Keywords: Baltic salmon, Salmo salar, delayed release, Carlin tagging, recapture pattern, straying.

Transcription

1 International Council for The Exploration of the Sea C.M. 2003/R:04 Danish delayed release experiments with Baltic salmon (Salmo salar L.) by Stig Pedersen and Gorm Rasmussen Danish Institute for Fisheries Research Department of Inland Fisheries Vejlsøvej 39 DK 8600 Silkeborg Abstract A release experiment with salmon (Salmo salar L.) was conducted in the Baltic Sea at the islands of Bornholm and Møn during the years A total of 600,000 salmon smolts originating from two Baltic Sea salmon stocks in Sweden and Finland were released implementing the delayed release technique where smolts were kept in net cages in saltwater for about 3 months before release in August-September. Just before release (approx. 2%) smolts were Carlin tagged. Observed mean recapture rates from all years were 15.8% and 9.0% from releases at Bornholm and Møn, respectively, but with substantial variations between years. Part of this variation could be ascribed to weather conditions at time of tagging and fish size (16-39 cm) of smolts at release. The major part of the stocked salmon were caught in their second and third year (total 82.2 %) of sea-life, the time of catch being a little later for salmon stocked at Bornholm than at Møn. The majority of recaptures were done in gill-nets, followed by long-line. Only a minor fraction of the catch was in fyke-nets or by recreational fishery. Approx % of the stocked salmon were recaptured by Danish fishermen, followed by Swedish, Polish, German and Finnish fishermen. The major part (97.5 %) of the catch was in the Baltic Sea, east and north of Bornholm. The pattern of recapture is compared for the two release sites. A small part of the catch was done outside the Baltic Sea, partly in the Kattegat and the Atlantic Sea and partly in freshwater. Possible straying to Swedish westcoast rivers with outlets to the Kattegat and with wild Atlantic salmon populations is investigated. The releases at Møn and Bornholm have reduced the number of wild salmon caught by Danish fishermen with approx specimen and reduced the pressure on wild salmon with about 11%. With the present Danish TAC (about 90,000) in the Baltic Sea releases (120,000 annually) from Bornholm would reduce the catch of wild salmon in this area with approximately 1,750 salmon annually corresponding to a 18.5% reduction. Keywords: Baltic salmon, Salmo salar, delayed release, Carlin tagging, recapture pattern, straying. Introduction The Baltic salmon was widespread with populations in more than 60 different river systems with outlets to the Baltic Sea (Karlsson & Karlström 1994), and the annual production of smolt were approx. 10 million specimens. Regulation of rivers including building hydropower stations, intensive fishing in fresh- and saltwater, eutrophication, and latest, M-74, have reduced the wild production of salmon. The lowest number was reached in 1995 when only million wild salmon smolt entered the Baltic Sea. The Salmon Action Plan ( ) by the International

