NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHOR

ICES CM 2003/R:03

An alternative strategy for coastal fish monitoring in the

Magnus Appelberg1, M. Holmqvist2 and G. Forsgren3 1) National Board of Fisheries, Institute of Coastal Research, Gamla Slipvägen 19, SE-740 71 Öregrund, Sweden 2) National Board of Fisheries, Institute of Coastal Research, Skällåkra 411, SE-430 24 Väröbacka, Sweden 3) County Adminstration of Västerbotten, SE-901 86 Umeå, Sweden

Abstract

The perspective of fish monitoring has extended from being mainly focussed on stock development to include anthropogenic effects on fish assemblage structure, biodiversity and protection. At present fish monitoring in the coastal areas of the Baltic Sea is performed in seven international and three regional reference areas according to HELCOM guidelines. Results are reported to COBRA (Co-ordination Organ for Baltic Reference Areas). The main objective of the programme is to determine trends in coastal fish populations and assemblages related to natural variation and large scale environmental changes. In 2002 the Swedish Environmental Protection Agency and the National Board of Fisheries initiated a study with the overall aim to develop a co-ordinated strategy for regional/national monitoring of the coastal fish fauna in Gulf of Bothnia. Among aspects dealt with is the geographical resolution of coastal fish sampling, revision of existing sampling strategy and co-ordination of regional and national resources to accomplish the national and international environmental quality goals. Stratified random sampling with Nordic multimesh gillnets was conducted simultaneously with the method adopted by HELCOM in the Gulf of Bothnia and Baltic Proper. A comparison between the two methods revealed that both sampling strategy and gillnet used affected the outcome of the fishing. The Nordic approach provided better estimates of existing fish species within a sampled area, enhancing the probability to detect changes in stock development and biodiversity over time. The new strategy also better reflected the true size spectrum of fish populations and assemblages, thereby increasing the ability to detect over harvesting and recruitment disturbances of coastal fish species.

Keywords: monitoring, coastal fish, Baltic Sea, sampling strategy, multimesh gillnets

Introduction

The perspective of coastal fish monitoring has extended from being directed towards stock development to include anthropogenic effects on fish assemblage structure, biodiversity and nature protection. At present monitoring of fish in the coastal areas of the Baltic Sea is conducted in seven international and three regional reference areas according to HELCOM guidelines (Thoresson 1993, Neuman et al. 1997). Data are reported to COBRA (Co- ordination Organ for Baltic Reference Areas), with a secretariat at the Provincial Government of Åland. The main objective of the programme is to determine trends in the coastal fish populations and assemblages related to natural variation and large scale environmental changes in the Baltic. In 2002 Swedish Environmental Protection Agency and National Board of Fisheries initiated a study with the overall aim to develop a co-ordinated strategy for

1 regional/national monitoring of coastal fish fauna in Gulf of Bothnia. Among aspects dealt with is the geographical resolution of ongoing coastal fish monitoring, co-ordination of regional and national resources to accomplish national and international environmental and ecological quality goals (Holmqvist et al. 2003).

An important issue of the work is to revise existing strategy for monitoring coastal fish fauna in order to facilitate the follow-up of the Swedish environmental quality goals as well as goals set by EU Water Framework Directive. In the present paper we summarize results from the first year of study with focus on a comparison of existing and revised sampling strategy.

Material and methods

In August 2002, six coastal areas in the Gulf of Bothnia, one coastal area in Sea of Åland, one in mid Baltic Proper and one in S. Baltic Proper were sampled with Nordic Stratified Random sampling. Four of these areas were also simultaneously sampled according to HELCOM guidelines (Table 1).

Sampling according to HELCOM guidelines was performed with Coastal Survey multimesh gillnets (Thoresson 1993, Neuman et al. 1997). These are 3 m deep and 35 long benthic gillnets with a 38.5 m sink line. Each gillnet consists of five mesh panels (meshes ranging between 17 and 50 mm knot to knot) each 7 m long (Table 2). Sampling is performed in naturally delimited bays, in this study with areas varying between 4 to 30 km2. After initial inventory sampling of a large number of stations, 6-10 stations are selected for establishing time series based on annual sampling. Sampling is performed from July 25th to August 15th at 2-5 m depth. At each gillnet station a minimum of six repeated samplings are carried out within two weeks. All stations are sampled with two coupled multimesh gillnets each night. Nets are set between 14:00 to 17:00 and lifted following day between 7:00 and 10:00.

