The quality of a plot is a sum of the local variation in food quantity (location relative to the sea holes and other plots), food quality, spatial variation in predators and/or competitors, current velocity, salinity, substrate (foundation), susceptibility to storms (depth) and ice cover. The quality of a mussel plot can be expressed in terms of production (mussel ton per ha.), average fish weight per plot, yield and suitable area. The yield of a mussel plot is the product of growth and survival. Mussel quality, in turn, can be expressed in terms of fish weight. Fish weight is expressed as the weight of cooked clam meat as a percentage of total fresh weight. Fish weight tends to be higher in locations where food quality and quantity is higher. Thus, the environment of a plot, as well as the density of mussels, have a major effect on growth and survival. For consumption mussels, the price at auction is mainly determined by the fish weight combined with the size of the mussels.
Food:
Food quality and quantity show wide variations in space and time. Quantity of food is greater near the sea holes, for example. The amount of inedible particles is greater in the Wadden Sea than in the Eastern Scheldt, and within the Wadden Sea the food quality decreases toward the afsluitdijk (Van Stralen, 1995). Mussels can also adapt to conditions. A study by Essink and Bos (1985) showed that mussels are more adaptable when moved from low to higher food quality conditions than vice versa. In addition to spatial differences in food quality and quantity, there are also temporal differences. Thus, food availability is often highest in the months of April through August. After summer, food availability decreases again as water temperatures drop. In winter, mussels can lose as much as 50 percent of their meat weight.
Current and wave action:
Flow across a plot should be sufficient to refresh the water just above the mussels and introduce new food. If flow rates are too low, the mussels can deplete food locally. In contrast, excessive current velocity, wave action or ice conditions can cause mussels to wash away (Widdows, Lucas, Brinsley, Salkeld, & Staff, 2002). The extent to which a mussel bed can withstand currents, wave action or glaciation depends on its physical and biological properties and the location of its establishment.
Waves
Especially in shallower plots, wave action can also cause mussels to be dislodged and washed away. Mussels located on exposed, shallow, or dry-falling plots protect themselves from washout by producing more byssys threads that allow them to attach better and are less likely to be knocked loose (Beadman, Caldow, Kaiser, & Willows, 2003). However, waves create what is known as orbital motion. In shallow water, this water movement creates a force on the bottom that may be capable of eroding whole sections of a mussel bed. The higher the waves the greater the force exerted on the bottom. The water near the bottom then moves strongly back and forth. In addition, waves can exert a lot of energy when they break causing erosion of both the sediment around and under the Mussels.
Current
Tidal currents along gullies can be very strong and can affect permanently submerged mussel beds in particular. Drying banks have this problem when gullies and gullies form within a bank. In addition, currents can also supply or drain predators such as Starfish.
Ice
It is not very common but drying mussel beds can suffer greatly from ice and icing in harsh winters. When ice surfaces and ice floes are set in motion with currents and winds, they can push everything in their path ahead of them. Within drying mussel beds, erosion marks of ice can be found regularly during severe winters.
Substrate:
Mussels cannot attach to substrate particles that are too small, such as sand or mud pellets. Lots of mud under the mussels therefore makes them more susceptible to being washed away, during storms. On a sandy or muddy bottom, the mussels will primarily attach to each other rather than to the surrounding substrate. Shells in the soil (gritty soil) allow mussels to attach well and crawl less on top of each other. As a result, local density is lower than on mud or sand (Leuchter, Hartog, & Capelle, 2015) and mussels are less likely to wash away (De Wit, Hartog, & Capelle, 2015). Mussels capture silt particles from the water and deposit them on the bottom. Especially in lee locations, this silt can form a thick layer under the mussels. The mussels then attach to each other and can remain as a mat on the mud layer. If the mussels are small, or in low densities on a plot, they may have a harder time forming a mat and sinking into the mud.
Predators:
Predators can account for quite a bit of mussel seed loss. On dry-falling plots, crabs in particular but also birds such as oystercatchers cause large losses. In sublittoral plots, starfish are particularly important predators.
Crabs
In an experiment on a dryland plot in Sand Creek, crabs were found to account for one-third of seed loss after seeding (Capelle, Scheiberlich, Wijsman, & Smaal 2016). The foraging activity of crabs in winter decreases sharply. In locations where mussels can grow rapidly, they can outgrow crab infestation (Murray, Seed, & Jones, 2007).
Starfish
The common starfish Asterias rubens is an important predator of sublittoral mussel seed banks. Starfish can occur in high densities (up to > 500 ind/m2) where they show high rates of consumption and growth under favorable conditions. Starfish can consume mussels larger than themselves, but there is a limit beyond which mussels are no longer a suitable food source.
Starfish are sensitive to environmental factors such as currents, temperature and salinity. Foraging activity of starfish in winter decreases to near zero at temperatures below 4°C. Starfish wash away quickly due to currents. On a soft bottom, as current velocity increases, the percentage of starfish decreases but the presence of mussels allows them to cling visibility and is less likely to wash away(Agüera, van de Koppel, Jansen, Smaal, & Bouma, 2015). At flow rates above 30 cm/s, the percentage of sea stars foraging on mussels was found to be approximately halved. Starfish are also very sensitive to low or fluctuating salinity levels. In areas where salinity is low or highly fluctuating, starfish are often absent or inactive.