2 Baltic Sea Fisheries Commission (IBSFC), and a number of strong year classes has changed this trend resulting in a fourfold increase in production of wild smolt by 2000 (Anon. 2003). The objective of the Salmon Action Plan is to rebuild salmon stocks in all original salmon rivers before 2010, and, at the same time to have a maximal fishery in fresh- and saltwater. Additionally stocking of salmon must consider genetical guidelines, meaning that releases can be carried out only under considerations of the principle of precautionary approach. For many years salmon have been stocked in the Baltic area, mostly as reared smolt in the lowermost end of the river during spring. Some smolts have also been stocked directly into the sea as coastal release or as delayed release. Compared to other methods of stocking salmon smolt, the delayed release method (i.e. smolt raised in net pens in the sea for 2 3 months after the smolt stage) originally described by Novotny (1980) and soon investigated in releases of Baltic salmon (e.g. Eriksson 1991, Anon. 1990), has resulted in higher survival rates and may result in better homing to the release area than releases directly into the sea. The higher survival and possible better return to the release area could be exploited by a fishery on released fish near the release area, thus reducing fishing pressure on wild salmon stocks. The delayed release technique has, however, recently been subject to criticism from Swedish authorities due to an observed increase in straying into western Swedish rivers, which may in turn result in an undesired genetic influence on wild salmon populations (Anon. 2001). During the years a large number of salmon smolt (120,000 salmon per year) were released by the delayed release technique from net-pens close to the coasts of Bornholm and Møn in a project managed by the Danish Fishermen s Association. A minor part of these salmon were Carlin tagged and released together with the rest of the non-tagged fish in order to monitor the releases. The purpose of the project was to examine the possibility for developing a terminal area, i.e. a fishing area close to Møn and Bornholm, where only or mostly stocked salmon would be targeted resulting in a shift from the traditional Danish off-shore salmon fishery, to a fishery in the release area (Terminal Fishery). This development in the Danish fishery could also reduce the fishing pressure (number of fish caught), on the wild salmon in the Baltic Proper. Preliminary results suggest the releases to be successful in terms of catches and also in terms of changing the composition of the catch towards a fishery on reared fish (Hansen & Pedersen 2002). Materials and Methods The salmon used in the experiments treated in this paper originated from the Finnish Iijokki strain (releases , and partly in 1997) and the Swedish Mörrum strain (releases in ), respectively. The salmon were raised from eyed eggs until smoltification at Bornholm Salmon Hatchery in Nexø. In the spring of ,000 salmon were transferred from the hatchery into three net-pens (approx. 18 m diameter) in the sea approx. 500 m from the shore near Tejn (north Bornholm) and 60,000 in three net-pens in the sea at Klintholm, Møn, Zealand, respectively. Fish were fed with pelleted commercial feed while in the net-pens. The fish were released during the second half of August after 3-4 months in the net-pens. Prior to release a minor part from each batch were anaesthetized (chlorobutanole or benzocaine) and tagged with Carlin tags, total-lengths were recorded for all tagged fish and weight recorded for sub-samples. Tagged fish were released together with the rest of the non-tagged fish immediately after tagging and recovery. Tagging took 2

3 place either at sea (i.e. fishing boat) where the net-pens were positioned or in the nearby harbour to where the net-pens were moved, depending on weather conditions. Both professional, recreational fishermen and anglers reported recapture of tagged fish. Reported tags were rewarded 30 DKK per tag. A total of 1,622 tags were returned until 30. April Only few recaptures are expected after this day. Information accompanying these tags is treated in this paper and it is evaluated to what extent the purpose of the releases has been fulfilled. The information accompanying the returned tags varied much in respect of details: year of recapture was known for 64% of the recaptures, while recapture date was known for only 55%. Some general information on recapture locality was know for 91% of the returned tags while more precise information on locality was available for only 44% of the tags. A total of 42% (i.e. 682) of the tags were caught in the Baltic Area but without indication of lacality and these tags were distributed into areas in accordance to the distribution of tags with known area within areas For 8.5% (i.e. 138) tags recapture area were unknown; these are, however, distributed in accordance to the distribution of tags with known area after distribution of the unspecified tags in the Baltic Area. These redistributed tags were all used when calculating the reduction in the Danish catch of wild salmon. Information on size at recapture was available for approximately 55%. Data treatment Variations in recapture rates were examined by examination of length, temperature and handling of the salmon. Average length at recapture was compared to average length at release of individual batches by linear regression, fraction of released salmon with L < 22 cm (Eriksson 1989) was compared to recapture rate. Influence of temperature was analysed by comparing recapture rate to water temperature at tagging. Effect of handling the salmon (towing net-pens to harbour) was analysed by comparing recapture rates with g-tests (Fowler & Cohen). Median day of recapture was compared for the two release sites by a Mann-Whitney W test and year of recapture by Chi-square test. Recapture gear and positions of recapture were compared for strain and release location by Chisquare tests for recaptures combined in types of gear. The reduction in the Danish catch of wild salmon was calculated in the following manner. During the period 1996 to 2002 when salmon from the Danish delayed releases were available for the fishery the average Danish catch of salmon was 87,700 annually. From knowledge of recruitment of wild and reared fish to the Baltic the number of wild salmon caught by Danish fishermen was calculated. Using information on the recapture of delayed released salmon the number of wild fish that would have been caught if no releases had taken place was calculated. From this the reduction in catch of wild salmon was calculated. The development in the Danish fishing effort near the release area (Bornholm) was obtained by calculating the fraction of salmon caught by the professional Danish fishermen filling in log-books, in the four ICES squares around Bornholm (38G4, 38G5, 39G4 and 39G5). The fraction of the catch by Danish fishermen not obliged to fill in log-books (vessels with length < 10 m) was added to this, since these small vessels normally do not operate far from the island. Results and Discussion Data on releases and recaptures are presented in Table 1. The average length of the tagged fish in the different batches varied between 21.3 cm and 30.2 cm, while the total variation in size (i.e. 3

