Marine Protected Areas and Common skate (Dipturus intermedia)
Position paper
Summary
1.
Common skate (Dipturus intermedia) are included on the MPA search feature list for
Scottish territorial waters (Marine Scotland, 2011a). Around the UK, the common skate
is found almost exclusively in Scottish waters. Currently there are no existing protected
area measures that help to safeguard common skate and they have been identified as
a search feature priority in the West MPA region (which lies within OSPAR Region III,
Celtic Seas). The species is widely spread across the region, but appears to be
concentrated in certain deep sea lochs and inshore deep channel areas.
2.
On the basis of the available evidence, it is recommended that two previously identified
search locations are subject to a more detailed assessment against the MPA Selection
Guidelines for common skate. These are Loch Sunart, and Loch Sween and the Sound
of Jura (mapped on Figure 1 overleaf). Replication in the MPA network for common
skate may be attained if both search locations are progressed as MPAs.
3.
Common skate is key to a third party MPA proposal by the Scottish Sea Angling
Conservation Network (SSACN). Encompassing both MPA search locations, their
proposal also covers the Firth of Lorn and Sound of Mull. There is evidence for resident
mature common skate in both of these other areas which could serve to provide
connectivity between the recommended search locations (see Figure 1). The merits of
progressing these additional areas should be discussed at workshop 4.
4.
Loch Sunart and the Loch Sween and the Sound of Jura search locations both
encompass multiple MPA search feature priorities but are considered here primarily in
relation to their potential to contribute to the protection of common skate. The reason
these locations are important for common skate is that the evidence suggests;
5.
6.

significant aggregations of large reproductively mature individuals

high site-fidelity (residency) of individuals to these locations
The evidence used to support Loch Sunart and the Loch Sween and the Sound of Jura
search locations for common skate is:

records of concentrations of large mature common skate as indicated by
recreational angling and trawl surveys

high site-fidelity as indicated by tag-recapture data and an acoustic tracking
study
Significant concentrations of common skate are also presently found in other areas,
notably a large offshore area to the west of Hebrides, an inshore area on the west side
of the Isle of Harris and the inshore waters of Orkney. However, there are currently
insufficient data to determine whether skate in these areas are transient or residential
or if these areas are especially important for a particular life-history stage, such as egg
case deposition.
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Figure 1
Map showing the two MPA search locations on the west coast of Scotland (within
the Celtic Seas OSPAR Region III) that are relevant to common skate, as well as
the two additional areas for common skate covered by the SSACN third party
MPA proposal
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Purpose of document
7.
This position paper has been produced to support identification of MPA search
locations suitable for conserving common skate.
8.
Common skate are a search feature priority in territorial waters of the OSPAR Celtic
Seas Region (fully encompasses the West and South-west MPA regions). Recently it
has been demonstrated that there are two species of what used to be classed
‘common skate’ or Dipturus batis; namely D. intermedia and D. flossada (Inglesias et
al., 2010). In the west of Scotland, the vast majority of individuals are D. intermedia
(Griffiths et al. 2010). The species is listed as ‘critically endangered’ and based on
evidence for site fidelity is considered to be a priority within the following MPA search
locations; 1) Loch Sween and the Sound of Jura, and 2) Loch Sunart. In addition, this
species is key to a third party MPA proposal by the Scottish Sea Angling Conservation
Network (SSACN) for five discrete areas that also include the Firth of Lorn and Sound
of Mull for common skate.
9.
Although not currently considered as MPA search locations, Orkney waters and West
Loch Tarbert on the Isle of Harris, also appear to hold important concentrations of
common skate. The species is also present in the following previously identified search
locations: Windsock; Geike Slide and Hebridean Slope; Skye to Mull; North-west sea
lochs and Summer Isles; Gairloch, Loch Torridon and Northern Inner Sound; Loch
Carron and Loch Kishorn; and, the Small Isles. However, no information is available on
site fidelity for these locations.
10.
Marine Scotland’s Marine Nature Conservation Strategy (Marine Scotland, 2011b)
advocates a three pillar approach to conservation in the marine environment; with the
three pillars representing species-specific measures, site protection and a wider seas
policy approach. There are fisheries restrictions on skate with a zero TAC and
mandatory release since 2009. It is important to emphasise that for the widely
distributed common skate, wider conservation measures (in addition to site protection)
will be required.
Distribution in Scottish waters
11.
