Indicator assessment
The indicator shows changes in the percentage,
by weight, of large individuals equal to or over 40 cm in length in
fish populations in the northern part of the North Sea, from the
Humber Estuary to the Shetland Islands. It shows a steady decline
in the percentage of large fish in the population, although there
is considerable year-to-year variability. The proportion of large
fish declined most rapidly from the mid-1980s to the mid-1990s but
stopped declining in the late-1990s. It is not yet clear whether
the higher proportions of large fish recorded in the last few years
reflect year-to-year variation or the start of a longer-term
increasing trend.
This broad pattern of general decline,
followed by a more recent period of apparent stability or possible
slight increase is repeated in other seas around the UK. These
other seas are discussed in the background section below. The North
Sea data is used in the indicator because it provides the most
detailed, precise and long-running data set.
There are high levels of variability in fish
size in trawl catches, so despite the apparent recent increasing
trend, the indicator is assessed as deteriorating in the longer
term but showing little or no overall change in the period
2000-2008.
Description of trends
Changes in the size composition of fish in
survey trawl catches will drive trends in this indicator. It
responds to changes in the proportion of larger fish of a given
species and changes in the sizes of different species.
During the 1980s, large fish in the North Sea
fish community included cod, haddock, saithe, ling, anglerfish and
rays. Recently, two strong year-classes of haddock have
dominated the community of large fish, and are thought to be
largely responsible for the recent increasing trend.
Background
The indicator is compiled using methods developed by the
International Council for the Exploration of the Sea for the
International Bottom Trawl Survey (ICES, 2007). Data on fish length
are taken from the UK part of the North Sea (shown in red on Figure
13 (ii))7. The survey is undertaken in the first
quarter of each year. All fish are measured as part of the survey.
To compile the indicator, the proportions greater than or equal to
40 cm are then estimated by weight. Further details of the methods
used to calculate the indicator are given in Cotter et al
(2008).
Data from other trawl surveys around UK are
available, although they are generally for shorter time periods or
cover less extensive geographic areas than those for the North Sea.
Two examples are given in Figure 13 (ii) for the southern North Sea
and Scottish Continental Shelf. The surveys are undertaken
independently and show a similar decline in fish length or
generally low values during the 1990s until around the year
2000. This is followed by a period of little or no overall
change, despite wide year-to-year variation or an apparent
increase. This pattern of apparent increase in recent years
may be linked to the predominance of occasional strong year-classes
of fish, or tighter fishing controls, although further research is
required to confirm this interpretation.
Data for other seas around the UK, as shown in
Figure 13 (ii) are available in a background technical paper
(Greenstreet et al, 2009), which can be downloaded using
the links below.
7. These trawl gears sample on and close to the seabed
using a net held open by steel ‘otter’ boards and towed behind a
research fishing vessel.
Figure 13 (ii). International Council for the Exploration of
the Seas survey rectangles in the seas around the UK
Northern North Sea – red; Southern North Sea –
yellow; Eastern Channel – blue; Western Channel and Celtic Sea –
green; Irish Sea – orange; Minches and Western Scotland – purple;
Scottish Continental Shelf – pink; Rockall Trough and Bank – grey;
Faroe-Shetland Channel – black). Red line indicates sub-division of
the Scottish Continental Shelf referred to in background technical
paper.
Figure 13 (iii). Proportion by weight of large fish for
the southern part of the North Sea and the Scottish Continental
Shelf.
Further development planned
This is the first time that this indicator has been published
and it is likely that further development, for example to integrate
data from different seas around the UK, will be required.
References
Cotter J, Rogers S, Ellis J, Mackinson S,
Dulvy N, Pinnegar J, Jennings S and Greenstreet S. 2008.
Marine Ecosystem Integrity: Development of a Marine Trophic
Index for UK waters and recommendations for further indicator
development. Final report for Defra, Centre for Environment,
Fisheries and Aquaculture Science (Cefas).
Greenstreet S, Fraser H, Cotter J and Pinnegar
J. 2009. Assessment of the state of demersal fish communities
in UK waters. Fisheries Research Services contract report for
Defra.
International Council for the of the Seas
(ICES) 2007. Development of EcoQO on changes in the proportion
of large fish and evaluation of size-based indicators.
International Council for the Exploration of the Sea, Copenhagen.
ICES ACE Report 2007.