An Assessment of the True Damages caused by Grey Seals, Halicoerus grypus, in the Swedish Baltic Net Fishery after Atlantic Cod, Gadus morhua

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1 An Assessment of the True Damages caused by Grey Seals, Halicoerus grypus, in the Swedish Baltic Net Fishery after Atlantic Cod, Gadus morhua Frida Sundqvist Masters Thesis in Marine Biology Department of Biology, Lund University Swedish National Board of Fisheries Supervisor: Sara Königson, Swedish National Board of Fisheries Secondary supervisor: Jep Agrell, Department of Ecology, Lund University The conflict between grey seals, Halichoerus grypus, and the Swedish cod fisheries in the Baltic Sea has steadily increased during the past 10 years. Seals locate the fishermen s nets, damage the catch and tear holes in the nets. In this study we investigated the damages caused by seals on the net fishery for cod, Gadus morhua, in the central Baltic Sea. Damage includes both the visible damage and the hidden damage i.e. the fish that are taken whole without leaving any visible trace or damages to the nets. To gather the required data an observer joined two professional fishermen for a period of tree months in During that period all catch were recorded together with any visible damages. On some experimental links of nets the caught fish were marked without being removed from the net. The net was reset and when retrieved again the number of damaged or lost marked fish was noted. Corrections to the data were made to account for fish that spontaneously fall off. On 101 out of 184 links set out damaged fish were found. Damaged fish was most often found as small remains or with only the head of the fish left. At 20 out of 31 occasions when fish were left in the net, marked fish were missing or found damaged indicating a seal visit percent of the marked cod were removed from the damaged links without leaving any visible trace, and an additional 10.1 percent were damaged adding up to a total damage of 74.4 percent of the catch. There was also a significant difference in catch per effort (cod caught per 100m*24h) between damaged and non-damaged experimental links. Our findings show that the damage to the cod fishery caused by seals is large and that damage can not only be estimated by observing and counting retrieved damaged fish. The hidden damage is significant and

2 should be taken into account when calculating expected catch losses due to seals. Key words: Halichoerus grypus, grey seal, hidden damage, Gadus morhua, Atlantic cod, net fishery. Introduction The fishery in the Baltic Sea is heavily subjected to damages caused by Grey seals, Halichoerus grypus (Fjälling 2004, Lehtonen and Suuronen 2004, Lunneryd et al. 2003). The conflict between seals and fisheries is based both on the competition for resources and the damages afflicted by seals on fish catch and gear. An increase of damages has been observed since the 1990 s. Passive gears such as gillnets and traps are especially affected, whilst active gear i.e. trawls receive less damage (Westerberg et al. 2000). There have been some advances made in the trapnet fishery, e.g. with the Swedish pontoon trap that has proven to be effective in catching salmon and decreasing seal damages, but for the net fishery no advances have been made (Suuronen et al. 2004). Due to the damage of catch and gear fishermen suffer great economical losses. Swedish fishermen can claim compensation from the county s administrative board for the visible damages. In 2004 the administrative board in Kalmar, the county of this study, compensated fishermen for their losses with approximately 1 million SEK, an increase with roughly 40 percent since the year before (Lst Kalmar 2005). The conflict has not always been such a big concern as it is today. To the late 18 th century seals were hunted in the Baltic for their oil, but they lost their economic value as photogene became available (Harding and Härkönen 1999, Westerberg et al. 2000). The seal hunt continued as bounties got paid for hunting seals between 1903 and 1967 (Harding and Härkönen 1999). One of the main reasons for the bounties was the damage to fisheries (Westerberg et al. 2000). The seal population in the Baltic decreased and in the late 1970 s the population was estimated to 4000 individuals (Harding and Härkönen 1999). Hunting was the main reason for population decline, but the numbers where then kept low due to sterility probably caused by excessive organochlorine levels (Harding and Härkönen 1999). Hunting of seals in the Baltic was prohibited in 1988 and the level of organochlorines is slowly improving and therefore the seal population has recovered (Bergman and Bäcklin 1999, Harding and Härkönen 1999). In 2004 an approximate number of grey seals were counted in the Baltic Sea during the fur change in May/June. Even though this number do not reflect the true population it reveals an annual population increase of 7.5 percent between 1997 and 2004 (Helander and Karlsson 2004). The fishery for Atlantic cod, Gadus morhua is one fishery that suffers from damages due to seals. Small-scale coastal fishery for cod in the Baltic use stationary nets that takes advantage of the fish swimming activity and are known to be selective i.e. only catching fish of certain size and species (Huse et al. 2000). The nets are usually set in a row consisting of about seven to ten 100 2

