The IUCN/SSC Shark Specialist Group
Shark News 7: June 1996
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Data storage tags: individual
behaviour-based approaches to
migration
Nick Dulvy and Julian Metcalfe, MAFF Fisheries Research, UK
Traditionally fish movements have been followed by mark-recapture
methods or by analysis of catch data (e.g. Pawson and Nichols 1994,
Rousset 1990). The former type of study can provide estimates of speed
and direction of movement during different seasons, as well as growth
rates, mortality estimates, rates of stock interchange, yield, and stock
replacement values (e.g. Smith and Abramson 1990, Holden 1972).
The disadvantages of simple studies are that large numbers of fish
must be tagged (1,000+), recaptures should be followed for as many
years as possible (2-15 years) and the data quality depends on the
level of cooperation by fishermen. More problematic, the estimates of
some of the above variables can be flawed. Mark-recapture methods
only indicate net movement; for instance in a recent study an
electronically tagged plaice showed a net movement of 80 km over 56
days, when in actual fact the fish had travelled 800 km and was
recaptured on its return migration. The key problem with conventional
tagging studies is that if any condition varies, e.g. size- or age-selectivity
of the fishery, fishing pressure or environmental change,
then the parameters of the population will change, necessitating a
costly re-estimation of parameters.
New methods of following fish movements
The Fisheries Laboratory of the Ministry of Agriculture, Fisheries
and Food at Lowestoft, UK, have developed an electronic "Data
Storage Tag" consisting of pressure and temperature sensors and a
1 megabyte chip to store the data. The tag can record data for over nine
months and store this data for five years. When the tag is returned by
fishermen the data is extracted using an infrared link.
The tag records vertical movement of the fish, which can be
translated into horizontal movements using a hydrographic simulation
model. The whole system is based on the knowledge that many
coastal benthic fish species rise into the water column to catch a 'tidal
conveyer', a form of movement known as "Selective Tidal Stream
Transport". It is assumed that horizontal movement is a function of the
period of time spent in the water column and the rate of tidal flow and
the vector of tidal current which the fish uses. Temperature profiles
can be used to validate broad scale movements, as can the tidal
profiles when the fish is at rest on the bottom.
One hundred of these tags were put on plaice in 1993 and I have
just put 100 tags on the thornback ray Raja clavata in the Irish Sea.
Preliminary results from the 20 returned plaice tags have demonstrated
surprising results (Mackenzie 1994). For the first time we can follow
the movements of individual fish in the wild for longer than a week or
so. Not only do these fish move ten times faster than previously
thought, but a number of distinct behaviour patterns have been
identified from this study, including:
- vertical movement linked to the night time period only
- vertical movement using both tides in the daily cycle
- vertical movement using night tides in one direction only.
These vertical migrations have direct consequences for
geographical movement.
Progress is currently being made towards determining the cues
triggering these vertical movements. Once this can be determined,
then individual behaviour-based predictive models can be constructed
to simulate population movement and distribution. The beauty of
these data are that the individual cues to migrate are independent of
fishing or environmental shifts, therefore such models could be used
to predict the spatial and temporal effect of changes in fishing effort
or habitat loss on fish populations (Sutherland 1996).
Relatively large numbers of electronic tags are required, as we are
dependent on returns from the fishery. Nonetheless, cost is comparable
to if not cheaper than the cost of large-scale conventional tagging
studies. In the future it is hoped that this basic form of tag will be
modified to download data to passing aeroplanes or even satellites,
avoiding the problem of obtaining fishery returned tags.
References
Holden, M.J. 1972. The growth rates of R. brachyura, R.
clavata and R. montagui as determined by tagging data.
J. Cons. int. Explor. Mer., 34: 161-168.
Mackenzie, D. 1994. In pursuit of plaice with chips. New
Scientist, 6.
Rousset, J. 1990. Population structure of Thornback rays Raja
clavata and their movement in the Bay of Douarnenes. J.
Mar. Biol. Ass. U.K., 70: 261-268.
Smith, S.E., and Abramson, N.J. (1990) Leopard shark Triakis
semifasciata distribution, mortality rate, yield, and stock
replenishment estimates based on a tagging study in San Francisco
Bay. Fish. Bull., 88: 371-381.
Sutherland, W.J. 1996. From individual behaviour to population
ecology. Oxford University Press, Oxford, UK, p. 213.
Nick Dulvy 1 & 2 and Julian Metcalfe 2
1 School of Biological Sciences, University of East Anglia,
Norwich, Norfolk, NR4 7TJ, UK. Email: N.Dulvy@uea.ac.uk
2 Directorate of Fisheries Research, Ministry of Agriculture,
Fisheries and Food, Pakefield Road, Lowestoft, Suffolk, NR33
OHT, UK. Email: J.D.Metcalfe@dfr.maff.gov.uk
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