4 range) at release varied between 16 and 39 cm. Recapture rates varied between years and batches from 2.8% to 25.2%, on average being higher from releases at Bornholm (15.8%) than Møn (9.0%). Variation in recapture rate Weather conditions at tagging varied between years; water temperature varied between 13.6 ºC and 23 ºC and strong winds and waves made it necessary to tow the net-pens to harbour for tagging in some of the years. In Figure 1 recapture rate is compared to sea temperature at tagging. A clear tendency to reduced recapture rate from releases at temperatures above 20 ºC is seen for releases at Bornholm. The one exception from this pattern is from the release in 1998, where average size of the salmon was the lowest in all years (21.3 cm). The same pattern is not found for releases at Møn. Towing the fish to harbour may impose additional stress on the fish, in particular at higher temperatures. In Table 2 recapture rate for fish being tagged in the harbour is compared to recapture rate for fish tagged at sea for the two release sites. The recapture rate from releases with additional handling is significantly lower (p<<0.001, g-test) at Bornholm, but not at Møn. Recapture rate is compared to the occurrence (frequency) of fish with L < 22 cm, found to be a critical size limit due to food availability by Eriksson (1989), in the different release batches in Figure 2. For releases at Bornholm a clear tendency to reduced recapture rate is found except for releases in 1997, where water temperature was very high (23 ºC). In Figure 3 the average length at release of individual batches of recaptured salmon is plotted against average length of the batches at release. Dots above the line y = x indicate size-dependant survival. A linear regression of length at recapture (L recapture ) against length at release (L release ) for releases at Bornholm gives the line (1) L recapture = L release (R 2 =0.95, p < 0.001) (regression dotted in the figure). This line crosses the line y = x (full line in the figure) at length 28.5 cm, indicating the minimum average length for released tagged fish, where size did not influence recapture rate. Recapture rate from releases at Bornholm in 1997 was much lower (high water temperature at tagging) than from releases of comparable sized fish in other years. It is believed that the low recapture rate is due to a high mortality in the handled and tagged fish only, since none of the fish being released directly from the net-pens were observed in a weak condition. In order to be able to calculate a reliable estimate of the yield from this release a relation on recapture rate (R) vs. length at release (L release ) was calculated for releases at Bornholm, excluding the release in The following relation was found (2) R = L release (R 2 = 0.69, p = 0.04) This relation was used to find the theoretical recapture rate (R 97 = 17.95% for average length cm) for releases of non-tagged salmon in Handling of the fish during tagging operating will influence stress levels (Sharpe et al. 1998, Hansen & Jonsson 1988) and subsequently survival. The observations of recapture rate being dependant on size at release are in accordance with previous findings (e.g. Eriksson 1989). The exceptions from this is found in the releases in 1997 at Bornholm, when tagging took place at an 4

