Lisa De Forest’s research while affiliated with Honolulu University and other places

The distribution of mesopelagic micronekton (small fishes, crustaceans, and cephalopods) is not uniform throughout the oceans. Seamounts are a feature that may influence the abundance, biomass, diversity, and taxonomic composition of a community of mesopelagic micronekton by introducing a hard substrate and benthic predators into a realm normally devoid of these. Cross Seamount, located roughly 295km south of the island of Oahu, Hawaii, has a summit that is 330m below sea surface and has a diameter of approximately 8km. Using a large, modified Cobb trawl, samples were taken both directly over and away from the summit of Cross Seamount to sample the deep scattering layer during the day and the shallow scattering layer during the night. Trawls were conducted during two cruises in the spring of 2005 and 2007. All organisms collected were identified to the lowest taxonomic level possible resulting in a description of the local assemblage of mesopelagic micronekton over and around Cross Seamount. Results indicate that there is a significant decrease in total abundance of organisms and an absence of certain diel vertically migrating taxa directly over the summit as opposed to away. While predation might be partly the cause, the taxa that are absent from the summit all have daytime depths that are deeper than the depth of the summit indicating that avoidance may be a major reason for the low abundance. The overall taxonomic composition of the community over the summit is dominated numerically by epipelagic juvenile fishes and stomatopod larvae. This is in opposition to that found away from the summit where the community is dominated numerically by mesopelagic fishes, mostly myctophid fishes, with the epipelagic juvenile fishes and stomatopod larvae contributing little to the overall taxonomic composition. The community over the summit also contains two species that appear to be found in higher abundance over the summit as opposed to away and may be considered as seamount-associated species. These are a cranchiid squid, Liocranchia reinhardti, and a myctophid fish, Benthosema fibulatum. This seamount is known to impact the mesopelagic micronekton community and tuna community, but the mechanisms behind these impacts are largely unknown at this time.

Deep-sea fishes prey on benthic fauna and scavenge on the carcasses of surface-living animals. Few studies have addressed the relative importance of each trophic pathway. Recent documentation of inter-decadal fluctuations in macrourid densities in the abyssal Pacific, related to changing food supplies, hastens the need for information. We conducted stomach content and stable isotope analyses of two abyssal macrourids, carrion sources, and benthic prey, collected concurrently at 4100 m off California. Squid and fishes were the most frequent prey for Coryphaenoides armatus, with small crustaceans numerous in the diet of small specimens. The diet of Coryphaenoides yaquinae included more benthic prey such as crustaceans, polychaetes, and the holothuroid Protankyra brychia. Carrion was present in both species and it was 69% of the mass of food of large C. armatus. Carrion d15N values were comparable to those in abyssal deposit feeders, but carrion was relatively enriched in 13C, giving it a unique isotopic signature compared to benthic prey sources. The highest d13C and d15N values were observed in benthic shrimps and large polychaetes. The two macrourids had intermediate values, with C. armatus having slightly but significantly lower d15N relative to C. yaquinae. Results of isotope mass balance suggest that carrion was the most important prey resource for both species but to a lesser extent in C. yaquinae. These two species bypass the conventional phytodetritus-based abyssal food web for much of their nutrition. Their population dynamics may be tied more closely to fluctuations in epipelagic nekton populations through fishing effects and direct climatic forcing on top trophic levels.