Following fish: how we learn about fish migration at sea

25 FOLLOWING FISH: HOW WE LEARN ABOUT FISH MIGRATION AT SEA. JULIAN METCALFE Fish Migration Many commercially important species of fish in European waters makc long distance seasonal migrations. Some, like cod, plaice and sole, migrate between summer feeding grounds and winter spawning grounds while others, like bass, appear to migrate in response to seasonal changes in their environment. In either case these migrations, which often cover several hundred kilometres, arc a crucial factor determining the seasonal changes in the distribution of fish stocks (see Plate 3 & Cover).

Understanding such changes in the distribution of fish stocks is important to effective fisheries management. It is a key to the proper understanding of the stock identity, as well as being relevant to understanding the spread of disease, and the possiblc effects of pollution and dumping at sea.

At Lowestoft, our aim in studying fish migralion is to gain a sound understanding of the basic biological (behaviour and physiology) and environmental (water currents, temperature etc.) processes which affect migrations. This knowledge is used to help us develop and improve predictive models that can aid in assessing the likely biological and socio-economic consequences of different management options, and the possiblc effects of environmental change.

Although understanding migratory processes is important to fisheries management, obtaining information about the behaviour of fish in the open sea is a difficult task; once a fish is released it disappears from view and cannot easily be followed. So our biological research has to be supported by an innovative technical development programme which provides the necessary "state-of-the-art" techniques to allow us to study the behaviour of free-ranging fish in the marine environment.

Electronic Fish tags Tagging, and other simple methods of marking, have been used since the middle of the 17th Century as a means of increasing our understanding of fish biology. By knowing where individual fish are at two times in their life (i.e. when it is caught and tagged, and when it is recaptured), often separated by months or even years, tagging a large number of fish can provide information on stock identity, movements, migration (both rates and routes), abundance, growth, and mortality. Such simple tagging may be adequate for describing fish migrations, but it teils us nothing about how fish migrate. Ii is by understanding the mechanisms fish use for moving about that allow us to bc predictive, rather than simply descriptive.

Since the early 1970s, acoustic tags have allowed us to track the movements of individual free-ranging fish for limited periods using ship borne sonar. Although this work has yiclded substantial advances in our understanding of how some species of fish migrate, the techniquc is limited because only one fish can be followed at a time, each fish can only bc followed

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for a short period (often only a few days), and sea-going work aboard research vessels is cxpcnsive. Morc rccently, we have overcome these limitations through the dcvclopment of electronic "data storage" or "archival" tags which record environmental and behavioural data. Because there is no need for human observers to follow the tagged fish, data storage tags now make it possible to monitor the behaviour and movements of many fish simultaneously over entire migrations. The first generation of data storage tag (Mk I DST) was developed at Lowestoft in the late 1980s. It is a small programmable data logger that can störe a minimum of 32,000 data samples and retain the information for at least 5 years. The tag is a dome-shaped device weighing about 55 g in air and 23 g in sea water. It has eight analogue sensor Channels, two of which are used to measure depth (derived from pressure) and water temperature. Programming and data retrieval are carried out with a PC and an electronic tag reader, via an infra-red optical link communicating through a clear window in the base of the tag. In our experiments with plaice, all the tags were programme to record pressure (depth) every 10 minutes, and temperature once each day, giving a rcording life of about 220 days (Metcalfe & Arnold, 1997).

Piaice tagging The first deployment of 50 maturing adult female plaice (>40 cm), each tagged with a Mk I DST, took place on 15 December 1993 about 50 km north-east of Lowestoft in the Southern Bight of North Sea. Subsequently, between March 1994 and February 1997, a further 253 tagged plaice have been released in the same area. To maximise the return of tags through the commercial fishery a directed poster campaign in four European languages (English, French, German and Dutch) was mounted both to inform the European fishing Community about the electronic tags, and to advertise a tag reward (£25).

