Local fisheries

Re-laying

Over-exploitation at the level of regional recruitment cells

Cultivation

Fisheries regulations

In virtually every cSAC location round Britain where mussel beds form mud-mound reefs, the mussels have been fished or are fished now. Such fisheries have been managed by Sea Fisheries Committee (SFC) byelaws for the past 100 years. In the Wash several thousand tonnes are fished by dredges in good years. In the past, intertidal mussel beds were often exploited by hand using a variety of simple hand tools. These artisanal fisheries still persist on a small scale and, in the absence of adequate recruitment, can significantly deplete the biomass on the most accessible beds. Where the beds extend into low water channels, as in the Conwy Estuary, long-handled rakes or long handled tongs may be used to lift clumps from the seabed into a boat. With hand-working, initial selection takes place on the beds, but when working against tide, weather or daylight constraints, the catch often has to be sorted again to remove undersized shellfish. The discards are returned to the beds but not always to the particular bed from which they came. When fished by hand at moderate levels by men with traditional skills the biogenic reefs will probably retain most of their intrinsic biodiversity. Many of the same species may even survive in good numbers under cultivation regimes. Natural mussel beds are, however, vulnerable to over-exploitation; indeed many sea fisheries committees have long had powers to close beds either when stocks fall to low levels or protect undersized shellfish.

Mussels are also taken on quite a large scale by hand for use as angling and long-line bait, although the latter is now in less demand than in the past. Anglers tend to have most impact where the beds are adjacent to roads leading to favoured shore fishing locations or to harbours where they board charter boats. A small mussel bed adjacent to a road causeway in Anglesey was virtually eliminated over a period of years by anglers collecting mussels for bait and digging over the gravel for ragworm, in spite of a Sea Fisheries Committee bylaw which has long prohibited digging in mussel beds.

Mussel dredging results in temporary resuspension of sediment in the vicinity of the activity. Rieman & Hoffman (1991) showed that 1470g / m² dredged was temporarily re-suspended, but that most of this was redeposited in the first 30 minutes and turbidity returned to background levels after 60 minutes. The elimination of sand from within the mantle cavities of mussels disturbed by dredging was measured by De Vooys (1987). Three phases were distinguished, rapid discharge in the first 15 minutes, an exponentially decreasing rate over the next 4 hours followed by slow release over 48 hours. Although this study was concerned with the harvested product, the information is relevant to disturbance in the field due to fishing and civil engineering projects. It should be noted that Mytilus reefs typically inhabit areas of high natural turbidity.

Mytilus is probably less affected by incidental damage due to fisheries for other organisms than are other biogenic reef communities. Reise & Schubert (1987) reported that reefs of S. spinulosa, lost from areas of the southern North Sea due to shrimp fishing, were replaced by M. edulis communities and assemblages of sand dwelling amphipods (Reise & Schubert, 1987). The mussels are also now exploited in addition to the shrimps.

The biggest yields from mussel fisheries in England and Wales now involve a measure of cultivation known as relaying, in which young seed mussels are transplanted onto plots (lays) in the low intertidal or very shallow sublittoral, where they grow well. This movement and the subsequent harvesting is now often done with quite large purpose built dredging vessels capable of carrying 12 tonnes of mussels or more. The entire enterprise is dependent on the availability of sufficient seed in circumstances where it can be dredged. Hence seed that grows so densely that it accumulates mud and lifts off the stones on which it settled is particularly valuable. The stony skear known as ‘South America’ within the Morecambe Bay cSAC is one such location. The Dornoch Firth is another good source of these seed mussels. There may be a narrow weather window between the build up of the seed into dredgeable condition in summer, and it being washed away by storms.

Nehls & Thiel (1993) make a distinction between persistent mussel beds and more dynamic beds in more exposed situations (such as the ‘South America’ skear) and suggest that the impact of mussel fishing will be quite different in persisting and dynamic beds. In particular, they considered that fishing of persistent beds could remove the crucial reserves which mussel feeding birds such as eiders and oystercatchers need in times of low mussel populations.

The operators of re-laid beds aim to spread the mussels out at around 50-100 tonnes / ha so that there is less density dependent depression of growth. The clumps build up some mounds in these on-growing plots but not to the extent found on undisturbed beds. Great differences can occur over short distances within cultivation plots in the rates of growth and the rates of predation. Carcinus maenas, Asterias rubens and oystercatchers are important predators both on natural mussel beds and on mussel lays, so in spite of the growth of the individual mussels the operators have to be satisfied with yields that are only about equal to the gross weight of seed laid. Various techniques have been used to ameliorate starfish predation, including the use of mop type tangles and roller dredges, as well as temporary movement of the mussels to higher intertidal locations where gulls can prey on the starfish.

The mussel cultivation industry is an established feature of several of the large embayments now listed as cSACs, notably in the Wash, Poole Harbour and the Dornoch Firth. The activity has increased the total area of ground covered by mussel beds, but these are often only temporary beds. The fishing of seed is usually controlled by systems of permits under Sea Fisheries Committee bylaws and sometimes through Regulating Orders. Protection for the on-growing cultivated stocks is achieved through leases of Several Fishery Order plots.