A large amount of data are available on the distribution of common skate from research
vessel surveys, tagging studies and catch records. Since the 1920s common skate
have been caught in Marine Scotland Science trawl surveys of the North Sea and west
coast of Scotland. In recent years an average of less than 1 individual is caught per
hour of trawling, although in the distant past at some locations large catches (up-to 150
fish per hour) were recorded. These data concur with Walker and Hislop (1998) that
the species was once common in the North Sea, but since the 1970s has become very
rare there. Across the West MPA region it is still widespread, albeit at relatively low
density. Consequently, it is practical to focus attention on the west region for this
species. Tag-recapture data from recreational sea anglers and recent research
projects suggests there are certain sea lochs and inshore deep channels where large
mature individuals are resident in relatively high numbers. There is also some evidence
that shallow reef areas are used by common skate for laying their egg-cases, however,
there is no evidence to point toward specific sites as nursery grounds for this species
(Ellis et al., 2011). In combination these data sources (Figure 2) have been used to
assess the distribution, abundance and degree of residency of common skate in the
West MPA region.
3
Figure 2
Map showing spatial extent of all sources of data used to assess distribution and
abundance of common skate in Scottish waters. Each dot represents a survey
station or observation record. Also shown are the previously identified MPA
search locations (see legend for details)
Perceived role of MPAs for common skate
12.
The common skate (D. intermedia) is a large (up-to 285 cm long and over 100 kg),
long-lived (in excess of 50 years), late-maturing species (> 11 years at first maturity)
with low fecundity. Around the UK it is found almost exclusively in Scottish waters
(Griffiths et al., 2010) and although once widespread across the northern North Sea is
now largely confined to the West MPA region. It is vulnerable to trawl and long-line
fisheries and has shown a marked decline and significant range contraction over the
past century, largely as a consequence of removal by fisheries (Walker and Hislop,
1998). Protection from fisheries is therefore a key means of conserving this species
and closing areas may confer significant benefits. The degree of protection arising from
closing an area, however, will depend on the level of site fidelity of fish throughout the
period they are vulnerable to fishing gears (Polunin, 2002). As common skate are
vulnerable to fishing gears from the time they hatch out of their egg case this means
that protection of the entire life cycle may be needed. There are no directed fisheries
for common skate. Landings data since 2008 suggest minor by-catch (a total of 18
tonnes between 2008-2011) of common skate, mainly from the sea area to the north
and west of the Hebrides and Orkney which is not within any of the search locations.
13.
An extensive tag-recapture programme first established by Glasgow Museum in the
1970s and more recent electronic tagging programmes (Wearmouth and Sims, 2009)
provides considerable evidence for high site fidelity of mature common skate in certain
sea lochs, inner firths and sounds. As such, area protection may be expected to
reduce fishing mortality on these mature individuals that may compromise a highly
significant component of the breeding biomass (OSPAR, 2010). Importantly there is
also evidence for occasional straying between areas suggesting that there is the
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potential for the species to re-colonise its former range. For a widespread species such
as common skate that is distributed far beyond all of the search locations, an effective
conservation programme should also include other fisheries management measures.
Evidence base
14.
MSS trawl survey data for the west coast is available from 1925 (a total of 1,580 hauls
recorded common skate). However the frequency, time of year and the types of survey
gears and methods were variable over this period until 1986. It is nevertheless useful
for establishing the distribution of the species and for giving a general insight into the
range contraction of the species over the past century (Figure 3).
Figure 3
Maps of the distribution and abundance (not standardised) of common skate
based on research vessel surveys for each decade over the past 90 years.
Green areas are the previously identified MPA search locations
15.
Since 1986, when the International Bottom Trawl Survey quality standard was
introduced for the west coast area, consistent data have been collected for the first and
last quarter of each year. These data allow for better definition of presence and
abundance and more reliable estimates of abundance and density (Figure 4).
16.
Other sources of recent data include a series of inshore trawl surveys undertaken
between 2003-2005 and an ad-hoc survey in the Firth of Lorn area undertaken in 2010
by SAMS (Figure 5). These provide important information in those inshore areas
(including sea lochs) not covered by the IBTS surveys.
5
Figure 4
Map showing the presence and absence of common skate on the west coast of
Scotland based on data from the International Bottom Trawl Survey during the
period 1986-2008. Yellow points = presence, green points = absence. Previously
identified MPA search locations also indicated
Figure 5
Map showing presence and absence of skate based on data collected in ad-hoc
surveys of the inshore areas of the west region in the 2000’s (green points =
absence, yellow points = presence)
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17.
