SEAL

BEHAVIOUR AROUND FISHING GEAR

AND ITS IMPACT ON

Sara Königson

SWEDISH

FISHERIES

Department of Marine Ecology, Göteborg University 2007

Seal behaviour around fishing gear and its impact on Swedish fisheries Abstract List of Papers Svensk sammanfattning Introduction Seals along the Swedish coast The conflict between seals and fisheries Direct interference Competition for fish Dispersal of parasites Accidental by-catch Food depletion through over-fishing

The present thesis Aims Harbour seals and the west coast fisheries Grey seals and the east coast fisheries Conclusions and recommendations Acknowledgements References

2 3 4 4 4 5 6 6 7 7 8 8 8 9 10 14 15 16

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Abstract The conflict between seals and fisheries along the Swedish coast is of serious concern. In the Baltic Sea, certain coastal fisheries, such as the herring gillnet fishery, are on the point of collapse because of this conflict. Damage caused by grey seals in this fishery is shown to be much more extensive than previously recognised. When seals raid the nets, a significant part of the catch is lost without trace. Seals pick out the fish from the nets without leaving any remains. In addition, the presence of seals around fishing gear scares fish away from the area, which can extend to a total loss of catch. The development of seal-safe fishing gear is crucial and is at present the only long lasting solution which would respect the needs of both seals and fishermen. However, development of new fishing gear is challenging and time-consuming and requires greater knowledge of both fish and seal behaviour. At present there is no suitable alternative fishing gear available for the gillnet fishery. If no solution is presented in the very near future, the only option for this fishery’s survival will be to reduce the seal population in areas subject to severe interference. In the eel fishery on the west coast of Sweden, seal-safe fishing gear (eel fyke nets) has been developed and is already in use in the commercial fishery along the west coast. Trials with seal-safe fyke nets were carried out over many years both by professional fishermen and the Swedish Board of Fisheries. A fyke net which resisted attacks and retained a comparable fishing efficiency was developed. In order to find a lasting solution to the problem, seal behaviour has been studied as well as the fyke nets' fishing efficiency. The issue of whether it is the eels or the by-caught species which attract harbour seals to attack fyke nets was raised. It was shown that harbour seals raiding fyke nets have a preference for eel. Certain harbour seals specialise in foraging at fyke nets and have developed different feeding preferences compared to other seals. Video observations of a fyke net baited with eel also showed that seals which have specialised in this behaviour may return repeatedly to attack the fishing gear. This is important knowledge which has been useful in the development of seal-safe fishing gear and in the management of harbour seals.

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List of Papers The thesis is based on the following papers, henceforth referred to by their Roman numerals (IIII) I

Königson, S., Lundström, K., Hemmingsson, M., Lunneryd S-G., and Westerberg, H. 2006. Feeding Preferences of Harbour Seals (Phoca vitulina) Specialised in Raiding Fishing Gear. Aquatic Mammals 32 (2):152-156

II

Königson, S., Hemmingsson, M., Lunneryd S-G., and Lundström, K. 2007. Seals and fyke nets: An investigation of the problem and its possible solution. Marine Biology Research 3: 29-36

III

Königson, S., Fjälling, A., and Lunneryd, S-G. 2007. Grey seal induced catch losses in the herring gillnet fisheries in the northern. In press. NAMMCO

Publications are reproduced with permission from the publisher.