Sampling with Nordic multimesh gillnets is performed using a stratified, random approach in predefined depth strata. Sampling is based on a standardised method used in freshwater lakes (Appelberg et al. 1995, Appelberg 2000). The multimesh gillnets are 1.8 m deep and 45 long with a sink line 50.5 m long. Each gillnet consists of 9 mesh panels (5 m long) with mesh sizes ranging between 10 to 60 mm (knot to knot). The ratio between mesh sizes is 1.25 and mesh size composition follows a geometric series. Panels are randomly distributed in the gillnet. In each sampling area, depth strata 0-3, 3-6, 6-10, 10-20 m are randomly sampled one night. Number of sampling stations within each depth strata is evenly distributed for the three uppermost strata (minimum 10 gillnet nights), and the deepest stratum is fished with 5 gillnets. In total 45 gillnet nights are used in most areas. Time of year and time for setting and lifting is similar as for the HELCOM guidelines.

For each sampling occasion position of the station, wind direction and speed, secchi disc depth and water surface temperature is recorded. Temperature at bottom is recorded at net setting and lifting at each station. Caught fish are determined to species, and individual length (cm) for all species is recorded per gillnet and mesh size. Total weight (g) is recorded for each species per gillnet and mesh size. Samples for age analyses (otoliths) are taken for dominating species.

Catch per unit effort (CPUE) was calculated as number of individuals or total weight per gillnet and night (mean, standard deviation and coefficient of variation) for all sampling

2 occasions. When using stratified random sampling with Nordic gillnets, CPUE and species distribution (percent of total) was calculated both separately for each depth strata, and pooled for all depth strata.

For the four areas where both sampling strategies were used, CPUE was also calculated as number of individuals per m2 net and night. To compare sampling strategy, only fish caught in mesh size range 15-47 mm in the Nordic gillnets was included as this range resembles mesh sizes in the Coastal Survey gillnets (17-50 mm). Coefficient of variation for fishing according to Coastal Survey and Nordic Stratified Random strategy was compared using paired sample t-test. When comparing type of gillnets, catches in the Nordic gillnets were restricted to the two uppermost depth strata (0-6 m), which resemble the depth zone sampled with Coastal Survey gillnets (2-5 m). Length distribution of perch and roach is given for fish caught in Nordic gillnets (0-6 m) and Coastal Survey gillnets.

Similarity in fish assemblages between the nine sampled coastal areas in the Baltic was analysed using Hierarchical Cluster Analysis based on number of individuals per unit effort in Nordic Stratified Random sampling.

Results

Number of species and species distribution In total, 30 different fish species were caught in all areas (Table 3). Out of these, 29 were caught in the Nordic gillnets, and 19 in the Coastal Survey nets. Number of species at each sampled area varied between 9 and 19 for Nordic gillnets and between 8 and 13 for Coastal Survey gillnets. Most species were caught in Örefjärden, Lagnöfjärden and Kvädöfjärden. Lowest number of species was caught at Holmöarna and in Rånefjärden. Number of caught species tended to be higher in southern Baltic. In average, 10 species were caught in the coastal area of Bothnian Bay, 13.5 species in N. Quark, 15.3 species in Sea of Bothnia, 18 in Sea of Åland and 15,5 species in Baltic Proper. The proportion marine species was related to the specific area sampled, and increased from north to south. There was a tendency to catch more species when using Nordic Stratified Random sampling compared to Coastal Survey sampling (average 12,5 and 10,5 species respectively, for the four areas sampled with both methods), however, the difference was not significant (p=0,116).

Perch was caught in highest number in all areas except when using Coastal Survey sampling in the northerly areas Råenå and Holmöarna, where roach dominated the catches. Other frequently caught species were ruffe and bream in Bay of Bothnia, ruffe, herring and zander in Sea of Bothnia and Sea of Åland, and , bleak, white bream in the Baltic Proper. Bay Örefjärden in N. Quark differed from the other areas due to an even distribution of ruffe, common dace, smelt, herring and whitefish.

