RESOURCE LIMITATION AND FISH PREDATION - THEIR IMPORTANCE TO MOBILE EPIFAUNA ASSOCIATED WITH JAPANESE SARGASSUM

The possibility that resource limits constrain the growth of mobile ep ifaunal populations associated with Sargassum patens plants was invest igated by placing plants and associated animals into field microcosms which excluded fish predators, and then comparing faunal abundance and size-structure changes in different microcosm treatments with field p opulations. Four different microcosm treatments were set up: two treat ments containing defaunated plants inoculated with caprellid amphipods , and two control treatments with natural faunas. The estimated second ary production of faunas enclosed in all microcosm treatments rapidly settled on a constant value (5 mg/day) which was similar to that deter mined in experiments conducted in Western Australia using the same mic rocosms but for faunas associated with a seagrass rather than a macroa lga. These results support the hypothesis that the secondary productio n of epifaunal communities associated with macrophytes is constrained by quantifiable food resource ceilings. Predation by the most common f ish species in the area, the wrasse Halichoeres tenuispinis, did not a ppear to alter macrofaunal production in the S. patens bed; however, i t did greatly affect the faunal size-structure by eliminating most of the larger animals. The majority of epifaunal animals greater-than-or- equal-to 2.0 mm sieve-size were consumed by H. tenuispinis, while negl igible numbers of 0.5-mm sieve-size animals were captured. We postulat e that food resource ceilings and predatory size-selectivity are wides pread phenomena, affecting epifaunal populations at a variety of locat ions. Predation is predicted to generally increase rather than decreas e faunal abundance because the consumption of each large invertebrate by a predator frees sufficient resources to feed several smaller indiv iduals.