Summary
Figure 13 (i). Proportion of large fish (equal to or larger
than 40cm), by weight, in the Northern North Sea, 1982 to 2009
Assessment of change in the proportion of large
individuals in North Sea fish populations
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Long term
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Since 2000
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Latest year
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Northern North Sea
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1982-2009 |
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Increased (2009)
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Note: The assessment for ‘Since 2000’ is made
by comparing a three year average for the baseline (1999 to 2001)
with the latest data point (2009).
- Changes in the size structure of fish populations and
communities reflect changes in the health of the fish
community.
- This indicator shows changes in the proportion of large fish
(40cm or greater in length) in the Northern part of the UK area of
the North Sea.
- The proportion of large fish, in the Northern North Sea fell
from around 15 per cent by weight of the fish community in 1982 to
around seven per cent in 2009; however this is an increase from a
low of two per cent in 2001. Large fluctuations in numbers between
years are typical features of the size of North Sea fish
populations.
- The measure for the Northern North Sea is used as the main
indicator because it is based on the largest data set and provides
the most reliable indicator of change. In addition, the North Sea
supports important fisheries and several are still recovering from
over-exploitation. In common with the Northern North Sea, there has
been a modest increase in recent years in other seas around the UK
since 2000.
Indicator description
The indicator shows changes in the percentage, by weight, of
large individuals equal to or over 40cm in length in fish
populations in the northern part of the North Sea, from the Humber
Estuary to the Shetland Islands. It responds to changes in the
proportion of larger fish of a given species and changes in the
sizes of different species. Changes in the size composition
of fish in survey trawl catches will drive trends in this
indicator.
It shows a steady decline in the percentage of large fish in the
population, although there is considerable year-to-year variability
in fish size in trawl catches. The proportion of large fish
declined most rapidly from the mid-1980s to the mid-1990s but
stopped declining in the late-1990s and increased between 2000 and
2009.
During the 1980s, large fish in the North Sea fish community
included cod (Gadus morhua), haddock (Melanogrammus
aeglefinus), saithe (Pollachius virens), ling
(Molva molva), anglerfish (Lophius
piscatorius) and rays (Batoidea). Recently, two
strong year-classes of haddock have dominated the community of
large fish, and are thought to be largely responsible for the
recent large fish recordings. The indicator is assessed as
deteriorating in the longer term but improving since 2000,
although it is not yet clear whether the higher proportions of
large fish reflect unusual years particularly for haddock, or
the start of a longer-term increasing trend.
This broad pattern of general decline, followed by a more recent
period of 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.
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))9. 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 40cm are then estimated by weight. Further
details of the methods used to calculate the indicator are given in
Cotter et al (2008).
Figure 13 (ii). Proportion of large fish (equal to
or larger than 40cm in length), in the Western Channel and
Celtic Sea, Irish Sea, Scottish Continental Shelf, and Southern
North Sea by weight, 1982 to 2009.
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. Four examples are
given in Figure 13 (ii) for Western Channel and Celtic Sea, Irish
Sea, 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 increase between 2000
and 2009. The 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.
9 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 (iii). 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.
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.