Common Skate Tagging
The following paper was produced to give information on the Common skate fishing in The Sound of Mull. A updated version will soon be available, as will the information on the full Skate and Tope tagging programme.
Sound of Mull Information as at 30/11/94
Tagging began in the Eastern end of the Sound of Mull in the spring of 1993 as part of the wider programme to study the behaviour of Dipturus batis off the West coast of Scotland. The Sound of Mull is ideal for a tagging study because it appears to hold a relatively small, localised and mainly resident population of skate. This means that, in theory at least, it should be possible to tag a high proportion of the population.
Release & Results
To date, mid-October 1994, a total of 57 Common skate have been reported tagged in the Sound of Mull. Of these, 19 individual fish have been recaptured (all to anglers) representing a recapture rate of 33.3% which is slightly higher than the overall recapture rate for the whole programme. Several fish have been recaptured on more than one occasion, i.e. one fish has been recaptured 4 times, two fish 3 times and 5 fish twice. Periods of liberty between captures have ranged from 1 day to 17 months
Composition of Catches, by Sex
A breakdown of the catches shows that the larger female skate outnumber the males by approximately 3 to 2. This simple ratio dose not tell the whole story however. In springtime, males and females are found on the local fishing grounds in fairly equal numbers but during the summer months the males seem to disappear from the immediate area before reappearing again in September / October. This pattern of behaviour has been very noticeable in both 1993 and 1994. It sees likely that the seasonal migration of males is in some way related to the breeding cycle. Most of the fish caught have been sexually mature, with only 7 immature specimens turning up, six of which were female. The majority of captures of these immature fish have been in April and May, after which they seem to disappear until November/December. No fishing has been carried out in January to March because of the unpredictably of the winter weather.
All recaptures of skate tagged in the Sound of Mull have been made within the release area suggesting that most Common skate show a strong site affinity i.e. remain within the same area. However, the programme since 1974 has shown some individual skate, a very small proportion of the total, have displayed a tendency to wander over quite long distances and it remains to see where some of the Sound of Mull specimens might turn up in the future. Approximately 56% of the recaptures have been female fish which have been made throughout the year. The recaptured males, however only turn up in the spring and autumn. This certainly seems to indicate that the mature females remain within the Sound of Mull area all year round but the males seem to travel away from the locale in the summer time. Where they go during this period is anyone's guess at present but hopefully with more tagging (and recapturing) this secret will soon be uncovered.
Growth Rates and Maximum weights
The growth rate for male skate in the Sound of Mull area has been difficult to determine because the majority of the males caught so far have been mature specimens, with most being very close to their maximum weight. The smallest male Common skate tagged so far has been 65lb (not recaptured), but the majority have been around the 'normal' ceiling weight of between 110 and 120lbs with one specimen being close to the 'absolute' ceiling weight at 134lb. Until some more smaller males have been tagged and subsequently monitored, it will be impossible to determine the growth rate of the male Common skate in the Sound of Mull area. The female skate are also presenting us with a bit of a problem. Their growth rates appear to be falling within the range determined by previous tagging work (i.e. 10 to 30lbs per year) but the growth is not constant. From data available to date, it appears that female skate remain at a given weight for a considerable period of time but then they put weight on quickly before the rate of growth slows down and it enters another period of zero growth. Just how long these periods of zero growth last for, is at the present uncertain. This 'stop-go-stop' growth pattern is possibly linked to the reproduction cycle. One fish put on 30ls in the space of four months in the summer of 1994, but then remained at 190lb for nine months. This particular specimen has not been caught for approximately six months and we eagerly wait its next capture to see what weight (if any) it has put on. Another fish remained at a constant weight of 150lbs for 3 months (summer 1993) but then over the winter she put on 35lbs. These dramatic increases in weight occur in both summer and winter and on several occasions have been accompanied by a heavy swelling towards the rear of the dorsal surface. this 'phenomena' has so far not been observed in male fish which suggest it is not connected by feeding but rather is connected by reproduction. The puzzling aspect of female growth rates will take a few more years to fathom out. It is wrong to place to much credence on one or two years figures because we may be dealing with something simple like a particularity good year for fodder species, or any one of the many variables that effect skate. Far better weight a few years and then look at the overall picture. This will overcome any 'blips' and take better account of the fact that Common skate breed only every other year. What the possible ceiling weight for female skate remains unknown, but as fish have been taken locally to 210lbs, we can reasonably assume that the 'absolute' ceiling weigh could be around 250lb, and perhaps even higher. It is known that Common skate have been taken commercially to over 400lbs in deep water to the north and west of Britain but a more realistic figure for inshore waters would probably be 300lbs. The smallest skate taken so far have been 3.04lb and 8.04lb suggesting that breeding takes place in the local area.
From the data collected to date, the size of the population of Common skate in the Sound of Mull is estimated at between 60 and 90 adult fish. This is not a large population in view of their slow reproduction rate and late maturing of the species. The loss of even a small number of fish could have serious consequences for the viability/ stability of the local population of Common skate. These fish are becoming rarer and indeed in some areas they have become if not biologically extinct, then very close to it.
THEY NEED OUR PROTECTION - NOW!
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