Tope & Common Skate tagging
off the West of Scotland -
39 Knockburnie Road
The Glasgow Museums tagging programme was originally set up in 1974 by Dr Deitrich Burkel as a result of concerns being expressed about the long term viability of common skate Raja batis stocks around Scotland. Fears had been raised due to the dramatic decline in stocks around the Shetland Islands caused by commercial fishing to supply a local fish meal plant. Simultaneously, the once prolific stocks off Ullapool and Orkney were dwindling due, in the main to angling pressure.
When the Shetlanders began their own tagging programme they soon confirmed that the skate were largely resident making them extremely vulnerable to any kind of exploitation. Dr. Burkel wished to see if a similar situation was evident in the rich waters around the Isle of Mull.
In addition, Dr. Burkel decided to initiate a tagging study of his own favourite sporting fish, the tope Galeorhinus galeus. These small sharks are summer visitors to the west coast of Scotland and most weekends would see Dr. Burkel searching the fierce tide races at the Mull of Galloway in the hope of contacting a hefty specimen or two.
Initially the fish, both skate and tope, were tagged using Dalton Rototags. However as more fish were recaptured it became apparent that these tags, with their large flat surfaces, were susceptible to barnacle encrustation. So much so that their use was abandoned in favour of Floy FT-1 Dart Tags in 1988. The lack of flat surfaces on these particular tags has now completely eradicated the problems.
Tope - Galeorhinus galeus
Initially the tope tagging was carried out at two sites, at Luce Bay / Mull of Galloway and off the Isle of Mull. In the last two years however, as more people became aware of the tagging study, tope have been tagged off Wales and the Isle of Wight.
To date, one hundred and eighty-two tope have been tagged, (73 off Galloway; 49 off the Isle of Mull; 30 of Aberystwyth; 18 off Tiree; 8 off the Isle of Wight and 4 off Pembrokeshire). Their weights ranged from 2.3kg (5lb) to 31kg (68lbs).
Fifteen tope have so far been recaptured ( see figures1a & 1b ) giving an 8.3% recapture rate. Of these 10 were released off Galloway and five off the Isle of Mull.
Only one of the recaptured fish turned up in the same area as its original release, a male fish which returned to the grounds to the west of Mull after 765 days. Four were recaptured off the north and west coasts of Ireland, two in the English Channel (one after 12 years at liberty ) and one each from Wales, Cornwall, the Isle of Man, Shetland and the Scilly Isles. The remaining three made long coastwise migrations before turning up off Lisbon, the Canary Isles and Algeria. The Shetland recapture is interesting because it was made in late January 1995 when popular belief would have had this fish many hundred or indeed thousands of Kilometres to the south.
Although very few recaptures have been made, it is already noticeable that fish tagged off the Isle of Mull never venture further south than the south coast of Ireland whereas the Galloway fish are much further travelled. Could this be linked to the fact the vast majority of tope taken off Mull are males which, unlike the females don't need to travel to known pupping areas to the south? Or is it possible that there are two distinct populations of tope, those found off the Isle of Mull being part of an Irish / Atlantic population and those off Galloway being from a Channel / Biscay population!
From conversations with professional west coast skippers, it appears that tope arrive in south western Scotland in mid-May when the first run of predominantly male fish arrive on the offshore grounds. Further north off Tiree, the first sign of tope occurs in mid-July when a run of males arrive. However it is usually late July or even August before these tope, again mostly males but with the odd larger female, arrive around the Isle of Mull. By this time the second run of larger females has also arrived inshore in south west Scotland.
So far there is no evidence of a 'mass migration' to the south in the winter. It has been suggested (Ray & Wilson, 1956) that tope in fact arrive from a westerly direction as the water warms up and not from a southerly direction. This hypothesis when combined with the comments of the skippers seems to indicate that tope arrive from the west but are progressively later in showing up, as the latitude increases. It also appears that the tope tend to hang around later in these northern climes with fish still being present in December / January.
As to whether tope actually breed in Scottish waters, the jury is still out at present. There are a few reports of tope pups in Scottish waters, but all are generally 'hearsay' or 'anecdotal' with no concrete evidence being put forward.
We have just receved a tag return for a male Tope that was tagged on 17/07/94 off the Isle of Tiree. The fish was recaptured 20 miles off the south east coast of ICELAND on 18/01/97 in 220 meters of water by an Icelandic trawler working the Sidugrunn area. More information to come.
Common skate Raja batis
The vast majority of skate tagged so far have been from the productive waters off the Isle of Mull but a few have been tagged off the Isle of Lewis and Lochinver.
To date, 719 common skate have been tagged, (420 to the west of Mull; 159 in the Sound of Mull; 121 in the Firth of Lorne; 11 in Loch Sunart; 7 off the Isle of Lewis and 1 off Lochinver. Their weights ranged from 1.5kg (3lb 5oz) to 102kg (225lb).
One hundred and forty-seven individual specimens have been recaptured giving a recapture rate of 20.5%. Of these, seventy-nine were released to the west of Mull, fifty-five in the Sound of Mull, twelve in the Firth of Lorne and one in Loch Sunart. Several skate have been recaptured on more than one occasion with one specimen having been recaptured 6 times.
Of the 147 recaptured skate, only five have moved more than 20 kilometers from the original release site. Of these, three were taken off the coast of Northern Ireland, one was recaptured 50km north of Lewis and the other turned up 130km off south west Norway.
Figures 2a & 2b shows the movement of tagged common skate recorded during the survey.
The results obtained so far do not lend any credence to the 'old chestnut' that skate move offshore 'en masse' in winter. In fact they appear to be just as thick on the grounds in the colder months as they are during the summer months.
Common skate are long lived, taking many years to reach sexual maturity. However they have relatively fast growth rates of 2-20lb per year for males and 9-31lb for females. Males generally grow up to 115-120lb, with a few growing to around 130lb but they never grow bigger than 140lbs. Females have been caught to 227lb but commercial fishing records show that they are capable of growing to twice that weight! As to what age skate live to, we can only guess at the moment. One fish which weighed 38lb (probably 5 years old) was recaptured after12 years and 4 months, meaning it was over 17 years old. This suggests a life-span of over twenty years. It is hoped that more will be found out about early growth rates of common skate, because several very small specimens have now been tagged. This should in turn enable us to put a more definite figure on the probable life-span of the species.
To further the cause of skate conservation, two charts , one for males and one for females have been produced allowing anglers to estimate the weight of common skate without the need even to boat the fish. Simply measure the length and the wingspan whilst the fish is still in the water, check the sex and refer to the chart. What could be simpler!
For further information about the tagging programme please contact
Art Gallery and Museum
Glasgow G3 8AG
Tel:- 0141 287 2660
Or contact the author ( Bill little) or Myself (Davy Holt)
Rae, B.B & Wilson, 1956. Rare and Exotic fishes recorded in Scotland during 1955 (including analysis of tope landings at Aberdeen). Scottish Naturalist, 68: 106-108