3 meter long nets creating a link approximately 700 to 1000 meter long. Each fisherman sets out two to six links and leaves them in the water for one to two days before retrieving the nets again. The Swedish Baltic Cod fishery is active from early fall until late spring. According to fishermen seal damages are the greatest from December to the end of February and decrease in March as the seals are giving birth to their cubs. After this period damages tend to increase again (Fiskeriverket 2001). During part of the summer grey seals spend a large portion of their time on land moulting and food activity is lowered (Westerberg et al. 2000). The aim of this thesis is to assess the total damage to the Swedish cod net fishery caused by grey seals. To assess the damage to the catch both the visible damage (i.e. fish remains left in the nets) and the hidden damage (i.e. fish that are completely consumed and leave no trace in the nets) was examined. However, seals also rip holes in the nets when taking fish and nets therefore do not last very long. In addition to examining damaged catch an assessment of the net damages was therefore included. Understanding the true effects that the grey seals have on the fishery will aid in trying to find the best solution for both parts in this conflict. Material and Methods Fieldwork The fieldwork was conducted on northern Öland, an island outside the southeast coast of Sweden. Data were collected from the 4 th of April until the 21 st of June An observer joined two local fishermen when fishing for cod using gill nets recording their catch and any damages that occurred as well as performing experiments to determine the quantity of the hidden damage. The fishermen were both located on northern Öland, but on different sides of the island. Fishing took place on locations marked on Figure 1. Fishing with cod nets in the Baltic is mainly one man fishing operation and smaller fishing boats than 12 m are most common. The fishermen usually set out between 2 and 6 links of nets, with 7 to 10 one hundred meter nets in each link. The links were left in the water for 1 or 2 days. Hidden damage On each fishing occasion an observer joined the fishermen on the boat recording data from all the links set out and retrieved. One of the links set out were on retrieval used as an experimental link to investigate the hidden damage. When this link was retrieved the first ten captured fishes were left in the net and marked with a number. The marking was done by attaching a piece of tape with a number to the net in close proximity of the fish. Before being marked each fish were surveyed to make sure that it was fresh and undamaged. All fishes were also measured and photographed using a digital camera. The experimental link was then set out again with the numbered fish still attached. When the nets were retrieved, after 1 to 2 days, the observer was able to see how many of the numbered fishes that were lost, damaged or left in the net. 3

4 Figure 1. A map of the study area. Experimental links are marked with fishes, regular links with squares and net damage links with circles. Links that were both experimental and net damage links are marked as experimental links (fishes). When the numbers of caught cod were less than ten on the net used as experimental net additional cod were manually tangled into the net and marked. To make sure that the fishes were properly attached to the net at least three meshes were put behind the gill cover, resembling the way the cod is naturally entangled. If no other cod were caught on other links I set out whatever number of fishes that were caught, therefore, on some occasions, less than 10 fishes were set out. To account for the possibility of fish falling off the experimental net while handling and incorrectly being recorded as seal damage, controls were made throughout the fieldwork period. Both fish that had entangled itself and fish that were manually entangled were marked in the net and set out. Once it was set out it was immediately retrieved again and the fish were counted to see how many had fallen off. During the setting of the link seal observations were made continuously to be able to exclude fish loss due to seals. These controls allowed us to determine if seals had been visiting the experimental nets or if lost fish were just due to falloff. Fisheries data Visible seal damage and catch data were collected from all links set for fishing. The damaged fish were recorded to belong to one out of four categories; remains, head, scratched, or bite marks (Figure 2). At the same time the fishermen kept a separate journal on catch and damaged fish. 4

5 A B C D Figure 2. Showing examples of the 4 different categories of visible damage. Picture A shows a remain, picture B a cod head, picture C a scratched cod, and picture D a cod with bite marks (bitemark location pointed out with a circle). It has been suggested in Königson et al., 2005 that the mere presence of seals can affect the catch. This also needs to be included in the assessment of the total damage and was therefore also investigated in this study. Each time a net was set or retrieved systematic seal observations were made around the boat. The observations were made by one person in all four directions of the boat for approximately 2 minutes in each direction, when necessary binoculars were used to verify seal presence. To minimize overestimation of seal presence all observations was considered as one seal being sited if not several seals were observed at the surface simultaneously. Net damages To try and quantify seal damages on the nets, a new net without holes was set out on one link at each fishing occasion. This net was 50 meters long instead of the usual 100 meters used by the fishermen to aid in handling. There was a total of 4 nets used, 2 with each fisherman, and they were individually marked to tell them apart. When retrieving one of these nets it was brought back to shore and the other net was attached to the link and set. This way there was always one of these nets in the water with each fisherman. The net that was brought back to shore where it was examined and every hole documented. Each hole were categorized, depending on the size of the hole, into one of 5 categories; 1 mesh

