Ten years of coastal fish monitoring in Estonia: dynamics of fish assemblages and populations

Transcription

1 ICES CM 2003/R:14 Ten years of coastal fish monitoring in Estonia: dynamics of fish assemblages and populations Toomas SAAT, Redik ESCHBAUM, Markus VETEMAA & Aare VERLIIN Estonian Marine Institute, University of Tartu, 10a Mäealuse Street, EE Tallinn, Estonia [tel: , fax: , Keywords: fish monitoring, Baltic Sea, fish assemblages, perch Summary. Standardized gill net monitoring (since 1992) has revealed rapid changes in abundance and distribution of freshwater and marine species, age composition of populations of model species, species richness and heterogeinity of fish assemblages in the Väinameri (Moonsund) and the gulfs of Finland and Riga. These changes are related both to human impact (mostly fishery) and environmental changes affecting reproduction and natural mortality (temperature, salinity, abundance of predatory fish and fish-eating cormorants, etc.). INTRODUCTION Altogether 79 fish and cyclostome species have been recorded in Estonia, and most of them can live in coastal waters of low salinity. Among native species, only Alburnoides bipunctatus, Thymallus thymallus, Misgurnus fossilis, Barbatula barbatula, Lampetra planeri have not recorded in the sea (Ojaveer, Pihu & Saat, 2003). Routine coastal fish monitoring using gill nets (Thoresson, 1993) in Estonia was initiated in the early 1990s in cooperation with the Institute of Coastal Research (Sweden) off the south-eastern coast of Hiiumaa Island (HELCOM COBRA warm water fish monitoring reference area) and in Küdema Bay (north to Saaremaa Island;

2 cold water fish community). Later this monitoring was extended to other permanent study areas including the gulfs of Riga and Finland. In addition, the same method has been applied to study seasonal and spatial distribution of fish (Saat & Kikas, 2002; Saat & Eschbaum, 2002; Saat & Taal, 2002), and it has been used in environmental impact assessments as well. Due to high selectivity of gill nets, only part of fish assemblages and not populations of all species can be monitored. STUDY AREAS Estonian coastal waters have low salinity (0-9 ppt, usually 5-8 ppt), they are often shallow (especially in the Väinameri and the Gulf of Riga area). Water temperature in shallow areas has reached 26-28º C in recent hot summers, and these areas are icecovered in winter. Location of permanent monitoring areas is indicated on Fig. 1, and further details in Table 1. In some cases (depending on weather conditions), the number of fishing efforts has been less than usual. S O O M E F I N L A N D S O O M E L A H T G U L F O F F I N L A N D 6 L Ä Ä N E M E R I B A L T I C S E A KÄRDLA HIIUMAA 7 PALDISKI VORMSI HAAPSALU 4 3 LIHULA MUHU LOKSA TALLINN MAARDU KEHRA KEILA SAUE TAPA TAMSALU RAPLA PAIDE TÜRI VÕHMA SUURE-JAANI 5 NARVA- JÕESUU KUNDA KOHTLA- SILLAMÄE NARVA JÄRVE JÕHVI KIVIÕLI RAKVERE MUSTVEE JÕGEVA KALLASTE PÕLTSAMAA V E N E M A A R U S S I A SAAREMAA 2 KURESSAARE KIHNU VILJANDI ABJA- PALUOJA KARKSI- MÕISA- NUIA KÜLA TÕRVA SINDI PÄRNU KILINGI- NÕMME 1 ELVA OTEPÄÄ TARTU RÄPINA PÕLVA RUHNU VALGA ANTSLA VÕRU L Ä T I L I I V I L A H T L A T V I A G U L F O F R I G A L Ä T I L A T V I A km Fig. 1. Location of monitoring areas (see also Table 1) REGIO 2000