Forty nine of the 303 tags released so far have been returned to date (January 1999), yielding over 2500 days of data. Three tags recorded continuously for over 200 days and one of these was returned after the fish had been at liberty for 300 days.

Plaice behaviour From our previous tracking studies using ship bome sonar, we know that plaice use the tidal streams to migrate. By synchronising their vertical movements with the local tidal currents, they are transported between their summer feeding grounds and winter spawning grounds (Greer Walker et al., 1978, Harden Jones et al., 1979; Metcalfe et al., 1992; Arnold & Metcalfe, 1996). Our knowledge of this behaviour has allowed us to reconstruct the geographical movement (Fig. lb, 2a) of the fish by combining the recorded pattems of vertical movement (Fig. la) with a simple two-dimensional model of the tidal streams for the European continental shelf (Amold & Holford, 1995). The model uses a linear interpolation from tidal stream data tabulated on British Admiralty Charts to calculate the tidal stream vector at any speeified position.

The first tag to be returned (DST 30) was caught at 50° 00' N 00° 34-8'W by a French trawler on 17 January 1994 in the eastern English Channel. The fish had been at liberty for 32 days and examination of its ovary showed that it

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had almost completed spawning when il was recapturcd. The distancc bctween thc reported recapture position and the cnd point of thc rcconstructcd track was 47 km; the net (straight-line) distancc bctween releasc and recapture positions was 285 km (Fig. ld).

The seventh fish (DST 08), which was caught on 10 February 1994 in thc southem North Sea 88 km from thc release position, was at liberty for 56 days.

Like thc first fish, its vertical movements (not shown) also exhibitcd prolonged pcriods of tidal periodicity. Thc reconstructed geographical track (Fig. 2a) indicates that the fish made substantial movements, first north to a position close to the Flamborough Off Ground and then south to thc eastern English Channel, both important plaicc spawning grounds. The fish finally moved north again towards feeding grounds in the central North Sca. The carcass of

30 C*-:

05 Jan

Figure 1. Vertical movements (Fig. la) of a maturing female plaice (length 53 cm) tagged with DST 30 and at liberty from 15 December 1993 to 17 January 1994. After the first 4 or 5 days at sea the fish adopted a tidal pattern of vertical movement which she maintained for about 15 days, during which excursions into mid-water were mostly synchronised with south-going tidal streams (indicated by black bars at bottom of Fig. lc). Subsequently, the fish changed to a diel pattern of vertical movement (Fig. ld), spending prolonged periods in mid-water at night (indicated by black bars at top of Fig. 1 d) but remaining on the seabed during the day. The recorded pattems of vertical movement (Fig. 1 a) have been used, in conjunction with a two-dimensional Computer Simulation model of the tidal streams to reconstruct thc gcographical track of the fish (Fig. lb). In the reconstruction, it has been assumed that, when off thc sea bed (<3 m), the fish swam down-tide at a speed of 0-6 body lengths per second through the water (Metcalfe et al., 1990; 1992) whilst simultaneously being transported by the tidal stream. The distance bctween the reported recapture position and the end point of the reconstructed track was 47 km; thc net distancc bctween release and recapture positions was 285 km.

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this fish was not returned with the tag, so we do not know whether or where it had spawned. However, the track was most unexpected as there have been no previous indications that North Sea plaice can visit more than one spawning ground within a Single spawning season and it is generally assumed from the results of conventional tagging experiments that plaice retum to the same spawning ground in successive years.

Although we have computed geographica! movement, rather than measured geographical position directly, we have been able to make independent checks on the overall pattern and extent of the computcd geographical movement in two ways. The First uses the hydrostatic data recorded when the fish remains on the sea bed throughout a füll tidal cycle (Fig. 2b-d). The second involves comparing the temperatures recorded by the tag with records of sea-surface temperature.

