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

Abstract

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 δ15N 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 δ13C and δ15N values were observed in benthic shrimps and large polychaetes. The two macrourids had intermediate values, with C. armatus having slightly but significantly lower δ15N 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.

Lipid composition and diet inferences of abyssal macrourids in the eastern North Pacific

“…; cnidarians: Bathyphelia nomados; crustaceans: Munidopsis sp., 1 caridean shrimp, 2 lysianassid amphipod species). Some of these animals have been found in the macrourids' stomachs and others are amongst the dominant megafauna at the abyssal station (Lauerman et al 1996, Ruhl 2007, Drazen et al 2008c). These samples were collected at the same site and at the same time as the macrourids.…”

“…As noted above we were only able to collect one squid species, Dosidicus gigas, to represent carrion sources for the macrourids. To augment the data, FA profiles of epipelagic fish and squid were obtained from the literature for species which have been found in macrourid stomachs from this site (Drazen et al 2008c). These include jack mackerel Trachurus symmetricus (Aranda et al 2006), pacific hake Merluccius productus (Exler & Weihrauch 1976) and the gonatid squid Gonatopsis borealis (Hayashi 1989).…”

“…Both species also had similar isotopic compositions, but stomach content analysis suggests differences in diet. From stomach contents, both species appeared to feed regularly on amphipods, but C. armatus fed more on squid, fish, and carrion and C. yaquinae more on polychaetes, other crustaceans and to some extent on the burrowing holothurian Protankyra brychia (Drazen et al 2008c). The differences may be the result of low sample sizes in the diet analysis and the fact that stomachs only record the last few feeding events.…”

“…Considering the disparate habitats of the macrourids and squid, the latter proposal is unlikely. Furthermore, isotopic evidence suggests dietary differences between the macrourids and Dosidicus gigas (Drazen et al 2008c), and diet studies of D. gigas and gonatids show that mesopelagic myctophids and other micronekton are the most important prey (Nesis 1997, Markaida et al 2008, animals which are unlikely available to macrourids on the abyssal plain. Even though the squid FA profiles (except for D. gigas) came from specimens captured in different North Pacific locations at different times, they all clustered closely together (Fig.…”

“…Macrourids have swimbladders which expand upon retrieval to the surface causing regurgitation or stomach eversion limiting conventional diet analysis. Stable isotope analyses have been used to augment traditional studies and have found that C. armatus and C. yaquinae rely heavily on epipelagic carrion and may be more closely tied to epipelagic food supplies (Drazen et al 2008c).…”