6 brake (1), 2-5 mesh brakes (2), 7-10 mesh brakes (3), part of net gone (4) and longer tear (5). The hole was also categorizes as being located at one of 6 net locations; bottom of net (1), m up (2), middle (3), top of net (4), fish hole (5), and stone hole (6). Each hole was repaired and marked so that the next time that net was used only new holes were documented. Calculations and Statistics All statistical calculations were performed using JMPIN version 4.0 computer software and with an alpha of Before any statistical tests were performed the data were tested for normality using the Shapiro-Wilk W- Test, and for equal variance using the Bartlett and Levene Tests. None of the data were normally distributed or transformable to normal. Confidence limits for catches and damages were therefore estimated by a bootstrap procedure. A visual basic macro was used in Microsoft Excel to simulate data collection procedure with repeated resampling. We used 2000 iterations and estimated the 95 percent confidence limits from the sample. Hidden damage To quantify the size of the hidden damage the percent of fish fall-offs first had to be calculated using data from the controls. For each control opportunity a percent of falloff was calculated for each way of entanglement (self entangled or manually entangled). Then an average percent was calculated for each way of entanglement with a bootstrapped confidence interval of 95 percent since the data was not normally distributed. For each experimental opportunity a percent of hidden damage (fish totally lost) was then calculated for each way of entanglement and the maximum percentage from the control calculation were subtracted. Occasion when less than the maximum control value were lost or when only one of ten fishes were lost and no other damages where recorded was excluded and considered as non-damage occasions. A calculation was made to receive an average percent hidden damage on occasions when damages occurred, taking into account the proportion of the ways of entanglement. Using the same data a calculation of the average percent of visible damages could be done. All these calculations were made in three different ways, one for each location and one for the data set in total. For each set of data (hidden, visible, and total damage) a Mann-Whitney U-test were performed to see if there were any significant difference in damages between the two locations (Byxelkrok and Böda). A statistical test was also performed to determine if there was any significant difference in catch per effort (cod caught per 100m*24h) between experiment links with or without damages, both visible and hidden. Fisheries data From the total fishery data an average percent of visible damaged fish of the total catch was calculated. These calculations were made per link. A calculation was also done to get the average number of caught cod per effort i.e. 100m and 24 hours. Then a Mann- Whitney U-test was done to detect any significant difference in catch per effort between links with and without visible damages. To be able to get a visual picture of how the number of cod caught varies with the amount of damages a scatter plot was done. 6

7 Net damages An average number of holes per effort i.e. 50m and 24hours were calculated from the new nets. Holes that had a high possibility not to be caused by seals were subtracted. These were the holes known to be caused by stones and holes on the bottom of the net (0-0.5 meter up). An average number for holes known to be caused by seals were also calculated. These were holes in the vicinity of where fish heads were found still attached to the net upon retrieval. The net damage data were also used for two different calculations: one to determine of what size the highest percentages of holes were, and a second calculation to see where on the net the highest percentage of holes were located. Results The gathering of fishery data was done continuously during the 12 weeks of fieldwork. The cod catch per effort (100m*24h) varied during the study as well as the damaged cod catch per effort (Figure 3). Hidden damage Control calculations showed that for the self entangled fishes (SEF) an average of 4.0 percent fell off with a 95% CI max/min of 8.9/1.1 percent. For the manually entangled fishes (MEF) the average percent fall off were 2.6 with a 95% CI max/min of 5.9/0.0 (Table 1). The maximum control values were used to make sure that fish that fell off on the experimental links were not considered to be eaten by seals. Since the maximum percent falloff differed between ways of entanglement this was considered when calculating the hidden damage (fish totally lost). Hidden damage was calculated from a total of 31 experimental links with marked fish. Damaged or lost fish were recorded on 20 links. The links were divided between the two locations Byxelkrok and Böda (Table 2). Total catch per effort April - 10 April 11 April - 17 April 18 April - 24 April 25 April - 1 Maj Damaged catch Undamaged catch 2 May - 8 May 9 May - 15 May 16 May - 22 May Week of fieldwork 23 May - 29 May 30 May - 5 June 6 June - 12 June 13 June - 19 June 20 June - 26 June Figure 3. Average catch of cod per effort caught during each week of fieldwork. The undamaged cod caught are shown in white whilst the damaged cod caught are shown in grey. 7