On the European Continental shelf the combined constraints of ocean basin geometry, and the influence of Coriolis force, result in the development of amphidromic systems in which co-tidal lines (lines linking points where the tide is at the same phase of its cycle) radiating out from each amphidromic point, cut across co-range lines (lines linking points with the same tidal ränge) approximately at right angles. This allows us to use the times of local high watcr and tidal ränge, estimated from the sinusoidal pressure cycle recorded when the fish remained on the sea bed throughout a füll tidal cycle, as a coarse bi-coordinate system for geolocation. We have used the Proudman Oceanographic Laboratory's (POL) numerical storm surge model (Fiather, 1991), which calculates tidal data on a 12 km grid, to identify areas of similar (to within ± 0-4 m) tidal ränge and similar (to within ± 10 minutes) times of high water to the tidal ranges and times of high water recorded by the tag (insets, Figs. 2b, 2c, & 2d) on the same date.

The estimated positions of the fish tagged with DST08 on 29 December and 17 January show reasonable agreement with the comparable positions derived from the tidal stream model, although there is a discrepancy on 4 February when the reconstructed track ends about 166 km short of the reported recapture position (53° 27' N 2° 26' E), and where the pressure record (inset Fig. 2d) shows the fish to have been close to an amphidromic point (the point of rotation of the tidal wave and where there is no rise and fall in sea level).

This is almost certainly a result of cumulative position errors associated with the track reconstruction. The ability to derive location directly from pressure measurements in this way means that it is possible to use data storage tags to deduce the geographical movements of any fish that periodically spends at least 7 - 1 2 h on the sea bed, irrespective of whether or not it exhibits a regulär pattem of vertical migration.

We have also been able to validate reconstructed geographical tracks by comparing the temperatures recorded by the tag each day with synoptic Charts of sea-surface temperatures for the appropriate dates. Figure 3a shows the temperature data for the fish tagged with DST 08. The temperature of 7 4°C recorded on the first day at sea is consistent with release near Smiths Knoll (Fig. 3b). The subsequent decrease in temperature is consistent with northward movement into cooler water (Fig. 3c) and this is followed by a rapid increase,

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i * >>

v

#Capture

\

Lowestoft )

O

Release

-1^

[ 15 D e c 1993 C to 1 0 F e b 1994

a { ° I

r w Longitude Figure 2. The reconstructed track (Fig. 2a) of a maturing fcmalc plaicc (48 cm length) tagged with DST 08, togethcr with indcpendcnt confirmation of location (+) at 3 positions (Fig. 2b-d) bascd on measurcments of tidal ränge (R) and times of high water (HW) recorded by the tag (insets in Fig. 2b-d) estimated using the Proudman Oceanographic Laboratory's (POL) numencal storm surge model (Fiather, 1991). Areas arc indicated of similar (± 0.4 m) tidal ränge (dark stipple) and similar (± 10 minutes) times of high or low watcr (light stipple) to those recorded by the tag. Congruent areas arc indicated in black.

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which is similarly consistent with thc fish moving south into the English Channel (Fig. 3d). Thc temperature record indicates that ihe fish remained in watcr warmer than 9째C for 12 days, before subsequcntly moving back into coolcr water. As with thc pressure data (Fig. 2d), the last recorded sea water temperature (5-4째C, Fig. 3e) shows that the fish had gone further north than indicated by the reconstructed geographical track (Fig. 2a). The ability to validatc location in this way is probably limited to winter and spring when there are pronouneed temperature differences between the various regions of thc Continental shelf and thc water column is well mixed.

Figure 3. The daily record of sea-water temperature from DST 08 (Fig. 4a) and sea surface temperatures in thc North Sea and eastem English Channel for selected periods during the track (Fig. 3a). The position (+) of the fish estimated from thc tidal stream model is shown for 4 selected days (16 December 1993, 29 Deccmbcr 1993, 27 January 1994, 10 February 1994; Fig. 4b-e, respectively), together with the recorded water temperature. Sea arcas with surface temperatures similar to that recordcd by the tag are indicated by grey stippling. Isotherms redrawn from Charts published by Bundesamt f체r Seeschiffahrt und Hydrographie, Bemhard-Nocht-Str. 78. D-2000 Hamburg 36).