Over 2000 presence records are available from a variety of sources amalgamated by
SNH (Figure 6). A further key source of information is the extensive tag-recapture
programme (~2000 individuals tagged and released) that the recreational sea angling
community have practised since the 1970s (Figure 7).
Figure 6
Map showing extent of records of skate gathered by SNH (presence only)
Figure 7
Map showing main area of tag-recapture data collected by Glasgow Museum
and SSACN. Green points are release sites, yellow points recaptures. See
legend for MPA search location symbology
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18.
This data derives mainly from the area that includes the Sound of Mull, Loch Sunart,
the Firth of Lorn and the Sound of Jura. The tag-recapture data provides strong
evidence for a high degree of residency with very occasional exchange between areas.
Only recently (since 2008) have landings been reported to the species level for skates
and rays and so catch statistics of common skate are limited to the past 3 years
(Figure 8).
Figure 8
19.
Map showing proportional size of landings of common skate per ICES rectangle
for the period 2008-2011 (total landings were 18 tonnes)
Table 1 summarises the evidence base for each of the types of areas that could be
used to help contribute to the protection of common skate and recommends which
should be considered further in terms of supporting identification of MPA search
locations for common skate.
Table 1
Summary of the evidence base for the types of areas that could be used to help
contribute to the protection of common skate in Scottish waters
Type of area
Evidence base
Recommendation
Concentrations of mature
residential individuals
Tag-recapture from anglers
and telemetry data shows
long term site fidelity
OSPAR recommends protection of the
reproductive component of stock;
reducing mortality on large mature
females may be the most effective way
of achieving this
Areas where egg cases
are found in high density
(equivalent of spawning
grounds)
Observational records from
divers
Protection of egg cases from physical
disturbance may increase survival to
hatching (vulnerable life-history stage)
Areas of high density
Trawl survey data indicates
some areas of higher than
average density
Until further information is available on
site fidelity, need to rely on wider
conservation measures
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Type of area
Evidence base
Recommendation
Areas of high juvenile
density (nursery grounds)
Trawl survey data do not
indicate that there are
geographically discrete
nursery grounds for common
skate (< 50 cm long)
Not considered further
MPA search location proposals
20.
The following search locations all fall within the distribution of common skate based on
survey data: Windsock; Geike Slide and Hebridean Slope; Skye to Mull; North-west
sea lochs and Summer Isles; Gairloch, Loch Torridon and Northern Inner Sound; Loch
Carron and Loch Kishorn; Small Isles; Loch Sunart; and, Loch Sween and Sound of
Jura. Table 2 summarises each area. Of these locations, Loch Sunart and the Sound
of Jura appear to be key sites for residential groups of mature common skate. The tagrecapture data provides the strongest evidence for this. In the Loch Sunart / Sound of
Mull area, out of over 500 skate tagged and released, 38 % were recaptured. Of the
recaptures 90 % were from the release area, with the remaining 10 % recaptured from
the neighbouring Firth of Lorn. In the Sound of Jura, out of 170 individuals tagged and
released, 10 % have been recaptured, all of which were tagged and released in this
area. Some individuals have been recaptured multiple times over many years providing
strong evidence for very limited home ranges in mature individuals.
21.
In October 2011, 20 large common skate were fitted with acoustic transmitters in the
Sound of Jura. A series of receivers were placed throughout the Loch Crinan area and
Sound of Jura. The first data were recovered in February 2012 from 2 of the receivers
in the area. Between these 2 receivers all 20 skate were detected in the first month
after release and approximately half of those were still being detected in February. This
indicates that those individuals have largely stayed within 2 kilometres of where they
were released. Further data will be needed to be recovered to confirm how far afield
the remainder of the tagged individuals had moved.
22.
The same tag-recapture programme provides similar strong evidence for residential
groups of common skate in the Firth of Lorn (an area covered by the SSACN third
party MPA proposal). It is possible that the Sound of Mull and Firth of Lorn are part of
the same population unit. The MSS inshore trawl survey data independently confirmed
the presence of common skate in Loch Sunart. It should, however, also be noted that
this survey sampled a number of other sea lochs including Loch Hourn where the
presence of common skate could not be confirmed. This suggests these large mature
skate are quite specific to particular areas rather than being generally associated with
deep sea lochs. For the remaining search locations, while there is evidence for the
presence of common skate, including large mature individuals, there is no information
on whether they are resident. It is possible that some groups of common skate are
vagrant and migratory while others are and residential and site-specific. Evidence from
Data from the Orkney Skate Trust suggests the Hoy MPA search location should also
be reconsidered for common skate.
