Morphology and density of mussels on natural and aquaculture structure habitats: implications for sea duck predators

Abstract

We compared density and morphology of wild mussels Mytilus trossulus growing naturally on shellfish farming structures to that of mussels from nearby intertidal areas, and considered the relevance to molluscivorous sea ducks. Mussel density on aquaculture structures greatly exceeded that of intertidal areas and farm mussels were larger, had lower shell mass, and had weaker byssal attachments. Differences in environmental conditions and predation regimes throughout the summer growing season were likely responsible for these differences. These morphological and density differences, in turn, have important implications for their primary winter predators, sea ducks, including surf scoters Melanitta perspicillata and Barrow's goldeneyes Bucephala islandica. Higher levels of mussel depletion in farm habitats and strong associations of ducks with shellfish farms indicate that sea ducks responded to this novel, profitable prey resource. Our results support recent research that indicates that artificial structures introduced into coastal areas create novel marine habitats that can support unique communities of epibiota. That sea ducks appear to profit from the introduction of aquaculture structures is evidence of a positive effect of anthropogenic modifications in a coastal environment.

Effects of off‐bottom shellfish aquaculture on winter habitat use by molluscivorous sea ducks

“…Introduced structures are known to support different epibiotic communities from adjacent natural intertidal areas (Glasby and Connell, 1999;Connell, 2001), and subsequently these habitats are differentially preferred by foraging predators. In the case of shellfish aquaculture, Kirk et al (2007) found that mussel densities on aquaculture structures in the Malaspina complex were much greater than in intertidal areas (9000 and 80 mussels m À2 respectively). Also, mussels on aquaculture structures had a number of morphological properties that made them a more profitable prey than intertidal mussels, such as weaker byssal attachment, and more fragile and thinner shells (Bustnes, 1998;Lovvorn, 2003, 2004;Kirk et al, 2007).…”

“…Alternatively, sea ducks might benefit from the presence of aquaculture structures that serve as substrate for fouling organisms including wild mussels (Mytilus trossulus), which in turn may offer abundant food resources. Kirk et al (2007) documented extensive recruitment of wild mussels on aquaculture structures in the Malaspina Complex, and suggested that these mussels constitute both abundant and profitable prey for sea ducks.…”

“…Mussel densities in intertidal habitats were estimated throughout the study site, as described by Kirk et al (2007). Using the Random Point Generator extension in ArcView 3.2 (ESRI, 1999;Jenness, 2005), two points were selected randomly in each survey polygon as start points for transects (for a total of 68 transects).…”

“…Within each large quadrat where mussels occurred, a subsample quadrat (0.1 Â 0.1 m) was tossed haphazardly and mussels were counted and measured to 5 mm size classes. An average intertidal mussel density was calculated for each survey polygon by pooling all size classes together, as all measured mussels fell within the size range potentially taken by Barrow's goldeneyes and surf scoters (Kirk et al, 2007).…”

“…To date, bird responses to the presence of shellfish aquaculture structures have been shown to vary, with the abundance and density of some species increasing in association with aquaculture and other species decreasing (Hilgerloh et al, 2001;Connolly and Colwell, 2005). Most studies have described the effects of shellfish aquaculture as being neutral (Roycroft et al, 2004;Ž ydelis et al, 2006) or even beneficial (Caldow et al, 2003;Kirk et al, 2007). The nature of the relationship depends on the bird species involved, the type and intensity of industrial activity, and the habitats affected.…”