BSEP116B Biodiversity in the Baltic Sea - Helcom

Recruitment (thousands)
8000000
7000000
6000000
5000000
4000000
3000000
2000000
1000000
0
1977
1980
hatching of cod eggs, has caused high cod egg
mortality (Köster et al. 2003). This, together with
very high fishing pressure, has resulted in historically
low values of the cod stock since the early
1990s (ICES 2008b, Figure 3.5.1) altough there
has been an increase in the eastern cod spawning
stock biomass during the past three years. This
observed increase since 2005 has been a result
of relatively strong year classes in 2003 and 2005
(ICES 2008d). Release of the predation pressure
by cod, accompanied by favourable hydrographic
conditions, has allowed the sprat stock to increase
since the late 1980s, which together with herring
has strongly dominated the Baltic fish communities
since then (Figure 3.5.2 and Figure 3.5.3). This shift
to domination by a pelagic fish community represents
a profound change in the marine ecosystem,
also called a ’regime shift’ (e.g., Alheit et al. 2005).
In addition, there are several non-commercial
marine fish present in the Baltic Sea, for example,
gobies Pomatoschistus spp., three-spined stickle-
Recruitment
SSB
1983
1986
1989
1992
1995
1998
2001
2004
2007
140000
120000
100000
80000
60000
40000
20000
Figure 3.5.2. Recruitment (R, age 1 in thousands) and spawning
stock biomass (SSB, in tonnes) of the Gulf of Riga herring during
1977–2007 (ICES 2008b).
0
SSB (tonnes)
back (Gasterosteus aculeatus), nine-spined stickleback
(Pungitius pungitus), and pipefish (Nerophis
ophidion). These species are adapted mainly
to low-salinity estuarine areas and play several
significant roles in the ecosystem. Despite their
importance, the knowledge on the spatio-temporal
population dynamics of these fish species is relatively
poor.
Freshwater fish
The most common freshwater species that are
found in a majority of coastal areas of the Baltic Sea
are perch, roach, ruffe (Gymnocephalus cernuus),
ide (Leuciscus idus), pike and whitebream (Blicca
bjoerkna) (Ådjers et al. 2006). The most recent
evidence suggests that freshwater species have
exhibited different population dynamics in various
parts of the Baltic Sea over the past 10–20 years for
which monitoring data are available. For example,
coastal fish surveys show a significant increase in
perch and roach abundance in the Archipelago Sea
(Figure 3.5.4, Finbo and Brunskär). This is thought
to be caused by the ongoing coastal eutrophication,
because these fish have been shown to be
favoured by moderate eutrophication, as well as
increased water temperatures (HELCOM 2006b). In
other areas, such as the west Estonian Archipelago
Sea (Hiiumaa), Daugava, Curoninan Lagoon and
Kvädöfjärden, the same species have decreased
significantly or even collapsed (Figure 3.5.4). This,
as well as the decline in pike and pikeperch stocks
in the Curonian lagoon, Daugava estuary and Pärnu
Bay which all are characterized as having a high level
of eutrophication, has occurred owing to a fishing
pressure that has been too high (Ådjers et al. 2006,
HELCOM 2006b).
56
Recruitment (millions)
300000
250000
200000
150000
100000
50000
0
1974
1977
Recruitment
SSB
1980
1983
1986
1989
Figure 3.5.3. Recruitment (R, age 1 in millions) and spawning stock
biomass (SSB, in thousand tonnes) of sprat in the Baltic Sea (ICES SD
22–32) during 1974–2007 (ICES 2008b).
1992
1995
1998
2001
2004
2007
2000
1800
1600
1400
1200
1000
800
600
400
200
0
SSB (thousand tonnes)
A number of community-level (number of species,
total biomass, species diversity, slope of size
spectrum, and average trophic level of catch) and
species-level (biomass, mean age, mortality, mean
length, and slope of size spectrum) indicators
for coastal fish were recently developed within
HELCOM. Some of these indicators have been used
in the pilot studies using the Biodiversity Assessment
Tool BEAT (see Chapter 5).
Migratory species
Several migratory species in the Baltic Sea are of
commercial value. These include salmon (Salmo