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Tagging of Common Skate (Dipturus Batis) off the west coast of Scotland between 1974 and 2000 Map of Main tagging Area Glasgow Museums Tagging Programme Preliminary observations December 2000 Map showing the skate fishing areas around the Isle of Mull. (Areas are coloured to assist with reading tables and diagrams contained in report) Tagging of common skate (Dipturus batis) off the west coast of Scotland between 1974 and 2000 Preliminary observations of data from Glasgow Museums Tagging Programme Introduction This tagging programme traces its roots back to 1974, when anglers in the Shetlands voiced concerns about the declining stocks of common skate (Raja batis) in their local waters. During the early 1970's, several hundred large skate had been killed, to supply a local fish meal factory, and there was a real danger that the local population would be decimated if the fish were found to be permanently resident in the area. Results from a tagging programme run locally indicated that the skate were, in the main, resident which in turn made them extremely vulnerable to over fishing. Unfortunately, for various reasons - including the age-old belief that the seas would keep on providing, little or no conservation effort took place. There was also the commercial aspect in that a charter skipper's reputation was only as good as his last big fish and there was considerable 'promotional mileage' to be gained by parading a large skate carcass in front of crowds of gawping tourists! In order to finally confirm (or refute) the findings of the Shetland tagging scheme, Dr. Deitrich Burkel initiated a tagging programme on behalf of Glasgow Museums. He chose to concentrate on the grounds off the Isle of Mull, which at that time were in the process of being opened up and explored by two very conservation minded angling brothers, Brian & Duncan Swinbanks working out of Tobermory. This was an ideal situation because there was no specific commercial fishing interests targeting the skate and the brothers would be out angling for them on an almost daily basis and most importantly - they were returning every skate to the water unharmed. Here we had almost virgin waters with fish, which were not going to be put under any additional pressure from the anglers who were specifically targeting them. Their only real threat would be to those caught as a commercial by catch by boats pursuing other target species. The scene was set! The Tagging System The Results To date, 30th November 2000, a total of 946 common skate have been tagged. Of these, 216 were tagged using the Jumbo Rototags and 730 using the Floy Dart Tag. Recaptures are 56 and 174 respectively giving a total number of recaptures of 230 or 24.31% of those tagged. The recapture rate for both phases was relatively similar at 25.93% and 23.84%. Skate have now been tagged at 9 specific locations on the west coast with by far the greatest concentrations being around the Isle of Mull. The locations are as follows: West of Mull; The Sound of Mull; East of Mull (Firth of Lorne); Loch Sunart; The Crinan Basin; Shieldaig; Oldany Island near Lochinver; Loch Roag on Lewis and Stornoway. The breakdown of capture/recaptures is as follows:
Tagged Recap %
West of Mull (Phase 1) 209 41 19.62% West of Mull (Phase 2) 138 18 13.04% Sound of Mull (Phase 1) 1 1 100.00% Sound of Mull (Phase 2) 290 97 33.45% East Mull (Phase 2) 213 52 24.41% Loch Sunart (Phase 2) 21 3 14.29% Crinan Basin (Phase 2) 21 0 0.00% Shieldaig (Phase 2) 4 0 0.00% Oldany Island (Phase 2) 6 0 0.00% Loch Roag (Phase 2) 1 0 0.00% Stornoway (Phase 1) 7 0 0.00% No Release Data 35 18 51.43% Totals 946 230 24.31% There are a couple of anomalies contained within these figures, which need to be explained before any serious discussion can take place. The most important one, in terms of numbers, is the high recapture rate for fish where no release data is available. This was simply because until we had received the recapture details, we had no idea this particular fish had been tagged. We have to rely on skippers/pleasure anglers to tag fish for us voluntarily and they sometimes lose them or quite simply forget either to take a note of the fish's details or to tell us about them! However, for the most part, they play a very important part in our studies and we cannot be too critical of them. The other anomaly is in the Sound of Mull figure for Phase 1 showing a recapture rate of 100%. However with only one fish tagged, there was reasonable chance that it would be recaptured at some point. A comparison of the phases 1 & 2 figures for west Mull skate shows that the former showed a recapture rate, which was 50% higher than the latter. Initially this figure disturbed us because we had been told locally that the commercial fleet had been active to the west of Mull, trawling for cod in the vicinity of Tiree & Coll, and our fear was that the population may have been damaged due to the skate featuring as a by catch of this fishery. However, closer examination of the facts showed that the Swinbanks brothers had given up their charter boat business in