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.  Recent experiences in Orkney and on the west coast at Ullapool had already suggested that this might be the case but now it appeared to be confirmed. During the 1960's, both Orkney and Ullapool had gained an international reputation for the quality of the skate angling to be had there. Visitors travelled from all over Europe to sample these prolific waters.

      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!  As we now sadly know, this situation couldn't continue for long and the Orkney and Ullapool grounds were soon 'fished out' but for once it wasn't commercial fishing which proved their undoing, it was so-called 'sports anglers'.

        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

          Initially, in Phase 1 (1974-1988), JUMBO ROTOTAGS (cattle ear tags) were employed to tag the skate but problems were to become evident when fish started being recaptured after a few years at liberty. Because the Rototag actually encloses the wing of the skate on both sides and can't expand after a certain amount of growth by the fish, the tag then cuts into the fish's flesh causing sores. In view of this, tagging was suspended in 1988 until a better tagging system could be obtained. The tag we then adopted in Phase 2 was the FLOY FT‑1 DART TAG. This is a purpose made fish tag which because it is attached at one end only causes no problems to the host specimen as it grows. At the time of writing this tag is still being employed.

The Results

         To date, 30^th 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. It should also be noted that the angling effort in this area is almost entirely carried out from dinghies, which imposes its own limitations due to safety issues. For example, anglers are loath to venture too far from their only safe haven, the slipway at Lochaline village.

        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:

  TAGGING BREAKDOWN

                        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.

  RECAPTURE RATES

                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

          The first noticeable fact when we examine the tagging numbers for both phases is that females tend to predominate in the catches. This is very much more noticeable in phase 2 where they outnumber the males by nearly 2 to 1. This is in all probability down to the type of fishing effort as the tagging is done entirely by anglers who tend to target the bigger fish (i.e. the females).  Similarly, the recapture rates for females are slightly higher than males, which again could be put down to the fishing methods employed.

        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. The slightly higher recapture rate for the fish where the sex is unknown is explained by the fact, as was mentioned before, that we have no release data (including sex) available for some of them and we only realised they had actually been tagged when we received their recapture details.

        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