BSEP116B Biodiversity in the Baltic Sea - Helcom

Table 6.1.1. Catch range during the past five years and the current state of selected major Baltic commercial
fish stocks (ICES 2008b,d). SSB: Spawning stock biomass; F: Fishing mortality.
Stock
Catch range in past
5 years (2003–2007,
10 3 tonnes)
SSB in relation
to precautionary
limits
F in relation to
precautionary limits
F in relation to high
long-term yield
Western cod 20–24 Increased risk Undefined Overfished
Eastern cod 50–71 Undefined Harvested sustainably Overfished
Herring in SD
91–116 Undefined Harvested sustainably Underfished
25–29 (excl.
GoR) and 32
Gulf of Riga
31–40 Undefined Harvested sustainably Overfished
herring
Sprat 308–405 Unknown Increased risk Overfished
Flounder 15–19 Unknown Unknown Unknown
94
to about 100 000 tonnes recently. In contrast,
the stock of Gulf of Riga herring has more than
doubled during the past two decades. Flounder
catches have increased since the early 1990s and
are currently over 15 000 tonnes annually. The
dynamics in the landings of eleven marine fish
species over time are shown in Figure 6.1.2 (ICES
2008b). Landings of the major Baltic fish stocks are
shown in Table 6.1.1.
6.1.2 Ecosystem effects of fishing
activities
The major impact of fishing is undoubtedly on
exploited fish stocks, but there are also impacts on
benthic invertebrate and fish communities, marine
mammals, seabirds and the abiotic environment.
The effect of fishing on various ecosystem components
other than fish is different in the open sea
from that in coastal areas mainly owing to the different
fishing gears employed, which depends on
the species composition of exploitable resources
and the different habitat characteristics.
Fish stocks
The main ecological impact of fisheries is the
removal of large quantities of fish, in particular
target species. The major effects include, amongst
others, a decrease in fish abundance and/or
spawning stock biomass, a decrease in the size of
individual fish at sea, and changed predator-prey
interactions. A dramatic reduction in a piscivorous
marine fish population may initiate a multi-level
trophic cascade and indirectly also affect zooplankton
and phytoplankton populations (Casini et al.
2008). Although the effect of fisheries on nontarget
species is of great importance from an eco-
system perspective, there is only very little known
on this topic. Effects of fishing on fish stocks and
fish communities are also discussed in Chapter 3 of
this report.
An important impact on the ecosystem is also
caused by by-catches and discards. Except in the
cod fishery, the extent of fish by-catch is unknown
in the Baltic Sea. Discard estimates for the two
Baltic cod stocks are available since 1996 when
sampling began. The western and eastern cod
discards have fluctuated during the period of
data availability (i.e., 1996–2007) from 5.0–26.6
million and 3.7–23.3 million individuals, respectively.
However, the discard estimates are relatively
uncertain (ICES 2008b). Calculations show that the
amount of cod offal discarded in the entire Baltic
Sea (i.e., sub-divisions 22–32) reached a peak in
the early 1980s (average ca. 58 000 tonnes annually)
and was relatively low in the 1990s, with an
annual mean of about 19 000 tonnes (ICES 1997).
Demersal communities
In general, bottom trawls have an impact on
marine biota in several ways including (i) a reduction
in structural biota; (ii) a reduction in the geographic
range of species; (iii) a decrease in populations
that have low rates of turnover; (iv) fragmentation
of populations; (v) alteration of the relative
abundance of species; (vi) sub-lethal effects on
individuals; (vii) an increase in populations that
have high rates of turnover; and (viii) favouring
populations of scavenging species (ICES 2000).
There are only a few studies available in the
Baltic region on bottom trawling impacts on the
marine ecosystem. It has been documented that