Wc have succcssfully reconstructed thc tracks of many of the other plaice for which working tags have bcen retumed, and have been able to make independent checks on the overall pattern and extent of the computed geographical movement using the recordcd depth and temperature data. The tracks of several fish showed two or more reversals, similar to thosc described above for thc plaice fitted with DST 08, although none of these fish appear to have moved beyond the boundary of the summer r채nge of Southern Bight plaice, as deduced from conventional tagging experiments (Veen, 1978).

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New tags for new problems.

Our tagging programme using Mk I DSTs has bccn vcry succcssful and wc arc learning a grcat dcal of ncw informalion about thc migratory bchaviour of plaicc. But wc still need further data if wc arc fully to undcrstand plaicc migrations in thc North Sea and thc intcrrclationships belwecn thc vanous stocks and sub-stocks that arc thought to exist. To this cnd wc arc cxtcnding thc work with plaice by releasing morc fish taggcd with D S T s in othcr parts oC thc North Sca as part of a E U fundcd research programme.

W c arc also extending thc work by studying thc bchaviour of othcr spccics.

Nincty six rays (Raja clavata), cach taggcd with Mk I DSTs, were rclcascd in thc eastern Irish Sca in 1996 and 13 of thc tags have bccn returned so far. Wc also plan to tag cod (Gadus morhua), a specics about which wc alrcady know a little by tracking with sonar and transponding acoustic tags in thc southern North Sea (Arnold et al., 1992; Arnold et al., 1994).

To help us expand our work, Our engineers have now devcloped a ncw data storagc lag which is smaller (16 g in air. 2-5 g in sca watcr) and less expensive but which has a larger memory and greater resolution. The tag, is manufacturcd and marketed under licence by L O T E K Marine Tcchnologies Inc. based in N e w f o u n d l a n d . Canada. In addition to pressure and temperaturc sensors, this tag also includes a light sensor which can be used to rccord day length (to givc an estimate o f l o c a l latitude) and the time of local noon (to givc an cstimatc of local longitude). This technique provides a crude geolocation system without thc added weight of the hardware required for satellite based systems such as A R G O S or GPS (Gunn et al., 1994). Since October 1997, 250 plaicc have bccn taggcd with these new tags and released in the North Sea as part of the EL funded work (above). Seventy have bccn returned to date, yielding over 8000 days of data. Thc longest data set for a Single fish is 457 days.

Future Developments Despite the benefits, the use of data storage tags with m a n n e fish remains limited because, for many of species, the prospect of the fish being caught and the tags returned is very low. To avoid thc need to rely on a commcrcial fishcry, and increase the probability of data recovery, development work has bccn initiated for a " p o p - u p " tag whose data could bc rccovered by airbomc radio or satellite (Nelson, 1978; Hunter, 1986). Similar development work is being undertaken by research groups studying large pelagic occanic spccics such as tuna and swordfish. Other development initiatives includc lurther miniaturisation. and thc addition of additional onboard sensors such as compass heading.

Conclusions , Our results illustrate how observations of individual fish shed ncw light on the bchaviour of exploited populations and providc a foundation for understanding and prcdicting the seasonal movements of fish stocks. This is an cssential precursor to any realistie assessment of thc potential effectivcncss of possiblc fisheries management mcasures.

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Acknowledgments This work is fundcd by the Ministry of Agriculture, Fisheries and Food.

Thanks arc also due lo my collcagues Trcvor Storeton West, Malcolm Fulcher, Michacl Challiss and Matthew Eagle, who developcd the data storage tags, Bruce Holford, who reconstructed the fish tracks, and John Aldridge, who provided assistance with the hydrographic models. I should also like to thank Alan Davies and collcagues at the Proudman Oceanographic Laboratory, for the POL tidal surge model, Graseby Microsystems Ltd, Newmarket, who built the tags Mk I DST. References: Arnold, G. P. & Walker, M. G. (1992). Vertical movements of cod (Gadus morhua L.) in the open sea and the hydrostatic funetion of the swimbladdcr. ICESJ.-Mar. Sei., 49(3): 357-372.

