Bypassing the abyssal benthic food web: Macrourid diet in the eastern North Pacific inferred from stomach content and stable isotopes analyses

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 delta N-15 values were comparable to those in abyssal deposit feeders, but carrion was relatively enriched in C-13, giving it a unique isotopic signature compared to benthic prey sources. The highest delta C-13 and delta N-15 values were observed in benthic shrimps and large polychaetes. The two macrourids had intermediate values, with C. armatus having slightly but significantly lower delta N-15 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.