3 Table 1. Coastal fish monitoring areas in Estonia No. on Fig. 1 and name Beginning of monitoring No. of efforts per year 1 Kihnu Remarks Western coast of Kihnu Island. Intensive coastal fishery 2 Vilsandi 1993 ca 20 3 Matsalu Hiiumaa Käsmu Vaindloo Sections a) in sheltered Kuusnõmme Bay, b) W and N to Vilsandi Island. Very limited fishery in the monitoring area but intensive in adjacent areas Inner, cental and outer part of bay are covered by test-fishing. Fishery in bay increased in 1993 but is declining in some recent years. HELCOM COBRA reference area including two sections (Saarnaki and Sarve). Both sections are fished in 6 fixed stations during 6 nights. Coastal fishery increased in the 1990s but is declining now. Käsmu Bay and adjacent bays. Coastal fishery mostly directed to salmonids and whitefish Western coast of a remote island. Coastal fishery almost absent 7 Küdema Cold-water community fished in autumn, depths m. Five fixed stations fished during 6 nights. METHODS Original methods (Thoresson, 1993) were slightly modified. In order to use data in assessment of commercial stocks, larger mesh size gill nets were included in stations (since 1995; in Hiiumaa reference area since 1998). In warm-water fish monitoring areas, each station is fished with 30 m long gill nets of 17, 22, 25, 30, 33 and 38 mm mesh size. In addition, whitefish stations are used in the Gulf of Finland and in Vilsandi area since 1999, consisting of 42, 45, 50, 55 and 60 mm mesh size nets. In Küdema area, each station is fished with 22, 30, 38, 50 and 60 mm mesh size gill nets. All fish are measured and weighed individually; data for growth studies are collected also from males (due to differences in growth rate between sexes).

4 Catch of every net is registered separately, which enables at least partial comparisons with other areas were different net sets are used. In most areas (except for Hiiumaa reference area and Küdema Bay) location of stations differs between years. Test fishing in Kihnu area is performed in late June - early July, in Vilsandi area in mid-july, in other warm-water fish monitoring areas between late July and late August. Cold-water fish community of Küdema Bay is monitored in autumn (when water temperature declines below 10º C). RESULTS AND DISCUSSION Most of the following analysis is based on data for Hiiumaa, Matsalu and Küdema areas. To allow longer data set, for warm water areas monitoring data for mm gill nets are used, if not indicated otherwise. Fish assemblages along the coast are rather dynamic both temporarily and spatially. Seasonal variations of fish assemblages There are significant seasonal variations in fish assemblages in Matsalu Bay which is important spawning area, nursery area and migratory route for fish spawning in the Kasari River (Erm, Kangur & Saat, 2002). Fish abundance and species composition of other shallow bays which represent spawning areas of several freshwater species are also highly variable. Permanent fish fauna of very shallow (up to 1 m) Käina Bay (southwestern Hiiumaa) is rather scarce. High abundance of fish is observed only during the spawning period in spring. Even in this period, most of fish can move out the bay when water temperature suddenly drops down (Fig. 2).

5 Fig. 2. Fish dynamics in Käina Bay in 1995 Spatial variation of fish assemblages Warm water fish assemblages in summer are different in near-shore and more distant areas (Table 2). Table 2. Species distribution, SPUE (mean number of species per effort), CPUE (number of fish per unit effort) and mean total weight (TW) of some fish species in relation to water depth in Matsalu Bay in 1995 (mean ± S.D.) (Saat & Eschbaum, 2002) Water depth (m) Indices SPUE 4.1± ± ±1.1 Distribution of species (% of stations): Perch Roach White bream Pike Ruffe Pikeperch Vimba bream Ide Bleak Flounder Herring

6 Burbot Total CPUE: No. of fish 70±51 148±87 186±108 Biomass (kg) 6.1± ± ±12.9 Perch: CPUE 23±12 24±12 21±19 Biomass (kg) 2.6± ± ±2.4 Mean TW (g) Roach: CPUE 28±31 74±51 123±104 Biomass (kg) 2.1± ± ±11.7 Mean TW (g) White bream: CPUE 14±25 45±35 18±23 Biomass (kg) 1.0± ± ±2.3 Mean TW (g) No. of stations Long-term variation of fish assemblages Since the early 1990s, fish assemblages and populations of commercial and noncommercial fish species along the Estonian coast have changed remarkably. Species richness The highest number of species (26) has been captured in gill nets in Küdema area followed by Hiiumaa reference area (24 species) and other areas (13-19 species). The number of species captured during a monitoring period has increasing trend in near-shore areas (Matsalu, Sarve). This trend is due to cyprinid species (gudgeon, gibel carp, rudd and some other) which have become more widespread and abundant

7 during some recent years. On the contrary, in the Saarnaki area and in the cold-water community of Küdema Bay the number of species shows decreasing trend (Fig. 3). Fig. 3. The number of species captured in gill-nets each year in Matsalu, Hiiumaa (Sarve and Saarnaki), and Küdema areas Comparisons of species richness between areas are complicated as the number of captured species depends on the number of efforts. Fish abundance Changes in fish abundance can be followed by changes of CPUE (number of fish per station and night). Hiiumaa reference area Fish community of the Sarve section is dominated by freshwater species; perch and roach strongly dominate in gill-net catches (Fig. 4). CPUE has varied between ~10 and 40 depending on abundance of these species. The bulk of the catch consisted of perch in the early 1990s and also during recent years. Between 1995 and 1999, this community was dominated by roach (Fig. 4).

