Food for the deep sea: utilization, dispersal, and flux of nekton falls at the Santa catalina basin floor

Abstract

The role of large food falls in the ecology of deep-sea benthos has been the topic of much speculation and little direct study. The submersible Alvin and free vehicles were used to assess experimentally the fate and flux of nekton falls at a depth of 1310 m in the Santa Catalina Basin. Parcels of dead fish (1 to 40 kg) placed on the basin floor rapidly attracted large aggregations of fish and invertebrate scavengers, which consumed the bulk of the carrion within hours to days. The most strongly attracted megafaunal scavenger was the ophiuroid Ophiophthalmus normani, the dominant megabenthic species in the background epifaunal assemblage. O. normani attained densities of 700 m −2 in aggregations containing thousands of individuals, and remained at elevated abundance around baitfalls for weeks. Six other megafaunal species also appeared to feed directly on carrion, including two of the next ten most abundant megabenthic organisms. Several of these species exhibited roosting behavior near baitfalls; this is probably an adaptation for exploiting rich but unpredictable food resources. Scavengers consumed bait parcels so rapidly and then dispersed so broadly that energy from nekton falls apparently reaches infaunal benthos only in very attenuated form, yielding at most minor community enhancement. Necrophagy was not the sole cause of megafaunal attraction to bait parcels; there is evidence that three predacious species were drawn to high concentrations of their ophiuroid prey. Benthic standing-crop and turnover-rate estimates for nekton falls suggest that perhaps 11% of benthic community respiratory requirements are met by nekton carcasses reaching the basin floor; the flux of energy to the deep sea through such fall events thus merits further study. These energy bonanzas occur frequently enough that O. normani, and other common necrophages, are likely to encounter at least one nekton fall per year. Such windfalls thus could influence the life histories of several dominant megafaunal species in Santa Catalina Basin.