Total marine fisheries extractions by country in the Baltic Sea

Total marine fisheries extractions by country in the Baltic Sea: 1950-present, Rossing, Booth and Zeller 67
lost from gear prior to being brought on board; b) ‗ghostfishing‘ due to lost or abandoned fishing gear that
continues to fish; c) ‗boat-based discards‘, usually resulting from fishers‘ catch retention behavior; and d)
‗seal-damaged discards‘ representing the fraction of catch lost because of seal damage.
‗Underwater discards‘: We only applied an underwater discard rate to herring
and sprat as few studies have been conducted on this kind of discard in the Baltic
(Kelleher, 2005). For herring caught with trawl-gear, Rahikainen (2004) related
underwater discard amounts to observed catches of herring. We transformed this
into a rate of approximately 9% for underwater discards of herring caught by
trawl (see ‗Methods‘ in Zeller et al., this volume). Since herring and sprat are both
caught in a mixed species fishery using similar gear-types, we applied the same
underwater discard rate to both species. However, herring and sprat landings for
Table 4. Boat-based salmon
discard rates for Estonia (ICES,
2008b) used in all subdivisions
except in subdivision 32 when sealdamaged
discard rates based on
sources (Königson et al., 2005;
FGFRI) were greater (1981-2007).
Values in italics indicate an
interpolated rate.
Year Boatbased
Sealdamaged
1950-1980 2.0 0.0
1981 2.0 6.9
1982 2.0 13.8
1983 2.0 20.7
1984 2.0 27.6
1985 2.0 34.4
1986 2.0 41.3
1987 2.0 48.2
1988 2.0 55.1
1989 2.0 62.0
1990 2.0 68.9
1991 6.0 75.8
1992 10.1 82.7
1993 14.1 89.5
1994 12.9 96.4
1995 13.9 103.3
1996 15.1 110.2
1997 14.9 117.1
1998 14.2 124.0
1999 14.8 130.9
2000 10.3 137.8
2001 15.0 203.4
2002 15.8 255.8
2003 15.4 337.3
2004 15.6 380.5
2005 15.2 113.2
2006 17.4 149.3
2007 14.2 102.7
Estonia are not reported by gear type, so the
underwater discard rate of 9 % for trawl
fisheries was reduced to a more conservative
estimate of 5% to account for the lack of catch
data by gear-type. This rate was then applied to
herring and sprat landings (i.e. ICES landings
statistics+ adjustments + unreported landings)
for all years between 1950 and 2007.
Table 3. Boat-based
discard rates (in %) for
taxa included in our
grouping ‗others‘
derived from Estonian
study for the period
1993-2007 (Anon.,
2006b, 2007a).
Common
name
Anchor
point
Trout 25.72
Perch 7.67
Pikeperch 4.88
Roach 6.36
Garfish 4.74
Smelt 4.32
Burbot 3.00
‗Ghostfishing‘: The estimated ghostfishing discard rate was based on
Tschernij and Larsson (2003), who estimated the amount of cod caught
by lost gear in Sweden and related it to commercial catches in Sweden.
Using these data, Brown et al. (2005) estimated the range of
ghostfishing rates by lost gear to be between 0.01% and 3.2%, and here
we used the average of 1.65% applied to all taxa, except herring and
sprat, for all years from 1950-2007.
‗Boat-based discards‘: Boat-based discard rates were compared to seadamaged
discard rates from 1980-2007 in subdivisions where sealdamaged
discard rates were reported. The higher rate between the two
categories was the only rate used to avoid the possibility of double
accounting as some seal-damaged discards may have already been
accounted for in estimates of boat-based discards.
From 1950-1990 a boat-based discard rate of 2% was applied to all taxa,
except herring and sprat, according to our default assumption-based
methodology for boat-based discards (see ‗Methods‘ in Zeller et al., this
volume). Our assumption that boat-based discards for herring and sprat
were zero over the entire study period was supported by an ICES report
indicating that boat-based discards for herring and sprat were almost
n0n-existent (ICES, 2005; 2007; 2008a). Rates for 1991 and 1992 were
derived through linear interpolation between the default assumptionbased
rate for 1990 of 2% and the first available anchor point in 1993.
Boat-based discard rates for the period 1993-2007 were derived from
three sources, the first being the Estonian government provided boatbased
discard tonnage for various taxa for 2005. The rates were derived from the total landings presented
in the same report, and were estimated for cod (3.7%), herring (3.9%), sprat (3.0%), European flounder -
the only reported flatfish - (17.2%), and several other taxa (Table 3) that were included in our grouping
‗others‘ (Anon., 2006b, 2007a). The 2005 rate was used as the discard rate throughout the 1993-2007
time period. The second source estimated boat-based discard rates of salmon from ICES stock assessment
working group data (ICES, 2008b), which presents Baltic-wide, boat-based salmon discards (in tonnes) as
a minimum, mode and maximum for the 1993-2007 time period (Table 4). The mode was used for
Estonia, following our default assumption-based approach for countries that do not report recreational
catches (see ‗Methods‘ in Zeller et al., this volume). These values were converted to rates using the total
landings presented in the same working group report. The third source for estimating discards for all
remaining individual taxa, included here in our group ‗others‘, was a boat-based discard rate of 6.2% for