A hierarchical cluster analysis indicated that geographically adjacent areas also showed similarities in fish fauna assemblages (Fig 1). Catches in Forsmark and Finbo in southern Sea of Bothnia, were similar to catches in Lagnö area close to Sea of Åland. At Långvindsfjärden, in mid Sea of Bothnia, the fish fauna showed some similarity to assemblages in southern Sea of Bothnia. The fish fauna in Kvädöfjärden and Torhamn in the Baltic Proper also resembled each other. Also fish assemblages at Holmöarna and Örefjärden in N. Quark were similar, whereas the catch of fish in the northernmost area in Bothnian Bay (Råneå) was separated from the other localities.

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Species composition differed between the two types of gillnets used in all areas, except Råneå. At Holmöarna, herring and viviparous blenny were caught only in Nordic gillnets, whereas pike was caught only in Coastal Survey gillnets. At Forsmark, bleak, smelt, flounder, black goby, viviparous blenny, and eel were caught only in the Nordic gillnets, whereas ide, rainbow trout, rudd, and vimba were caught only in Coastal Survey gillnets. At Finbo four horned sculpin, bleak and smelt were caught only in Nordic gillnets whereas rudd was caught only in Coastal Survey nets (Table 3).

Comparison of sampling strategy To compare the two sampling strategies, CPUE were recalculated to catch per m2 gillnet and night, including only mesh sizes between 15 and 47 mm in the Nordic gillnets. CPUE (numbers) were about double using Nordic Stratified Random sampling compared to Coastal Survey sampling (Table 4). An exception is Holmöarna, where CPUE was slightly higher for the Coastal Survey sampling.

Standard deviation and coefficient of variation for the mean CPUE using Coastal Survey sampling was based both using station as an independent sample (mean value for all nights), and using each gillnet night as independent sample. In paired-sample t-test coefficient of variations (CV) did not differ significantly between the two sampling strategies, irrespective method used for calculating CPUE for the Coastal Survey method (p=0,127 and p=0,682, respectively).

Total CPUE decreased with increased sampling depths in most areas. One exception was noted in Örefjärden, where total CPUE was evenly distributed down to 20 m. Species adapted to warm water, such as perch and roach, generally decreased with increasing depth, whereas species such as herring, whitefish, vendace, ruffe and smelt increased, especially in the deepest stratum fished (10-20 m). Also size structure was related to depth, with increasing mean size of perch with increasing depth.

Comparison of type of gillnet To compare catches in the two types of gillnets, CPUE was calculated as number of caught individuals per m2 net and night at the depth strata in the Nordic fishing (0-3 and 3-6) which corresponds best to the depth strata sampled with Coastal Survey gillnets (2-5 m). CPUE was significantly higher using Nordic gillnets compared to Coastal Survey gillnets (paired-sample t-test, p=0.046). In three out of four areas the catch per m2 net was nearly three times higher in the Nordic gillnets compared to the Coastal Survey gillnets (Table 5). Exception is Holmöarna, where CPUE was just slightly higher in the Nordic gillnets compared to the Coastal Survey gillnets. Although there seemed to be a higher CV using Nordic gillnets, CV was not significantly higher than for Coastal Survey gillnets.

Size distribution Size of fish caught in multimesh gillnets is strongly dependent on mesh size combination used. Differences in size distribution between gillnets used could be illustrated by the catch of perch and roach in the four areas where both sampling methods were used simultaneously. In Nordic gillnets, size range of perch was 4 to 350 mm with a peak at 90-120 mm, but also with smaller peak at 70-80 mm total length in some areas. In the Coastal Survey gillnets perch sizes ranged between 110 and 350 mm with one peak at about 130 mm and one at 170 mm in most areas. The peak of small perch noted in Nordic gillnets was not apparent in the Coastal Survey gillnets. The pattern in length distribution is similar for roach, which in the Nordic

4 gillnets showed a peak at 110 mm, whereas it is about 140-180 mm in the Coastal Survey nets (Fig 2). Size range of other common fish species caught in the Nordic gillnets was for herring 40-250 mm, and for whitefish between 110 and 450 mm.

Kurkilahti et al. (2002) established gillnet selectivity correction factors for Nordic multimesh gillnets used in freshwater monitoring. These gillnets are similar to the ones used in the present study, except that the smallest mesh sizes (5-8 mm) are excluded in the coastal modification. Correction factors were established for six freshwater species. When adopting the correction factor for perch caught in the Nordic gillnets in Forsmark area, it is apparent that size spectra of perch will be even more skewed to the left, indicating that small individuals are underrepresented in the catch. It becomes even more pronounced when comparing the size distribution based on Coastal Survey gillnets.

