The IUCN/SSC Shark Specialist Group
Shark News 12: November 1998
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Fisheries Effects and Management of North Sea Rays
Paddy Walker, Netherlands Institute for Sea Research, Texel
Long-term trends in abundance and changes in growth and maturation
have been identified in the ray populations in the North Sea (Walker
1998). Although it is not possible to attribute these unambiguously to
effects of fishing, there is certainly circumstantial evidence that
exploitation has played a major role.
Species Composition, Growth and Maturation
Photo: P. Penning
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The species composition of ray populations has changed in such a
way that those species most sensitive to enhanced mortality (e.g.
common skate Raja batis) have severely declined in numbers, while
more resilient species (e.g. starry ray R. radiata) have increased
(Walker and Hislop 1998). Changes in growth and maturation have
been ascertained for thornback R. clavata and starry rays which are
indicative of a decrease and increase, respectively, in density (Walker
and Witte, submitted). These observations fit expectations predicted
by matrix modelling (Brander 1981, Caswell 1989).
Size
Additional evidence for fisheries effects can be seen in the change in
length distributions. The length distributions of all species, with the
exception of the starry ray, have shown a shift over time, with a
paucity of fish above 80 cm now, whereas individuals of 100 cm and
above used to be common (Walker & Hislop 1998). This indicates a
major increase in total mortality, which is probably attributable to
fishing. Moreover, this has meant a loss of all or some of the
reproducing females for the larger species (common skate and
thornback ray).
Catch Composition
Official catch statistics decreased during the 1930s and again between
1955 and 1975, following a period of recovery during the Second
World War. Since the mid-1970s catches have remained stable
(Walker & Heessen 1996). As the common skate has become virtually
extinct, this catch supposedly consists largely of thornback rays and
to a lesser extent of spotted R. montagui and cuckoo rays R. naevus.
According to the surveys these three species have very limited
distributions, which suggests that the fishery is able to maintain
stable catches due to local strongholds in the population. Neverthe-less,
large areas have become void of the larger rays, which may be
primarily caused by the extensive demersal fisheries in those areas.
Strongholds as Sources of Recruitment
The idea of strongholds, or sources, within the North Sea, where
mortality (or emigration) is lower than natality (or immigration)
(Pulliam 1988) is an appealing one from the point of view of
replenishment of exploited stocks. The topography of the North Sea
is highly heterogeneous and there are areas which are difficult to fish.
Thornback rays, for example, are still found between the banks off the
east coast of Britain and in deep, stony pits (i.e. Silver Pit). These and
similar areas could function as sources of recruitment to the more
exploited areas, but the very characteristic of a source (birth rate >
death rate) makes it difficult to identify with classical methods.
This concept of sub-populations with different demographics,
within the entire area of distribution of a species, needs careful
evaluation before it can be ascertained to play a role in the North Sea
ray stocks. The successful dispersal of individuals from one sub-population
to the other is a necessary condition for the continued
existence of the species in all suitable habitats.
In the case of individuals of the source population being removed
faster than they disperse to the sink (e.g. by fishing), stocks decline
(Pulliam 1988, Dias 1996). Moreover, the rate of movement from
source to sink may be dependent on population size in the source
(Holden 1974, Pulliam 1988). Therefore, if the source population is
reduced then dispersal (and immigration to the sink) will also be
reduced. Moreover, if the balance of birth and death rates is tipped,
the source may become a sink and the entire stock can decline, unless
another source is close by.
This illustrates the importance of identifying sources and sinks,
and protecting at least the sources to maintain the spatial mosaic of
habitat-specific demographic rates. Although abundance and
migration data may indicate where particular sources may be found,
information on local demographics is needed to identify the
relationship between local sub-populations.
Fisheries Management Options
Tagging experiments indicate that rays are quite sedentary and form
local sub-populations with limited exchange of individuals (Walker
et al. 1997). This suggests that these local populations can be
effectively protected by restricting fishing activities (closed areas).
The current information on distribution and movement of rays may be
used in selecting particular hotspots (Walker et al. 1997, Walker
unpublished).
This will not, however, bring back the ray populations in other
areas, where they once occurred regularly, because their decline or
disappearance is mainly due to the total effort of the demersal
fisheries. Consequently, to boost ray populations in the entire North
Sea and improve conditions for the ray community, a significant
reduction in fishing effort would be required.
In January 1998 a precautionary TAC (total allowable catch) for
skates and rays (all species) of 6,060 metric tonnes was introduced in
the North Sea. This level is based on landing statistics from the past
5 years. The precautionary nature of the TAC is from the point of view
of allotment of fishing rights in the North Sea, and not necessarily from
a biological perspective (ICES, in press). It is unlikely that the TAC will
have a positive effect on ray population size. In this respect an
embargo on ray landings would be the most effective way of reducing
the mortality on the species.
Need for Urgent Action
The current level of mortality experienced by skate, thornback and
spotted rays, is higher than that of the replacement mortality. The
status of ray and skate stocks in the North Sea as identified by the
ICES Advisory Committee for Fisheries Management (ICES 1998) ranges from almost extirpated (common skate) to within safe biological
limits (starry ray). The thornback ray and spotted ray stocks are
estimated to be outside safe biological limits, and the cuckoo ray only
marginally above (ICES 1998). Time is pressing, because if these
stocks are not managed soon, some species may disappear completely.
References
Brander, K. 1981.Disappearance of Common Skate Raia batis from Irish Sea.
Nature, 290: 48-49.
Caswell, H. 1989. Matrices in Population Biology. Sinaeur Associates,
Sunderland, MA, USA, 328 pp.
Dias, P.C. 1996. Sources and Sinks in Population Biology. TREE 11: 326-330.
Holden, M.J. 1974. Ray Migrations - Do Bigger Eggs Mean Better Dispersal?
Proc. Challenger Soc. 4(5), p. 215.
ICES 1998. Report of the ICES Advisory Committee on Fishery Management,
1997. ICES Coop. Res. Rep. No. 223.
ICES, in press. Report of the Study Group on Elasmobranch Fishes.
Pulliam, H.R. 1988. Sources, Sinks and Population Regulation. Am. Nat.
132(5): 652-661.
Walker, P.A. 1998. Fleeting Images: Dynamics of North Sea Ray Populations.
PhD Thesis, University of Amsterdam, 145 pp.
Walker, P.A. and H.J.L. Heessen, 1996. Long-term changes in ray populations
in the North Sea, ICES Journal of Marine Science, 53: 1085-1093.
Walker, P.A. and J. Hislop, 1998. Sensitive skates or resilient rays? Spatial and
temporal shifts in ray species composition in the central and north-western
North Sea between 1930 and the present day. ICES Journal of
Marine Science 55: 392-402.
Walker, P.A., Howlett, G. and Millner, R. 1997. Distribution, Movement and
Stock Structure of Three Ray Species in the North Sea and Eastern English
Channel. ICES Journal of Marine Science, 54: 797-808.
Walker and Witte, submitted. Variation in Age and Size at Maturity of North
Sea Rays in Relation to Growth.
Paddy Walker, Netherlands Institute for Sea Research,
PO Box 59, 1790 AB Den Burg, Netherlands.
Fax: +31 222 319674. Email: paddy@nioz.nl
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