Electronic fish tagging is vital for management
Based on an article published in Fishing News, 29
October 1999
Ewan Hunter and Julian Metcalfe describe how the Cefas
electronic data tag has given new insights into plaice behaviour -
plus they describe how to win £1,000!
The second annual tag lottery draw for a £1,000 reward took
place in October at the Centre for the Environment,
Fisheries & Aquaculture Science (Cefas) laboratory in
Lowestoft.
The prize was collected by a Peterhead-based fishing company
which returned one of 60 state-of-the-art electronic data storage
tags (DST) received back in the last 12 months.
Last year's prize-winner was drawn by Fishing News'
editor Tim Oliver, and was won by a Lowestoft-based fisherman. This
year the prize was drawn by Hugh Sims, the Chief Executive of the
Lowestoft Fish Producers Organisation.
The lottery is part of a joint MAFF/EU-funded research programme
designed to gain a better understanding of the seasonal migrations
of plaice in the North Sea. The study is being carried out with
other fisheries scientists in the UK, Holland, Belgium and
Denmark.
Since October 1997 nearly 500 DST tagged plaice have been
released and so far over 100 have been returned from European ports
all around the North Sea.
As with all tagging experiments, we rely totally on the help and
co-operation of the commercial fishermen who catch the fish and
return the tags. The lottery was set up as an additional incentive
to help encourage fishermen to return these very special tags.
It was not just the winner of this year's £1,000 cheque who was
smiling. Those of us involved in the research programme also have a
great deal to celebrate. We are now getting tags back which have
been out long enough to collect over a year's data.
But it's not just record breaking tags we're after; the
information they contain is the real prize for us. Tagged fish that
have been at liberty for over a year reveal how fish move over a
full annual cycle of feeding, migration and spawning.
This information, together with that from all the other tags
that have been returned, is giving us a detailed insight into the
mechanisms by which plaice move around on the European Continental
Shelf, which was unimaginable even a decade ago.
The electronic tags measure water pressure (to give depth), sea
water temperature and daylight. The tags can log data for up to 900
days and then store the data for up to 25 years.
Unlike conventional tagging experiments, which only provide
information about the distance between release and recapture
positions, the data from the electronic tags provide continuous
information about the behaviour of each fish. We use this
information to calculate their geographical movements.
From earlier work we know that in areas like the western side of
the central North Sea, the Southern Bight and English Channel,
plaice use a tidal "conveyor belt" to move around. They do this by
coming up into mid-water when the tide is flowing in one direction,
and go back to the seabed when the tide turns and flows back. The
fish are transported along the tidal stream path in a series of
"hops".
This transport system helps them save energy during their
long-distance migrations between summer feeding grounds in the
central North Sea and winter spawning areas in the Southern Bight
and the eastern English Channel. Energy saved by using the conveyor
belt allows fish to devote more energy to egg production and
growth, and this in turn means more plaice.
For the tagged fish, we are able to calculate geographical
movement (the ground track) by combining the fish's pattern of
movements on and off the seabed recorded by the DST, with a Cefas
computer-simulation model of the tidal streams.
This "reconstructed" ground track is independently confirmed
using the records of sea-water temperature, and tidal data (times
of high and low water, and tidal range) recorded when the fish
stayed still on seabed for a period of 12 hours or more:
Movement of plaice in relation to tidal
flow
In areas of fast tidal flow, North Sea plaice use a tidal
"conveyor belt" to move around. The DST depth record above (black
line) shows a plaice move up into mid-water on three consecutive
south-flowing tides, and return to the seabed when the tide turns
and flows northwards.
The final portion of the record shows a period where the fish
rested on the bottom for a period of 12 hours. During this period
the tag records the rise and fall of the tide over a full tidal
cycle.
Using times of high (HW) and low water (LW) and tidal
range (TR), the location of the fish can he estimated with a Cefas
computer simulation model of the tidal streams.
But what happens where the tidal currents are slower?
Calculations show that in such areas plaice cannot save energy by
using the tidal conveyor belt, and would do better just to swim
between feeding and spawning grounds (it's a bit like deciding to
get off the bus and walk when you're stuck in a traffic jam).
To answer this question, MAFF/EU-funded research has
concentrated on the behaviour of plaice in the central North Sea
and German Bight where the tidal streams are slower than in the
Southern Bight.
The 110 tags already recovered have yielded over 14,000 days of
behavioural data.
The longest record so far is 512 days, which came from a tag
returned by a Peterhead fisherman. This fish was released on the
Outer Rough in the central North Sea in December 1997, and was
recaptured in May this year [1999] in the vicinity of the
Ekofisk field.
We are still in the early stages of analysing the new data, but
initial results are intriguing. They show that even in areas where
the tides are relatively slow, plaice may still move large
distances but spend much less time swimming in mid-water than
plaice in areas where the tides are fast.
Instead, they appear to spend more time swimming close to the
seabed, making less use of the tidal currents. However, if they
then reach an area of faster tidal currents, they change their
behaviour accordingly and get back on the conveyor belt.
These records will be invaluable in addressing as yet unanswered
questions, such as whether plaice remain loyal to the same spawning
grounds each year. Data from DSTs have already shown that plaice
are capable of visiting more than one recognised spawning ground
within a single spawning season. Furthermore, not all plaice
migrating from the central North Sea to the eastern Channel return
there after the end of spawning.
Two out of 34 plaice migrating from the Southern Bight into the
eastern English Channel remained within the vicinity of the Channel
spawning grounds before being recaptured there in June and August
respectively. Such information has important implications for the
structure of North Sea plaice stocks, and could not have been
determined from conventional tagging experiments.
Such information will lead to better stock assessments and a
more informed debate on the use of closed areas for fisheries
management. By returning tags therefore, fishermen are not only
helping scientists to understand and predict the annual movements
of fish stocks, but they are also playing a valuable part in
helping us to formulate biologically realistic strategies which
will ensure the long-term sustainable harvest of North Sea fish.
They may also be taking one step closer to winning £1,000!