8 Table 1. Results from control trials were number of fish that fall off the net naturally were calculated. Number of trials done, total number of marked fishes set out and how many of the total number of set out fishes that fell off are shown (fall off fishes) as well as the calculated average percent fall off with a bootstrapped 95% confidence interval (CI) are shown for both ways of entanglement. Way of entanglement No. of control trials Total no. of marked fishes No. of fall off fishes Self entangled Manually entangled Average fall off % (95% CI max/min) 4.0 (8.9/1.1) 2.6 (5.9/0.0) Table 2. Grey seal damage to cod fishery nets. Results from experimental nets with marked fish including average percent totally lost (hidden damage), damaged (visible damage), and total damage (hidden + visible) with a 95% bootstrapped confidence interval (CI). Location Total (Byxelkrok + Böda) Total no. of links No. of damaged links (N) Byxelkrok Böda 13 6 Hidden damage % (95% CI max/min) 64.3 (78.3 / 52.6) 70.4 (86.2 / 52.6) 50.2 (74.2 / 48.9) Visible damage % (95% CI max/min) 10.1 (18.5 / 4.0) 6.7 (16.7 / 2.1) 17.3 (40.0, 4.8) Total damage % (95% CI max/min) 74.4 (85.1/ 57.8) 77.3 (90.3/ 60.2) 67.6 (90.5,/38.6) The hidden damage in percent was calculated for experimental links with damages, giving how many percent of the catch that is taken whole by the seals when damages occur. Results show that for both the locations in total an average of 64.3 percent of the catch is lost. Average loss was also calculated for the two different locations separately yielding a loss of 70.5 and 50.2 percent for the Byxelkrok and Böda locations respectively (Table 2). A Mann-Whitney U-test was carried out showing that there is no significant difference in percent hidden damage between the two locations (P = 0.20). For the experimental links calculations were also made on how large the visible damage was. Results showed that the average loss for both locations were 10.0 percent of the marked fishes. A Mann- Whitney U-test showed that the two locations were not statistically different (P = 0.31) (Table 2). Catch per effort For the experimental links statistical tests were conducted to determine if there was any significant difference in average catch per effort between links with or without damages. Results show that both fishing locations (Byxelkrok and Böda) had a higher average catch/effort when no damages were found than when links had been visited by seals. However, this difference was significant on the Byxelkrok location, but not on the Böda location (average catch/effort and 95% CI max/min for Byxelkrok: no damage = 5.0 and 6.8/4.0, damage = 1.2 and 2.5/0.4, P = 0.01, Böda: no damage = 5.1 and 12.9/0.8, damage = 1.3 and 1.8/0.8, P = 0.78) (Figure 4). 8

9 14 12 N = 11 N = 7 Average catch / effort N = 4 N = 20 N = 14 N = 6 Damage No damage 0 Total Byxelkrok Böda Location Figure 4. Average catch per effort (cod caught per 100m*24h) on experimental links with or without damages on the different locations. Average catch/effort on links with damage are shown in grey whilst average catch/effort on links without damages are shown in white. The error bars show the bootstrapped 95% confidence interval (CI). Stars indicate the locations were a significant difference was found between damaged and non-damaged links. When comparing links from both locations a Mann-Whitney U-test showed that there were significantly more cod caught on links without damages (average catch/effort and 95% CI max/min for no damage = 5.2 and 11.4/2.0 damage = 1.2 and 2.1/0.7, P = 0.01), see Figure 4. Fisheries data During the time of the study a total of 2667 cod were caught on 184 links set out and retrieved. On 101 links, out of the total 184 set out, seals had attacked the nets and damaged part of the catch (Table 3). Seals were observed nearby a net on only ten occasions. All these observations were made at Byxelkrok during a total of 1488 minutes (8 min*93 links*2 occasions, setting and retrieving) of observation time. No seal was observed at Böda during 1456 minutes (8 min* 91 links*2 occasions, setting and retrieving) (Table 3). At no time was more than one seal observed at the surface simultaneously, therefore all seal sightings were considered to be of one single seal even if it was observed several times. Visible damage A total number of 184 links were set out on the different locations. Out of these 184 links 101 turned out to have visible damages. On links with damage an average percent visible damage of the total catch was calculated for the different locations. For all damaged links the average damage was 41.2 percent (Table 3). If separating the two locations the Byxelkrok location had a larger percent damaged fish than the Böda location, however, a Mann-Whitney U- test showed that the difference was not significant (P = 0.25) (Table 3). 9