There is concern that it is possible to so overexploit the stocks of mussels within particular embayments that this reduces subsequent recruitment. The sizes of areas from which stocks may come for recruitment is unknown, and relationships between stock and recruitment are poorly understood. There is nevertheless some historical evidence to support intuitive views that a precautionary approach needs to be taken. All delays in development and settlement of larvae decrease chances of survival.

Gross declines in stocks within intertidal mussel beds either through repeated recruitment failure, over-exploitation, or a combination of circumstances can result in knock-on consequences over wider areas. For example in the Dutch Wadden Sea in 1990 mussel stocks declined to unprecedentedly low levels resulting in eiders dying or leaving the area, and oystercatchers seeking alternative prey such as first year and older cockles, adult Macoma balthica and juvenile Mya arenaria, which then suffered particularly high rates of mortality (Beukema, 1993). There were also consequences for fishing pressures in other areas, including Britain, through international market demand and prices.

A useful review of cultivation was produced by Bayne (1991) in a volume published following an international symposium on the biology and cultivation of mussels. A review on cultivation by Hickman (1992) shows world production of 1.1 million tonnes in 1988 most of which is of farmed mussels.

Raft-and-line cultivation of mussels within the UK is limited mainly to Scotland. Despite early predictions of enormous potential, and the high quality of product, tonnages have not increased anywhere near as dramatically as once anticipated, being generally less than 1,000 tonnes per year (McKay & Fowler, 1997), mainly from sheltered sea lochs in the Strathclyde and Highland regions.

Raft-and-line mussel farms may have impacts on the local benthos (see McKay & Fowler, 1997 for a summary) but do not generally impact directly onto Mytilus biogenic reef areas.

There is evidence from Scotland and Europe that, where shellfish rafts cover 10% or more of sea lochs in areas of poor water exchange, the filtering activity of mussels can severely reduce plankton levels, with potential effects on wild filter feeders (McKay & Fowler, 1997). McKay & Fowler also point out that in the one instance in Scotland where such reduced plankton levels were suspected (Caol Scotnish, Loch Sween), the production of the farm itself suffered to such an extent that cultivation was discontinued. They also pointed out that most Scottish mussel farms are small and unlikely to have serious effects on plankton levels.

The possibility of the inadvertent transfer of toxic or nuisance phytoplankton with shipments of bivalve spat was demonstrated in a preliminary study by Scarratt et al. (1993).

Fisheries regulations vary greatly in different parts of the UK. The regulatory considerations in terms of mussel fisheries management are complex and a reasonably comprehensive summary is presently in preparation (Wilson, in  prep). Useful information on Scottish Legislation is given in McKay & Fowler (1997). The following very brief summaries draw on these plus personal communications from D. McKay (Scotland, England & Wales) P. Dare (England & Wales), J. Andrews (England & Wales) and B. Magorrian (N. Ireland).

Mussel fisheries may be removed from the public fishery by the granting of Several and Regulating Orders¹ by MAFF to commercial companies or to Sea Fisheries Committees, who in turn grant licenses to commercial companies or individuals. These may extend for up to 60 years in some instances, although 30 years is more usual. Several Orders grant the rights to mussel fisheries to individuals or companies and are issued principally in those areas where relaying and ongrowing of seed mussels is important. Regulatory Orders allow the granting of numerous licenses to individuals or companies to fish an area for mussels but give no control over where, or how much, any individual fishes, although they may stipulate the method of fishing. Hybrid orders - essentially Regulating Orders where the grantee has the power to assign Several fishery plots within the area - can also be assigned.

A fundamental consideration with Several and Regulating Orders is that the holder is obliged to manage the fisheries in such a way as to develop the fishery.

¹Several, Regulating and Hybrid Orders may be assigned for any shell fisheries including crustacea

In many places Sea Fisheries Committees may also pass by-laws which can cover public fisheries and regulated fisheries, for example to regulate the size of mussels taken, to regulate physically damaging activities such as pipelaying, or to prevent fishing altogether at times of low stocks.

The majority of all mussels are managed by the Crown Estate Commissioners who grant leases for their exploitation. Few such licenses have been issued in recent years, and SNH have been consulted on all applications. In many cases the Crown has ceded rights of ownership to others such as local landowners, for example in the Dornoch Firth where the rights to manage and exploit mussels is now held by the Highland Council. There is no right of Public Fishery for mussels. Although Several and Regulated Fisheries Regulations were extended to cover Scotland in 1986, it is thought that these may have little, if any, relevance to mussel fisheries in Scotland due to the absence of a public fishery.

Any registered fishing vessel is entitled to fish for mussels in NI waters. DANI awards licences to individuals or companies to relay seed for ongrowing, and to subsequently dredge the mussels. Closure of wild mussel beds because of low stocks is not a management option which is presently pursued, mainly because there is no legal mechanism currently in place.

In all cases there are also stringent public health regulations with regard to the sale of mussels for human consumption, but these are not considered here.

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