The IUCN/SSC Shark Specialist Group
Shark News 5: October 1995
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Sensitive Skates or Resilient Rays? - a North Sea Perspective
Paddy Walker, NIOZ, P.O. Box 59, 1790 AB Den Burg, Texel, NL
At the beginning of the century rays and skates were considered quite
common in the North Sea. For example, between 1907 and 1909 the
biomass of rays and skates caught by Dutch trawlers was as high as that
of whiting, haddock, cod and small plaice. More than 600 tonnes were
landed annually, of which half were thornback rays Raja clavata and
half common skates Raja batis. Nearly 5,000 fishing boats were in
operation; small sailing boats which operated locally. The estuaries of
the rivers Schelde and Maas were regularly fished and in the summer
stingrays Dasyatis pastinaca, as well as thornback rays, were often
caught. There was a lively standing net fishery for stingrays in the
Wadden Sea, which lasted until the 1930s. The stingrays were caught
for their liver oil, a guaranteed cure for ailments such as rheumatism.
Fishermen even soaked their underwear in the oil to protect themselves
frombitterweather. Thornback rays were regular visitors in the Wadden
Sea in the summer months, although there is no evidence that this
species spawned there.
Figre 1. Landings of rays and skates as reported to ICES in thousand metric tonnes.
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The present situation is quite different.The common skate is seldom
caught in the southern and central North Sea and has not been caught
in Dutch coastal waters since the mid 1950s. With the exception of a
few individuals, no rays (thornbacks or stingrays) have been caught in
the Wadden Sea since 1966.In the 1930s fishermen started complaining
about the lack of rays in the Wadden Sea which was then attributed to
the extensive fishing of the stocks in the North Sea. Thornbacks and
stingrays have been rare off the northern coast of Holland since the
1970s, although stingrays are still caught in the estuaries in Zeeland.
The situation in the North Sea as a whole is not much more
optimistic and there has been concern about the status of the stocks of
rays and skates in the North Sea, following a steady decline in landings
since the 1960s. Analysis of landing statistics from the International
Council for the Exploration of the Sea(ICES), which have been published
annually since 1903, shows that landings of rays and skates have
decreased significantly since the beginning of the century, both between
the two World Wars and after WWII (see Figure 1). Present landings
(approximately 4,000 tonnes) are a quarter of what they were in the
1920s. Landing statistics are obviously a poor indication of the actual
abundance of fish. However, considering the increase in fishing effort
which has occurred in the past three decades, the returns would seem
to be decreasing. Unfortunately, because the majority of the landing
statistics are not detailed to species level, it is difficult to identify the
current status of the skate and ray stocks in the North Sea. Although the
North Sea represents only part of the area of distribution for most
species, it is unlikely that local depletion of stocks will be alleviated
from elsewhere at the current level of fisheries exploitation, as most
species appear to be quite sedentary. The Dutch coastal waters form
an example.
Rays and skates represent a by-catch in the beam trawl fisheries
for other demersal fish species such as sole and plaice. Although usually
only the large individuals will be landed for consumption, the size and
shape of the juveniles and their thorniness means that they have a large
chance of being caught. Normally they would be discarded, but
juvenile thornbacks and spotted rays Raja montagui have been known
to be sold at fish markets. There are nine different species of rays and
skates, of which the most abundant (the starry ray Raja radiata) is not
landed for consumption but is discarded. The chances of survival of
discarded rays are unknown. The fact that these species are a by-catch
makes it difficult to take management measures which will protect
them, without affecting the catch of commercially more important fish
species.
The level of exploitation that rays and skates can withstand is
unknown and is species-specific. Possibly the situation is less dramatic
than it appears; some scientists believe that rays and skates are quite
resilient because they are still around (with a few notable exceptions)
despite intensive fisheries. It's a complex problem, and one which
deserves more attention.
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