No abstract

Sessile biota can compete with or facilitate each other, and the interaction of facilitation and competition at different spatial scales is key to developing spatial patchiness and patterning. We examined density and scale dependence in a patterned, soft sediment mussel bed. We followed mussel growth and density at two spatial scales separated by four orders of magnitude. In summer, competition was important at both scales. In winter, there was net facilitation at the small scale with no evidence of density dependence at the large scale. The mechanism for facilitation is probably density dependent protection from wave dislodgement. Intraspecific interactions in soft sediment mussel beds thus vary both temporally and spatially. Our data support the idea that pattern formation in ecological systems arises from competition at large scales and facilitation at smaller scales, so far only shown in vegetation systems. The data, and a simple, heuristic model, also suggest that facilitative interactions in sessile biota are mediated by physical stress, and that interactions change in strength and sign along a spatial or temporal gradient of physical stress.

Summary1. Monocultures of mussels might alter the infaunal benthic community of adjacent and interstitial sediments through provision of a complex habitat, input of organically rich material and larval removal through filter feeding. At a site of commercial seabed mussel cultivation, we aimed to determine the effect of mussels on the infaunal community of an intertidal mudflat at different spatial scales and under different stocking strategies. 2. Mussels were laid at four different densities (2, 3, 5 and 7·5 kg m − 2 ) on 400-m 2 plots in a 4 × 4 Latin square. Benthic samples were collected within and 10-100 m distant from the cultivation area c . 7 months prior to and 18 months after seeding the plots with blue mussels. Benthic community characteristics were related to initial seeding density and to the actual surface area of mussels associated with each set of samples collected within replicate plots. 3. The presence of mussels significantly changed the occurrence of some species of the infaunal community within the cultivated area. The infaunal communities supported fewer individuals and species than control treatments at all but the lowest mussel cover. 4. Species richness and the abundance of individuals per unit area also declined with increased area of mussel cover. The abundance of cirratulids and amphipods declined strongly with increasing mussel surface area. 5. Although the species composition and abundance of individual invertebrate species were altered by the presence of mussels, the distribution of individuals among species remained relatively unchanged. 6. Synthesis and applications . Overall, mussel beds changed the infaunal community, but the effects were localized (0-10 m) and not detectable at larger scales (10-100 m). Changes in benthic community composition could be reduced (but not eliminated) by lowering the stocking density of mussels to either 2 or 3 kg m − 2 . Given the small edge effects associated with cultivated mussel beds, the use of larger mussel beds would be preferable to many smaller mussel beds.

Bottom cultivation of mussels on intertidal flats is practiced throughout the world. This often generates conflicts between commercial interests and competing birds such as oystercatchers. At the Menai Strait, United Kingdom, the overwinter consumption of 242 tonnes (1 metric tonne = 1000 kg) of commercially harvestable mussels (>40 mm) by oystercatchers in 1999–2000 was worth £133 000 ($226 000 U.S. dollars). This represents 19% of the value of the landings. We used a behavior‐based simulation model to predict the extent to which such losses can be reduced by novel commercial management practices, and to explore the consequences for the oystercatcher population. Simulations of novel lay management practices indicated that the losses of commercially harvestable mussels to oystercatchers can be considerably reduced by altering the shore level and/or extent of the commercial lays. We propose a novel management strategy for the bottom cultivation of mussels in intertidal areas. Seed mussels (15–20 mm) should be laid relatively far upshore, where losses to oystercatchers will be minimal. As the mussels grow over the next 2–3 years, they should be moved progressively further downshore such that the largest mussels spend their last season prior to harvest in a relatively small area, lower on the shore than all mussels earlier in the cultivation cycle. Support for the effectiveness of this proposed management strategy can be found in the reports of commercial operators who have incorporated this management strategy in new management practices in the last few years. They report an increase in the ratio of the live mass of harvested to seeded mussels from the previous norm of 1:1 to 4:1. By accepting greater losses of mussels earlier in the cultivation cycle, rather than later, the feeding conditions for oystercatchers might even be improved under this system. With appropriate management, the interest of shellfish growers and competing shorebirds need not conflict.

Mussel Mytilus edulis cultivation on intertidal flats affects the invertebrate community, often adversely, and this may have detrimental consequences for shorebirds. Here we present the results of an experimental study to quantify the effects of intertidal mussel cultivation on shorebirds. A study area of 4.32 ha, comprising experimental plots and control plots, was laid out in summer 1999 on the mudflats of the Menai Strait in Wales. Regular counts throughout winter of 1999/2000 established pre-cultivation patterns of bird usage. Mussels were laid in the experimental plots in April 2000 and bird usage in these plots and the controls was monitored over the 2 subsequent winters. Although no species were lost from the experimental plots, the bird assemblage in them changed. This reflected variation in the distribution of the 5 most abundant species. However, none of these key species declined in abundance following the laying of mussels. Curlew Numenius arquata and redshank Tringa totanus increased in abundance, although, unexpectedly, oystercatchers Haematopus ostralegus did not. At this study site, commercial mussel cultivation may have beneficial effects, not just for the birds that eat mussels, but also for other species that can take advantage of the associated changes to the benthic fauna and habitat complexity. However, features of conservation interest at other localities may mean that bottom cultivation of mussels will have detrimental rather than beneficial effects. The environmental effects of proposals to initiate or expand bottom cultivation of mussels need to be assessed on a case-by-case basis.

Mussels are extensively cultivated worldwide and are of growing economic importance. However, constraints on the exploitation of wild mussel resources have necessitated the need for tools to improve the management of mussel cultivation towards increased production. Ecological models are increasingly being used as a management tool, and therefore the existing approaches to modelling mussels have been reviewed with respect to their possible application to the improvement of shellfish management strategies. We suggest that dynamic energy budget (DEB) models have the greatest potential in this area, and discuss the mussel DEB models that have been developed to date in terms of their physiological complexity, accuracy of prediction of individual mussel growth and ability to predict mussel population production. Individual mussel production has been predicted; however, the focus of many of the models has been on the growth and reproduction of a single mussel and therefore population effects generally have not been included. Other models at the population level have included only limited population effects, and this has reduced the capacity of many of the models to accurately predict mussel production at the population level. Interactions at the population level (self-thinning and predation) are discussed and the models that describe the consequences of these processes are examined. In future DEB models will need to include the ability to parameterise population level processes if we are to have greater confidence in their application to shellfish management.