the trophic SIAR model, the results indicated mixed but lower trophic-level feeding by D. gigas relative to previous diet-based studies, with greatest con- tributions from macrozooplankton, ichthyoplankton, and nekton such as juvenile rockfish, market squid, sand lance, and juvenile Pacific hake. Sensitivity analyses of the SIAR model based on vary- ing isotopic fractionation factors of δ 13 C and δ 15 N showed that proportional contributions of prey to squid diets were resilient to change.