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Svensk sammanfattning Det kustnära fisket i Sverige har under de senaste årtiondena genomgått en negativ utveckling. En av orsakerna är att sälskadorna ökar längs stora delar av Sveriges kust. Det är framförallt det småskaliga kustnära fisket som drabbas. I Östersjön, där gråsälen är den främsta skadegöraren, har fisket med fasta redskap efter lax och sik varit mest utsatt men på senare år är nätfisket ett av de fisken som drabbats allvarligast. Med en gråsälpopulation som under 2000-talet ökat med mer än 10 % årligen, riskerar vi, om inte några direkta åtgärder vidtas inom en snar framtid, att flera mindre kustnära fisken längs kusten utrotas. Ett av de hotade fiskena är fisket efter strömming med skötar. De ekonomiska förlusterna som syns i form av skadad fångst och förstörda redskap är endast en liten del av de totala förlusterna. När säl attackerar skötarna försvinner stora delar av fångsten utan att sälarna lämnar några spår. Sälarna plockar ut hela fiskar ur näten och dessutom tyder data på att fisken skräms iväg från redskapet när sälarna patrullerar längs näten. Om sälar varit i närheten när skötarna lagts kommer skötarna upp tomma utan varken skadad eller hel fångst. Att utveckling av sälsäkra redskap är den mest hållbara och långsiktiga lösningen på problemet är de flesta intressenter överens om. Detta kräver kunskap om både fiskens och sälens beteende i förhållande till redskapet. För tillfället finns inga fungerande lösningar på sälproblemen i nätfisket. Om inga lösningar presenteras inom en snar framtid så är det enda alternativet för att strömmingsfisket ska fortleva en reducering av sälstammen. På västkusten är det knubbsäl som dominerar och även de orsakar stora ekonomiska förluster i det småskaliga fisket. Ålfisket med ryssjor är det fiske som drabbats värst av sälskador och i detta fiske har det skett en riktad insats för att minska skadorna. Sälsäkra ålryssjor har utvecklats där både sälens beteende och redskapens fiskeeffektivitet haft betydelse för ryssjornas utformning. Ålryssjor med starkt material i fiskhuset, där ålen samlas, används idag i det kommersiella yrkesfisket. Det diskuterades under lång tid vilken fisk i ryssjorna som attraherar sälarna. Var det ål eller var det den bifångade torsken? Det visade sig att sälar som attackerar ålryssjor prefererar ål framför torsk, tånglake och platfisk. Trots detta visar flera studier att ål är ytterst sällsynt i sälarnas diet. Detta talar för att vissa sälar specialiserar sig på att attackera redskap och att de sälarna har en preferens för ål i ryssjorna. Filmer, filmade med en undervattenskamera placerad bredvid en ryssja betad med ål, visade också att samma säl återkom flera gånger till ryssjan för att söka föda. Denna kunskap har haft stor betydelse i utvecklingen av sälsäkra ålryssjor.

Introduction Seals along the Swedish coast There are three species of seals along the Swedish coast; the grey seal (Halichoerus grypus), the ringed seal (Phoca hispida) and the harbour seal (Phoca vitulina). On the west coast of Sweden the prevalent species is the harbour seal. Grey seals do occur in the area, but only very rarely (Härkönen & Lunneryd, 1990). In the early 20th century harbour seals were seen as a competitor to the commercial fisheries and a bounty system for hunting was introduced in Sweden in 1902. During the 1960s the population was estimated at 2-3,000 animals and harbour seals were considered endangered (Härkönen & Lunneryd, 1990). Hunting ceased in Sweden in 1967 and after that the population increased dramatically. In 1988 an epizootic occurred and a large part of the population died. After the epizootic, harbour seal numbers had increased from around 5,000 to 19,000 animals, when in summer of 2002 another severe epizootic hit the west coast population. By the end of September 2002, the death toll in the Kattegat and Skagerrak was over 6,000 individuals (Harding et al., 2002). The recovery of the population after the second epizootic is thought to have proceeded on the same lines as after the first, with an annual increase in numbers of over 10 %. Today the harbour seal population in the Kattegat and Skagerrak is estimated to be around 9,500 (NRM). 4

In the Baltic, all three species of seals are present. Ringed seals are found mainly in the northern Baltic, where their numbers were estimated to 4,100 in 2005 (NRM). There is a small population of harbour seals south of Öland. This population is considered endangered and was estimated at 390 individuals in 2004 (Härkönen et al., 2005). Grey seals are the dominant species in the Baltic overall. It is mainly the grey seal that causes damage and losses in commercial fisheries, although reports of damage caused by ringed seals also occur in the northern part of Sweden (Hemmingsson & Lunneryd, 2005). In the middle of the 20th century high levels of organochlorines were found in both grey seals and ringed seals. These compounds caused damage to the reproductive organs of the females, which led to a strong decline in the grey seal population (Helle et al., 1976; Bergman & Olsson, 1986; Olsson et al., 1994). Since around 1980, organochlorine pollution in the Baltic has been reduced and this has allowed the grey seal population to recover (Hårding & Härkönen, 1999; Halkka et al., 2005; Karlsson & Helander, 2005). In the seal census carried out in 2004, the number of grey seals counted in the Baltic was 17,640 (Halkka et al., 2005). In 2006 the number of counted grey seals in the Baltic was 20,700 (RKTL). Photo ID studies indicate that the count covered 80 % of the total population which means that the population is now well over 25,000 animals. The conflict between seals and fisheries Small scale fisheries are widely scattered around the Swedish coastline and are of great importance to the local population in many coastal villages. Inshore fishing is often carried out by a single fisherman, making daily fishing trips and returning every night to harbour. Inshore fisheries, which are often small scale fishing operations, tend to suffer from diminishing fish stocks and structural problems. In addition to these problems, the inshore fisheries are also the fisheries most subjected to damage caused by seals. The grey seal population in the Baltic has increased dramatically during the last decade (Karlsson & Helander, 2005). The seals-fisheries conflict in the Baltic has escalated in parallel with the population explosion (Baltscheffsky, 1997; Kauppinen et al., 2005; Westerberg et al., 2000; Lunneryd, 2001; Fjälling, 2004). If no effective mitigation measures for seal-induced damage are developed, the conflict between the protection of seal stocks and the existence of the coastal fisheries will become very serious (Suuronen et al., 2006). This conflict is reciprocal in that seals affect the fisheries and the fisheries affect the seals. The negative impacts on the fisheries include direct interference with fishing gear and catches, competition for the food resource and the dispersal of parasites. Fisheries also have a negative impact on seals. Accidental by-catch directly affects seals and occurs in many different fisheries. Food depletion through over-fishing could also have a negative effect on seals. Table 1. Interactions between fisheries and seals.

Seals → Fisheries

Fisheries → seals

Direct interference Competition for fish Dispersal of parasites

Accidental by-catch Food-depletion through over-fishing

Direct interference When a seal attacks fishing gear, fish remains are some times left as evidence that direct interference has occurred. In several reports concerning the seals-fisheries conflict, the loss of catch caused by seals has been measured by looking at the percentage of fish remains found in the catch, see (Wickens, 1995). However, the visible damage such as fish remains only represents a minor part of the catch loss caused by seals (Königson, 2007; Stridh, 2006; Sundqvist, 2005). In addition to the visible catch loss, there is a part of the catch which is completely removed from the fishing gear by the seals, leaving no trace (Fjälling, 2005; 5

Königson, 2007; Sundqvist 2005). Seals can also scare fish away from the fishing area, creating additional hidden losses (Königson, 2007). Damage to fishing gear is also an economic loss for the fishermen. The visible gear damage such as holes and tears in the nets is only a small part of the total economic losses. Indirect expenses include costs for new material, time for repairing fishing gear and reduction in gear durability. Additional costs such as increased time and fuel consumption due to emptying the gear more often should also be included in the total. Altogether, the cost of the visible catch losses and damage to fishing gear are just a minor part of the actual total costs. The total economic losses for fishermen after a seal attack are listed in table 2. Direct catch losses and indirect costs together amounted to at least 15-20 % of the total catch value in the inshore fisheries in 1997 (Westerberg et al., 2006). Seal interference is an important reason for the current poor viability of the fisheries concerned, and for the ongoing low recruitment and gradual decline in the number of active fishermen (Anon, 2001). Table 2. The visible and direct losses to catch and gear are only a small part of the total losses fishermen are subjected to. Invisible and indirect losses also need to be taken into account when estimating total losses.

Total losses due to seal attacks Visible and direct losses

-Damaged catch -Damage to fishing gear

Invisible direct losses -Catch removed completely from the fishing gear -Fish scared away from the fishing area Indirect losses due to damaged fishing gear

-Loss of catch due to damaged fishing gear -Costs of new material -Time spent repairing fishing gear -Reduced life of the fishing gear

Indirect additional losses

-Increased time and fuel consumption due to checking and hauling the fishing gear more often -Longer fishing trips to areas where there is less seal interference -Lost fishing opportunities, due to both fishing grounds and fishing gear not being worth using any more as a result of seal interference

Competition for fish The question has been discussed, whether fish consumption by seals affects fish stocks in the long run. The Baltic Sea is considered to be a "top-down" ecosystem in which consumers such as seals control the abundance and diversity of species at lower trophic levels. In such a system there is a competition between fisheries and apex predators for the food resource, and reducing the top predators is likely to increase the availability of fish biomass to the commercial fisheries. Österblom (2006) indicated that the Baltic Sea is indeed one of only a few areas in

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the world where reduced top-down control has been responsible for changes in the fish community. Kaschner & Pauly (2004) also indicated that competition between marine mammals and fisheries is likely in the Baltic, while Hjerne et al. (in prep) have preliminary results which show that the grey seal has the same impact on fish populations as the Swedish fisheries have for certain commercial species in the Baltic Sea. Commercial fisheries have significantly affected fish stocks so it is likely that seals can also affect fish stocks. Österblom (2006) has described and come to conclusions about the changes in the Baltic ecosystem through time. In the early 20th century the seal population in the Baltic was high, which Österblom (2006) suggests resulted in low prey abundance of cod. The Baltic then changed from a seal to a cod-dominated state when the seal population decreased. After that it changed from an oligotrophic to a eutrophic state, and finally from a cod-dominated to a spratdominated state. We are now at the clupeid-dominated state where the seals' prey species are low and seal numbers are increasing. Even though the decline of the cod population cannot be correlated with the increase in the seal population, it is likely that the recovery of the fish stock will be affected by that increase. Chouinard et al., 2005 also suggested that grey seal predation increases natural mortality in prey species. Natural mortality played a role in the collapse of fish stocks and their failure to recover in the North West Atlantic. Jounela et al. (2006) suggests that because of the growing grey seal population it would be useful to assess the effect of natural predation by seals on salmon stocks in the Baltic. Österblom (2006) also suggests that an ecosystem approach should be implemented in fisheries management to integrate knowledge of seabird consumption requirements with the ICES definitions of “safe biological limits” for the Baltic Sea clupeid stocks, and thereby take account of the role which common guillemot play in clupeid consumption. Maybe fish consumption by grey seals should be taken into account in the same way in the management of these fisheries. Dispersal of parasites Three parasitic nematodes are found in the stomachs of Swedish seals. Two of these, Pseudoterranova decipiens (sealworm) and Contracaecum osculatum have seals as a true final host. The seal worm occurs in the flesh of the fish and is therefore a problem to the commercial fisheries. Both harbour seals and grey seals get infected by seal worms but the latter commonly have larger infestations and are therefore a more important vector for the parasite. It is alarming that the seal worm is now commonly found both in benthic fish species and grey seals from the Baltic (Lunneryd, pers. com.). With the increasing grey seal population this could become a serious problem for the fishing industry. The seal worm has a complex lifecycle including intermediate hosts such as crustaceans and fish. The seal worm could infect humans if raw or marinated fresh fish was ingested, although the likelihood of infection is low (Adams, 1997). The main problem is that infected fish look unattractive to consumers. This brings added costs for the processing industry but also for the small scale fishermen who prepare their own fish for market. Accidental by-catch Lunneryd et al. (2004) suggest that about 450 grey seals were by-caught in the Swedish commercial fisheries in the Northern Baltic in the year 2001. In the Gulf of Bothnia, about 50 ringed seals were by-caught, while more than 400 harbour seals were by-caught off the west coast of Sweden. This study was based on a telephone survey of 16 % of all Swedish commercial fishermen. The fisheries for cod and flatfish with bottom-set nets are the fisheries where most seals get by-caught, and the species most affected is the grey seal. Many grey seals also get by-caught in various traps used to catch salmon and whitefish. Before the introduction of the so-called 'push-up trap', the Baltic Sea trap fishery was also the fishery most subject to

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seal damage in Sweden. Harbour seals, on the other hand, are often by-caught in eel fyke nets, the fishery most subject to damage by seals on the west coast. The current levels of by-catch cannot be considered a serious threat to the Swedish seal populations, as all three seal species have demonstrated strong population growth in recent years. Nevertheless, by-catch of seals is not considered ethical and fishermen do have problems handling the by-caught animals. Bycaught seals are a part of the seals-fisheries conflict. A way to solve the by-catch problem is to develop seal-safe fishing gear which is uninteresting for seals and reduces seal presence in the area. A decreased presence of seals around the gear would naturally reduce the amount of bycatch. Food depletion through over-fishing There is little conclusive evidence that prey depletion caused by human activities such as fishing can reduce marine mammal stocks, although Trites et al. (2006) suggest that intensive fishing may lead to large and long-lasting ecosystem changes. Heavily fished ecosystems do not always have the ability to support abundant, healthy populations of apex predators. Marine mammals can even be affected by fisheries when prey and species caught do not overlap. They indirectly compete for the primary production required to sustain the prey and species caught, so-called food-web competition (Trites, 1997). However food depletion can also create responses such as migration, with predators moving from an area where fish stocks are depleted to other areas. In turn, shifting their distribution can subject the animals to increased incidental mortality such as by-catch in bottom set gillnets. This occurred in the 1980s in the Barents Sea when the capelin stock collapsed due to over-fishing in offshore waters. Harp seals migrated towards the Norwegian coast and got by-caught in inshore fishing nets (Nilssen et al., 1992). However, there is no evidence that depletion of fish stocks by commercial fisheries affects the survival rate of seals in Swedish waters.

The present thesis Aims The aim of my work at the Swedish Board of Fisheries is to find mitigation methods to reduce the conflict between seals and fisheries. To accomplish this, knowledge about both seal behaviour and the fishery concerned is needed. The aim of this thesis was to study the conflict in two different fisheries where two different seal species are the cause of the problem: harbour seals in the west coast eel fishery and grey seals in the Baltic Sea herring gillnet fishery. Harbour seal behaviour in relation to fishing gear has been of importance in the search for a lasting mitigation method in the eel fishery along the west coast. (Papers I and II) presents some aspects of harbour seal behaviour around eel fyke nets, how they affect the eel fishery negatively and finally a solution to the conflict. In the gillnet fisheries along the Baltic coast, fish caught in gillnets are effectively impossible to protect from grey seals. Many methods have been tried to reduce losses and damage in these fisheries, but without any successful results. To move forwards towards a solution, the conflict in the Baltic herring gillnet fisheries has been studied in detail as well as grey seal behaviour around herring gillnets. Knowledge gained from these studies will be useful in any future grey seal management plans. It is also vital information in the development of a lasting mitigation method. Harbour seals and the west coast fisheries On the west coast of Sweden it is the eel fishery which suffers the greatest losses due to seals and harbour seals are responsible for the bulk of the damage (Königson et al., 2003, Königson et al., 2006). The west coast eel fishery is economically one of the most important segments of the Swedish small-scale fishing industry. Fyke nets, pound nets and traps are the types of 8

fishing gear commonly used, with fyke nets being the principal gear in use and also the fishing gear most subject to damage. Damage consists of tears or small holes in the net, mainly in the fish bag where the fish gather. Eels are also dragged through the mesh itself and bitten in half. Seals that attack fyke nets prefer eels, in other words eels are the main reason why the fyke nets get attacked, even though the fish bags of a fyke net often contain many fish species that could be tempting to a seal (Königson et al., 2006). A consequence of this is that the frequency of damage incidents will intensify with increasing eel catches, which in turn will increase the fishermen's economic losses. During an experimental cull in 2001, eight seals were killed in two areas subjected to high incidents of net damage in order to see if gear damage decreased subsequently. Stomach contents from two of the seals culled in close proximity to the fyke net consisted mostly of eel (11 undigested eels from one of the seals' stomachs, figure 1).

Figure 1. The stomach contents of a seal consisting of 11 undigested eels. The seal was shot near to an eel fyke net in an area where fishing with fyke nets is common.

Several studies have shown that harbour seals feed on a variety of prey. The main prey species are herring, codfish and flatfish. However, a scat analysis during a three-year study in the Skagerrak identified 40 different species, although eels were rarely found (Härkönen, 1987; Härkönen & Heide- Jørgensen, 1991). In conclusion, eels are not normally part of a typical harbour seal's diet. This raised the question as to whether there were a few seals that had developed this feeding preference and were returning repeatedly to the same area to forage. It has been concluded in many studies that individual seals have a regular pattern of visiting the same feeding areas and then returning to their haul-out sites (Bjørge et al., 1995; Tollit et al., 1998). Tollit et al. (1998) also found that individual seals had different foraging habitats. They suggested that this might indicate an individual specialisation in certain predation and foraging techniques. The preference for eels in fyke nets and the results from the experimental cull showing stomach contents with eels as the dominant species indicate that certain individuals have indeed specialised in this way. To study seal behaviour around fyke nets and to analyse whether it might be the same seal or seals attacking the fyke nets repeatedly, we placed an underwater camera next to a fyke net containing eels to tempt the seal (Königson et al., 2003). Seals were observed on six occasions out of 521 filmed hours both night and day. On five of these occasions, the seal could be identified as the same seal (figure 2) repeatedly coming back to the fyke net during a period of almost two months.

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Figure 2. Pictures of two of the five occasions when seals were identified attacking the baited fyke net. The pattern circled makes it possible to identify the seal.

How did this conflict intensify to the level that it is at today? Researchers have suggested that declining fish stocks might be one of the reasons why the conflict has increased. However, the abundance of juvenile cod, which is one of the harbour seals' main prey items on the west coast, did not decrease during the 1990s (Lagenfeldt & Svedäng, 1999), the period when seal damages increased rapidly. Herring, another prey item which is highly significant in the harbour seals' diet, actually increased. Therefore declining fish populations is not a probable explanation for the increased conflict. It is more likely that certain seals have developed the behaviour of raiding fyke nets because it is energy saving and easy for the seal to find food in the fishing gear. An increasing number of seals might have developed this behaviour in proportion to their increasing population. Grey seals and the east coast fisheries The most severe damage and losses in Swedish fisheries are reported from the Gulf of Bothnia, where grey seals are responsible for most of the conflict. The fisheries most affected are the set trap fishery for salmon, sea-trout and whitefish and the gillnet fisheries for herring and whitefish. The trap-net fishery in the Baltic is in many respects a model fishery - being selective, energy saving and harmless to the benthic environment. The traps used are a huge construction including a leader arm, a trap and a fish chamber where the fish gather. Seals have learnt to get hold of the catch in the fish chamber by entering the trap either via the main entrance, over the net panels or through a hole torn in the side of the fish chamber. They also use the net panels of the trap to hunt fish on their way into the fish chamber. Many solutions have been tried to reduce the seal damage, including a seal scarer or so-called Acoustic Harassment Device (Fjälling, 2005). However the most successful solution has been the recently developed 'pushup' fish chamber which keeps the caught fish protected from the seals (Suuronen et al., 2005; Lunneryd et al., 2003) (Figure 3). This fish chamber, in combination with a large mesh trap, which prevents seals from using the trap to hunt fish by allowing fish to escape through the net when being chased, is a successful development of seal-safe fishing gear (Lunneryd et al., 2003). The traps are now used by 86 % of the Swedish salmon trap fishermen along the northern Baltic coast (Hemmingsson & Lunneryd, in prep).

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Figure 3. The 'push-up' trap, here seen on its way up to be emptied, consists of a fish chamber connected to a large mesh trap.

The gillnet fishery on the other hand is an unsolved problem. Seals locate the nets where fish have been caught and help themselves as if it was a dinner table. This herring fishery is probably the fisheries with the highest frequency of seal damage along the Swedish coast. However the conflict is also increasing in gillnet fisheries further south and seals are the cause of very high economic losses in the cod fisheries of the Baltic proper (Sundqvist, 2005). Fishing for herring with gillnets is carried out both in the spring and in the autumn in the northern Baltic. Fishermen have claimed that damage by seals is most severe in the autumn when the accessibility of herring has decreased. Our results, based on both the daily EUlogbooks and on voluntary reports from fishermen, also showed that levels of damage increased in the early spring and in the autumn. Seal interference was the least during May to July, and reached a maximum by the end of the year (figure 4). This reflects the life histories of both the herring and the seals (Königson et al., 2007). In the late spring, herring spawn and aggregate in shallow waters near the coastline. This supplies seals with an abundant food source, thus decreasing any motivation to visit fishing gear. In the early spring, adult Baltic grey seals focus on mating; this begins right after weaning the pups in February-March. During this time the adults, at least the males, do not eat at all (Bonner, 1972). Somewhat later, in May and June, the Baltic grey seals moult and spend most of their time on land for some weeks (Söderberg, 1974).

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Proportion of entries with seal interaction

100 % 90% 80% 70% 60% 50% 40% 30% 20%

Daily log bVoluntary k log

10%

b k 0% 1

2

3

4

5

6

7

8

9

10

11

12

Month

Figure 4. Relative frequency of seal interactions in the herring gill-net fisheries throughout the year, based on entries in the voluntary log-books (mean per month for 2003 and 2004), and in the daily log-books (mean per month for 2000 to 2005) in the northern Baltic, north of 60º00´N were use. Error bars showing s.e. bars. For January and February, data from the daily log-books were excluded since there were no data in the voluntary logbooks to compare with.

Damage by seals includes visible damage such as damaged fish and fishing gear, but also hidden losses. The hidden losses include fish that are removed completely from the fishing gear, leaving no traces, and losses of fish that are scared away from the fishing area by seals patrolling the nets. These hidden losses are of great significance in the herring gillnet fisheries (Königson et al., 2007). Placing a net in an area where there are seals around is pointless, according to the fishermen. When this happens by mischance, in most cases the nets are retrieved with no catch at all, or only a few damaged fish remaining in the net. Seal interference represents a great threat to the herring fisheries and may lead to a collapse of the entire fisheries. There is therefore an urgent need for mitigation methods, especially for non-stationary gear such as gillnets. To develop such mitigation methods, we need to know more about seal foraging behaviour in general and about how seals locate and exploit fishing gear in particular. Do seals for example use the marker buoys on the fishing gear to find the nets? Fishing grounds are often located in the open sea many nautical miles off the coast, where no landmarks are visible to enable the seals to find the fishing gear, and yet the seals seem to arrive at the gear very quickly. Königson et al. (in prep.) and Königson (2002) found that damage did not decrease when fishing gear was hidden under the surface with a submersible buoy which surfaced when it was time to empty the nets. In another investigation, Fjälling et al. (in press) set out different sizes of buoys within a defined area, baited with herring underneath. A positive correlation between the buoy diameter and the proportion of bait missing was found. Most likely therefore, seals can indeed use their eyesight at short ranges for locating buoys above the surface. However in the normal course of events it seems that seals locate the fishing gear without help from the surface marker buoys, so hiding the fishing gear is not an effective mitigation method for gillnet fisheries.

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Other attempts to mitigate the impact of seals on the herring gillnet fisheries have been made, but without success. However the studies have added to our understanding of seal behaviour. In another trial, an Acoustic Harassment Device (AHD) was submerged together with the nets in hope of keeping seals at a distance. AHDs have shown positive results when used in the trapnet fishery in the Baltic (Fjälling et al., 2006). Catches increased and damages decreased when AHDs were in use next to the salmon traps, which are set in shallow water near the mouths of estuaries. However damage levels did not decrease when the AHD was used in the herring gillnet fishery. This could be because the device had many technical faults and did not always emit sound at the level it was supposed to. It is of great importance that any AHDs do emit sound at the right level, or else there is no seal deterrent effect. With the herring gill nets, set in deeper waters, we came to suspect that the device was working more as a dinner-bell than a harassment advice. To test the dinner-bell theory, we placed the AHD close to the coast, with an observer on shore, to see if any seals would be attracted to the sound, having got used to associating this with the fishing gear in the earlier experiments. On four out of five occasions seals did indeed approach the AHD after only half an hour. The AHD did work as a dinner-bell and we concluded that the seals had been using their hearing to more quickly locate the fishing gear. So maybe seals can also learn to associate the noise of the fishing boat with a feeding opportunity and then follow the boat to the fishing area. Another factor to consider is that for generations past fishermen have set their nets on banks where fish aggregate. Sjöberg & Ball (2000) suggested that grey seals maximise their feeding effort in areas where the sea floor is steeply sloped and herring schools concentrate. These are often the same sites where fishermen set their nets. If the fishermen fish on the banks where the seals usually forage, the probability that the seals will eventually find the nets and return to these feeding grounds must be large. As a possible solution to the gillnet fisheries conflict, large cages filled with herring were placed some distance from the fishing grounds in order to distract the seals away from the fishing grounds. The results showed no apparent decrease in the damage levels but some interesting aspects of seal behaviour were revealed. By filming one of the cages we noticed that the same seals came back repeatedly every day for a week. Two seals were identified feeding from the cage (figure 5) and by supplying a measured amount of herring in the morning, filming the two seals and then weighing the remains, we concluded that it was possible for the two seals to eat in average 32kg of herring in 10 hours (Königson et al., in prep). The cage was filmed and the remains weighted at three occasions. The amount of fish that a grey seal can eat has been much debated. The most common view is that grey seals consume between 3kg and 7kg per day (Rae, 1960; Innes et al., 1987; Ronald et al., 1984 and Mansfield & Beck, 1977). Königson et al., (in prep.) showed that it is possible for a seal to eat as much as 16 kg per day during certain circumstances, which is a higher amount consumed fish than commonly known. The following year, after the feeding cage project was finished, a second experiment was carried out in the same location. Feeding cages were again filled with herring and a cull was carried out to eliminate the seals feeding from them. The idea was that the seals which had specialised in feeding from the provisioning cage might be the same ones which were raiding the fishermen's nets. By eliminating these seals, maybe damage to the fishery would decrease. The experiment did not in fact produce any decrease in damage levels; however other interesting information did come up. One previously identified seal did indeed return to the cage a year later and was then shot. It has been discussed whether individual grey seals have specialized in the behaviour of attacking fishing gear; and whether it is the same seals returning

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to the fishing gear all the time. The results from the feeding cage project provide the first example that the same seals do return to a specific feeding site, and over a long time period.

A. B. Figure 5. A. The seal identified in 2004 at the feeding cage which returned every day to feed on herring. B. The same seal which returned to the feeding cage in the same area in 2005 and was culled. The pattern which made it possible to identify the seal is circled.

Conclusions and recommendations The most successful mitigation method for solving the conflict to date has been the development of seal-safe fishing gear. Passive fishing gear such as traps or fyke nets are possible to make seal-safe. In the eel fishery along the west coast, seal-safe fyke nets have been developed. These fyke nets have a stronger material in the fish chamber where the fish gather. Replacing standard fyke nets with seal-safe fyke nets not only decreases damage by seals but can also lead to increased catches, in areas where seal attacks are a problem. This is now an approved solution for decreasing the damage caused by harbour seals in Sweden. Modification of gear can now be subsidised by local authorities, using funds from the Environmental Protection Agency. A general goal is that fyke nets all around the coastline will be replaced with seal-safe fyke nets in order to prevent seal damage in all fishing areas. Filming has shown that seals put in a lot of effort when raiding a standard fyke net. With only seal-safe fyke nets to attack, the possibility that the seals will not get rewarded when attacking fyke nets is higher and the energy cost for trying to tear through the strong material will be very high. Therefore those seals will hopefully abandon the tactic of attacking fishing gear and it will certainly be harder for new seals to learn the behaviour. There are harbour seals which have specialized in attacking fyke nets and have developed a certain feeding preference for eels. Because of that, if the hunting of seals should be permitted in order to reduce the levels of damage to fisheries, it should be done in a targeted way, close to fyke nets which have been attacked, and not by randomly culling seals in a haul-out area. It is reasonable to expect that the culling of a limited number of seals could still achieve a considerable positive effect. In the herring gillnet fishery, as in other gillnet fisheries, there is no simple solution to the present conflict. The only reasonably seal-safe alternative fishing method presently available is trawling, which has several other negative implications such as high energy consumption, reduced selectivity as regards the size of herring, and a generally larger by-catch of unwanted fish species. Development of new fishing gear such as large seal-safe herring traps takes time and knowledge and may be impossible as a practical and economical solution. Therefore more knowledge about both fish and seal behaviour is urgently needed. The herring gillnet fishery is on the verge of collapse and it has come to the point where a substantial cull at haul-out sites might be necessary if no other mitigation methods are developed very soon. Contrary to the 14

situation with harbour seals along the west coast, culling of specific individuals is more difficult or impossible as the nets are often placed far out in deep water. In the long term, mitigation methods such as the development of new fishing gear represent the most sustainable solution. Seals affect the gillnet fishery negatively and seriously and this must be taken into consideration in any management plans for the Baltic grey seal. The total losses caused by seals, both visible and hidden losses, also need to be taken into account when estimating the damage caused by seals. Acknowledgements I would like to thank my colleagues Sven-Gunnar Lunneryd, Malin Hemmingsson, Arne Fjälling, Karl Lundström and David Börjesson in Project Seals and Fisheries. Without their support I wouldn’t have been here today. This also includes Håkan Westerberg, the initiator of Project Seals and Fisheries. Thank you, Graham Timmins for always being honest and critical when reading my reports. In all our projects we have been working closely with fishermen and their knowledge and help has been most vital. So I would like to thank all fishermen who participated in this project. Bengt Börjesson introduced me to the eel fishery on the west coast and Dennis Westlund taught me everything there is to know about the herring gillnet fisheries. I would also like to thank my family for encouraging me, and Lena Granhag the founder of “Blända kollektivet” for all her good advice which has been very useful. Thanks also to our volunteer and our students, Irene Weinberger, Anna Söderlind and Marie Mårtensson, for helping out during long hours at sea. Finally, Henkan, who is the best part of my life. The project was funded by the Swedish Board of Fisheries, the Swedish Environmental Protection Agency and the European Union.

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