the early 1990's and as a consequence no one was targeting the west Mull skate any more. Until someone decides to fill the Swinbanks' shoes and work the grounds to the west of Mull, we can only speculate on the real reason for the much lower recapture rate. Probably a combination of both reasons is responsible but if so, we would be disappointed with the commercial fishing boat skippers for not reporting any tagged fish they may have landed. The recapture rate in the Sound of Mull in phase 2 is much higher than that experienced to the west of Mull. This is as expected because the skate angling grounds in the Sound are more concentrated and are mainly confined to the deep trench running from just west of Lochaline village to the lower end of Loch Linnhe (just east of Rudha an Ridire). Almost all the skate angling effort in the Sound of Mull takes place in this one small area. The recapture rate to the east of Mull in the Firth of Lorne, between Kerrera Island and the entrance to Loch Spelve, is in line with the overall average figure for the programme. These grounds are arguably the most accessible around Mull, being within range of both dinghies and charter boats working out of Oban and dinghies, private boats working out of the various marinas, dive centre slipways, etc. The figure for Loch Sunart is well below the average but the fishing effort here is very patchy and seasonal. All the fish have come from two small localised areas but until a more consistent fishing effort is put in this figure will remain lower than the overall average. The Crinan Basin fish have only just been tagged and no recaptures have been recorded so far. At the time of writing, we are not sure what the level of fishing effort is likely to be in this area but it has already turned up an interesting recapture of a skate from another area. A skate tagged off Lochaline in the Sound of Mull turned up 10½ months later at the mouth of Loch Craignish. We await further news from this area. Skate tagged off Shieldaig in Loch Torridon, Oldany Island near Lochinver and Loch Roag on Lewis were taken during one-off trips and it remains to be seen whether any more will be tagged in these areas! The seven skate tagged at Stornoway were tagged way back in 1974 and nothing has been seen or heard of them since. Variation in results between the genders The overall breakdown by gender is as follows: Males Females Unknown
Phase 1 96 (44.4%) 103 (47.7%) 17 (7.9%) Phase 2 250 (34.3%) 464 (63.5%) 16 (2.2%) Overall 346 (36.6%) 567 (59.9%) 33 (3.5%) * Percentages shown are of the total catch. Males Females Unknown Total
Ph 1 23 (24.0%) 28 (27.2%) 5 (29.4%) 56 (25.9%) Ph 2 56 (22.4%) 113 (24.4%) 5 (31.3%) 174 (23.9%) Overall 79 22.83%) 141 (24.9%) 10 (30.30%) 230 (24.31%)
* Percentages shown are recapture rates for each category. i.e. (Number Recaptured/ Number Tagged) x 100 However, the recapture rates in all categories fall within a couple of percent of each other, which would suggest that there are no gender related differences. If we next look at an area-by-area breakdown taking gender into account, there is one glaring difference, which needs to be examined. A full table of the area-by-area results is printed below
If we look at the west of Mull figures, the recapture rates in phase 1 are very similar between the sexes (19-20%). However if we look at phase 2, there is a dramatic difference between the recapture rates of the sexes. Both of these figures are well down on phase 1 (for reasons explained earlier) but the recapture rate for males is 50% lower than females. If we accept the reasons for the 50% drop in recapture rate between phases 1 and 2, then the added 50% drop in male recaptures is a real mystery. Is it because of the fishing method (i.e. targeting larger female fish)? Are commercial boats picking up the smaller males and not reporting them? Are the males moving out of the area?
Below is reproduced a full print out of individual fish that have moved away from their release site. Also reproduced on the following pages is a map showing tagging locations and a schematic diagram showing movement between the fishing areas. Number 1 is tag number 00643. The skipper of the trawler did not report this fish; it was discovered in a consignment of fish that came from Northern Ireland. The fish however could have come from anywhere, including off the west coast of Mull where it was tagged. Number 2 is tag number 00697. This fish was reported as having been taken off the north west coast of Lewis by a French trawler. However, anecdotal evidence at the time suggested that French trawlers had been active to the west of Mull. Much of this activity was of dubious legality! Number 3 is tag number 00729. This fish was reported as having been taken from off the southwest corner of Norway. In view of all the other fish's reluctance to travel any great distance, this record must be regarded as extremely doubtful! In conclusion, the weight of evidence suggests that skate live in static mostly discrete populations with only occasional migrations taking place between them. This makes them very vulnerable to exploitation.
Map of the area around the Isle of Mull showing the spots where skate have been tagged or recaptured. Schematic diagram showing movements between populations Common skate appear at times to form up in small groups, which are made up of fish of both sexes and of all sizes. There are times when angling for skate can become quite chaotic and several fish are hooked at once. One day in particular saw 5 fish (3 females and 2 males) ranging from 36"to 62" in length coming to the boat in very quick succession. This is not an isolated case either, when you catch one skate several others tend to come quickly. Whether these groups are permanent or transitory due to prey abundance is not yet known.
There was one instance where two fish were tagged, from the same boat, within 40 minutes of each other and 25 months later they were recaptured (again from the same boat) within minutes of each other. Interestingly, the tagging location and recapture site were within a few hundred meters of each other. Confirming again the static nature of skate behaviour.
Individual females have been known to put on as much as 33lbs (15 kg) per year and to stretch by between 4 and 6 inches (10-15 cm) per year. The average though is closer to 9lbs (4.1kg) and 2½ inches (6.3 cm) respectively. With the males though it is very noticeable that when they reach approximately 110-120lbs (50-55kg) and 72-73 inches (183-186 cm), their growth rate slows and stops. The graph below clearly confirms these total length limits. Individual specimens, however, will continue to grow beyond these sizes. Males have indeed been caught to approximately 140lbs and females have been caught to around 230lbs. Tag Number 00713 Date Weight Weight gain/Year 24/06/85 124.0 (56.3kg) 29/07/87 158.0 (71.7kg) 16.32lb (7.4kg)/year Date Weight Weight gain/Year 24/06/85 109.0 (49.4kg) 29/07/87 141.0 (64.0kg) 15.36lb (7.0kg)/year Although as mentioned earlier, it is only two fish in 946, it does raise an interesting question. Does this confirm that perhaps these fish did stick together for the intervening 25 months? A similar feeding pattern, leading to a similar growth rate perhaps? One thing to briefly bring up when talking about growth rates, is the fact that a female are always more thickset than males of the same dimensions (i.e. length and wingspan). They are not markedly different up to about 69 inches (170cm) long thereafter the gap widens noticeably until they eventually weigh around 25-30% heavier than similarly sized males. This fact came to light during data collection and calculation of the weight for size tables now used by the majority of anglers in our target area.
Conclusions The major conclusion that must be drawn from all the data gathered so far is that most common skate live their whole lives in static populations, which remain in relatively small geographical areas. Based on this sample of 946 skate, only 23 of the 230 recaptured skate have moved any significant distance. Even this figure includes 3 reported fish whose recapture locations that are dubious. This does not bode well for the regeneration of areas that have previously been 'fished out'. In Orkney (Scapa Flow), Shetland (Whalsay Island & Dury Voe) and Ullapool (Summer Isles & Little Loch Broom) where the stocks were wiped out in the late 60's and early 70's, they show no sign of re-establishing themselves. This after 30 years! This may be a case where man's intervention is necessary. If the species can't regenerate 'fished out' areas for themselves, then it may be necessary to 'seed' areas by either moving small immature fish or fertilised eggs to protected zones within these areas. The days of angling pressure causing damage to local skate populations now appear to be over. The vast majority of anglers would not consider killing them nowadays. The Scottish Federation of Sea Anglers now accept estimated weights for their specimen awards, based on the 'ready reckoner' tables devised from the data derived from Glasgow Museums Tagging Programme. Whether or not there is any need to follow the Irish Specimen Fish Committee example and remove the species from the lists is debatable. Several fish, which were close to or over the existing record, have been landed in recent years but the anglers concerned preferred to return the fish and accept the table's figure. The problem nowadays will stem from commercial fishing interests. With dwindling whitefish stocks there will be increasing pressure on owners, skippers to 'make a living' and this will increase the chances of common skate being targeted. If sandeels aren't safe, what is? The future of common skate is not assured. They have already disappeared from 90% of the British coastline, as have the white skate (Raja alba) from the waters off southern England. However there is a viable population in the waters around the Isle of Mull and with care it can be preserved. It represents the common skate's last, best chance.
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Copyright: Davy Holt 1996 - 2005
