Over-exploitation at the level of regional recruitment
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
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
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.
Over-exploitation at the level of regional recruitment
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).
England & Wales
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
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
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.