10 Table 3. Results from all links with damages caused by grey seals, showing the average percent visible damage of the total catch with a 95% bootstrapped confidence interval (CI). Table also gives the total number of cod caught on all links (damaged + undamaged) as well as on links with visible damages. The total number of damaged cod retrieved throughout the field study is given and the number of days when grey seals were observed during the systematic seal observation is listed. Location Total (Byxelkrok + Böda) Total no. of links No. of damaged links (N) No. of caught cod (all links) No. of caught cod (damaged links) No. of damaged cod Byxelkrok Böda Visible damage % (95% CI max/min) 41.2 (47.5 / 34.6) 44.1 (51.7 / 36.7) 30.5 (38.9 / 23.2) No. of seal observations Scratched 14% N = 61 Head 35% N = 153 Bite marks 2% N = 10 Remains 49% N = 211 Figure 5. Visible damages to cod caused by grey seals from all links of nets retrieved and proportion of the visible damages belonging to the four different categories. Type of damage The visible damage observed on the 435 damaged cods was divided into 4 categories; remains, head, scratched, and bite marks depending on how the fish was damaged. For the links in total the largest part of damages consisted of remains left in the net, followed by heads, scratched, and fish with bite marks (Figure 5). Visible damage data were very similar between the two locations so only the total result is presented. Catch per effort A total of 2267 cods were caught on both locations. Out of those 1464 were caught on links with catch damaged by seals (Table 3). Both locations showed a difference in average catch/effort between damaged and non-damaged links, but only on the Byxelkrok location was the difference significant (Figure 6) (average catch and 95% CI max/min for Byxelkrok: no damage = 3.4 and 5.1/1.9, damage = 1.6 and 2.1/1.2, P = 0.04 for Böda: no damage = 1.1 and 1.4/0.8, damage = 0.8 and 1.1/0.5, P = 0.83). When pooling data from the two locations the difference was nonsignificant (average catch and 95% CI max/min for no damage = 1.5 and 1.9/1.1, damage = 1.4 and 1.8/1.1, P = 0.73) (Figure 6). The proportion of damaged cod on each link was plotted against the numbers of caught cod. A trendline illustrate that the larger the catch the smaller is the proportion of damages (Figure 7). 10

11 6 Average catch / effort N = 101 N = 83 N = 79 N = 14 N = 22 N = 69 Damage No damage 0 Total Byxelkrok Böda Location Figure 6. Average catch/effort (cod caught per 100m*24 h) on all links of nets retrieved, with or without damages, on the different locations. The error bars are showing the bootstrapped 95% confidence interval (CI). Stars indicate the locations were a significant difference was found between damaged and nondamaged links. Proportion damaged cod y = x R 2 = Number of cod caught Figure 7. Scatter plot showing the number of caught cod on all damaged links plotted against the proportion of damaged cod on the same link. The trend in damages is illustrated by a trendline. Table 4. Showing on how many net settings that there were holes in the net on retrieval, and how they were divided between the two locations. Holes (Yes/No) Total (no. of net settings) Byxelkrok (no. of net settings) Böda (no. of net settings) Yes No Sum Net damages Nets were set out a total of 50 times on the two locations, and on 30 occasions holes were found (Table 4). In total the 200 m of net that were set in the water for a period of 70 days yielded 128 holes caused by seal i.e. more than one hole per two meters of net. On occasions when holes were found the average numbers of holes/effort were 4.1 (95% CI max/min = 5.6/2.9) (Figure 8). For the two different locations (Byxelkrok and Böda) the average number of holes/effort differed quite a bit but there was no significant difference (average and 95% CI max/min for Byxelkrok = 4.6 and 6.7/2.8, Böda = 3.3 and 5.2/2.0) (Figure 8). Amongst the holes there were some that by all certainty were caused by seals. These were the holes in the vicinity of where cod heads were found still attached to the net (Fish head holes). 11

12 Average number of holes / effort N = 18 N = 30 N = 12 Total Byxelkrok Böda Location Holes total Fish head holes Figure 8. Average number of holes per effort (50m*24h) on nets with damages for the different location, showing the average number of holes/effort known to be caused by seals (fish head holes) in white and other holes in grey. The errors bars are showing the bootstrapped 95% confidence interval (CI). For all the nets the average number of fish head holes/effort were 0.7 (95% CI max/min = 1.4/0.0) (Figure 8). Neither for the fish head holes was there any significant difference between the two locations (average and 95% CI max/min for Byxelkrok = 0.2 and 1.9/0.0, Böda = 1.4 and 2.2/0.0, P = 0.29) (Figure 8). Holes found were of different sizes and divided into 5 categories. From all the holes made on a location a certain percent were of each category. The most common type of hole found was of the category two to five mesh breaks (Figure 9). The two separate locations had almost the same distribution of categories, therefore only the total is presented. The holes found were also located on different parts of the net, with the largest number of holes found in the middle and the least amount of holes in the top of the net. The same pattern was found on both locations mesh breaks 13% N = mesh breaks 49% N = 63 1 mesh break 26% N = 33 Hole w ith net gone 3% N = 4 Longer tear 9% N = 11 Figure 9. Average percent of holes made in the nets throughout the study period belonging to each size category. The total number of holes added up to 128. Discussion Hidden damage / Fisheries data The results indicate that on fishing occasions when seal damages occur the fishermen loose 64.3 percent of their catch without ever seeing it. Only a 12

