by Erika Steyn
The brown mussel Perna perna is an important filter feeder on the intertidal rocky shores of KwaZulu-Natal (KZN). For many this inconspicuous mussel may be little more than a convenient foot hold when negotiating slippery rocks or a refreshing salty snack that goes well with garlic and lemon juice. In reality, this humble bivalve (marine snail with two lidded shell) forms the basis of a highly diverse and healthy intertidal zone.
As filter feeder, the brown mussel forms an important link between terrestrial and marine food webs. It not only feeds on phytoplankton (microscopic marine algae), but also feeds on terrestrial plant material washed down rivers and broken down to small particles by wave action. In turn mussels are eaten by several reef fish, octopus and the East coast rock lobster. Mussel beds are also a home and feeding ground for numerous other animals such as amphipods, polychaete worms, anemones and barnacles.
Mussels are also collected by people and due to the ease of collecting these animals, it has a high risk of been overexploited. The Oceanographic Research Institute, therefore, initiated the invertebrate catch statistics programme in 2002 to monitor the status of mussels and other popular nearshore shellfish. This programme monitors the sport collection of shellfish along the KZN coastline by asking collectors to report their catches either by means of a postal or online questionnaire.
These surveys provided valuable information about the mussel fishery in KZN over the last decade. On average ~3800 annual mussel permits are sold each year at KZN post offices (2010-2015 permit sales data). Permit holders are mainly residents from coastal areas with high population numbers, where it is easy to access the beach and there is a good possibility of finding large mussel beds. Most brown mussel beds are confined to the southern and central regions of KZN. Thus, it is not surprising that ~50% of permits are bought by people residing in the highly populated eThekweni metropolitan municipality, which includes Durban, and extends from Clansthal in the south to Westbrook in the north. Large numbers of permits are also bought by coastal residents of the uGu (Port Edward to Scottburgh: ~600 permits) and iLembe municipalities (Ballito to Amatikulu Nature Reserve: ~400). A further ~200 permits are bought by the inland uMgungundlovu residents (Pietermaritzburg and midlands), as well as from residents of the King Cetshwayo municipality (Richards Bay) where there are few mussel beds. Visitors from Gauteng account for a further ~200 of the permits bought.
Mussel collection is still dominated by men (84% of permit sales) and are particularly popular under the middle aged (30-59 years old: 66%). Few young people seem to be interested in this activity (<30 years: 12% of permit sales) and decreasing health and agility would explain why only 21% of permit holders are >60 years old. However, an amazing ~30 octogenarians still buy mussel permits each year.
The survey results suggest that the way people use the mussel permit have changed very little over the last decade. People are more likely to collect mussels during summer when the weather is warm, and the majority of South Africans are on holiday, than in winter. Very few permit holders are active mussel collectors and conduct at least one mussel outing a year. Despite living close to the coast and good mussel beds, only half of the permit holders living along the KZN south and central coasts collect mussels (iLembe, eThekweni and uGu). Those living further inland (uMgungundlovu) or along the north coast (King Cetshwayo) where there are few mussel beds are even less likely to use their permits; and as expected only a quarter of Gauteng permit holders collect mussels.
In combination with the low number of people active in the fishery, active mussel collectors conduct only 3-4 mussel outings during a 3-month period. This low level of activity is the same throughout the year and for all active collectors, regardless of their age or where they come from. Clearly, some other factors limit mussel collection activity. Most mussel trips occur over weekends or holidays, as it is a sports fishery, but mussel beds are only exposed during spring low tides and favourable weather conditions. The combination of these factors is thus most likely to limit the number of mussel trips.
In most cases, collectors can collect the full daily bag limit of 30 mussels per collector. This consistency in catches are largely due to mussels occurring in beds and does not necessarily indicate that mussel numbers have remained stable over the last decade. Field surveys have shown great fluctuations in mussel populations during the same period. Recording how long it took to collect the full bag limit or measuring the size of mussels collected would be more useful in determining the status of mussel beds.
For now, the mussel fishery seems to be sustainable and if collectors adhere to the daily bag limit, remove individual mussels and do not clear large patches in the mussel bed, this fishery will remain so.