Disappearing Blue Mussels – Can Mesopredators Be Blamed?

Abstract

Despite many theories, the recent evident decreases in blue mussel (Mytilus edulis) abundance in southern Norway and western Sweden (eastern North Sea) have not yet been explained. To test the possible role of increased predation, an ongoing mesocosm experiment exploring general effects of two mesopredators on the structure of littoral macroalgal and macrofaunal communities was used. These mesopredators were the green crab (Carcinus maenas) and the goldsinny wrasse (Ctenolabrus rupestris) which were distributed in a crossed manner to 12 large mesocosms containing diverse rocky shore communities. For the purposes of this study, boulders covered with recently recruited blue mussels and barnacles (Balanus improvisus) from a natural shore were brought in to the mesocosms during two seasons (August and October), and the coverage of the animals (just blue mussels in summer, both mussels and barnacles in autumn) was registered repeatedly over 24 h. The mussels were rapidly consumed by crabs and wrasses, whereas high survival was recorded on boulders in the controls without predators. The barnacles were only eaten by the crabs and not before most of the mussels had been consumed. As both the green crab and the goldsinny wrasse have been reported to increase in abundance, probably related to overfishing of top predators, the resulting higher predation pressure on especially small blue mussels (recruits) may contribute to the mussel decline along these temperate rocky shores.

Declining Populations of Mytilus spp. in North Atlantic Coastal Waters—A Swedish Perspective

“…The paradox has also been documented from Norway where Mytilus disappeared from hard bottoms, but not from the underside of floating jetties. Speculations about potential reasons include changed behavior of settlement (Andersen et al 2017, Frigstad et al 2018, Christie et al 2020.…”

“…Crabs of 20 and 80 mm carapace length prefer Mytilus of 10 and 32 mm length, respectively. Predation of Mytilus by Carcinus as a possible interfering factor for the decrease of Mytilus along the Skagerrak coast of both W Sweden and S Norway was tested by Christie et al (2020). In a rocky mesocosm environment and using natural densities of Carcinus and newly settled Mytilus (2-3 mm), they found that the mean Mytilus cover decreased by 30% within 5 h and to 0% within 24 h indicating an efficient predation.…”

Contrasting management regimes indicative of mesopredator release in temperate coastal fish assemblages

“…They are exposed to the same coastal current flowing east to west in Skagerrak. Thus, although some of the variation in species abundance and occurrence in our data could be due to different seascape properties, it seems likely that significant differences in top‐predator size are mainly due to high fishing pressure, as have been found in the eastern Skagerrak (Baden et al., 2010 ; Christie et al., 2020 ; Sköld et al., 2022 ; Svedäng, 2003 ).…”

Incorporating facilitative interactions into small‐scale eelgrass restoration—challenges and opportunities

“…Here we focus on two ecosystem engineers, eelgrass Zostera marina and blue mussels Mytilus edulis/M. trossulus, that are widely distributed and often co-occur across the temperate North Atlantic, but are in decline in many areas (Short et al 2011;Christie et al 2020). Seagrass cover has decreased by at least 30% globally over the past 50 years (Waycott et al 2009), and many northern European eelgrass populations were eliminated in the 1930s due to eelgrass wasting disease (Muehlstein et al 1988;Muehlstein 1989).…”

“…Eutrophication, algal blooms, and coastal development further reduced eelgrass cover (Erftemeijer & Lewis 2006; Burkholder et al 2007), while trophic cascades caused by overfishing, climate change, and heat waves threaten survival and growth (Baden et al 2010; Duarte et al 2018). Similarly, overfishing has led to mussel population declines (Christianen et al 2017; Christie et al 2020), while extreme climatic events can cause mass mortality events, especially in conjunction with pathogens such as Vibrio spp. (Polsenaere et al 2017).…”