Potential indirect effects of shellfish culture on the reproductive success of benthic predators

1. Environmental assessments of coastal aquaculture are concerned mostly with direct impacts on natural assemblages in the vicinity of shellfish or fin-fish farms. As the size and density of farmed sea space increases, there is greater potential for indirect effects on food webs beyond the immediate culture area. 2. We investigated the potential indirect effects of long-line mussel Perna canaliculus farms on the demography of an important benthic predator, the sea star Coscinasterias muricata. Surveys beneath four active farms, an abandoned farm and three unfarmed areas of seabed in Pelorus Sound, New Zealand, described the direct effects of mussel culture on the distribution and abundance of sea stars and other benthic consumers. These data were used to calibrate a model that simulated the fertilization success of sea star populations in farmed and unfarmed areas of the bays. 3. Deposits of living mussels and mussel shells covered up to 55% of the seafloor beneath farm sites, but were absent from soft sediments at unfarmed sites. Mean densities of sea stars were up to 39 times larger at active farm sites than in unfarmed areas and were correlated with the abundance of living mussels on the seafloor. 4. Within individual farms, the distribution of sea stars was highly aggregated at small spatial scales, with most (63%) individuals occurring within 2 m of their nearest neighbour. In unfarmed areas, sea stars were widely dispersed (< 1 80 m(-2)). 5. Our simulations indicate that, because of the extremely clumped distributions of sea stars, spawning individuals at farm sites would on average have substantially greater fertilization success (c. 90% of eggs) than those foraging in areas where farms are absent (<< 2% of eggs), and total zygote production could be as much as 1500 times greater than in unfarmed areas. 6. Synthesis and applications. This study demonstrates the potential for significant bottom-up effects of aquaculture on surrounding ecological assemblages. If, as has been suggested, sperm limitation is a major constraint on recruitment of asteroids and other invertebrate predators, supplemental provisioning from increased farm development could result in occasional outbreaks of populations, over a broader area than the farmed location. Without appropriate monitoring such events are likely to be dismissed as rare, natural phenomena rather than a consequence of shellfish culture.