Listed as Critically Endangered by the IUCN due to overfishing in the first half of the 20th century, Giant Sea Bass (Stereolepis gigas) appear to be showing signs of recovery. Giant Sea Bass are high-level generalist predators, and are generally thought to forage in the kelp forest. Serving in this role, the return of Giant Sea Bass is likely to influence kelp forest trophic dynamics. To begin to understand these implications, we first need a deeper understanding of the trophic ecology of Giant Sea Bass. As part of an ongoing study we are leveraging stable isotope and gut content analyses to contribute to this understanding.

Results

Bulk stable isotope results showed a strong positive relationship between δ^13 C and fish size, with capture location accounting for about ~30% of model variability. We did not find strong relationships between δ^15 N and fish size from bulk isotopes; however, a subsample of tissue analyzed using compound specific stable isotope analysis of amino acids showed an increase in trophic position with size. We also found that larger/older Giant Sea Bass occupy a larger trophic niche area than smaller/younger Giant Sea Bass. Using a Bayesian stable isotope mixing model to parse apart basal carbon sources by fish size, we found that Giant Sea Bass of all sizes rely heavily on macroalgal-derived carbon sources, but smaller individuals derive more of their carbon from phytoplankton sources. Gut contents are limited, because of difficulties obtaining stomachs for a protected species. Combining results from a small sample of stomachs and anecdotal reports in the literature, Giant Sea Bass appear to be generalist predators that feed on a wide array of prey items. Most documented Giant Sea Bass prey are benthic organisms that reside in kelp forest and adjacent sandy bottom habitats.

Conclusions

Giant Sea Bass trophic ecology is more nuanced than initially thought. The differences in basal carbon sources with size could be indicative of spatial partitioning of foraging locations or ontogenetic shifts in prey. There is likely some effect of size limitation, with larger Giant Sea Bass able to exploit larger prey items. This would account for the increase in trophic position with size and the larger trophic niche area associated with larger individuals. Pairing results from initial gut content analyses with reports in the literature, it is likely that smaller individuals feed more on smaller transient prey items, such as anchovies or sardines. Gut contents also suggest that Giant Sea Bass frequently forage outside of the kelp forest environments, with a large portion of their diet being made up of sand-associated prey.

Next Steps This work is currently being prepared for publication.