BSEP116B Biodiversity in the Baltic Sea - Helcom

Baltic Sea, most of the 105 recorded by-caught
porpoises in the years 1990 to 2001 were
reported from bottom-set gillnet fisheries or were
stranded with characteristic net-marks (Siebert
et al. 2006). While these numbers are considered
to be minimum figures, Rubsch & Kock (2004)
estimated the annual by-catch in the German
set-net fishery to be 57 individuals in the western
Baltic Sea and 25 in the German part of the Baltic
Proper. A total of 45 by-caught animals were
reported from Polish waters between 1990 and
1999 (Skóra & Kuklik 2003). In Latvia, two porpoises
were found entangled in fishing nets in
the Gulf of Riga in October 2003 and in January
2004, respectively (ASCOBANS 2004). A modelling
study by Berggren et al. (2002) estimated
the potential limits to anthropogenic mortality
for harbour porpoises. To achieve the goal of a
population recovery to more than 80% of carrying
capacity, by-catch limits of two individuals
per year in the Baltic Proper and three individuals
per year in the Kiel Bight–Mecklenburg Bight area
should not be exceeded.
Seabirds
Several studies from different parts of the Baltic
Sea have shown that set-net (gillnet) fisheries in
the Baltic Sea cause the death of tens of thousands
of birds every year. The by-catch problem is of
special relevance where gillnet fisheries are practiced
in areas with high concentrations of resting,
moulting or wintering seabirds. The conflicts are
usually seasonal.
Finland, especially eider, black guillemot (Cepphus
grylle), razorbill (Alca torda) and red-throated and
black-throated divers (G. stellata and G. arctica)
are the species most affected (Hario 1998). The
most recent Swedish by-catch study covering the
Swedish fishery as a whole (Lunneryd et al. 2004)
showed that the cormorant was the species predominantly
affected, followed by eider, guillemot,
merganser, and long-tailed duck. The threat of
drowning in fishing gear is larger for piscivorous
species than for benthophagic ducks, although in
most areas the total number of piscivorous birds
drowned is lower owing to their smaller populations.
The studies available mainly investigated
bird by-catches in near-coastal waters. Information
about the by-catch on fishing grounds
further offshore is scarce, although it is known
that high densities of birds and high fishing intensity
may overlap seasonally in these areas also.
Another effect of fisheries on birds is related to
the discarding of unwanted catch and offal at
sea. Gulls especially benefit from this food source.
Furthermore, the increase in sprat stocks as a consequence
of cod overfishing is expected to have a
positive effect on piscivorous birds, especially auks.
Abiotic environment
In general, the effects of bottom-towed gears on
abiotic habitats are primarily (i) removal of physical
features, (ii) reduction in complexity, and (iii)
alteration of the physical structure of the seafloor
(ICES 2005b).
Piscivorous birds (divers, grebes, mergansers,
auks, cormorants) and benthophagic ducks may
become entangled and die in fishing gear. At
the southern coast of the Baltic Sea, the longtailed
duck (Clangula hyemalis) is the most
numerous species caught in gillnets, followed by
black scoter (Melanitta nigra), common scoter
(Melanitta fusca) and red-throated diver (Gavia
stellata), while in some areas, eider (Somateria
mollissima), greater scaup (Aythya marila), guillemot
(Uria alge) and cormorant (Phalacrocorax
carbo) are also found in large numbers (I.L.N.
& IfAÖ 2005, Schirmeister 2003, Stempniewicz
1994, Kirchoff 1982, Kowalski & Manikowski
1982). In the coastal waters of Lithuania, losses
of Steller’s eiders are of concern owing to the
rareness of this species (Dagys & Žydelis 2002). In
Towed demersal fishing gear can also substantially
affect the abiotic environment in the Baltic
Sea area, where this fishing practice is applied.
For example, in the intensely fished Kiel Bight, the
total area disturbed was estimated at 630 km 2
per year causing a mobilization of nutrients
estimated to be 0.6–2.5 tonnes silicate, 0.3–1.3
tonnes nitrogen and 0.1–0.5 tonnes phosphorus
per km 2 and year (Krost 1990). The increase
in nutrient concentrations was followed by an
increase in oxygen consumption of 1.5–7.8 tonnes
per km 2 and year. This points to a considerable
impact of fishing activities on the marine environment
(Krost 1990). There is no documented
evidence that traditional coastal fisheries (e.g., by
means of trapnets and gillnets) have affected the
abiotic environment.
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