5 extremely high temperature and at Møn where the reason for observed lower recapture rates is likely to be influenced from lack of tradition for fishing salmon in this area. Length of the released fish has often been related positively to recapture rate (e.g. Eriksson 1989). The present results suggest that other factors may also influence recapture rate. Due to varying conditions at tagging (combined variation in size at release, temperature at release and handling) the variations in recapture rate cannot be unambiguously assigned to one of the possible parameters but is a function of the parameters. The results indicate all of the parameters to be influential, and in addition to these the pattern of fishing (see below) is likely to be important as well. For releases at Møn the pattern of fishing seems to be a likely cause for varying recapture rates. This pattern may include both early recapture of not yet legal sized salmon, leading to additional mortality and reduced recapture rates, if tags are not reported, and it may include lack of targeted fishing for mature salmon near the release area. Both circumstances are believed to be possible for the releases at Møn, where a relatively large fraction of the recaptures happen soon after release (Figure 4). In this area there is not a tradition for salmon fishing and in the area near the point of release fishing for among other eel in large pound and fyke nets is well developed. In general the recapture rate is relatively stable for releases at Bornholm, with the exception of releases in This is in contrast to observations in tagging experiments in rivers in other countries during the same time period, where recapture rates have been declining constantly (Anon. 2003). Time of recapture Date of recapture is known for 690 salmon recaptured from releases at Bornholm and 208 salmon from Møn. The recapture pattern over time of these salmon is illustrated in Figure 4. Initial recaptures tend to happen sooner for releases at Møn, where half of the recaptures were done after 505 days compared to 520 days for releases at Bornholm. Median day of recapture was, however, not significantly different for the two release sites (p=0.35, Mann-Whitney W). Year of recapture for releases in different years are presented in Table 3. The majority of recaptures took place in the second (Møn average 31.6%, Bornholm average 38.0%) and third year after release (Møn average 41.3%, Bornholm average 53.7%). On average a larger proportion of recaptures from releases at Møn take place during the year of release (5.1%) compared to Bornholm (1.1%) (p=0.0014, Chi-square test). Overall, the time of recapture does not differ significantly from previous Danish tagging experiments with salmon in the Baltic (Glüsing & Rasmussen 1996), depending both on the size (catchability) of the fish and fishing season of the Baltic salmon fishing fleet. Gear Recapture gear for the two release sites and strains is presented in detail in Table 4 and for combined types of gear for Møn and Bornholm in Figure 5. For a large part of the recaptures (approx 30%) the gear is not known and for another 20-25% it is only know that recaptures were done by professional fishermen. The composition of gear was quite different between the two locations (p<<0.001, Chi-square test). Salmon released at Møn was reported caught more frequently by sports fishing, in traps & fyke nets and by drift nets, and salmon released on Bornholm was reported caught more frequently by professional fishermen (gear not specified). 5

6 The catch in traps and fyke nets as well as sportsfishing would be related to location of recapture (see below), where recapture of the salmon released at Møn is frequently observed in the inner Danish waters. The reason for the rest of the differences is not known. A large part of the reported recaptures were not accompanied with precise information on gear used. However, in the Baltic Proper the gear by professional fishermen is generally either drift nets or long lines, while there is more uncertainty in the inner Danish waters, where a number of different tackle types are used. Nationality The recaptures were primarily done by Danish fishermen (Table 5) catching on average 45.3% of the tagged fish. Substantial numbers were also caught by Swedish, Polish and Finnish fishermen, while other nationalities caught only minor fractions of the tagged fish. Distribution of recaptures on the two release sites is presented in Figure 6. Recapture locality Recapture localities for releases in the different years at the two localities are presented in Table 6. Recaptures in ICES fishing area 25 dominate the recaptures, also illustrated as an example in Figure 7, where the distribution of the recaptures from releases at Bornholm is found. For 548 recaptures from the releases at Bornholm the position of recapture is known with sufficient accuracy to determine in which ICES square the fish was recaptured. In the four ICES squares positioned around the island of Bornholm (i.e. 38G4, 38G5, 39G4 and 39G5) a total of 192 recaptures (35% of all recaptures) were recorded. A Chi-square comparison of the distribution of recaptures inside the Baltic sea of the two strains of salmon from releases at Bornholm proved differences on a larger scale (ICES fishing areas combined, 25,26, and combined) to be not significant. The distribution of recaptures from Møn was slightly different (p=0.03): the Mörrum strain being caught more frequently to the west of Bornholm (area 22-24) than the Iijokki strain. Comparison of the two release localities showed that the Møn releases to take place more to the west of Bornholm, than the Bornholm releases (p<<0.001). Freshwater recaptures constituted only a minor fraction of all recaptures where location was reported (1.5% of all recaptures). This low fraction was, however, higher than what has been reported previously (0.3%) (Eriksson & Eriksson 1991). The reason for this difference is not known, but could be related to release location, combined with lack of fishing near the release area during spawning migration. Yield An estimate of the total yield in term of numbers from the releases is presented for combined results from both Møn and Bornholm in Table 7. When calculating the yield it was assumed that all recaptures during the second year at sea after release were legal-sized (L 60 cm), and that survival from the releases in 1997 followed a linear length recapture curve determined from other releases. The total estimated yield was 62,765 salmon, varying considerably between years. 6