13 small portion of the catch is left as damaged catch in the nets. This means that the fishermen s economical losses are greater than we expected and that the hidden damage should be considered when calculating the economical compensation. Other studies conducted by Königson et al., (2005) and by Söderlind (2004) investigating the hidden damages in the gillnet fishery for herring and pike-perch in the Baltic also both showed that the hidden damage has a negative impact on the fishery. The corrections of the data that was made after calculating the maximum control values are important for the accuracy of the data. Control links were never left in the water for a longer period of time due to that seals might be present. This might influence the control values, but since the gear is subjected to most stress during setting and retrieval of the net this error is assumed to be minimal. When looking at the results for the visible damages on all links (not only the experimental ones) an average of 41.2 percent of the catch is damaged. Given that for each damaged fish over six disappear (10.1% vs. 64.3%) indicate that the losses are 5 times the catch during a seal visit. How large the catch is might also influence how large the damage is. When looking at the scatter plot it indicates that the larger the catch is the smaller is the proportion of damages, or if viewed from another perspective, the lesser the damage the greater the catch. So there is two ways of looking at the scatter plot. Firstly, if the number of seals that visit the net varies the fewer number of seals the smaller is the proportion of damages and therefore the larger the catch. Secondly, if the numbers of seals visiting the net are constant there seems to be a saturation limit and therefore a larger catch yields less damage. Considering that the seal might have a saturation limit it would be interesting to know how the damages change with cod availability. During the course of this study the damages seemed to change as availability of cod increased, so that more scratched fish and less fish heads were found. Perhaps the seals only take certain parts of the cod when availability is high? Conducting this study again during a different time of the year might answer this question. Another aspect that has to be considered is how much damaged cod that fall out of the net and is left at the bottom and how many seals that creates the damage? Is it always the same seals that visit the nets or is it different individuals each time? These are just some of the questions that need to be studied in the future. When calculating the amount of visible damage on all links it constitutes 41.2 percent of the catch in comparison with 10.1 percent on the experimental links. This shows that the visible damage also constitute a large portion of the catch even though the limited number of fishes and experimental links gave a lower number. When examining the damages per link and not per fishing occasions some links probably ended up being more similar than others, thereby risking pseudo replication. However, this similarity is probably insignificant since the links were placed some distance apart and fishing often occurred on consecutive days. Therefore all links were independent of each other. There was a significant difference in catch per effort between damaged and non-damaged links on the Byxelkrok location irrespective if looking at all links or only on the experimental links. 13

14 On the Böda location, however, there was no significant difference, which may at least in part be explained by a small sample size. The difference between damaged and non-damaged links on the Byxelkrok location demonstrates that the presence of seals influences the fishermen s catch by increasing the proportion of damaged fish. Possibly the seals also scare fish away when they are in close proximity of the nets as has been suggested by Königson et al. (2005), however this is hard to see in this study since the difference in catch is not very large between occasions when seals have been sighted and occasions when they have not been seen. The fact that so much remains are found in the nets might indicate that the cod are not scared away and that any difference in catch is solely due to that the seal take a large portion of the caught cod. In general, the results revealed some differences between the two locations. The average damage was larger at the Byxelkrok location than at the Böda location, although this difference was not always significant. One explanation for this pattern is that there are fewer links set at the Böda location. This increased the confidence interval and probably made the difference between damaged and non-damaged links nonsignificant. It could also be that there are more seals present on the west side of the island (the Byxelkrok side) since seals were only observed on the Byxelkrok location. Furthermore, the Byxelkrok location is closer to the mainland and in closer proximity of the nearest seal haulout site (BIOMAD 2005). The fact that seals are not sighted more frequently even though damages are found could be due to several factors. Whether conditions such as wind or rain make the seals harder to spot in the water and if the seals are scared they might avoid coming up to the surface in close proximity of the boat. The fact that Böda is located on the east side on the island and is more exposed to waves might be one reason for why seals are not observed there. It could also be due to the fact that the links set out are so long that a seal can feed off the net at one end without being detected even when the net is being retrieved from the other end. Finally, it is possible that the seals are more active during the night and approaches the nets in the dark, thereby limiting the chance of being seen. Most damaged fish that were found were remains left in the net. This of curse indicates that when seals are taking fish from the nets the most frequent visible damage left in the net is remains, but perhaps the numbers presented is a little misleading since it is not known how many visible damaged fishes that fall out of the nets. However, remains would probably be the damage that would fall off most frequently from the nets, since it is the smallest. Therefore, the remains portion should still be the most frequent visible damage, and may even be larger than estimated in this study. Net damage Net damages occurred more frequently at the Byxelkrok location than at the Böda location. This is probably explained by that the Byxelkrok location also has more fish damages, a fact that strengthens the idea that seals are responsible for the holes. The only holes that are certain to be caused by seals are the fish head holes, which were located where a fish head was still attached. 14

