Trends in mussel cover, density and size at exploited and unexploited intertidal reefs in eastern South Africa

Abstract

The brown mussel Perna perna is the dominant indigenous mussel along the east coast of South Africa, where it is harvested by recreational and subsistence fishers. High fishing pressure near metropolitan areas has led to declining abundance, and closure of some reefs to fishing in 1998. We estimated trends in mussel population dynamics at exploited and unexploited sites over a 27-year period (1993-2019) along fixed transects. Trends in recreational fishing effort were inferred from yearly permit sales and existing catch statistics.At high fishing effort, short-term trends in mussel cover and densities were inversely related to fishing effort at three of four sites, while a fourth site was influenced by intermittent breaching of a nearby estuary. Mussel size was inversely related to densities. The effects of longer term harvesting bans were partially obscured by sharp declines in fishing effort across the entire recreational fishery. Seasonal and interannual patterns in cover and density were partially synchronised among sites, indicating environmental forcing at similar time-scales. The longterm dataset was invaluable in disentangling the relative effects of fishing and environmental factors on mussel population dynamics and should be continued as a baseline for assessing future climate-induced effects on rocky shore biota.