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-12618

Tallinn, Estonia [tel: +372 6267 402, fax: +372 6267 417, e-mail: tsaat@sea.ee]

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;

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 ice-

covered 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.

Fig. 1. Location of monitoring areas (see also Table 1)

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Table 1. Coastal fish monitoring areas in Estonia

No. on Fig. 1 and name

Beginning of

monitoring

No. of efforts

per year Remarks

1 Kihnu 1997 20 Western coast of Kihnu Island. Intensive coastal fishery

2 Vilsandi 1993 ca 20

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

3 Matsalu 1993 40-50

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.

4 Hiiumaa 1992 36+36

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.

5 Käsmu 1997 20 Käsmu Bay and adjacent bays. Coastal fishery mostly directed to salmonids and whitefish

6 Vaindloo 1997 4 Western coast of a remote island. Coastal fishery almost absent

7 Küdema 1992 30 Cold-water community fished in autumn, depths 10-15 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).

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 17-30 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).

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 2-2.5 2.5-3 3-4 SPUE 4.1±1.6 5.0±1.4 6.5±1.1 Distribution of species (% of stations): Perch 100 100 100 Roach 95 100 100 White bream 81 100 67 Pike 33 36 50 Ruffe 33 55 67 Pikeperch 14 59 100 Vimba bream 19 23 83 Ide 14 5 0 Bleak 29 18 17 Flounder 0 9 17 Herring 0 0 33

Burbot 0 0 17 Total CPUE: No. of fish 70±51 148±87 186±108 Biomass (kg) 6.1±4.0 12.8±7.2 21.1±12.9 Perch: CPUE 23±12 24±12 21±19 Biomass (kg) 2.6±2.1 2.0±1.1 3.5±2.4 Mean TW (g) 112 83 112 Roach: CPUE 28±31 74±51 123±104 Biomass (kg) 2.1±2.3 6.5±4.1 13.2±11.7 Mean TW (g) 76 88 108 White bream: CPUE 14±25 45±35 18±23 Biomass (kg) 1.0±1.6 3.2±2.5 1.7±2.3 Mean TW (g) 69 71 96 No. of stations 21 22 6

Long-term variation of fish assemblages

Since the early 1990s, fish assemblages and populations of commercial and non-

commercial 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

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).

Fig. 4. CPUE dynamics and proportion of perch, roach and other species in

the Hiiumaa reference area (Sarve) during 1992-2002

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 1992-93. This community was originally dominated by

roach but perch has been more abundant in recent years (Fig. 5).

Fig. 5. CPUE dynamics and proportion of perch, roach and other species in

the Hiiumaa reference area (Saarnaki) during 1992-2002

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 1993. At the beginning of

the study period (1993-94), perch was dominating in gill-net catches. Both CPUE and

proportion of perch declined rapidly in 1995-99. 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 2002-03, CPUE of some other cyprinids

(rudd, gibel carp) has also been much higher than earlier.

Fig. 6. CPUE dynamics and proportion of perch, roach, white bream and other

species in Matsalu Bay during 1993-2002

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 1992-97 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.

Fig. 7. CPUE dynamics and proportion of flounder, herring and other species

in Küdema Bay during 1992-97 and 2000-02

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

vimba bream in 1997, very strong 2000 year-class of perch in 2001 and 2002, and

herring in 2000.

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 1997-2002 (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).

Fig. 9. CPUE dynamics of perch, herring and other species in the Vaindloo

area during 1997-2002 (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 1993. 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.

Fig. 10. Perch age distribution and abundance (CPUE) in Matsalu Bay during 1993-2002

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

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

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Väinamere kalastik ja kalandus (Fishes and Fisheries of the Väinameri). Tartu, Tartu

University Publishers, 122-158 (in Estonian, with English summary).

Eschbaum, R. & Veber, T. 2003. 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

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Eschbaum, R., Veber, T., Vetemaa, M. & Saat, T. 2003. 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), 72-83.

Ojaveer, E., Pihu, E. & Saat, T. (editors) 2003. Fishes of Estonia. Tallinn, Estonian

Academy Publishers (in press).

Saat., T. & Eschbaum, R. 2002. 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).

Saat, T. & Kikas, L. 2002. Käina lahe kalastiku sesoonsed muutused (Seasonal

dynamics of fish in Käina Bay). In: Saat, T. (editor), Väinamere kalastik ja kalandus.

Tartu, Tartu University Publishers, 90-102 (in Estonian, with English summary).

Saat, T. & Taal, I. 2001. Saunja lahe kalastikust Silma looduskaitsealal (About the

fish of Saunja Cove, Silma Nature Reserve). Estonia Maritima 5: 269-278 (in

Estonian, with English summary).

Thoresson, G. 1993. Guidelines for coastal monitoring. Kustrapport, 1993: 1, 1-35.