Discussion

The sampling method for coastal fish monitoring in the Baltic Sea adopted by HELCOM have been used in several Baltic countries for monitoring coastal reference areas during several years (e.g. Ådjers et al. 2001). It is designed to establish time series based on index of abundance of warm water adapted coastal fish species. Results are to be used to indicate impact of exposure to eutrophication and toxic substances (Neuman and Sandström 1996, Neuman et al. 1997). However, recent years some less attractive characteristics of the method have been evident. The sampling strategy, which is based on repeated sampling of the same stations over six nights, have in some areas showed to cause depletion of local fish populations during fishing. As this reduction in catches over time is not constant for all stations and areas, it is difficult to handle in the further analyses. Additionally, in Swedish coastal monitoring there has been indication that repeated sampling on a station over several nights may attract seals, thereby interfering with the results. This problem will possibly increase in pace with an increasing seal population in the Baltic. Furthermore, the mesh size combination in the used multimesh gillnets are selecting for fish in certain size groups and does not mimic true population size distributions. This characteristic may bias sampling for age and growth as well as actual size distribution. Although this problem to some extent can be treated using correction factors, this is usually not done.

Number of species and species distribution. There was a tendency to catch more species using the Nordic Stratified Random sampling compared to the Coastal Survey sampling. In three out of four areas where both methods were used, more species was caught in the former. However, the material is too small to detect any tendency in type of species, as both marine and freshwater species were caught using both methods. By increasing area sampled, both in horizontal and vertical direction, the number of species caught ought to increase according to the species-area relation. When changes in biodiversity are to be monitored, it is important to cover as many habitats as possible in the selected area. This is especially important in habitats with high heterogeneity.

Catch per unit effort. CPUE was strongly affected by both sampling strategy and by multimesh gillnets used. Despite that the range of mesh sizes was similar when comparing effects of sampling strategies, CPUE was higher using Nordic sampling strategy in three out of four areas. This difference could have several explanations. When using Nordic Stratified Random sampling the sampled area is larger and more habitats are included. It is also likely that the repeated sampling at each gillnet station in the Coastal Survey method will cause

5 local depletion of the fish stock in some areas, and thereby reduce mean CPUE. Although only the same mesh size interval was compared, it is probable that the Nordic gillnets are more effective to catch fish of all sizes within the size range. Variation around the mean seemed to be higher for the Nordic sampling, irrespective way of calculating variance for the Coastal Survey gillnets. Although this difference was insignificant, it may affect the possibilities to detect trends negatively.

CPUE was also affected by type of multimesh gillnet used. Fishing nearly the same depth strata, Nordic gillnets caught almost three times more individuals. This is possibly due to that small individuals are caught to a much higher extent in the Nordic gillnets than in the Coastal Survey gillnets. This also affect the variance (expressed as coefficient of variation), which seemed to be higher for the Nordic multimesh gillnets, however, not significant.

Size distribution. It is often desirable to describe population and assemblage size structure as correct as possible. Changes in size structure have become an important indicator of population and assemblage status. A low, or decreasing, proportion of large individuals may indicate high adult mortality (e.g. over fishing), whereas low abundance of small (young) individuals may indicate recruitment disturbances. Comparing size distribution of fish between the two sampling strategies, differences obtained may be related to both strategies and gillnet used. As size of some fish species increased with increasing depth, the Nordic Stratified Random strategy would provide a wider size range of fish compared to the Coastal Survey strategy. By including small mesh sizes (10, and 12.5 mm) size distribution of the most frequently occurring coastal fish species are represented during their second summer in the Nordic multimesh gillnets. In the Coastal Survey gillnets, the sample of the second summer perch was skewed to the right, thereby increasing the risk for misinterpreting growth of young fish.

As gillnetting is a strongly size selective method for catching fish, mesh size combinations will to a high extent determine size distribution in the catch. The combination of mesh sizes in the Coastal Survey gillnets will result in an overrepresentation of fish in some size classes due to that mesh panels 22 and 25 mm is quite close. For perch, this was apparent for individuals of about 17 to 20 cm total length. The combination of mesh sizes in the Nordic multimesh gillnet are based on a geometric series, with the ratio between meshes of about 1.25. This mesh size combination reduces some of the effects caused by wider selection curves for larger individuals (Kurkilahti 1999). Still, the applied correction for perch indicate that there is need for gillnet selectivity corrections.

