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THE ABILITY OF STANDARD DEMERSAL SURVEYS
TO EVALUATE THE FISHERIES IMPACT ON
THE BARENTS SEA ECOSYSTEM
by
Knut Korsbrekke and Sigbjørn Mehl
Institute of Marine Research, P.O.Box 1870 Nordnes
N-5024 Bergen, Norway
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Introduction and Background
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The Barents Sea covers an area of about 1.4 x 106 km2, with an average depth of 230 m.
Climatic variations depend mainly on inflowing Atlantic water and variation in ice extension
from year to year may exceed 500 km.
Annual catches have ranged from about 1 to 3 million tonnes and the main commercial
species are capelin, herring, cod, haddock and deep sea shrimp.
There are strong species interactions among these and other species and in later years
recruitment, growth, and mortality have varied considerably for some of these species.
Commercial species have been heavily exploited through most of the last 20 years and in this
study we are using data from standard demersal trawl surveys to get an impression of the
impact of exploitation on the ecosystem, including non-target fish species.
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Demersal surveys have been conducted annually in February (since 1981) and in July-August
(since 1995). Trawl stations taken in February 1996 and July-August 1996 are shown in the
Figures to the right. The July-August survey covers a larger area and includes the mature part
of the cod stock which is not covered in February due to spawning migrations.
Stomach samples of cod from the years 1988-1998 were also used in this study.
Analysis
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All results exept the cod/capelin/temperature figure below are based on swept area estimates
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2 key species in the Barents Sea:
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The estimated biomass of cod in the period 1985-1998 peaked at
more than 2.4 million tonnes in 1993 which followed an
impressive recovery from the the all time low of 0.8 million tonnes
in 1988.
Individual growth of cod has been closely related to the variations
in capelin biomass.
A large inflow of Atlantic water starting in 1989 led to an increase
in temperature and plankton production. The increase in
temperature can be seen in the mean temperatures of 4
hydrographical sections, first in the warmest areas in the west
and later in the coldest eastern part of the sea.
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Some ecosystem metrices
Based on trawl survey data, the ecosystem metrices Hills N1 and
N2 and Species evenness were calculated. Results from the
February survey are shown with solid lines, while the short time
series of results from July-August are given with dotted lines.
No clear trends can be seen, but after 1993 there seems to be
less “noise” in the data due to improved standardisation of trawl
handling and instrumentation. The “noise” in the early 1990’s
could also be related to the strong yearclasses produced by some
species (due to better temperature and feeding conditions). The
July-August series are too short for any comparisons, but in the
two last years they also show an upwards pointing trend.
Treating the cod as an observer
Using stomach contents from cod as samples from the
ecosystem (February survey) and calculating the ecosystem
metrices Hills N1 and N2 and Species evenness, these results
are apparent.
There are some clear trends. The low values in 1991-1995 are
caused by a domination of capelin in the stomachs.
When capelin are not present cod switch to other prey and
utilises a higher number of species. There is a time lag in this
process; the capelin were quite abundant in 1989, but the
increase had no effect on the metrices until 1991. And after the
capelin depletion in 1994, the change in metrices was observed
in 1996.
Size spectrum
The slopes of the size spectra (below and over 50 cm) are shown together
with the estimated fishing mortality. The minimum landing size for cod is 47
cm and the selectivity of the trawls indicate that little effect of fishing should
be observed below 50 cm. The flat size spectrum above 50 cm in 1988
changed as the strong 1983 yearclass of cod entered the size range in 1989
and 1990. High growth and low fishing mortality led to a higher number of
larger individuals up to 1994. The 1990 yearclass was also quite strong, but
was not able to affect the size spectrum in a similar way. This could be
caused by a high fishing effort targeting fish around 50 cm.
CONCLUSIONS
Species richness
Generally a higher number of prey species could be identified in
the cod stomachs than the total number of species caught in the
trawl. The overall stomach contents were very low in 1988, which
could explain the low number of species observed. The low
number of species observed in stomachs in 1993 could be
caused by the dominance of capelin as a prey item. The figure
clearly demonstrates the inabillity of the trawl to sample more
than a small fraction of the species in the ecosystem. The slightly
increasing trend could be caused by the technical staff gaining
more experience throughout this period.
ICES/SCOR Symposium on Ecosystem Effects of Fishing
Montpellier, France, 15-19 March 1999
The standard demersal sampling trawl can only
observe a modest fraction of the ecosystem.
The effect of reducing the fishing effort in 1989-1992 is
difficult to quantify and interpret due to the concurrent
changes in environmental conditions.
The high fishing mortalities in the latest years have
perhaps, caused a reduction in the slope of the size
spectrum, but could also be due to low recruitment
flattening the spectrum.
The strong fluctuations in environmentaly dependent
recruitment makes interpretation of changes in the size
spectra difficult.
The cod can not be trusted as an observer of the
ecosystem.