Trophic Ecology and Parasitism of a Mesopelagic Fish Assemblage

Guy Harvey Oceanographic Center Facility

Mesopelagic (open ocean, 200-1000 m daytime depth) fishes are important consumers of zooplankton and are prey of oceanic predators. Some mesopelagic fishes (e.g. lanternfishes and dragonfishes) undertake a diel vertical migration where they ascend to the near-surface waters during the night to feed and descend into the depths during the day to avoid predators. Other mesopelagic fishes (e.g. Sternoptyx spp.) do not vertically migrate and remain at deep depths throughout the day. While in the epipelagic zone (surface – 200 m depth), vertically-migrating fishes can become prey to upper-trophic level predators, such as: tunas and billfishes. Demersal fishes (e.g. rattails) often vertically-migrate as well, ascending into the pelagic zone to feed on holoplanktonic organisms. Hypothetically, a connection between the surface and the benthos can exist if migratory patterns overlap. Fishes of different depths and vertical migration habit likely have a different ecological role in the food web. The relationship between parasites and gut contents provides insights into ecological processes occurring within assemblages, as prey items are often vectors for parasites. This study examined the differences between the prey contents and parasites of 26 mesopelagic fish species in the Gulf of Mexico. Results showed that based on the proportionally dominant prey items per species, six main feeding strategies were differentiated within in this assemblage: copepodivores, copepods and other zooplankton, copepods and euphausiids, gelatinivores, generalists, shrimpivores, and upper-trophic level predators. Larger fishes preyed on larger prey items and harbored more parasites. An ontogenetic diet shift was detected for Sigmops elongatus at 75 mm standard length, progressing from eating primarily copepods at small sizes to eating primarily euphausiids at large sizes. Compared to similar studies, this study revealed a preponderance of trematodes, an endoparasite (parasite within the host) class often restricted to nearshore hosts. Helicometrina nimia, the dominant parasite of the snake mackerel Nealotus tripes, has not previously been recorded in hosts below 200 m depth, suggesting a food-web pathway that transitions from nearshore to offshore. These data can be used to develop and refine models aimed at understanding ecosystem structure and resilience to large-scale disturbances such as the Deepwater Horizon oil spill.