15 Holes could of course theoretically be caused by things other that seals i.e. stones on the bottom or the fishermen them selves. However, holes caused by things besides seals were most likely very limited since the holes at the bottom of the net were excluded before analysis. Hole size was also studied and results showed that holes with 2-5 mesh brakes were most common. This is probably due to that the seals have to tear the fish away from the net and that the fish is perhaps commonly attached to 2-5 meshes. On some occasions the fish are more or less entangled and the seal then makes a larger or smaller hole to get the fish. If the net damage study was to be repeated recordings should be made of the location of the fish head holes, since this would give an indication of if there is any particular area of the net that seals prefer to take fish. There is always a chance that the fish damage or net damage can be caused by other predators, such as cormorants or other sea birds (Lunneryd et al. 2004, Saulamo et al. 2001). It is, however, hard to imagine how these species could have taken such large parts of the fish or torn holes in the nets. The nets are also often set at such large depth that most seabirds cannot reach them. The fact that seals on some occasions were sited in the area when setting or retrieving the nets supports the conclusion that the damages are primarily caused by seals. Conclusions This study gives a good indication of how extensive seal damages are to the fishery in this region. During the study there was also a pattern in time of how large the damages where. During a period of 1-2 weeks in the beginning of June there were comparably fewer damages and catch/effort increased. An explanation could be that the high peak of annual moult occurs in late May, beginning of June and the grey seals gather at traditional hauling sites (Sjöberg and Ball 1999). The movement of the nets is also something that needs to be studied. During this study there appeared to be less damages to the catch at the first fishing opportunity on a new location. Does this indicate that moving the nets more frequently will decrease the seal damages? Königson (2001) showed that damages in the cod fisheries outside Västervik in the Baltic increased if they were fishing at the same locations over a long time. This study will help to understand how large the seal inflicted damages are and shed a light on how large the conflict between seals and fishermen really is. The findings in this study show that it is not possible to only count the observed damages. The hidden damages constitute such a substantial portion of the total damage that it is important to take it into account when calculating the catch loss. Better models for determining fish loss may help the fishermen to get a more accurate economical compensation. The study also shows how far south in the Baltic Sea this problem has spread. A few years ago there were hardly any reports of seal damage in this region. Today this study shows that the problem has grown substantially. To know how large the damages are and understanding the seals behavior is a start, something that this study hopefully helps in achieving. There are no easy solutions the problem of seal damage. Some believe that seal hunt is the answer, but before 15

16 starting to hunt seals again it would be good to know if many seals actually hunt in and around fishing gear or if it is only a small part of the population (Lunneryd 2001, Sand and Westerberg 1997). The restriction in the Swedish legislation of the grey seal hunt means that hunting from boats is not allowed and that there is a demand of retrieving any seals killed (Lunneryd 2003). This legislation makes seal hunt an impossible way of solving the problem with specialized seals. Another solution that scientists are working on is to improve the gear and trying to make it seal proof (Lehtonen and Suuronen 2004, Lunneryd et al. 2003). This is work that takes a long time, since all new gear has to be tested, remade and then tested again. This may solve the conflict in some fisheries, but for the cod net fishery it will be hard to find a good seal proof gear that can measure up to the nets. One solution in the future may be to change the fishing methods to either static gear such as traps or fyke nets that are possible to make seal proof. An alternative used today is longlines with hooks, but according to the fishermen this method only works during part of the year. The worst alternative could be that in the future trawling is the only way to catch sufficient amount of fish, an alternative that will exclude the majority of the fishermen, increase the fuel consumption, and damage the bottoms. Perhaps there are other solutions waiting to be discovered, but in the mean time the conflict is growing at a rate that will make it hard it to keep both a healthy grey seal population and a profiting small scale fishery for cod in the central Baltic Sea. Acknowledgements Thanks are extended to everyone that made this study possible. My primary supervisor Sara Königson at the Swedish National Board of Fisheries for her help and support, and to everyone else involved in the project seals and fisheries who has always extended a helping hand whenever I needed one. Thanks also to Jep Agrell my supervisor at Lund University for his help. A special thanks to the fishermen Tore Johnsson and Magnus Sandin for their help, smiles and understanding of all crazy things they thought I was doing. Finally, thanks to my family and friends without them this paper would have never been written. References Bergman A., Bäcklin B.M Gråsälarnas hälsa - bättre men inte bra. Swedish report on the environmental state of the Baltic proper (Miljötillståndet i egentiliga Östersjön). Stockholms marina forskningscentrum. Contaminant research group, Swedish museum of natural history. BIOMAD Top consumers monitoring. Database on marine biological monitoring data, administered by the department of Systems Ecology, Stockholm University. Fiskeriverket Småskaligt kustfiske och insjöfiske - en analys. 16

