Disappearing Blue Mussels – Can Mesopredators Be Blamed?

Hartvig Christie,Eli Rinde,Lucinda Kraufvelin,Niklas Niemi,Patrik Kraufvelin

doi:10.3389/fmars.2020.00550

Abstract

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 hours. 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.

Highlights

There has been a concern about decreasing abundances of the blue mussel Mytilus edulis in coastal waters of southern Norway and western Sweden (Skagerrak and eastern North Sea) that has been reported by both scientists and people collecting mussels for recreational purposes (Andersen et al, 2016; Frigstad et al, 2018; Lundström, 2020)
In 2019, the project CRABFISH started in early June and ended in mid-October, being set up for testing predatory top-down effects by the goldsinny wrasse (C. rupestris) and the green crab (C. maenas) on the entire seaweed ecosystem
The green crabs were most efficient in removing the blue mussels, while the wrasses only predated on the mussels

Summary

Introduction

There has been a concern about decreasing abundances of the blue mussel Mytilus edulis in coastal waters of southern Norway and western Sweden (Skagerrak and eastern North Sea) that has been reported by both scientists and people collecting mussels for recreational purposes (Andersen et al, 2016; Frigstad et al, 2018; Lundström, 2020). The same factors mentioned above in international studies, including hybridisation between the three occurring Mytilus species, have been suggested as potential causes of M. edulis losses in South Norway. Observations of high mussel abundance on ropes in harbors and normal recruitment and growth in mussel farms do not support the disease/parasite theory, nor a negative role of higher water temperatures. The observations of high mussel abundances on ropes support an increased predation pressure as being a potential cause of reduced M. edulis populations

Methods

Results

Conclusion