7 From recaptures with known length and weight average weights at different ages was calculated. This information was used to estimate total yield in terms of weight in the different years after release. The estimated total yield by weight from releases at Bornholm constituted approx (min 278.0, max 328.2) tonnes or on average kg per 1000 salmon released. From the releases at Møn the estimated total yield was (min 97.0 max 121.0) tonnes or on average kg per 1000 salmon released. The results are presented in Figure 8. All figures presented are without application of any raising factor (compensation for tag loss, tag mortality, non-reporting of tags). The values found are comparable to yields reported in the literature (Eriksson 1989), however being in the lower range of reported values (765 2,258 kg per 1000 released). After applying a raising factor of 1.65 (Anon. 1988) the average value (Bornholm 1,387 kg; Møn 749 kg) is fully comparable to previous Danish experiments with delayed release reported by Glüsing & Rasmussen (1996) where a value of 1,230 kg per 1000 was estimated. Straying A total of 7 tagged fish from the delayed release experiments has been reported from salmon rivers on the Swedish west coast: 2 from Bornholm and 2 from Møn in River Göta, 1 from Møn in River Lagan, 1 from Bornholm and 1 from Møn in River Ätran. Scaling (i.e. weighing) these recapture numbers to the total number released assuming a 50% catch rate in rivers and no tag loss gives an estimate of approximately 790 salmon entering the Swedish west coast rivers from the delayed releases during the period , most of which would have entered the River Göta. The number is, however, due to the low number of observations, very uncertain. Due to criticism from Swedish authorities releases in 1999 were moved to Bornholm only and an additional experiment established. This experiment involved release of CWT tagged salmon in 2000, combined with targeted sampling in selected Swedish rivers. The results from this, including genetic analysis of samples from rivers on the Swedish west coast, is presently in preparation. Influence on wild salmon catch by Danish fishermen and fishing effort near Bornholm The main objective of the releases was to reduce the pressure on the wild salmon, and furthermore to investigate if Danish fishing effort could be moved, from an off-shore fishery to a fishery close the release areas. The Danish commercial salmon catches in ICES areas in the period was on average 87,700 salmon annually, of which approx. 6,510 salmon were delayed released from the stockings at Møn and Bornholm. During the same period the proportion of wild salmon in the area varied between 6.1 % and 19.7 % (Anon. 2003). Due to catch of salmon from the delayed releases at Møn and Bornholm the mean annual catch of wild salmon in this seven years period by Danish fishermen was approx. 8,150 fish whereas it would have been 9,150 salmon if the fishery had not in part been a fishery on Danish stocked fish, corresponding to a reduction of approx. 11%. This figure is based on a contribution of wild salmon to the entire stock of 10.44% (average value , Anon. 2003). The recapture rates of the releases at Møn are about half the recapture rates from Bornholm, so if steady releases of 120,000 smolts annually at Bornholm is considered, the pressure on wild salmon would be reduced with approx specimen (18.5%) still assuming a recruitment of 10.44% wild and a TAC of 90,000. 7

