Performance of cages as large animal-exclusion devices in the deep sea

Sedimentary, deep-sea communities include megafaunal animals (e.g., sea cuc umbers, brittle stars, crabs) and demersal fishes, collectively termed the large, motile epifauna (LME). Individuals of the LME are common, and their biomass approximates that of the macrofauna. Based on analogies with shallo w-water animals, they are likely to be sources of mortality for the infauna and to create spatial and temporal heterogeneity in the community. Given p resent theories of deep-sea community organization, such effects could be i mportant. Unfortunately, this hypothesis has not been tested because of the difficulty of conducting experiments in the deep sea and because tools for manipulating the LME have not been developed. We studied the suitability o f exclusion cages for this purpose at 780 m depth in San Diego Trough. We p laced 16 cages of two mesh sizes for 4.5 months over regions of the seafloo r that appeared free of LME. Time-lapse photographs of a cage and a control plot coupled with observations of all cages at the end of the experiment i ndicated that small (1.27-cm X 1.27-cm square)-mesh cages were effective at excluding LME. Further, the cages were essentially foe of cage artifacts t hat have been reported in shallow-water studies. Large, mobile and disrupti ve animals (e.g., fishes, crabs) did not establish long-term residence adja cent to or on the cages. Bio-fouling slightly reduced the open surface area of the cage mesh, potentially reducing flow through the cage, but the comp osition of surface sediments in terms of organic C and N, phytoplankton-der ived pigments, and grain size was indistinguishable between cages and contr ol areas. Activities of excess Th-234 were significantly higher (average = 37%) inside of small-mesh cages, which might suggest enhanced particulate d eposition inside cages. However, this measurement was an artifact of experi mental manipulation. Particles that accumulated on the cage during the expe riment were dislodged acid settled to the seafloor when the cage was opened just prior to sampling. These particles would have been highly enriched in Th-234, and their inclusion in core samples artificially inflated the calc ulated sediment accumulation rates inside cages. Therefore, the cages perfo rmed well; they excluded the targeted LME without causing artifacts and thu s should be useful fur experimental study of a group of animals that may ha ve substantial impact on the structure and organization of deep-sea communi ties.