8 Fig. 4. CPUE dynamics and proportion of perch, roach and other species in the Hiiumaa reference area (Sarve) during Fish community on the western coast of Saarnaki Island is also dominated by freshwater species but marine species (mostly flounder, in windy summers e.g. in 1998 also herring) are more abundant in gill-net catches than in the Sarve section. The total CPUE has declined since This community was originally dominated by roach but perch has been more abundant in recent years (Fig. 5).

9 Fig. 5. CPUE dynamics and proportion of perch, roach and other species in the Hiiumaa reference area (Saarnaki) during Matsalu Bay Alltogether 45 fish species have been recorded in Matsalu bay and its inflows (Erm, Kangur & Saat, 2002). Freshwater fish dominate in summer gill-net catches (Fig. 6); marine species (flounder, herring) are more common in the mouth area of the bay (Table 2). Fishery in Matsalu Bay was strongly limited until At the beginning of the study period ( ), perch was dominating in gill-net catches. Both CPUE and proportion of perch declined rapidly in In the recent years, perch CPUE has increased but the proportion of perch remains low in catches. CPUE of roach has varied significantly but that of white bream (one of a few species of no commercial interest) has been continously increasing. In , CPUE of some other cyprinids (rudd, gibel carp) has also been much higher than earlier.

10 Fig. 6. CPUE dynamics and proportion of perch, roach, white bream and other species in Matsalu Bay during Küdema Bay Cold-water community of Küdema Bay is inhabited predominantly by marine species (herring, flounder, cod, sculpins, etc.); the proportion of freshwater species (mostly perch, ide and dace in this area) has declined since the early 1990s, and abundance of migratory species (salmon, sea trout, whitefish, smelt) has been low during all years. CPUE declined between due to decrease of herring CPUE. In recent years, CPUE has been higher due to significant increase of flounder abundance. Also herring CPUE has increased (Fig. 7), but herring is nowadays much smaller than in earlier years.

11 Fig. 7. CPUE dynamics and proportion of flounder, herring and other species in Küdema Bay during and Other areas In the Vilsandi area, test fishing is performed in sheltered Kuusnõmme Bay and in open areas off the western and northern coast of Vilsandi Island. The former is dominated by freshwater species (perch, roach, white bream), the latter by marine species (mostly flounder and herring). Since 1993, the following changes could be detected. Recruitment of perch has been low (and lower than in other monitoring areas) since 1993, when very strong 1992 year class dominated in catches. CPUE of cyprinids (mostly roach and white bream) has increased remarkably, especially in Kuusnõmme Bay but also in open areas. CPUE of whitefish has increased since the early 1990s. Data sets for other areas are shorter. Catches in the Kihnu area have been very variable, CPUE has fluctuated between 13 and 146. Catches were dominated by

12 vimba bream in 1997, very strong 2000 year-class of perch in 2001 and 2002, and herring in Käsmu Perch is the dominating species in this area. Fishing pressure for perch has been lower than in Western Estonia, and no dramatic decline in perch population occurred (Fig. 8). At the same time, Gulf of Finland is less favourable area for perch as water temperature is lower; this affects growth rate (which is much slower than in the Väinameri) and possibly recruitment Fig. 8. CPUE dynamics of perch, smelt, flounder, herring and other species in the Käsmu area during (17-38 mm mesh size gill-nets) Vaindloo Vaindloo Island in the central part of the Gulf of Finland has no permanent human population, and this is the only monitoring area where coastal fishery is almost absent. Perch is very abundant and dominating in catches (Fig. 9). In this area, CPUE of whitefish is higher than in other areas (up to 3.2 specimens per station and night).

13 Fig. 9. CPUE dynamics of perch, herring and other species in the Vaindloo area during (17-38 mm mesh size gill-nets) Changes in perch populations We consider perch in Matsalu Bay as an example of changes in commercially exploited fish populations (Fig. 10). Coastal fishery in Estonia increased dramatically in the late 1980-s-early 1990s. In protected Matsalu Bay it happened in In summer 1993, perch population of Matsalu Bay was in rather healthy condition and fishes up to 14 summers old were captured during test-fishing (Fig. 10). By 1994, abundance of perch older than 6 years had declined. In spite of strong 1992, 1993 and 1994 year-classes, population of perch collapsed. Very strong 1999 year-class and (according to 2003 data) relatively strong year-classes of the most recent years have improved the situation. This is facilitated by fishery regulations (limiting of gear, introduction of minimum legal size for perch). Similar changes in perch population can be followed in adjacent Hiiumaa area. In the Gulf of Finland where coastal fishery is traditionally directed to other species, perch populations have been more stable.