8 In Figure 9 the proportion of the Danish catch taking place near Bornholm (ICES Squares 38G4, 38G%, 39G4 and 39G5) is presented for the period Values are extracted from the Danish catch database (log-books) and included in the catch near Bornholm is also catch from vessels without log-books since this catch is almost certainly by small boats fishing close to the shore. A clear increase in the proportion of the catch being done near Bornholm is observed. However, during the same time period the number of larger vessels has been reduced from approx. 50 in to 28 in 2002, thus reducing the potential for fishing off-shore. The trend in an increase in the fishery near Bornholm could be attributed to other causes than just releases. However, unanimous reports from the fishery claims that the number of salmon near the island of Bornholm did increase during the period with delayed releases, and so did the interest for fishing in the area. Consequently it seems reasonable to believe that releases did indeed have some effect on the pattern of fishing. References Anon. (1988). Report of the Baltic Salmon and Trout Assessment Working Group, ICES C.M. 1988/Assess:19. Anon. (1990). Delayed release and closed areas - salvation of salmon and salmon fishery in the Baltic (In Swedish: Fördröjd utsättning och fredningsområde - räddningen för laxen och laxfisket i Östersjö området). Referensgruppen för projektet "Vidareutveckling av fördröjd utsättning av lax i Östersjön". Anon. (2001). Environmental surveying in Hallands län 2000, increase in Salmonids - but overfishing of a number of species (in Swedish: Miljöövervakning i Hallands län 2000, Laxfiskan ökade - men överfiske av en rad arter i Kattegatt), Meddelande 2001:26. Länsstyrelsen Halland. Anon. (2003). Report of the Baltic Salmon and Trout Assessment Working Group. ICES C.M ICES, Copenhagen (Denmark). Eriksson, C. (1989) Delayed release of young Baltic salmon (Salmo salar L.) in the Baltic area. Comparative releases of salmon from different salmon river stocks. Nordic Journal of Freshwater Research, 65, Eriksson, T. (1991) Sea releases of Baltic salmon: Increased survival with a delayed-release technique. American Fisheries Society Symposium, 10, Eriksson, T. & Eriksson, L.O. (1991) Spawning Migratory Behavior of Coastal-Released Baltic Salmon (Salmo-Salar) - Effects on Straying Frequency and Time of River Ascent. Aquaculture, 98, Fowler, J. & Cohen, L. (1995) Practical statistics for field biology. John Wiley & Sons, Chichester. Glüsing, H., Rasmussen, G. (1996). Stocking experiments with Baltic salmon (In Danish: Udsætningsforsøg med Østersølaks), DFU-rapport nr. 6-96, 48 pp. Danish Institute for Fisheries Research. Hansen, F.I. & Pedersen, S. (2002). Danish salmon fishery in the Baltic 2001 and staus on the experiments with delayed relese at Bornholm and Møn (in Danish: Dansk laksefiskeri i Østersøen 2001 og Staus for forsøg med forsinket udsatte laks ved Bornholm og Møn). DFU-rapport Danish Institute for Fisheries Research. Hansen, L.P., Jonsson, B. (1988) Salmon ranching experiments in the river Imsa: Effects of dipnetting, transport and chlorobutanol anaesthesia on survival. Aquaculture, 74, Karlsson, L. & Karlström, Ö. (1994) The Baltic salmon (Salmo salar L.): its history, present situation and future. Dana, 10, Novotny, A.J. (1980). Delayed release of salmon. In Salmon Ranching (ed J.E. Thorpe). Academic Press, New York. 8

9 Sharpe, C.S., Thompson, D.A., Blankenship, H.L., & Schreck, C.B. (1998) Effects of routine handling and tagging procedures on physiological stress responses in juvenile chinook salmon. Progressive Fish-Culturist, 60,

10 Table 1. Release and recapture data for salmon experiments with releases at Møn and Tejn (Bornholm) Stocking Batch No. No. % Length No. Release date location number Tagged Rec. Rec. at release. stocked Strain Møn ,51 27, Iijokki ,22 26, ,50 22, Mörrum ,80 27, Tejn ,52 30, Iijokki ,90 25, ,80 24, ,20 28, ,67 23, Mörrum ,95 27, ,27 27, Iijokki ,80 21, Mörrum & ,96 24, Table 2. Recapture and release numbers of salmon receiving additional handling (net-pens being towed to harbour for tagging). Additional handling Location Number Yes % No % Bornholm Recaptured Released Møn Recaptured Released Table 3. Number recaptured in different years for releases of salmon at Møn and Bornholm. Only recaptures where year of recapture is known are included. Location Recapture Møn Release Bornholm