23.
Common skate are found in association with ‘shelf deeps’ habitat, e.g. Sound of Jura.
They may also use shallow reef habitat for laying their egg-cases as is reported by the
Orkney Skate Trust and SNH. However, there is little further evidence for functional
relationships with other priority feature habitats or species.
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Table 2
Summary of MPA search locations and third party MPA proposals within which
the status of common skate was assessed; rows in bold text are those proposed
search locations for common skate, rows in plain text are areas not
recommended for progression on the basis of common skate alone
Location
Other mobile species
MPA search features
Comments/evidence
Loch Sunart
n/a
Strong evidence for resident
mature individuals
Loch Sween and Sound of
Jura
n/a
Strong evidence for resident
mature individuals
Firth of Lorn and Sound of
Mull (third party proposal)
n/a
Evidence for resident mature
individuals
Windsock
n/a
Presence of mature individuals
Geike Slide and Hebridean
Slope
Basking shark
Presence of mature individuals
Skye to Mull
Basking shark and minke
whale
Presence of mature individuals
North-west sea lochs and
Summer Isles
n/a
Presence of mature individuals
Gairloch, Loch Torridon and
Northern Inner Sound
n/a
Presence of mature individuals
Loch Carron and Loch Kishorn
n/a
Presence of mature individuals
Small Isles
n/a
Presence of mature individuals
Hoy
n/a
Presence of mature individuals
Issues and opportunities
24.
Two further areas, the waters around Orkney and West Loch Tarbert on the Isle of
Harris have been suggested by recreational anglers and conservation organisations as
areas of particular significance for common skate.
25.
Replication in the network for common skate may be attained if both Loch Sween and
Sound of Jura, and Loch Sunart MPA search locations are progressed. The third party
proposal for the larger area that also encompasses Firth of Lorn and the Sound of Mull
could provide linkage between these site locations. A greater degree of replication
around Scotland would be achieved if the Orkney and West Loch Tarbert sites turn out
to contain critical habitat for common skate or contain mature resident groups of fish
and are therefore progressed in the future.
Next steps
26.
The concurrent acoustic tracking and data storage tag project is monitoring the
movements of 20 common skate in the Loch Crinan / Sound of Jura area. This will give
better definition of the degree of residency and individual home ranges of the common
skate found in this area. Preliminary results have been obtained but full results will not
be available until October 2012. Further electronic tagging using data storage tags will
be undertaken in other areas, e.g. Loch Sunart, Isle of Harris, Orkney and more
offshore areas over the coming year. An extensive analysis of the SSACN tagrecapture database is currently being undertaken to elucidate individual movement
patterns, whether dispersal is sex biased and to assess factors such as growth rates
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and survival. Further analysis is needed of the survey data so that area-weighted
densities can be computed and spatial and temporal trends in abundance mapped.
Data from other EC member states research vessel surveys (via ICES DATRAS
database) will be collated to further add to the distribution database. Information on
estimation of discards of common skate will also be extracted from the MSS observer
programme database.
References
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McHugh, M., Neat, F.C., Pade, N.G., Queiroz, N., Serra-Pereira, B., Rapp, T.,
Wearmouth, V.J., Genner, M.J. (2010). Molecular markers reveal spatially-segregated
cryptic species in a critically endangered fish, the common skate Dipturus batis.
Proceedings of the Royal Society B, 277, 1497-1503.
Iglésias, S.P., Toulhoat, L. and Sellos, D.Y. (2010). Taxonomic confusion and market
mislabelling of threatened skates: important consequences for their conservation status.
Aquatic Conservation: Marine and Freshwater Ecosystems, 20: 319-333.
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pp. 1-18.
Polunin, N.V.C. (2002). Marine protected areas, fish and fisheries. In: Hart, PJB; Reynolds,
JD, ed. Handbook of Fish and Fisheries. Oxford: Blackwell, pp. 293-318.
Walker, P.A. and Hislop, J.R.G. (1998). Sensitive skates or resilient rays? Spatial and
temporal shifts in ray species composition in the central north-western North Sea
between 1930 and the present day. ICES Journal of Marine Science, 55, 392-402.
Wearmouth, V.J. and Sims, D.W. (2009). Movements and behaviour patterns of critically
endangered common skate Dipturus batis revealed by electronic tagging. Journal of
Experimental Marine Biology and Ecology, 380, 77-87.
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