| Summary |
Several studies have sought to quantify the contribution of mesopelagic fishes to the ocean carbon cycle and the biological pump. However, to determine fish-mediated carbon transport, it is necessary to understand the behavior and ecology of mesopelagic fishes, including their foraging ecology. In this study, stable isotope analysis (SIA) was used to gather insight into mesopelagic fishes' contribution to marine food webs and their feeding behavior by interpreting [delta]¹³C and [delta]¹⁵N isotope signatures obtained from white muscle tissue. [delta]¹³C and [delta]¹⁵N represent the ratio of 13C/12C and 15N/14N isotopes relative to an international standard in a sample, in this case mesopelagic fish white muscle tissue. Mesopelagic fishes were sampled from the North Pacific Subtropical Gyre at station ALOHA. A MOCNESS net was used to sample mesopelagic fishes from depths of 0-1,000 m. From SIA results, I compared [delta]¹³C and [delta]¹⁵N isotope signatures based on fish migratory status (fishes who vertically migrate to feed or those that do not), time of day, depth sampled, variations in fish size (length/weight), and interspecific variation in isotope signatures. From this thesis, I sought to gather insight of fish dietary sources and trophic structure based on the studied parameters. In addition, how SIA results fluctuate with the chosen factors will help scientists learn how to properly interpret SIA results. The results of my study indicated that [delta]¹³C and [delta]¹⁵N isotope signatures are higher in fishes caught at deeper depths and in those classified as non-migrators in comparison to migratory fishes. In addition, [delta]¹³C was slightly higher for migratory fishes in shallower depths (<400 m) at day-time compared to other time and depth categories. An increase in [delta]¹⁵N at night at deeper depths (>400 m) for non-migratory fishes was found to be significant. I also observed a positive trend in [delta]¹³C and [delta]¹⁵N isotope signatures with increasing fish biomass (both length and weight) for migratory and non-migratory fishes. Variation in [delta]¹⁵N was observed amongst different species and genera of mesopelagic fishes; yet there was little fluctuation in [delta]¹³C between mesopelagic fish genera/species. Based upon [delta]¹⁵N isotope signatures, my data indicate that non-migratory fishes, who feed at deeper depths, likely feed from a detrital based food web and fishes who feed near the ocean surface feed from pelagic based food web. These sources of carbon are important to account for when quantifying fish mediated carbon contribution to the biological pump. The depth at which carbon is transported by physical or biological processes impacts the duration for which carbon is sequestered in the ocean. The deeper carbon is transported, the longer it remains sequestered and increases the chances of escaping bacterial remineralization and being released back into the atmosphere through wind and wave activity. In addition, biological factors such as fish size and their diet also impact how much carbon is injected into the ocean interior by functions such fecal pellet egestion, respiration, and mortality. |