11 Table 4. Recapture gear for releases of salmon at Møn and Bornholm Møn Bornholm Møn & Bornh. Iijok Møn Total % Iijok Møn Total % Total % Proff. fishermen unspec Gill net unspec Salmon drift net Gill net other Trap net (incl. floating) Trap net herring Trap net other (incl. fyke net) Trawl Salmon ling line Trolling / hook unspec Sports fishery Sports fishery lure Sports fishery fly Sports fishery nat. bait Found dead / tag found Predated by bird Electric fishing Unknown Total Table 5. Proportion of recapture by different nationalities. Release year Denmark Sweden Poland Finland Germany Latvia Lithuania Norway Unknown Average

12 Table 6. Recapture area. ICES Fishing Area if known (22 32) or other including recaptures in freshwater inside and outside the Baltic. ICES Fishing Area / Møn Bornholm Grand Recapture area Total Total Total Baltic Area unspecified Kattegat Skagerrak Atlantic and North Sea Freshwater inside Baltic Freshwater outside Baltic Unknown

13 Table 7. Yield by number from combined releases at Møn and Bornholm of 120,000 salmon annually. It is assumed that all recaptures in the year after release are legal-sized salmon (L > 60 cm). No raising factor applied to the figures. Year of Year of recapture release Total ,123 7, , ,682 8, , ,571 8,975 1,994 21, ,440 1, , ,223 5, ,406 Total 3,123 12,610 19,545 11,316 9,317 6, ,785 13

14 Figures Rec % Temp. at tagging ºC Figure 1. Temperature at tagging plotted against recapture rate. Squared marks are releases at Bornholm; triangles are releases at Møn. Numbers are average lengths of batch released Rec % Bornholm 1997 Møn Freq L < 22 cm Figure 2. Frequency of small (L < 22 cm) tagged fish plotted against recapture rate. Squared marks are releases at Bornholm; triangles are releases at Møn. Dotted line shows regression line for releases at Bornholm (excluding releases in 1997). 14

15 34 Avg. length at recapture (cm) Avg. length at release (cm) Figure 3. Average length of individual batches at recapture plotted against average length at release. Squared marks are releases at Bornholm; triangles are releases at Møn. Dotted line shows regression line for releases at Bornholm. Full line shows y = x. 1 Proportion of catch 0.5 Bornh. (N=690) Møn (N=208) Days Figure 4. Cumulative fraction of recaptures over time after release Møn and Bornholm. 15

16 % of recaptures Møn Bornh. 0 Proff. Unsp. Salm. D.N. Other. G.N. Trap / Fyke LL Sports Other Figure 5. Recapture gear for recaptures where gear is known releases at Bornholm and Møn. Categories of gear: Proff. Unsp.: Professional fishermen gear not specified, Salm. D.N.: Salmon drift net, Other G.N.:Other gill net, Trap / Fyke: Trap or fyke net, LL.: Salmon long line / hook, Sports: Sports fishery Møn Bornholm 10 0 Denmark Sweden Poland Finland Germany Latvia Lithuania Norway Unknown Figure 6. Distribution of recaptures on different nationalities for releases at Møn and Bornholm respectively. 16

17 Releases at Bornholm Legend Figure 7. Distribution of recaptures from releases at Bornholm in different ICES Fishing Areas. Mörrum and Iijokki strain combined Total catch (kg) Bornholm Møn Year of release Figure 8. Cumulative total yield by weight from releases in different years at Møn and Bornholm. Numbers in the figure is average length at release. Total number released each year was 120,000 salmon. 17

18 Proportion of DK catch near Bornholm Figure 9. Proportion of total Danish catch taken near Bornholm (ICES Squares 38G4, 38G5, 39G4 and 39G5). Included is Danish catch from vessels without log-books. 18