Marine Mussels. Elizabeth Gosling
ducks) on mussels on bouchots can be reduced by using nylon threads to prevent their landing. When sea stars and mollusc‐drilling snails (Nucella lapillus) are present in high densities and predation levels are high, they need to be manually removed.
Predation may exert a top‐down limitation on production, especially in bottom culture, since mussel plots are accessible for benthic predators as well as for fish and birds. Intertidal mussels are preyed upon by shore crabs and birds (oystercatchers, herring gulls), while subtidal mussels are preyed upon by shore crabs, sea stars and diving ducks. The number of sea stars on culture plots is reduced by freshwater treatment and there is a selective fishery on sea stars with sea star mops in The Netherlands, the United Kingdom, Germany, Ireland and purse‐seines (large walls of netting deployed around an entire area or school of fish) in Denmark (Petersen et al. 2016). Freshwater treatment is applied before seeding when mussels are in the vessels’ hold; the process consists of the joint exposure of mussels and associated sea stars to freshwater for several hours. Mussels will keep their shells shut, while sea stars are unable to protect themselves against osmotic stress and will not survive. Sea star mops are made of fuzzy rope entwined around small chains that are towed over the mussel plots ensnaring the sea stars, thereby enabling removal. Calderwood et al. (2016) estimated the efficiency of sea star removal by mops in Belfast Lough in Northern Ireland and found a large variation in the catch efficiency (4–78%), while the mean sea star reduction when applying this method was 27% (±SE 3.2.)
Studying the effect of exclusion of shore crabs in newly formed intertidal mussel beds on a scale of 800 m2, Davies et al. (1980) found a 400–500% increase in yield over a period of two years. Rope or net culture of mussels has the advantage over bottom culture in that benthic predators cannot reach the mussels directly. Predation by mobile predators on mussels in raft or longline culture is therefore limited to diving birds and fish. However, predators with pelagic larvae (e.g. sea stars) commonly settle in long‐line farms. Ducks such as eider ducks that primarily feed on mussels can cause extensive damage to longline mussel cultures (Dunthorn 1971; Žydelis et al. 2009). In Maine, United States, mussels are protected by nets placed around the mussel rafts (Newell & Richardson 2014). Mussel ropes and nets are very attractive for a range of fish species. Due to a dramatic decline in Croatian shellfish production, Šegvić‐Bubić et al. (2011) investigated the most abundant fish species at a mussel farm situated along the eastern coast of the Adriatic Sea. Over a period of two years, the most abundant species observed were gilthead seabream (Sparus aurata), sand smelt (Atherina hepsetus) and mullet (Mugilidae species) in summer and autumn, A. hepsetus and bogue (Boops boops) in winter and the sand smelt (A. hepsetus), mullet (Mugilidae species) and saddled bream (Oblada melanura) in spring. At control locations, characterised by significantly lower fish assemblage abundances, the most abundant species were A. hepsetus and mullet (Mugilidae species) in summer, A. hepsetus and O. melanura in autumn, A. hepsetus and picarel (Spicara flexuosa) in winter and A. hepsetus and bogue (B. boops) in spring. Gilthead sea bream was extremely abundant at the mussel farm, with 5936 individuals censused in 155 of 192 fish counts (80%) and a maximum abundance of 285 individuals per 5000 m3. Stomach content analysis confirmed the presence of M. galloprovincialis as the dominant prey. During a single month, monitoring of 423 ropes revealed that approximately 828 kg of mussels with an average shell length of 34.3 ± 2.54 mm were destroyed within the first week of mussel deposition into the sea, highlighting the degree of fish predation on mussels at this farm. Šegvić‐Bubić et al. (2011) suggest that shifting farm concessions toward locations with greater depths may reduce fish predation.
Bivalves provide an excellent substrate for the settlement of many fouling organisms. Biofouling appears to be a significant cause of mortality in intertidal mussels, mainly due to dislodgement caused by the increased weight, especially from barnacles and seaweed. Fouling is a particular problem in suspended mussel culture, and almost 100 invertebrate species, including gastropods, crustaceans, bivalves, polychaetes, ascidians, sponges and hydroids, have been identified on mussel ropes (Hickman 1992). These organisms cause reduced growth and productivity through competition for space, but are not a major cause of mortality in suspended culture. See Chapter 10 for details on biofouling.
Mussels are the most prominent competitors for space in mid‐ to low‐shore areas on gently sloping rocky shores, but on steeper shores they tend to be replaced by barnacles or algae. Generally, where two mussel species coexist, there is competition but rarely elimination of one by the other. There are a few notable exceptions. M. galloprovincialis was accidentally introduced on the west coast of South Africa in the late 1970s, where it outcompetes the indigenous mussels Aulacomya atra and Choromytilus meridionalis by reason of its superior reproductive output, faster growth rate and greater tolerance to desiccation (Hockey & van Erkom Schurink 1992; Figure 3.13), while it exhibits partial habitat segregation with the local mussel, P. perna, on the south coast (Rius & McQuaid 2006). Because of weaker attachment strength in M. galloprovincialis, however, the species will be largely excluded from open coast sites, where wave action is generally stronger, although its greater capacity for exploitation competition through recolonisation will allow it to outcompete P. perna in more sheltered areas (especially in bays) that are periodically disturbed by storms (Erlandsson et al. 2006). Currently, South Africa is experiencing a second mussel invasion, with detection of the Chilean Semimytilus algosus in 2009 (de Greef et al. 2013). Both invasive species are now much more abundant intertidally than either of the indigenous mussels, Aulacomya atra or Choromytilus meridionalis, which have become largely confined to sublittoral and sand‐inundated habitats, respectively. The two invasive mussels display strong spatial segregation, with M. galloprovincialis dominating the midshore and S. algosus blanketing the lower shore. Alexander et al. (2015) predict that S. algosus will become established along the south coast of South Africa and that M. galloprovincialis will maintain dominance on the south and west coasts.
The mussels S. algosus and P. purpuratus cohabit most of the Chilean rocky shores, with the former inhabiting the low intertidal zone and the latter dominating the mid and mid to high zones. Field and laboratory experiments show that S. algosus is a weak competitor with respect to P. purpuratus, and post‐settlers present high mobility to relocate in the intertidal (Brante et al. 2019). Under this scenario, Brante et al. (2019) evaluated the dispersal behaviour of juveniles and adults of S. algosus as a potential response to competition with P. purpuratus. They also measured the attachment strength of S. algosus in the presence of its competitor as a measure of its escape response ability. Their results show that the presence of P. purpuratus increased the movement activity of juveniles and adults of S. algosus and decreased their attachment strength. Field experiments carried out with marked individuals on a Chilean rocky shore showed that S. algosus exhibits higher local dispersion in the zone where P. purpuratus is present. Mussels' high dispersal ability throughout the whole benthic phase may serve not only to reach the optimal physiological position in the intertidal, but also to reduce interspecific competition.
Figure 3.13 Performance of three indigenous South African mussels, Aulacomya atra, Perna perna and Choromytilus meridionalis, relative to Mytilus galloprovincialis. (a) Growth rate (mm in the first four years). (b) Total annual reproductive output as a percentage of body mass. (c) Survival rate after 24 months at a shore height experiencing 50% exposure to air per tidal cycle.
Source: From Branch & Steffani (2004). Reproduced with permission from Elsevier.
Another example of interspecific competition is the contribution