Conclusions. A comparison between two methods for sampling coastal fish revealed that both sampling strategy and gillnet used affected the outcome of the fishing. The Nordic approach provided better estimates of existing fish species within a sampled area, enhancing the probability to detect changes in stock development and biodiversity over time. Furthermore, the Nordic sampling strategy possibly provides a more reliable depiction of fish community and specific species size structure, thereby increasing the ability to detect over harvesting and recruitment disturbances of coastal fish species. The Nordic sampling strategy relies on a statistical sound base, and allow for integration between different monitoring programmes.

The suggested sampling strategy will be further assessed coming years. By simultaneous sampling in ongoing monitoring programmes a base for conversion of earlier collected data will be established. Although repeated gillnetting has proven to be suitable for establishing time series, it is a size and species selective sampling method, which needs to be

6 accomplished by alternative methods such as fyke nets and seines in order to catch all fish species present in a habitat. Future studies will focus on optimising the effort in relation to statistical power in time series. The relation between recruitment areas and index of abundance obtained in gillnet sampling will also be studied.

Acknowledgements

This study was funded by Swedish Environmental Protection Agency and National Board of Fisheries.

Literature Cited

Appelberg, M. 2000. Swedish standard methods for sampling freshwater fish with multi-mesh gillnets. Fiskeriverket Information 2000:1, 27 p

Appelberg, M., H.M. Berger, T. Hesthagen, E. Kleiven, M. Kurkilahti, J. Raitaniemi, and M. Rask. 1995. Development and intercalibration of methods in Nordic freshwater fish monitoring. Water, Air and Soil Pollution 85:401-406.

Holmqvist, M., M. Appelberg and G. Forsgren, 2002. Strategi för ett samordnat nationellt/regionalt övervakningsprogram för kustfisk i Bottniska viken. [English summary: Strategy for co-ordinated national/regional monitoring of the coastal fish fauna in Gulf of Bothnia]. Fiskeriverket Informerar 2003:5. 47 p.

Kurkilahti, M. 1999. Nordic Multimesh gillnet – Robust gear for sampling fish populations. Ph.D. thesis, University of Turku, Finland

Kurkilahti, M., M. Appelberg, T. Hesthagen and M. Rask, 2002. Effects of fish shape on gillnet selectivity: a study with Fulton’s condition factor. Fisheries Research 54:153- 170.

Neuman, E. and O. Sandström, 1996. Fish monitoring as a tool for assessing the health of Baltic coastal ecosystems. Bull. Sea Fish. Inst., Gdynia, 3 (139):3-11.

Neuman, E., O. Sandström and G. Thoresson 1997. Guidelines for coastal fish monitoring. National Board of Fisheries, Sweden. 44 p.

Thoresson, G. 1993. Guidelines for coastal monitoring. Fishery biology. Kustrapport 1993:1. 36 p.

Ådjers, K., M. Appelberg, R. Eschbaum, A. Lappalainen and L. Lozys, 2001. Coastal fish monitoring in Baltic reference areas 2000. Kala- Ja Riistaraportteja nro 229. 15 p.

7 Table 1. Areas sampled, type of coastal zone and sampling method, related monitoring programmes.

Sampled coastal Main basin Type of coastal Type of Monitoring area area samplinga programme Rånefjärden N. Bay of Bothnia Closed N, CS Regional Holmöarna N. Quark Semi closed N, CS National Örefjärden N. Quark Closed N This project Långvind Mid Sea of Bothnia Open N This project Forsmark S. Sea of Bothnia Closed N, CS Recipient control Finbo S. Sea of Bothnia Semi closed N, CS Reference area Lagnö Sea of Åland Closed N Regional Kvädöfjärden Mid Baltic Proper Closed N National Torhamn S. Baltic Proper Closed N National a) N=Stratified random sampling with Nordic multimesh gillnets. CS=Sampling according to HELCOM guidelines using Coastal Survey multimesh gillnets.

Table 2. Summary of sampling strategy and type of gillnet used for Coastal Survey sampling according to HELCOM guidelines and Nordic Stratified Random sampling.

