Bathyal feasting: post-spawning squid as a source of carbon for deep-sea benthic communities.

In many oceanic carbon budgets there is a discrepancy between the energetic requirements of deep-sea benthic communities and the supply of organic matter. This suggests that there are unidentified and unmeasured food sources reaching the seafloor. During 11 deep-sea remotely operated vehicle (ROV) surveys in the Gulf of California, the remains (squid carcasses and hatched-out egg sheets) of 64 post-brooding squid were encountered. As many as 36 remains were encountered during a single dive. To our knowledge this is one of the largest numbers of natural food falls of medium-size deep-sea nekton described to date. Various deep-sea scavengers (Ophiuroidea, Holothuroidea, Decapoda, Asteroidea, Enteropneusta) were associated with the remains. Although many of the 80 examined ROV dives did not encounter dead squids or egg sheets (n = 69), and the phenomenon may be geographically and temporally restricted, our results show that dead, sinking squid transport carbon from the water column to the seafloor in the Gulf of California. Based on food fall observations from individual dives, we estimate that annual squid carcass depositions may regionally contribute from 0.05 to 12.07 mg C m-2 d-1 to the seafloor in the areas where we observed the remains. The sinking of squid carcasses may constitute a significant but underestimated carbon vector between the water column and the seafloor worldwide, because squid populations are enormous and are regionally expanding as a result of climate change and pressure on fish stocks. In the future, standardized methods and surveys in geographical regions that have large squid populations will be important for investigating the overall contribution of squid falls to regional carbon budgets.