MARINE RESERVES ENHANCE ABUNDANCE BUT NOT COMPETITIVE IMPACTS OF A HARVESTED NONINDIGENOUS SPECIES.

Marine reserves are being increasingly used to protect exploited marine species. However, blanket protection of species within a reserve may shelter nonindigenous species that are normally affected by harvesting, intensifying their impacts on native species. I studied a system of marine reserves in the San Juan Islands, Washington, USA, to examine the extent to which marine reserves are invaded by nonindigenous species and the consequences of these invasions on native species. I surveyed three reserves and eight non-reserves to quantify the abundance of intertidal suspension-feeding clam species, three of which are regionally widespread nonindigenous species (Nuttallia obscurata, Mya arenaria, and Venerupis philippinarum). Neither total nonindigenous nor native species' abundance was significantly greater on reserves. However, the most heavily harvested species, V. philippinarum, was significantly more abundant on reserves, with the three reserves ranking highest in Venerupis biomass of all 11 sites. In contrast, a similar, harvested native species (Protothaca staminea) did not differ between reserves and non-reserves. I followed these surveys with a year-long field experiment replicated at six sites (the three reserves and three of the surveyed non-reserve sites). The experiment examined the effects of high Venerupis densities on mortality, growth, and fecundity of the confamilial Protothaca, and whether differences in predator abundance mitigate density-dependent effects. Even at densities 50% higher than measured in the field survey, Venerupis had no direct effect on itself or Protothaca; only site, predator exposure, and their interaction had significant effects. Analyses incorporating environmental variables tracked at each site indicated that crab biomass most heavily influenced clam responses, causing lower growth of both species and higher mortality of Venerupis, whose annualized loss rate was 50% when exposed to predators. A laboratory prey choice experiment indicated that Cancer productus, an influential intertidal crab predator, favored small adult Venerupis at least I .7 times over Protothaca. Venerupis' high susceptibility to excavating crab and human pred- atorst as well as its faster growth compared to Protothaca can be explained by its shallower burial depth. By growing quickly and residing near the surface, Venerupis apparently absorbs the brunt of harvest pressure while Protothaca maintains high biomass even outside of reserves. [ABSTRACT FROM AUTHOR]