Coastal Survey Nordic Stratified Random No depth zones 1 4 Sampled depth zones 2-5 0-2,9 (m) 3-5,9 6-9,9 10-20 Sampling strategy Repeated sampling Randomised sampling within during 6 nights at fixed depth strata using single set stations with two gillnets coupled gillnets

Number of stations 6-10 ≈ 45 Effort One net night-1 One net night-1 (mean value for six (one independent sample) nights) Number of mesh sizes 5 9 in multimesh gillnets Order of meshes and 17 (0,17) 30 (0,15) thread thickness (mm) 22 (0,17) 15 (0,15) 25 (0,17) 38 (0,15) 38 (0,20) 10 (0,15) 50 (0,20) 47 (0,17) 12 (0,15) 24 (0,15) 60 (0,20) 19 (0,15)

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Table 3. Species caught in relation to coastal area, main basin and sampling strategy.

Bay of Sea of Northern Quark Sea of Bothnia Baltic Proper Bothnia Åland Öre- Lång Tor- Råneå Holmöarna Forsmark Finbo Lagnö Kvädö fjärden vind hamn Na CSb N CS N N N CS N CS N N N Perch (Perca fluviatilis) X X X X X X X X X X X X X White bream (Abramis bjoerkna) X X X X X X X Bream (Abramis brama) X X X X X X X X Ruffe (Gymnocephalus cernnus) X X X X X X X X X X X X X Pike (Esox lucius) X X X X X X X X X X X X Zander (Sander lucioperca) X X X X X X Grayling (Thymallus thymallus) X Fourhorned sculpin (Triglopsis quadricornis) X X X Ide ( idus) X X X X X X Bleak (Alburnus alburnus) X X X X X X X X X X X Roach (Rutilus rutilus) X X X X X X X X X X X X X Smelt (Osmerus eperlanus) X X X X X X Rainbow trout (Onchorynchus mykiss) X Crucian carp (Caracius caracius) X Rudd (Scardinus erythrophtalmus) X X X X X Whitefish (Coregonus lavaretus) X X X X X X X X X X X Vendace (Coregonus albula) X X X X Sprat c (Sprattus sprattus) X X X X X X X Flounder c (Platichtys flesus) X X X X X X X Bullhead (Cottus gobio) X Stickleback (Gasterosteus aculeatus) X X X Herring c ( harengus) X X X X X X X X X X X Common dace (Leuciscus leuciscus) X X X X X Snake pipefish c (Entelurus aequoraeus) X Tench (Tinca tinca) X Black goby c (Gobius niger) X X X X Viviparous blenny c (Zoarces viviparus) X X X X X Vimba (Abramis vimba) X X X Eel (Anguilla anguilla) X X B. Trout (Salmo trutta) X X # species per area 10 10 9 8 18 15 17 13 14 11 18 19 12 Total # species caught per basin 10 18 24 18 20 Mean # species per area and basin using 10 13,5 15,3 18 15,5 Nordic sampling Prop. Marine species 0,10 0,10 0,22 0 0,22 0,27 0,29 0,08 0,21 0,27 0,28 0,21 0,17 a N = Nordic gillnets b CS = Coastal Survey gillnet c Marine species

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Table 4. Number of individuals caught per m2 net and night (meshes 15-47mm in Nordic gillnets).

GILL- DEPTH NUMBER m-2 NET NIGHT-1 BASIN AREA NETa (m) nb MEAN STD CVc Bay of Bothnia Rånefjärden N 0-3 15 1,63 0,63 38,48 N 3-6 20 0,93 0,30 32,15 N 6-10 10 0,94 0,30 32,38 N Total 45 1,16 0,54 46,54 CS 2-5 42 0,44 0,19 43,35 CS* 2-5 7 0,44 0,15 33,40 N. Quark Holmön N 0-3 20 0,92 0,42 45,95 N 3-6 10 0,75 0,37 49,78 N Total 30 0,86 0,41 47,34 CS 2-5 30 0,91 0,28 30,64 CS* 2-5 5 0,91 0,19 21,30 Sea of Bothnia Forsmark N 0-3 14 0,80 0,37 46,23 N 3-6 16 1,01 0,31 30,84 N 6-10 15 0,63 0,38 59,14 N Total 45 0,82 0,38 46,13 CS 2-5 48 0,40 0,21 51,78 CS* 2-5 8 0,40 0,16 41,13 Finbo N 0-3 12 1,09 0,57 51,89 N 3-6 18 0,96 0,55 56,72 N 6-10 10 0,78 0,39 50,11 N 10-20 5 0,46 0,41 90,14 N Total 45 0,90 0,53 58,71 CS 2-5 48 0,44 0,22 50,03 CS* 2-5 8 0,44 0,18 41,56 a N = Nordic gillnets, C = Coastal Survey gillnets b Sample size c STD = Standard deviation, CV = Coefficient of variation (%) * calculated per station (mean value over 6 nights)

