the gaping valves of the mussel or ‘hammer’ through the shell on the dorsal or ventral side (Zieritz et al. 2012 and references therein). Stabbers and dorsal hammerers open the mussel in situ, but ventral hammerers usually tear the mussel off from the substrate and carry it to a suitably firm patch, where the shell is broken at the ventral region. They invariably select thin‐shelled mussels to hammer through because these are easier to crack than thick‐shelled mussels; it is the thickness of the prismatic layer that largely determines the vulnerability of mussel shells (Le Rossignol et al. 2011). Wintering oystercatchers feed extensively on M. edulis in the estuaries of southern Britain. They show a marked preference for brown‐shelled mussels over the commoner black‐shelled morph, and show that this enables them to maximise their rate of energy gain over a longer period than a single foraging bout (Nagarajan et al. 2002a). The brown and black mussels do not differ in ventral thickness or energy content, which are the main criteria for mussel selection and the most important for short‐term optimisation. The brown mussels contain significantly less moisture, so by selecting them, oystercatchers can pack more mussel flesh into their limited oesophageal storage capacity. This enables them to increase their overall consumption during a feeding bout and increases their long‐run energy gain rate, to an extent that is large enough to be significant for survival, especially during the short exposure of the mussel beds in winter. All birds show size selection within the prey species; this is because flesh content increases more steeply with prey size than handling time (Zwarts et al. 1996b). Oystercatchers are highly selective toward mussels of between 35 and 50 mm shell length, and fewer than 5% of mussels taken are below 35 mm or above 50 mm (Nagarajan et al. 2002b). The oystercatchers select ventrally thin‐shelled mussels, especially if the length is more than 35 mm. Removal of the largest mussels may reduce protected refuge for younger mussels, but may also allow younger mussels to grow at a faster rate – although, as already mentioned, gulls preferentially prey on small mussels (Goss‐Custard et al. 1996).
Figure 3.11 The American oystercatcher, Haematopus palliatus, a significant predator of bivalves, eating a surf clam (Spisula solidissima) at Nickerson Beach, Long Island, New York.
Source: ©Arthur Morris, www.birdsasart.com. (See colour plate section for colour representation of this figure).
Different diving duck species, including eiders (Somateria spp.), scoters (Melanitta spp.) and scaups (Aythya spp.), also predate extensively on mussels. In the case of eiders, mussels often constitute as much as 60% of their diet (Nehls & Ruth 1994). Eiders select mussels of smaller than optimal size, because this minimises shell ingestion, even though larger available prey would provide greater net energy gain per prey item (Bustnes & Erikstadk 1990; Hamilton et al. 1999). In the process of zoning in on their prey, the ducks may remove whole mussel clumps, thus causing mussel mortality over and above that produced by direct predation (Raffaelli et al. 1990).
Numerous studies have been undertaken to provide actual data on the impact of oystercatchers and other bird predators on commercial mussel beds. Analysis of faeces or regurgitated pellets provides information on the sizes of mussels selected by bird species. In the laboratory, the ash‐free dry weights (AFDW) of different sizes of mussels are determined in order to calculate the biomass eliminated (Hilgerloh 1999). According to exclosure experiments on the Danish tidal flats, oystercatchers and eiders eliminated 116 g AFDW m−2 (Egerrup & Høegh Laursen 1992), while on the Dutch tidal flats, oystercatchers alone eliminated 48 g AFDW m−2. Eiders consumed 360 g AFDW m−2 on the west cost of Denmark. In a regional study in the Danish Wadden Sea, 300 g AFDW m−2 emerged as a potential consumption, assuming that 100% of the food of all three predator species (eiders, oystercatchers and herring gulls) consisted of mussels and that eiders did not feed on mussels that lived submerged or on hard substrates (Faldborg et al. 1994). In the Wadden Sea of Lower Saxony, the same three bird predators consumed a total of 32 and 71 g AFDW m−2 in two different years (Hilgerloh 1997). On a newly settled mussel bed in the same area, oystercatchers and herring gulls consumed 71 g AFDW m−2 in five to six months (Hilgerloh et al. 1997). Predation pressure differs according to the available size classes and the predatory bird species, as birds are size selective.
It is well established that sea ducks feed in mussel aquaculture sites, but whether they are able to identify those mussels as being of higher quality or are only attracted by farms because of better accessibility of mussels (i.e. higher densities and convenient suspension in the water column) is not known. Varennes et al. (2015) showed that when detectability is controlled, eiders still choose the cultivated mussels. Preferences for cultivated mussels and their foraging advantages have important implications for sea ducks and habitat management.
Other birds that feed on mussels include knots, Calidris spp. (Alerstam et al 1992), and crows, Corvus spp. (Berrow et al. 1992a). Indeed, crows are significant predators of mussels in the intertidal zone and show several interesting adaptations. They frequently cache mussels during low tide, and recover them during high tide some two to three days later. This behaviour is believed to be a response to short‐term, daily fluctuations in food availability (Berrow et al. 1992a). In order to break them open, the crows drop mussels and other hard‐shelled prey on to hard surfaces such as roads or rocky shores (Berrow et al. 1992b). This behaviour peaks during October–February and usually involves only large‐sized mussels, no doubt an adaptation to food shortages in winter.
Mussel farms, with their very high densities of small, thin‐shelled mussels, can be foraging hot spots for diving ducks, particularly during spring and autumn when birds are building up their energy reserves for reproduction, migration or overwintering. For example, in spring 2011 in Baie de Chaleurs, Quebec, Canada, all mussel growers were severely hit by scoter predation, losing almost all their collectors and over 30% of their one‐ to two‐year‐old mussels on ropes (Varennes et al. 2013). Acoustic and visual deterrents have been tried with little success (Dionne et al. 2006). Provincial aquaculture authorities in Prince Edward Island, Canada proposed the use of protective socking material as a potential solution to the problem of diving ducks. The material consists of the standard polypropylene sock with a biodegradable protective layer stitched around it (Figure 3.12). When mussels are put into socks and hung in the water, they start migrating toward the outside of the sock in order to filter feed properly, making them more vulnerable to predation by diving ducks. The purpose of the second layer, with its smaller mesh openings, is to prevent their migrating outside the sock, keeping them between the layers until the bays freeze up in winter and the predation threat is over. Currently, in Canada, the United States and Europe, the only effective method that provides a complete and long‐term control of bird predation in culture facilities is the use of exclusion nets deployed around longlines and rafts of suspended mussel ropes (Varennes et al. 2013).
On the west coast of North America, the sea otter (Enhydra lutris) is an important predator of M. californianus. This species removes large clumps of mussels, which it sorts and consumes on the sea surface by pounding them on a flat stone on its chest or against other mussels. So, although sea otters are selective in terms of the size of prey they consume, they have profound effects on all size classes of mussel (Seed & Suchanek 1992). In addition, otters have substantial, indirect effects on the biomass of mussel bed‐associated communities. The total biomass of species associated with mussel beds was found to be more than three times higher where otters were absent (Singh et al. 2013). Other predators of mussels include sea urchins (Strongylocentrotus droebachiensis), lobsters (Panulirus interruptus and Homarus americanus), flatfish (Platichthys
