Changes in Benthos Associated with Mussel (Mytilus edulis L.) Farms on the West-Coast of Scotland

Abstract

Aquaculture, as a means of food production, is growing rapidly in response to an increasing demand for protein and the over-exploitation of wild fisheries. This expansion includes mussels (family Mytilidae) where production currently stands at 1.5 million tonnes per annum. Mussel culture is frequently perceived as having little environmental impact yet mussel biodeposits and shell debris accumulate around the production site and are linked to changes in the benthos. To assess the extent and nature of changes in benthos associated with mussel farming grab and video sampling around seven mussel farms was conducted. Grab samples were analysed for macrofauna and shell-hash content whilst starfish were counted and the shell-hash cover estimated from video imaging. Shell-hash was patchily distributed and occasionally dominated sediments (maximum of 2116 g per 0.1 m2 grab). Mean shell-hash content decreased rapidly at distances >5 m from the line and, over the distance 1–64 m, decreased by three orders of magnitude. The presence of shell-hash and the distance-from-line influenced macrofaunal assemblages but this effect differed between sites. There was no evidence that mussel farming was associated with changes in macrobenthic diversity, species count or feeding strategy. However, total macrofaunal count was estimated to be 2.5 times higher in close proximity to the lines, compared with 64 m distance, and there was evidence that this effect was conditional on the presence of shell-hash. Starfish density varied considerably between sites but, overall, they were approximately 10 times as abundant close to the mussel-lines compared with 64 m distance. There was no evidence that starfish were more abundant in the presence of shell-hash visible on the sediment surface. In terms of farm-scale benthic impacts these data suggest that mussel farming is a relatively benign way of producing food, compared with intensive fish-farming, in similar environments.

Magnitude, spatial scale and optimization of ecosystem services from a nutrient extraction mussel farm in the eutrophic Skive Fjord, Denmark

“…Petersen et al 2008, Cranford et al 2014, Newell & Richardson 2014, which will enhance light penetration and water quality (Schröder et al 2014 and reduce the occurrence of anoxic events (Newell 2004). Mussel ropes and anchors can also provide a temporary habitat for other species (Murray et al 2007, D'Amours et al 2008, Wilding & Nickell 2013. Beside these ecological services, the mitigation mussels provide goods for human consumption (Lindahl 2011, Petersen et al 2014, feed for husbandry (Jönsson & Elwinger 2009, Jönsson et al 2011, Nørgaard et al 2015 and fertilizer (Olrog & Christensson 2008).…”

Influence of mussel aquaculture on the distribution of vagile benthic macrofauna in îles de la Madeleine, eastern Canada

“…This spatial variation within the farm also suggests that the distribution of vagile species varies temporally as a function of harvesting and as the crop is growing. As for the line-scale effects observed for lobsters in the present study, Wilding & Nickell (2013) Abundance (mean ± SE, n = 4) of common species recorded along transects in reference (in white: R1, R2, R3) and farm areas (in grey) with 1 yr old blue mussels Mytilus edulis (M1), 2 yr old mussels (M2) and where mussels had been recently harvested (MH). Species: American lobster Homarus americanus, Atlantic rock crab Cancer irroratus, common starfish Asterias rubens, and winter flounder Pseudopleuronectes americanus.…”

“…Romero et al (1982) observed a greater abundance of various crab species in areas with mussel rafts than in areas without them in the Ría de Arousa, Spain, and stomach content analysis of crabs in the same region showed crab diets shift to include more mussels in farm areas (Freire et al 1990, Freire & González-Gurriarán 1995. Likewise, mussels are the preferred prey of many starfish (Penney & Griffiths 1984, Gaymer et al 2001, and aggregations of starfish have often been noted with fallen mussels at mussel farm sites (Olaso Toca 1982, D'Amours et al 2008, Wilding & Nickell 2013, suggesting that they are attracted to mussel farms for the abundant prey they offer. The importance of mussel drop-off to the distribution of rock crabs and starfish is further supported by the observation that both of these organisms were most abundant in the area where mussel fall-off is greatest (the area with 2 yr old mussels).…”

“…Studies on larger and more mobile macrofauna species have observed that scavengers and other preda-tors (crabs, starfish, and gastropods) are more abundant in aquaculture areas (Romero et al 1982, Inglis & Gust 2003, D'Amours et al 2008, Wilding & Nickell 2013 or found no difference in macrofauna community structure between farmed and unfarmed areas (Chesney & Iglesias 1979, Clynick et al 2007). Simple trophic mechanisms, whereby scavenging spe cies are seen associated with fallen farmed bivalves, are usually invoked to explain the observed patterns of greater abundance in farm areas relative to non-farm areas.…”

“…A trophic mechanism also supports observations of variation in the distribution of macrofauna within farms. Mussel fall-off is concentrated directly below mussel lines, where starfish and other vagile macrofauna were observed to be more abundant than in areas between mussel lines (D'Amours et al 2008, Wilding & Nickell 2013. Such mussel line-scale variation in the distribution of macrofauna could explain why some studies failed to observe spatial patterns in the abundance of vagile macrofauna.…”