Table 5. Number of individuals per m2 net and night (depth 0-3 m and 3-6 m in Nordic gillnets)

GILL- DEPTH NUMBER m-2 NET AND NIGHT BASIN AREA NETa (m) nb MEAN STD CVc Bay of Bothnia Rånefjärden N 0-6 35 1,33 0,74 55,20 CS 2-5 42 0,44 0,19 43,35 CS* 2-5 7 0,44 0,15 33,40 N. Quark Holmön N 0-6 30 0,98 0,70 70,74 CS 2-5 30 0,91 0,28 30,64 CS* 2-5 5 0,91 0,19 21,30 Bothnian Sea Forsmark N 0-6 30 1,17 0,49 42,30 CS 2-5 48 0,40 0,21 51,78 CS* 2-5 8 0,40 0,16 41,13 Finbo N 0-6 30 1,40 1,63 115,81 CS 2-5 48 0,44 0,22 50,03 CS* 2-5 8 0,44 0,18 41,56 a N = Nordic gillnets, C = Coastal Survey gillnets b Sample size c STD = Standard deviation, CV = Coefficient of variation (%) * calculated per station (mean value over 6 nights)

10 0 5 10 15 20 25 Area +------+------+------+------+------+

Finbo SB ØÞ Forsmark SB ØÚØØØØØÞ Lagnö SÅ ØÝ ß ØØØØØØØØØØØØØÞ Långvind SB ØØØØØØØÝ ß ØØØØØØØØØØØØØØØØØÞ Kvädö BP ØØØØØØØØØØØØØ8 ØØØØØØØÝ ß ØØØØØØØØØÞ Torhamn BP ØØØØØØØØØØØØØÝ Ù Ù Råneå BB ØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØÝ Ù Holmöarna NQ ØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØ8 ØØØØØÝ Örefjärden NQ ØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØØÝ

Fig 1. Hierarchical Cluster Analysis based on number per unit effort (excluding rare species) in sampled coastal areas in the Baltic. BB= Bay of Bothnia, NQ=Northern Quark, SB= Sea of Bothnia, SÅ=Sea of Åland, BP=Baltic Proper.

11 Råneå, Perch Råneå, Roach 20 30

25 15 20 Nordic Nordic Coastal Survey % 10 Coastal Survey % 15

10 5 5

0 0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 Length (cm) Length (cm)

Holmön, Perch Holmön, Roach 25 20

20 15

15 Nordic Nordic % Coastal Survey % 10 Coastal Survey 10

5 5

0 0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 35 Length (cm) Length (cm) Forsmark, Perch Forsmark, Roach 30 35

25 30

25 20 Nordic 20 Nordic % 15 Coastal Survey % Coastal Survey 15 10 10

5 5

0 t 0 5 10 15 20 25 30 35 40 45 5 10 15 20 25 30 Length (cm) Length (cm) Finbo, Perch Finbo, Roach 20 25

20 15 Nordic Nordic 15 Coastal Survey % 10 Coastal Survey % 10

5 5

0 0 5 10 15 20 25 30 35 40 5 10 15 20 25 30 Length (cm) Length (cm)

Fig 2. Length distribution of roach and perch in Nordic multimesh gillnets and in Coastal Survey multimesh gillnets in four coastal areas where both types were used simultaneously.

12 # Perch

500 Coastal Survey net Nordic multimesh gillnets 400 Selectivity corrected

300

200

100

0 5 10 15 20 25 30 35 40

Length (cm)

Fig 3. Length distribution of perch in Coastal survey nets and in Nordic multimesh gillnets and selectivity corrected lengths based on Nordic gillnets from Forsmark area.

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