Publication Abstract

Electronic fish tagging is vital for management

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 £1000!
(Based on an article first published in Fishing News, 29 October 1999)

The second annual tag lottery draw for a £1000 reward took place in October at the Centre for the Environment, Fisheries and 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 £1000 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 sea bed 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 sea bed 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 sea bed 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 sea bed 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 14000 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 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 sea bed, 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 £1000!

Related Information:

What to do if you catch a tagged fish

Ewan Hunter is a senior research associate at the School of Biological Sciences, University of East Anglia, Norwich.
Julian Metcalfe is head of the Behaviour & Physiology team at The Centre for Environment, Fisheries and Aquaculture Science, Lowestoft, Suffolk.