Musselling Up: Assessing the large-scale effects of introduced source populations of Mytilus galloprovincialis

Explaining the distribution and abundance patterns of organisms in the natural environment is a central objective of ecological theory. Predation and competition have long been seen as major drivers of diversity and abundance, while more recently ‘supply-side’ ecology has developed as a potentially important factor explaining populations dynamics. Marine sessile and sedentary species comprise a large component of rocky shoreline biodiversity. For many benthic organisms, broadcast spawning is the primary mode of reproduction with extremely high larval output potentially making larval supply a key step in their ecology. Large-scale studies (over 100’s of km’s) of supply-side ecology are uncommon. Aquaculture, however, provides a perfect opportunity to conduct large-scale ‘supply-side’ research by substantially increasing source populations and the opportunity for increased larval supply. The smoothshelled mussel Mytilus galloprovincialis is farmed in two open coastal embayments along the NSW east coast, Twofold Bay and Jervis Bay. This study used a combination of three observational surveys and one experimental manipulation to investigate the effect of a potential altered larval supply on the surrounding patterns of species distribution and assemblage composition. While additionally assessing the significance of post-settlement processes of predation and competition. Two embayments exposed to aquaculture, Twofold Bay and Jervis Bay, were compared to two control embayments, Botany Bay and Batemans Bay (which did not have mussel aquaculture). The rocky intertidal mussel populations were quantified at all four bays using photo quadrats randomised along 100 m transects. Mytilus galloprovincialis populations were significantly greater in Twofold Bay than the reference embayments, while Jervis Bay had intermediate levels of mussel abundance. To assess the influence of the distance from a potential larval source on recruitment patterns of Mytilus galloprovincialis, 24 subsurface recruitment ropes were deployed ‘close’ (<3500 m), ‘near’ (<7000 m) and ‘far’ (>7000 m) from the potential larval source in Jervis Bay. Greater recruitment of Mytilus galloprovincialis was observed ‘near’ than ‘far’ to the potential larval source. Post-settlement predation was investigated to understand the influence of predation on potential population establishment. Predation of juvenile M. galloprovincialis inside and outside of mussel patches was used to explore this. To test the hypothesis that there will be greater predation of Mytilus galloprovincialis outside of mussel patches, a manipulative staked goat experiment was conducted at two locations in Twofold Bay. Contrary to III predictions, greater predation of mussels was detected inside of mussel patches at one location, with no difference in predation inside or outside patches at the second location. To infer the competitive ability of Mytilus galloprovincialis and its effect on surrounding species diversity, species richness was measured with photo quadrats inside and outside of mussel patches in Twofold Bay. Species richness was found to be reduced inside of mussel patches. This study provides strong evidence that Mytilus galloprovincialis larval supply influences the population abundance and distribution in embayments along the NSW coastline. Twofold Bay was found to have the highest mussel abundance, which was consistent with predictions as it has been exposed to an increased larval source for the longest period. Within Jervis Bay, there is evidence to support that the mussel culture is resulting in an increased larval supply, as greater subtidal recruitment of mussels was found closer to the mussel culture site. It is possible that the continued supply of larvae from the culture of mussels in Jervis Bay into the future may continue to seed new populations and may result in similar trends to Twofold Bay. Post-settlement predation and competition do not appear sufficient to control mussel population increases resulting from an increased larval supply, as evident by significantly greater shoreline abundances in Twofold Bay. Once populations establish, study results indicate Mytilus galloprovincialis can outcompete other epifaunal organisms for space, suggesting that an increase in Mytilus galloprovincialis populations due to increased larval supply may have long term impacts on the ecosystem structure through the reduction of species richness. Overall, this study demonstrates that post-settlement predation and competition are not capable of controlling large-scale supply-side ecological shifts that significantly alter the population distribution and abundance of the mussel Mytilus galloprovincialis. These findings provide new evidence of the effects of supply-side ecology, with a need for these findings to be incorporated in the management of anthropogenic influences on the marine environment.