Download Ten years of coastal fish monitoring in Estonia: dynamics of fish

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: [email protected]]
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 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
G U L F O F
6
L A H T
F I N L A N D
5
LOKSA
LOKSA
LOKSA
LOKSA
LOKSA
TALLINN
TALLINN
PALDISKI
PALDISKI
PALDISKI
PALDISKI
PALDISKI
PALDISKI
KEHRA
KEHRA
KEHRA
KEHRA
KEHRA
SAUE
SAUE
SAUE
SAUE
SAUE
KEILA
KEILA
KEILA
KEILA
KEILA
KEILA
TAPA
TAPA
TAPA
TAPA
TAPA
TAPA
RAKVERE
RAKVERE
RAKVERE
RAKVERE
RAKVERE
KOHTLAKOHTLAKOHTLAKOHTLAKOHTLAKOHTLAJÄRVE
JÄRVE
JÄRVE
JÄRVE
JÄRVE
NARVA
SILLAMÄE
SILLAMÄE
SILLAMÄE NARVA
NARVA
NARVA
NARVA
SILLAMÄE
SILLAMÄE
SILLAMÄE
JÕHVI
JÕHVI
JÕHVI
JÕHVI
JÕHVI
JÕHVI
KIVIÕLI
KIVIÕLI
KIVIÕLI
KIVIÕLI
KIVIÕLI
KIVIÕLI
TAMSALU
TAMSALU
TAMSALU
TAMSALU
TAMSALU
VORMSI
RAPLA
RAPLA
RAPLA
RAPLA
RAPLA
RAPLA
KÄRDLA
KÄRDLA
KÄRDLA
KÄRDLA
KÄRDLA
KÄRDLA
PAIDE
PAIDE
PAIDE
PAIDE
PAIDE
HAAPSALU
HAAPSALU
HAAPSALU
HAAPSALU
HAAPSALU
HIIUMAA
4
7
3
LIHULA
LIHULA
LIHULA
LIHULA
LIHULA
LIHULA
MUHU
JÕGEVA
JÕGEVA
JÕGEVA
JÕGEVA
JÕGEVA
JÕGEVA
VÕHMA
VÕHMA
VÕHMA
VÕHMA
VÕHMA
VÕHMA
KALLASTE
KALLASTE
KALLASTE
KALLASTE
KALLASTE
KALLASTE
PÕLTSAMAA
PÕLTSAMAA
PÕLTSAMAA
PÕLTSAMAA
PÕLTSAMAA
SUURE-JAANI
SUURE-JAANI
SUURE-JAANI
SUURE-JAANI
SUURE-JAANI
SUURE-JAANI
SAAREMAA
PÄRNU
PÄRNU
PÄRNU
PÄRNU
PÄRNU
2KURESSAARE
KURESSAARE
KURESSAARE
KURESSAARE
KURESSAARE
KIHNU
SINDI
SINDI
SINDI
SINDI
SINDI
KILINGIKILINGIKILINGIKILINGIKILINGIKILINGINÕMME
NÕMME
NÕMME
NÕMME
NÕMME
1
20
ELVA
ELVA
ELVA
ELVA
ELVA
MÕISAMÕISAMÕISAMÕISAMÕISAKÜLA
KÜLA
KÜLA
KÜLA
KARKSIKARKSIKARKSIKARKSIKARKSINUIA
NUIA
NUIA
NUIA
NUIA
OTEPÄÄ
OTEPÄÄ
OTEPÄÄ
OTEPÄÄ
OTEPÄÄ
OTEPÄÄ
PÕLVA
PÕLVA
PÕLVA
PÕLVA
PÕLVA
RÄPINA
RÄPINA
RÄPINA
RÄPINA
TÕRVA
TÕRVA
TÕRVA
TÕRVA
TÕRVA
ANTSLA
ANTSLA
ANTSLA
ANTSLA
ANTSLA
VALGA
VALGA
VALGA
VALGA
VALGA
L I I V I L A H T
G U L F O F
R I G A
0
TARTU
TARTU
TARTU
TARTU
TARTU
VILJANDI
VILJANDI
VILJANDI
VILJANDI
VILJANDI
ABJAABJAABJAABJAABJAABJAPALUOJA
PALUOJA
PALUOJA
PALUOJA
PALUOJA
RUHNU
L Ä T I
L A T V I A
MUSTVEE
MUSTVEE
MUSTVEE
MUSTVEE
MUSTVEE
MUSTVEE
TÜRI
TÜRI
TÜRI
TÜRI
TÜRI
TÜRI
V E N E M A A
R U S S I A
L Ä Ä N E M E R I
B A L T I C S E A
NARVANARVANARVANARVANARVANARVAJÕESUU
JÕESUU
JÕESUU
JÕESUU
JÕESUU
JÕESUU
KUNDA
KUNDA
KUNDA
KUNDA
KUNDA
MAARDU
MAARDU
MAARDU
MAARDU
MAARDU
VÕRU
VÕRU
VÕRU
VÕRU
VÕRU
L Ä T I
L A T V I A
40 km
Fig. 1. Location of monitoring areas (see also Table 1)
© REGIO 2000
Table 1. Coastal fish monitoring areas in Estonia
Beginning No. of
No. on Fig. 1
of
efforts
and name monitoring per year
1 Kihnu
1997
20
Remarks
Western coast of Kihnu Island. Intensive coastal
fishery
2 Vilsandi
1993
3 Matsalu
1993
Sections a) in sheltered Kuusnõmme Bay, b) W and
N to Vilsandi Island. Very limited fishery in the
ca 20 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
40-50 declining in some recent years.
1992
HELCOM COBRA reference area including two
sections (Saarnaki and Sarve). Both sections are
fished in 6 fixed stations during 6 nights. Coastal
36+36 fishery increased in the 1990s but is declining now.
4 Hiiumaa
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 1015 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)
Indices
SPUE
2-2.5
4.1±1.6
Distribution of species (% of stations):
Perch
100
Roach
95
White bream
81
Pike
33
Ruffe
33
Pikeperch
14
Vimba bream
19
Ide
14
Bleak
29
Flounder
0
Herring
0
Water depth (m)
2.5-3
5.0±1.4
3-4
6.5±1.1
100
100
100
36
55
59
23
5
18
9
0
100
100
67
50
67
100
83
0
17
17
33
Burbot
0
0
17
Total CPUE:
No. of fish
Biomass (kg)
70±51
6.1±4.0
148±87
12.8±7.2
186±108
21.1±12.9
Perch:
CPUE
Biomass (kg)
Mean TW (g)
23±12
2.6±2.1
112
24±12
2.0±1.1
83
21±19
3.5±2.4
112
Roach:
CPUE
Biomass (kg)
Mean TW (g)
28±31
2.1±2.3
76
74±51
6.5±4.1
88
123±104
13.2±11.7
108
White bream:
CPUE
Biomass (kg)
Mean TW (g)
14±25
1.0±1.6
69
45±35
3.2±2.5
71
18±23
1.7±2.3
96
No. of stations
21
22
6
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
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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