Arnold, G. P„ Walker, M. G„ Emerson, L. S.& Holford, B. H. (1994).

Movements of cod (Gadus morhua L.) in relation to the tidal streams in the southem North Sea. ICESJ.-Mar. Sei., 51(2): 207-232.

Arnold, G. P. & Holford, B. H. (1995). A Computer Simulation model for predicting rates and scales of movement of demersal fish on the European Continental shelf. ICESJ. -Mar. Sei., 52: 981-990 Arnold, G. P. & Metcalfe, J. D. (1996). Seasonal migrations of plaice (Pleuronectes platessa) through the Dover Strait. Marine Biology, 127: 151-160.

Flathcr, R., Praetor, R. & Wolf, J. (1991). in Computer Modelling in the Environmental Sciences (eds Farmer, D. G. & Rycroft, M. J.) 15-30.

Oxford: Clarendon.

Greer Walker, M„ Harden Jones, F. R. & Arnold, G. P.(1978). The movements of plaice (Pleuronectes platessa L.) tracked in the open sea. J. Cons. int.

Explor. Mer, 38: 58-86.

Gunn, J„ Polacheck, T„ Davis, T„ Sherlock, M.& Betlehem, A. (1994). The development and use of archival tags for studying the migration, behaviour and physiology of southern bluefin tuna, with an assessment of the Potential for transfer of the technology to groundfish research. ICES-CM1994/Mini: 2.1 (21) Harden Jones, F. R„ Arnold, G. P., Greer Walker, M. & Scholes, P. (1979).

Selective tidal stream transport and the migration of plaice (Pleuronectes platessa L.) in the southern North Sea. J. Cons. int. Explor. Mer, 38: 331-337.

Hunter, J. R., Argue, A. W„ Bayliff, W. H., Dizon, A. E., Fonteneau, A., Goodman. D. & Seckel, G. R. (1986). The dynamics of tuna movements: an evaluation of past and future research. FAO Fish. Tech. Pap., 277. 1-78.

Metcalfe, J. D„ Arnold, G. P. & Webb. P. W„ (1990). The energetics of migration by selective tidal stream transport: an analysis for plaice tracked in the southern North Sea. Journal of the Marine Biological Association, 70: 149-162.

Metcalfe. J. D„ Fulcher, M. & Storeton-West, T. J„ (1992). Progress and developments in tcletmetry for monitoring the migratory behaviour of plaice in the North Sea. In: Wildlife Telemetry: Remote Monitoring and Tracking of Animals. eds. I. G. Priede & S. M. Swift. Ellis Horwood. New York, London, Toronto, Sydney, Tokyo, Singapore. pp.359-366.

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Metcalfe, J. D. & Arnold, G. P. (1997). Tracking fish with clcclronic tags.

Nature, 387: 6 6 5 - 6 6 6 .

Nelson. D. R. (1978). Tclcmctering techniques for thc study of frcc-ranging sharks In Sensory Biology ofSharks, Skates, and Rays, (cds. Hodgson, E. S. & M a t h c w s o n , R. F.) pp. 4 1 9 - 4 8 2 . O f f i c c of Naval Rcscarch, Department of the Navy, Arlington. Va. Veen, J. F. de, On selective tidal transport in thc migration of North Sea plaicc (Pleuronectes platessa L.) and other llatfish species. Neth. J. Sea Res., 12: 115-147.

Julian D Metcalfe The Centre for Environmental, Fisheries & Aquaculturc Scicncc The Lowestoft Laboratory Pakeficld Road Lowestoft Suffolk N R 3 3 OHT

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