17 Fjälling A Assessment and reduction of the conflicts between commercial fisheries and grey seals (Halicoerus grypus) in Swedish waters. Licentiate Thesis, Linköping University Harding K.C., Härkönen T.J Development in the Baltic grey Seal (Halicoerus grypus) and Ringed Seal (Phoca hispida) Populations during the 20 th century. Ambio 28:7. Harvey C.J., Cox S.P., Essington E.T., Hansson S., Kitchell J.F An ecosystem model of food web and fisheries interactions in the Baltic Sea. ICES Journal of Marine Science 60: Helander B., Karlsson O Gråsäl. Bottniska viken 2004 distributed by Umeå Marina Forskningscentrum p ing/miljorapporter.htm Huse I., Løkkeborg S., Soldal A.V Relative selectivity in trawl, longline and gillnet fisheries for cod and haddock. ICES Journal of Marine Science 57: Königson S., Fjälling A., Lunneryd S.G Impact of grey seals in the herring gillnet fishery along the Swedish Baltic coast. Institute of Coastal Research, Swedish Board of Fisheries. Working paper cited with prior reference to the author. Königson S Torsk och strömmingsfiske med pop-up boj som vaktare. Slut rapport pop-up Skärså och Västervik. Fiskeriverket. Lehtonen E., Suuronen P Mitigation of seal-induced damage in salmon and whitefish trapnet fisheries by modification of the fish bag. ICES Journal of Marine Science 61: Lunneryd S.G Fish preference by the harbour seal (Phoca vitulina), with implications for the control of damage to fishing gear. ICES Journal of Marine Science 58: Lunneryd S.G Resultat från uppföljning av skador I svenska yrkesfisket relaterat till 2001 och 2002 års skyddsjakt efter gråsäl. Rapport från Projekt Sälar & Fiske. Mars Lunneryd S.G., Fjälling A., Westerberg H A large-mesh salmon trap: a way of mitigating seal impact on a coastal fishery. ICES Journal of Marine Science 60: Lunneryd S.G., Königson S., Sjöberg N.B By-catch of seals, harbour porpoises and birds in the Swedish commercial fisheries. Fiskeriverket informerar 2004:8. Lst Kalmar County administrative board of Kalmar. Seal damages 2003 and September Sand H., Westerberg H Försumbar effekt av begränsad jakt vid fiskeredskap - resultat av forskningsjakt på gråsäl Rapport från Instutitionen för Naturvårdsbiologi, Sveriges Lantbruksuniversitet och Kustlaboratoriet (Institute of Coastal Reseach, Sweden). 20 december

18 Saulamo K., Andersson J., Thoresson G Skarv och fisk vid svenska Östersjökusten. Fiskeriverket informerar 2001:7. Suuronen P., Siira A., Ikonen E., Riikonen R., Kauppinen T., Aho T., Lunneryd S.G., Hemmingsson M., Königson S., Fjälling A., Westerberg H., Larsen F Mitigation of seal damages by improved fishing technology and by alternative fishing strategies. FGFRI (Finish Game and Fisheries Research Institute). National Board of Fisheries, Institute of Coastal Research, Sweden. DIFRIS (Danish Institute for Fisheries Research). Final report of project Journal No: /02. Sjöberg M., Ball J Grey seal, Halichoerus grypus, habitat selection around haulout sites in the Baltic Sea: bathymetry or central-place foraging? Canadian Journal of Zoology 78: Söderlind A Estimation of the seal-inflicted hidden damage in the net fishery for pike-perch and whitefish. Master thesis in Marine Zoology. Department of Marine Ecology. Göteborg University Westerberg H., Fjälling A., Martinsson A Sälskador i det svenska fisket. Beskrivning och kostnadsberäkning baserad på logboksstatistik och journalföring Fiskeriverket Rapport 3: