Fishing gear in the Baltic is often raided by grey seals (Halichoerus grypus). The seals remove the fish and damage the nets, or entangle themselves and drown. In order to develop ways of mitigating the seals-fisheries conflict, it is important to know exactly how the seals locate the fishing gear. A field experiment was conducted in order to clarify whether seals use their vision above water to do this. Bait (herring; Clupea harengus) was attached to the anchor lines of buoys of the type that is commonly used to mark the position of fishing gear. In all, 643 buoys were set. Some of the buoys (210) were also fitted with camera traps. Weather data were collected from official weather stations nearby. Bait loss (mean 18%) was significantly correlated with buoy size (P = 0.002) and wind speed (P = 0.04). There was a significant association between bait loss and seal observations near the buoys (P = 0.05). Five photos of grey seals were obtained from the camera traps. No fish-eating birds, such as cormorants or mergansers, were ever observed near the buoys or caught on camera. It was concluded that a main cause of missing bait was scavenging by grey seals, and that they did use above-water vision to locate the buoys. It was also concluded that wind strength (i.e. wave action) contributed to the bait loss. The camera trap buoys had a somewhat lower bait loss than the other buoys (P = 0.054), which was attributed to a scaring effect. Neither the number of seal observations nor the bait loss differed significantly between the 2 study areas in the experiment (P = 0.43 and P = 0.83, respectively). Bait loss was not affected by the buoy colour (red, white, or grey; P = 0.87). We suggest that the findings of this experiment could be put into practice in a seal-disturbed area by deploying a number of decoy buoys, or by hiding live buoys below the surface of the water. This would increase the cost of foraging for the seals, and hence discourage them from exploiting fishing gear as a feeding place.

The interaction between grey seals (Halichoerus grypus) and the Baltic fishery for herring (Clupea harengus) was investigated for the period 2000-2004, using a three level perspective. Data from the official EU log-book system, data from a voluntary log book system, and data from field studies were combined. It was found, based on records from the official log-book and using a method with paired data compensating for temporal variations in catches and seal activity, that catches were significantly higher for fishing days when there were no seal interactions recorded during setting or lifting the nets, compared to days when there were such notations (0.59 and 1.03 kg /m net * day respectively; p< 0.001). It was found that the frequency of seal-disturbed fishing efforts, encompassing 30 % of all records of herring gillnet fishing in the official log-book, was probably an under-estimation and explained by the fact that making notes on seal interactions are optional for the fisherman. The corresponding figure of the occurrences of seal-disturbed fishing efforts was 60% in a voluntary log book system, which requires the contracted fishermen to record all occurring seal-interactions, in addition to detailed data on the whole of the fishing operation. There was a pronounced variation in the frequency of seal-disturbed fishing efforts in relation to the time of the year. The interaction was least in the early summer, and reached a maximum at the end of the year. The variation is alleged to be dependent on the life cycle of the seals and its prey, herring. It was found that the calculated seal-induced losses were larger than the occurring number of seals in the area reasonably could have consumed. It was therefore deducted that there was a significant hidden catch-reducing scaring effect from seals’ presence near the nets. The catches in the herring gillnet fishery decreased over the investigated period, whereas the catches in the trawling fishery increased, as revealed by the official log-book data. The variances in the catches were however too large to allow for an analysis of possible effects of seal interactions. The method that worked best for estimating the catch losses was using paired data which compensates for temporal variations in catches and intensity of seal interaction. A method using nets baited with marked fish for estimating the hidden losses was tested, but did not work well since seals removed more fish than the method could accept. Seals visited the experimental herring nets in 14 of the 19 trials. In 11 cases, more than 95% of the marked fish went missing. It is argued that the herring gillnet fishery in the north Baltic is severely affected by the seals-fisheries conflict.

Acoustic harassment devices (AHDs) were deployed at salmon-trap nets in the Baltic Sea to reduce gear and catch damage by grey seals (Halichoerus grypus). The AHDs emitted pulses of 250-500-ms duration, worked at a frequency of 15 kHz, and a source level of 179 dB re 1 μPa rms at 1 m. AHDs were deployed during three consecutive fishing seasons. Catches were significantly higher in traps with AHDs (25.5 kg d^-1) than in controls (12.0 kg d^-1), and catch damage was less (3.5 vs. 6.7 kg d^-1). These results persisted over and between fishing seasons, but late in the season damage to the catches was common also in traps with AHDs. This study shows that the AHD may be a complementary mitigation tool in the seal-fishery conflict in certain types of fisheries, even though it is technically demanding, and for environmental reasons should be used with great care.

There is a problematic interaction going on between grey seals and the small scale coastal fisheries in the Baltic. A large number of seals are by-caught and drowned each year, and the viability of the fishery is threatened by catch losses caused by the seals. Traditional mitigation methods are not sufficient, or have in some cases not been properly evaluated. Available methods of quantifying and analysing the catch losses are also insufficient. This thesis consists of three parts, each studying a different angle of this conflict.

In the first part, new models for estimating catch losses are presented. In addition to the commonly used method of counting the number of damaged fish in the nets, the new models also allow for an estimation of the hidden losses. Hidden losses may be fish that are completely removed from nets without leaving any traces, fish that escape through holes in the net torn by the seals, or even fish that are scared away from the fishing gear. Such losses were found to be significant, and hence it is now clear that the traditional models seriously underestimate the total losses. The new models also allow for a deeper analysis of the interaction process. The first presented model compares catches between adjacent days (day-pairs), the second uses nets that are pre-baited before deployment, and the third relies on a detailed inspection and repair of all seal-induced damage to the net meshes.

In the second part, some traditional methods of mitigating the conflict are evaluated. A commercially available Acoustic Harassment Device was tested in a field trial. AHDs were deployed at several set-traps for salmonids for three consecutive years. The damage reducing effect was persistent throughout a season, as well as over the full three-year test period, and no “dinner bell” effect was observed. When seal attacks became frequent in the 1980´s, several of the traditional salmon traps were reinforced with newly developed extra strong net materials. These materials dramatically reduced the damage to the nets, and to some degree also the catch losses. However, the losses were still substantial, and the traditional gear was gradually phased out when better solutions emerged.

In the third part, new methods of mitigating the conflict are evaluated. A salmon trap was built, using net meshes which were large enough to allow seal-chased fish to escape through, but which would still guide and confine non-stressed fish. The trap was fitted with a fish chamber with a double wall of very taut netting, separating the catch from the surroundings by a fixed distance. Interference by seals was significantly reduced with this construction. Field experiments revealed that seals used their above-water vision to locate and search out buoys of the type that are used in the fisheries. Larger buoys were more readily found than smaller. A set of trials was initiated where certain geographical areas were made unattractive for seals prior to their seasonal arrival to the region, by deploying stationary AHDs. Finally, aquarium experiments demonstrated that underwater vision and hearing were equally important in seals’ detection of fish in a test box. It was also found that there was a “near zone”, within which seals stayed focused on a fish and attempted to catch it by a quick thrust of the head. These studies strongly suggest that new seal-safe fishing gear and mitigation methods should be based on, and would benefit from, an in-depth understanding and analysis of natural seal behaviour.

A database has been constructed using detailed information on catches and seal-disturbance from contracted commercial fishers in the northern Baltic Sea. A model was developed for the calculation of seal-induced losses in set traps for salmon. The model compared catches on consecutive days or day-pairs. It was found that the total losses in set traps were high: 61% of the potential catch in a trend-adjusted sample of paired data. A significant part of these losses, such as fish wholly removed from gear was hidden. The traditional method of assessing losses by counting the remains of fish underestimated losses by 46%. The scaring effect of seal visits was not included. The model was also used for an analysis of the damage process. There were significant negative after-effects of seal visits on catch levels. It was also found that seal visits co-occur with salmon runs. It seems that seals prefer smaller to larger salmon when raiding traps. It is suggested that the traditional method of estimating losses by counting fish remains should be calibrated when used and that the new model with day-pairs should be tried in analyses of seal interference in other fishing operations.

Along with the recovery of the Baltic Sea grey seal population from very low figures in the late 1970 's, there were clear signs of a rapidly increasing conflict between grey seals and fisheries. The set trap fisheries for salmonids seemed to be suffering the worst, but such data as were available were bard to evaluate. This lack of usable data fuelled a heated debate between commercial fishermen and seal biologists; it also made it difficult to take administrative decisions on compensatory programs to fishermen and inhibited research into mitigation methods. Available methods for the calculation of lost catches were imperfect. It was evident that the usual procedure, of simply counting the number of damaged fish found, would not reveal the total losses. It seemed likely that as well as the damaged and stolen fish, there could be other factors which would be negative for the outcome of the fishing operation. Finally, mitigation methods described were few and were said to be largely ineffective.

A database was therefore built with detailed information on catches and seal disturbance from contracted commercial fishermen. A model was developed for the calculation of seal-induced Josses in salmon set traps. The mode] compared catches on consecutive days, i.e. in day-pairs. It was found that the total catch losses in set traps were high, at above 50% of the potential catch. A significant part of these losses was 'hidden', for example in the form of fish wholly removed from the gear. The mode! was also used for an analysis of the damage process. It was found that there were negative after-effects of seal visits and that increasing seal damage coincided with rising catches. It was suggested that the traditional estimation of damage by counting fish remains should be calibrated when used and that the new mode! with day-pairs should be tried in analyses of seal interference in other fishing operations. Data from the new model, together with logbook data, was then used in an estimation of seal damages to the Swedish fisheries as a whole. The overall losses, of catch plus gear damages, were estimated at over 40 million SEK in 2003.

An attempt was then made to design a seal-safe salmon trap on the basis of previous observations of seal behaviour. In this experiment a set trap for salmonids was built using a larger mesh than the standard design and then compared with a standard trap. The basic idea was to use meshes small enough to guide an undisturbed fish further into the gear, towards the fish chamber, but large enough to allow a panicking fish being chased by a seal into the net panel to pass through, leaving the seal behind. Seal activity in and around the test trap was found to be only 6% of what it was around the standard trap; the activity level decreased further the following year when only large-meshed traps were used in the area. An estimate indicated that 65% of the potential catch was lost in the standard trap owing to seal predation, while the escape rate through the large meshes in the test trap was 52%. The standard trap had a total of 269 holes owing to seal damage, while only six holes were found in the test trap. A strategy that deprives seals of a reward makes the gear uninteresting to seals and may have long-term mitigation effects.

Samtidigt med återhämtningen hos gråsälbeståndet i Östersjön från mycket låga tal i slutet av 1970-talet syntes tydliga tecken på ökande sälskador i yrkesfisket. Fisket med fasta redskap efter lax tycktes vara värst drabbat men tillgängliga data var få och svåra att värdera. Bristen på kvantitativa data ledde till en het debatt mellan yrkesfiskare och sälforskare, den försvårade också administrativa beslut rörande ersättning för sälskador och satsning på forskning inom området. I litteraturen visade det sig finnas få användbara data på sälorsakade fångstförluster, liksom metoder för sådana beräkningar. Det tycktes dock klart att antalet skadade fiskar som återfinns i redskapet inte motsvarar hela förlusten. Det verkade också sannolikt att även andra faktorer än skadad och stulen fisk kunde vara negativa för resultatet av fisket. I övrigt var beskrivna skyddsmetoder få och uppgavs ha begränsad effektivitet.

En databas med detaljerade data om fångster och sälskador från kontrakterade yrkesfiskare byggdes upp. En modell utformades för beräkning av sälinducerade skador i laxfällor. Modellen jämförde fångsten under angränsande dagar, dagpar. Resultatet visade att totalförlusten i fällorna var hög, över 50% av den potentiella fångsten. En signifikant del av förlusten var "dold", det gäller till exempel fisk som fullständigt avlägsnats från redskapet. Modellen användes också för en analys av skadeprocessen. Det visade sig finnas en negativ inverkan på fångsten även dagen efter ett sälbesök samt att sälskador samvarierar i tiden med ökad fångst. En rekommendation var att den traditionella metoden att beräkna sälskador från antalet funna skadade fiskar eller fiskrester kalibreras. En annan rekommendation var att den föreslagna metoden med dagpar prövas vid beräkningar av sälskada i andra fiskeslag. Data från den nya modellen användes sedan tillsammans med loggboksdata för en uppskattning av de sammanlagda sälskadorna i det svenska fisket. De sammanlagda förlusterna, fångst och redskapsskador, uppskattades till över 40 miljoner SEK 2003.

Ett koncept provades att utforma sälsäkra fiskeredskap utifrån observationer och sälars beteende. I detta experiment byggdes en stormaskig fälla för fångst av laxfisk. Den grundläggande iden var att använda nätmaskor som var tillräckligt små för att leda en ostörd fisk vidare inåt redskapet mot fiskhuset, men stora nog för att tillåta en säljagad och panikslagen fisk som simmar emot nätet att passera igenom detta och undkomma sälen. Fällan var försedd med ett cylindriskt fiskhus med dubbelväggiga och sträckta nätpaneler. Fällan testades i ett område där sälattacker är vanliga. Sälaktiviteten i och intill ett försöksredskap var endast 6% av den vid ett standardredskap. Aktiviteten avtog ytterligare året därpå då endast stormaskiga redskap användes i området. En uppskattning visade att 65% av den potentiella fångsten förlorades i standardfällan på grund av sälpredation medan förlusten genom att fisk rymde genom de stora maskorna var 52%. Standardfällan hade totalt 269 hål på grund av sälen medan endast sex hål hittades i försöksredskapet. En strategi som hindrar sälen att få något utbyte gör redskapet ointressant för sälen och kan ge långsiktiga skadeförebyggande effekter.

A new design for a salmon trap aimed at minimizing damage to catch and gear caused by grey seals was tested. The traditional trap design used in the northern Baltic permits an efficient hunting strategy by seals, whereby chased fish entangle themselves in the side panels and can then easily be taken, with associated damage to the net. The side panels of the test trap (excluding the fish chamber) are made of large-mesh (400 mm) netting compared to ≤200 mm in traditional traps. This should allow seal-chased and panicking salmon to pass through, while less stressed individuals should still be guided efficiently towards the fish chamber. Trials with the two trap types were performed at the mouth of the river Indal (northern Sweden) in a comparative test programme. Catches of salmon and trout in the test trap were larger than in the standard trap. We estimated that 65% of the potential catch was lost in the standard trap owing to seal predation, while escape rate through the large meshes in the test trap was 52%. The standard trap had a total of 269 holes owing to seal damage, while only six holes were found in the test trap. Seal activity in and around the standard trap was up to 16 times higher compared with the test trap and decreased considerably during the following year when only large-meshed traps were used in the area. We suggest that seals are difficult to deter from fishing gear as long as they get a reward in terms of food and propose that a strategy that deprives seals of a reward will make the gear uninteresting to them and may have long-term mitigation effects.