Skates
Fishery
Skates are an important by-catch in otter
trawl, beam trawl and gill net fisheries all around the UK. There
are also some directed skate fisheries (e.g. gill net, tangle nets
and long line) in some coastal areas, including in the Thames
Estuary, and off the coasts of Norfolk, Cornwall, Devon and Wales.
These directed fisheries may be seasonal, targeting skates during
the spring and early summer when they may aggregate in inshore
areas for spawning. Skates are also important target species in
some recreational and charter boat fisheries. Currently, skates are
landed either gutted or as skinned or unskinned wings. Though some
fisheries may target certain species, landing statistics do not
currently differentiate between the various species.
Biology
Over 20 species of skate occur on the UK shelf in the
surrounding deep-water areas, though several deep-water species are
only known from a few specimens. Skates are long-lived, slow
growing, and are oviparous, with females laying egg cases (mermaids
purses) on the sea floor. Fecundity varies between species, though
is less than 100 eggs per year. The embryos develop over the
following months, and then hatch at lengths of 10-20 cm, depending
on the species. Many species are relatively large, growing to about
1 m in length, with the largest species (e.g. common skate Dipturus batis and
white skate Rostroraja alba) growing to over 2 m, and the
smaller species (e.g. blue pygmy skate Neoraja caeruleai)
growing to about 30 cm.
Juvenile and small-bodied skates tend to feed predominantly on
small crustaceans (e.g. amphipods, shrimps and small crabs), with
larger skates feeding on either large crustaceans or fish. Some
skate species, such as the cuckoo skate Leucoraja naevus
develop sharply pointed teeth as they grow and then feed on fish,
whilst other species, such as spotted skate Raja
montagui, tend to retain their molar-like teeth and
feed mainly on shrimps and crabs.
Several species of skate have been tagged and released.
Juveniles tend to be recaptured quite close to the release
position, which would indicate that they have a restricted home
range, though larger fish may move more widely, which has also been
observed in studies using electronic tags. More targeted
tagging experiments could usefully be undertaken to clarify the
nature of skate stocks.
Within inshore waters, the most abundant skates are thornback
skate Raja clavata, spotted skate R. montagui and
blonde skate Raja brachyura, and these species may occur
around much of the British Isles. Some species have a more
restricted distribution, with smalleyed skate Raja
microocellata most abundant in the Bristol Channel and English
Channel, and undulate skate Raja undulata observed most
regularly in the English Channel. Further offshore, cuckoo skate
L. naevus are common in the North, Irish and Celtic Seas
and off North-west Scotland. The related shagreen skate
Leucoraja fullonica and sandy skate Leucoraja
circularis also prefer offshore grounds, occurring along the
edge of the continental shelf. Starry skate Amblyraja
radiata is one of the more common skate species in the North
Sea.
Some skates have declined in European seas. For example, common
skate is now observed only occasionally in the Irish Sea, southern
North Sea and other inshore waters where it was once encountered
regularly, though it is still caught on offshore fishing grounds.
Historically, white skate was targeted in coastal fisheries in
southern and western areas, though there are few recent records of
this species.
Management and Advice
Since 1999, the various species of skate in the North Sea (ICES
sub-area IV) and EC waters of the Norwegian Sea (ICES Division IIa)
have been managed under a single TAC for "skates and rays".
ACOM also provides advice on skates in this area
Some Sea Fisheries Committees have bylaws giving a minimum
landing size (MLS) for "skates and rays".