Rationale

While great strides have been made modelling the food webs supporting the Hawaiʻi longline fishery, the ecological interactions around the Main Hawaiian Islands (MHI) are poorly understood. This research seeks to improve the knowledge base for Ecosystem-based Fisheries Management (EBFM) of tunas in nearshore waters, with the goal of enhancing sustainable fisheries for skipjack  (aku, Katsuwonus pelamis) and yellowfin (ahi, Thunnus albacares).                  We are quantifying the trophic links of these focal tuna species (aku and ah) with three prey groups (epipelagic mollusks, epipelagic fish, crustaceans) in nearshore waters around O'ahu.    Our results will help operationalize EBFM, by identifying species-specific foraging differences,     ontogenetic shifts in diet and trophic position, and key prey functional prey groups.      Central Pacific pelagic food web, showing energy flow from micronekton to top predators, where the numbers in parentheses indicate the number of pooled functional groups. Only important (>5%) diet components are shown, with the thickness of the lines indicating relative contributions. Colored lines highlight trophic paths from "other crustaceans" (red) to "juvenile tunas" (red), and from "epipelagic mollusks" to "adult tunas" (blue).    Approach  To this end, we will develop a dynamic picture of the food-webs supporting these tuna species using samples collected by collaborating with fishers and three complementary analyses:   (1) skipjack / yellowfin tuna diet using undigested prey remains, Stomach contents from an aku illustrate the diverse and generalist diet of this predator.   This specific fish contained 23 fresh prey items: 16 fish, 2 squid, and 5 crustaceans.   (2) stomach contents using barcoding and environmental DNA methods.         DNA barcoding identifies individual prey items in tuna stomachs.            Environmental DNA identifies digested prey in tuna stomachs.         (3) trophic position using stable isotopes from muscle tissue of tuna and their prey.                                        Tissue samples are dried in a convection oven and grinded with a mortar and pestle.                 .                                              C13 indicates the source of carbon and N15 indicates the trophic position.   Results In summer of 2020 (May - September), we sampled 121 aku, ranging in size from 40 to 84 cm fork-length (FL).   A locally estimated scatterplot smoothing (LOESS) fit to the data suggests that, as the sumer progresses           from early may to early September, the tunas increase in size.        We analyzed white muscle samples from the caudal peduncle for C13 and N15 stable isotope ratios.     A locally estimated scatterplot smoothing (LOESS) fit to the del C13 data suggests there is an ontogenetic                  shift in diet, with the larger specimens having a heavier isotopic ratio.          A locally estimated scatterplot smoothing (LOESS) fit to the data does not suggest an ontogenetic shift in trophic position, with no increase in the nitrogen ratio for the larger specimens.                      Next Steps To fully characterize the diet of the smaller-sized fish (fork length < 50 cm). Work with fishers to sample more specimens during in the summer of 2022. Can you help us ?   Pelagicos Contact David Hyrenbach   Other Pelagicos People Ali Islas Vazquez  Sydney Luitgaarden     Partners Dr. Matthew Iacchei Dr. David Horgen  Alex Filardo  Hawaiʻi Fisheries Extension Agent  Kailua Back Home