14 Fig. 10. Perch age distribution and abundance (CPUE) in Matsalu Bay during

15 Other remarkable changes in coastal fish populations (revealed during monitoring but not illustrated here) include 1) very low abundance (as compared to earlier times) of pike in the 1990s and its increase during the most recent years, 2) increase in abundance and more wide distribution in the coastal sea of some cyprinids (white bream, crucian carp, gibel carp, bream, gudgeon, rudd), 3) increase in flounder and (in some areas) whitefish CPUE. Factors affecting fish assemblages and populations Both anthropogenic and natural factors are responsible for observed changes in fish assemblages and populations. Dramatic increase in coastal fishery in the late 1980s-early 1990s affected seriously stocks of the most valuable species (perch and pikeperch). At least in the Väinameri region, even roach population was overexploited by (mostly gill-net) fishery, as suggested by extraoribarily high mortality rates. On the other hand, recruitment of most freshwater species was very fluctuating in the 1990s. These species include at least perch, pikeperch, pike, roach, vimba bream. In the case of warm-water cyprinids this is rather unexpected as summers in the 1990s were very warm. Also, abundance of main cyprinid predators (pike, large perch and pikeperch) was low during this period. Increase of flounder stocks is probably due to saline water inflow from the North Sea to the Baltic Sea. Conditions for cod reproduction in the eastern Baltic have probably also somewhat improved by this salinity shift, as abundance of juvenile cod in Küdema Bay is higher than in eralier years. There has been a dramatic increase of cormorant population in the Estonian coastal areas, especially in the Väinameri, since the first half of 1990s. Diet analyses have indicated that cormorants in the Väinameri region consumed 481 t of fish in 1998 (commercial catch 1292 t, and 220 t without herring). Food composition of cormorants varied seasonally; the most important prey were slow-swimming demersal

16 species (viviparous blenny, burbot) but also roach, herring, perch, pikeperch and (by the numbers) sticklebacks. Commercial catch of burbot in the Väinameri was 2 t in 1998, but the amount consumed by cormorants was estimated at 112 t (Eschbaum & Veber, 2002; Eschbaum,, 2003). Obviously, cormorants can seriously affect fish assemblages competing effectively with predatory fish (main prey of which consists of viviparous blenny and sticklebacks) and populations of some species. Literature Erm, V., Kangur, M. & Saat, T Matsalu märgala kaladest ja kalapüügist aastatel (Fishes and fishery of the Matsalu wetland in the 1980s). In: Saat, T. (editor), Väinamere kalastik ja kalandus (Fishes and Fisheries of the Väinameri). Tartu, Tartu University Publishers, (in Estonian, with English summary). Eschbaum, R. & Veber, T Kormoranide mõju Väinamere kalavarule (Effects of cormorants on fish resources of the Väinameri). In: Saat, T. (editor), Väinamere kalastik ja kalandus (Fishes and Fisheries of the Väinameri). Tartu, Tartu University Publishers, (in Estonian, with English summary). Eschbaum, R., Veber, T., Vetemaa, M. & Saat, T Do cormorants and fishermen compete for fish resources in the Väinameri (eastern Baltic) area? In I.Cowx (ed), Interactions Between Fish and Birds: Implications for Management. Blackwell Science Ltd. (Fishing News Books), Ojaveer, E., Pihu, E. & Saat, T. (editors) Fishes of Estonia. Tallinn, Estonian Academy Publishers (in press). Saat., T. & Eschbaum, R Väinamere kalastik ja selle muutused viimastel aastakümnetel (Fishes of the Väinameri; changes during recent decades). In: Saat, T. (editor), Väinamere kalastik ja kalandus. Tartu, Tartu University Publishers, 9-45 (in Estonian, with English summary).

17 Saat, T. & Kikas, L Käina lahe kalastiku sesoonsed muutused (Seasonal dynamics of fish in Käina Bay). In: Saat, T. (editor), Väinamere kalastik ja kalandus. Tartu, Tartu University Publishers, (in Estonian, with English summary). Saat, T. & Taal, I Saunja lahe kalastikust Silma looduskaitsealal (About the fish of Saunja Cove, Silma Nature Reserve). Estonia Maritima 5: (in Estonian, with English summary). Thoresson, G Guidelines for coastal monitoring. Kustrapport, 1993: 1, 1-35.