ICES CM 2005/X:12 Working paper may not be cited without prior reference to the authors Impact of grey seals in the herring gillnet fishery along the Swedish Baltic coast Sara Königson, Arne Fjälling and Sven-Gunnar Lunneryd Institute of Coastal Research, Swedish Board of Fisheries, Contact author: Sara Königson, Institute of Coastal Research, Swedish Board of Fisheries, Box 423, 401 26 Göteborg, Sweden tel: +46 31 7430422 email: [email protected] The gillnet fishery for herring Clupea harengus in the Baltic Sea is subjected to intense damages by grey seals, Halichoerus grypus. Fishing takes place mainly during spring and autumn. Damage caused by seal peak in late autumn and for which reason fishery during this part of the year has more or less disappeared. In this study we investigated the hidden damage, i.e. the amount of fish completely removed from nets without a trace as well as the catch losses due to the presence of seals. Observers joined a commercial fisherman on his regular fishing trips. Catch and damaged fish were noted and systematic observations for seals were carried out during every fishing occasion. On some fishing trips herring was caught in a 30 meter long net of commercial type. All fish was then marked, without being removed from the net. The net was thereafter reset and linked to the local fisherman’s gear. The percentage of marked fish that was damaged or missing the next day, was used as an indication that seals had visited the nets. If a large part of the marked fish were missing it was concluded that seals had visited the nets. Adjustments were made for the marked fish that fell off during the handling of nets. 14 occasions of 19 rounds were considered as occasions with seal damages. In those cases more than 86 percent of the marked fish was missing and 5 percent was found left but damaged. The overall catch, including the commercial fisherman’s during occasions with marked fish, did decrease significantly when seals had been present. The catch losses (difference in catch) were much larger than the few observed seals possibly could have consumed in that time period. Observations from all set links showed that on those occasions when seals had been observed while setting the nets the catch was much lower than when seals had not been observed. The low catch when seals are close by and the very few remains of herring found, indicates that the mere presence of seals affected the catch level. This supports the commonly held view among fishermen that seals “patrol” along nets and thus scare fish away. Keywords: conflict, catch loss, hidden damage, herring fishery, grey seal, Baltic Sea Introduction The grey seal Halichoerus grypus population of the Baltic Sea has recovered from a severe decline. The population has increased dramatically during the last decade. The increased population has escalated the conflict between seals and commercial fisheries (Kauppinen et al, 2005). For many years the gillnet fisheries in the Baltic Sea have been subjected to damages caused by seals (Anon, 1909). Seals steal fish damage the catch (leave half-eaten fish) and tear large holes in the nets. Fishermen also suspect that the mere presence of seals can scare fish away from the fishing areas and thereby decrease their catches. This means great economic losses for the fishermen and in some areas the conflict with seals is the main reason for a decline in the number of active fishermen. Fishing for herring Clupea harangus with small meshed gillnets is carried out by coastal small scale fisheries. It is often combined with other fisheries, for example trap net fishing after Salmoniformes. The main herring population spawns in the spring and gather in shallows water near the coast. This is when a larger part of the fishing takes place. In the autumn, fishing is carried out in deeper waters farther away from the coast. This is when the main damage caused by seals occurs. Damage caused by seals has its peak in spring and autumn. In the summer grey seals spend a lot of time on haul-out areas moulting and their food-intake is low (Westerberg et al, 2000). During winter fisheries activity are low. The staple food for grey seals in the Baltic Sea is herring (Lundström et al, 2005 and Söderberg, 1975). Sjöberg, 2000 suggests that seals might maximise their feeding effort in areas where the ocean floor is steeply sloped and herring schools are stationary. Fishermen have been fishing for generations with gill nets on these slopes and this will of course increase the conflict between seals and fisheries. In this study we wanted to investigate the losses caused by seals to the Swedish herring gillnet fisheries in the Baltic. This includes the damages on catch and the hidden losses. Hidden damage is defined as the amount of fish wholly removed from nets without