The consumption rate of mussel-eating starfish can reach 0.34 Mussels per day per gram of starfish. At an average sea star density of 300 g/m2, a mussel seed bank (mussel density 1000 mussels/m2 with shell length between 15 to 33 mm) can be completely consumed by sea stars within 10 days. Such starfish densities (up to about 500 ind/m2) and mussel densities (up to about 3000 ind/m2 ) are regularly observed in the Wadden Sea. In normal winters, however, temperatures are low and these consumption rates will not be achieved (Agüera 2012).
Birds
Oystercatchers mainly eat the larger mussels on the dry-falling littoral mussel beds. Oystercatchers can be quite dominant and rob other oystercatchers of their prey. This ensures that as the density of foraging Oystercatchers increases, the rate of food intake decreases. As a result of this interference, there is a limit to the density at which Oystercatchers search for food on the mussel beds. As a result, the predation pressure during the winter of Oystercatchers on dry-falling mussel beds rarely exceeds 20% of the supply at the beginning of the winter.
Herring gulls are true opportunists, not shy about stealing prey from other animals. Herring gulls can often forage in very large groups on young mussel beds. They prefer young mussels that are not yet so firmly attached and have a still somewhat thinner shell.
The predation pressure of eider ducks on sublittoral mussels can be very high. So high, in fact, that mussel farmers used to hire staff to chase the ducks away from the growing plots. Eider ducks show clear distribution patterns, these birds often feed on mussel plots in the Wadden Sea and cause considerable losses among half-winter and consumption mussels. For birds that swallow shellfish with shell and all, such as the Eider Duck and Herring Gull, mussels with lots of meat and little shell are, of course, most beneficial.
Competition:
Water where some of the food has already been filtered out contains less food for the mussels. This is called re-filtration and can cause mussels on the edges of plots to often do better than in the middle (Knights, 2012). Competition for food also causes juvenile mussels to often form band-like patterns as they either die or move away from sites with less food in the water (Van de Koppel, Rietkerk, Dankers, & Herman, 2005). It can also be expected, for example, that plots located downstream or in the middle of a plot block in a gully will be less nutrient-rich than plots located at the beginning of a gully. Not only competition for food between mussels can play an important role, but other filterers such as oyster, clams or barnacles growing on the mussels can also cause competition.
Barnacles
Barnacles are small crustaceans that can settle as larvae on shells of live mussels. This is detrimental to the mussel because it becomes less streamlined, making it more likely to wash away. In addition, competition for food may occur because, like mussels, smallpox filter plankton from the water. It is known that barnacles are not a problem everywhere, this varies between areas and plots. A certain amount of silt seems to have an inhibiting effect on smallpox development, possibly due to low oxygen levels.
Thus, the quality of a plot is largely determined by its location. For both the Eastern Scheldt and the Wadden Sea, landings and monitoring data show that the quality of mussels is improving toward the sea holes. Practice and research show that survival of mussel seed, especially MZI seed, is better on plots in quieter areas than in more exposed locations. Preferably also in places where the seiment is a bit grittier so that the mussel seed can attach to something. However, at the exposed locations near the sea holes, the supply of food is often better and these are therefore good locations for the somewhat stronger half-grown mussels that are less susceptible to washout than seed. This is where the half-grown mussels can grow into good quality consumption mussels.
Mussel plots are often positioned in or along channels, where flows are relatively high, mixing is high and it is not too muddy. In the gully, downstream flow rate decreases and water is increasingly refiltered. Thus, the amount of food in the water gradually decreases, reducing growth and quality.
Local conditions can also play a role in the quality of a plot. Drainage gullies from a slab, for example, can locally affect food supply to the plot. Also, benthic diatoms from the plate during the ebb phase can lead to additional food in the water. Growth of a plot in the mouth of such a trench may be better than an adjacent plot as a result. There are several ways a grower can improve the yield of a plot, depending on the location. Growers can spread the seed extra well and match the density to the site to lower competition among the mussels but at the same time ensure that the mussels can clump together. Sowing along shells can also increase survival. Because the soil is made grittier, the mussels can adhere better and are less likely to wash away. In addition, it is important to consider local food dynamics. So to move mussels at the right time to a more suitable location to grow into consumption mussels. However, the quantity and quality of food varies greatly from year to year and by season. There are also large differences between plots. To optimize their mussel seed yield, growers move their mussels between the different growing plots they have available during the culture cycle. They make trade-offs between expected growth and survival in the respective plots. On the plots in the North Branch of the Eastern Scheldt, growth is generally limited and these plots are mainly used for storage of mussel seed and half-grown mussels. From here the mussels are moved to plots in the middle and western part of the Eastern Scheldt where the mussels continue to grow into consumer mussels.