a trace. We also wanted to study if there are any catch losses due to the mere presence of seals. Methods Fieldwork was conducted during September and October, 2003 and 2004 in cooperation with a local fisherman in Skärså, a small fishing village north of Söderhamn in the north Baltic Sea, latitude N 61º 23. Fishing for herring was carried out with one to four 180 meter long links of nets. The individual nets were 60 meter long, nine meters deep and the stretched mesh size was 39 mm. The links were set at a distance of around 3 nautical miles between and at 7 nautical miles of the coast. Nets were most often set in the afternoon and emptied the following morning. One or two observers joined the fisherman on his regular fishing trips. The catch and damaged fish were noted on every set link at every fishing occasion. Systematic seal observations were made while setting the nets and lifting them. One person was carrying out observations for one minute in forward, backwards, starboard and port direction of the boat. On some occasions when the nets were lifted, one 30 meter long net was hauled in and left with catch intact instead of being emptied. The herring was marked without being removed from the net by pressing out the eyes of the fish. A maximum of 13 kg and a minimum of 2.5 kg of herring were marked on the net with an average of 20 herrings per kilo. The 30 meter long net with marked herring was then linked to the local fisherman’s nets and reset on the following fishing trip. When the nets were lifted again, the number of marked and unmarked fish, damaged or undamaged were counted. It was concluded if seals had visited the nets or not depending on the percentage of recovered marked fish. Calculations of the percentage of damaged and missing marked fish was performed both for cases with or without seal visit. The total catch on the link was also noted and compared with regard to seal visit or no seal visit. The data are neither normally distributed nor transformable to normal. Confidence limits for catches were therefore estimated by a bootstrap procedure. A visual basic macro was used in Excel to simulate data collection procedure with repeated re-sampling. We used 2000 iterations and estimated the 95 percent confidence limits. Controls were carried out in order to estimate the amount of marked fish that fell from the nets when handled. Nets with marked fish were set in the same manner as on ordinary fishing trips and then picked up shortly thereafter. The number of fish that spontaneously fell off or were damaged in the process was counted. A criterion of a seal visit is then when the missing part of the marked fish is larger than the missing part of marked fish in the controls. Results In 2004 seven control nets with marked fish were set and then retrieved after a short period to measure the amount of fish that fell off spontaneously. The maximum amount of fish that fall off was 19 percent, mean 15.2± 4.1 (95 % c.f.). Occasions when more than 19 percent of the marked fish was missing were considered as occasions with seal damage. The extra net with the marked fish was attached to the ordinary link on 19 net links during 17 fishing trips in 2003 and 2004. A total of 2 601 herrings were marked on the nets and connected to the ordinary links. Of those 1 823 herring were lost without a trace. At only five occasions seals had not been in the vicinity of the nets and more than 19 percent of the marked fish were retrieved. During the other 14 occasions seals had been present. In those cases more than 86 percent of the marked fish was lost. Only 5 percent of the marked fish was retrieved as damaged. Seals were observed while setting the nets on five occasions. On eleven of all of these occasions more than 19 percent of the marked fish was lost i.e. seals had certainly visited the net. During the seal visits more than 86 percent of the marked fish was lost. Only few remains of damaged fish were left in the net (5 %). The catch of the total link on all fishing occasions with test nets is presented in table 2. Table 1. All occasions when test nets have been connected to the commercial net links, occasion considered to have had seal visits and occasions regarded as no seal visits. No of % of lost Kg herring/60 No observed No observed marked marked meter net and day seals while seals while fish fish for the total link setting lifting Occasions with no seal visits 61 0 177.6 146 15.1 70.7 1 85 15.3 52.7 71 15.5 49.8 54 18.5 2.46 Occasions with seal visits 193 25.4 2.7 1 42 40.5 74.9 1 256 52.0 6.3 1 146 96.6 4.4 1 1 264 97.0 29.7 1 201 99.0 0 1 205 99.0 0 2 102 100.0 0 1 78 100.0 0 123 100.0 0 1 265 100.0 0 66 100.0 0 1 80 100.0 0 1 163 100.0 4.2 If we compare the total catch from links with and without seal visits there is a significant difference in catch.