Designing tagging programmes. Metcalfe et al.

NOT TO BE CITED WITHOUT PRIOR REFERENCE TO THE AUTHORS

International Council for Use of data storage tags to reveal aspects Exploration of the Seas of fish behaviour CM2006/Q:03

Designing fish-tagging programmes to understand fish movements and population dynamics

Julian Metcalfe, D.A. Righton and E. Hunter

Centre for Environment, and Aquaculture Science, Lowestoft Laboratory, Lowestoft, Suffolk, England UK. tel: + 44 1502 524352, fax: + 44 1502 5513865, e-mail: [email protected]

Large-scale fish tagging programmes are becoming more popular as managers realise the importance of including spatial structure in assessment and management models. Two recent EU-funded projects on plaice and cod have shown how information from electronic tags can be used to gain new insights and add value to historic tagging data. Highlights have been the demonstration of unexpected population sub-structuring in plaice, and the realisation that cod behaviour is very variable in response to regional environments. Success does not come without planning and management; of staff, of data and of expectations. We share our experiences from the last 10 years of electronic tagging, and review those of others, to provide an up-to- date analysis of what makes a good tagging programme, and how to get the most from it.

Keywords: Data storage tags, migration, behaviour, tagging

Designing fish tagging programmes. Metcalfe et al.

INTRODUCTION

From a management perspective, a fish stock may be defined as that part of a population within which the effects of exploitation on population structure are recognisable (Pawson & Jennings, 1996). If possible, these management (and assessment) stocks should also have a high degree of biological integrity. The biological identification of stocks, and knowledge of their spatial distribution and dynamics, are therefore important aspects of any strategy.

Many commercially exploited fish species like cod and plaice in European waters are migratory (Metcalfe et al. 2002), making regular seasonal movements between, for example, winter spawning grounds and summer feeding grounds. Theses migrations result in substantial changes in fish stock distribution and so understanding is clearly important to rational management and conservation of . However, obtaining this understanding at appropriate temporal and spatial scales is frequently problematic. Fishermen can provide useful information about seasonal appearances and disappearances of fish, and fisheries statistics can give a general picture of seasonal changes in population distribution. However, such information rarely suffices to describe the migrations of a particular species, and much more detailed information about the behaviour and movements of individuals and populations are needed. But following fish is difficult, particularly in the open sea where, once released, they quickly disappear from view and cannot easily be followed.

Mark-recapture studies using various methods of tagging, have been used since the mid 17th century (Walton and Cotton, 1898) as a means of increasing our understanding of fish biology. Mark-recapture methods tell us where individual fish are at two times in their life (i.e. when caught and tagged, and when recaptured). If tagging and recapture are separated by a suitable amount of time (months or even years), it can provide information on stock identity, movements, migration (both rates and routes), abundance, growth, and mortality. Some littoral species, like blennies (family Blenniidae), make seasonal inshore and offshore movements that extend no more than a few kilometres. For such species, large-scale mark-recapture programmes are unlikely to be cost-effective ways of studying their movements. However, species like herring ( Clupea harengus ), mackerel ( Scomber scombrus ), cod (Gadus morhua ), plaice ( Pleuronectes platessa ), Atlantic salmon (genus Salmo), Pacific salmon (genus Oncorhynchus) and eels (Anguilla species), various species of tuna, billfishes and large sharks make more extensive movements over several hundreds or thousands of kilometres. For these species, mark-recapture studies can provide very valuable information about population structure and dynamics.

There are many methods for marking or tagging fish (see Parker et al. 1990; Jennings et al. 2001). Branding or fin clipping is a quick and simple way to mark large numbers of fish, and chemical tags such as the tetracycline (an antibiotic which is deposited specifically in calcified areas and fluoresces under ultraviolet light), can easily be applied to large numbers of fish and Designing fish tagging programmes. Metcalfe et al. remain as a permanent mark. Alternatively, various types of tag can be attached to, or placed in, the fish. External tags include Petersen discs used to tag plaice and other flatfishes, while internal tags include the tiny coded wires that are used for the mass marking of young salmon.

Simple mark-recapture studies can be very useful for describing gross patterns of population movement, but the method tells us very little about how fish migrate. Population movements derived from mark-recapture studies rely on commercial fishermen reporting details of the time and location of recapture of tagged fish and the results of such studies are therefore inevitably an integration of both fish behaviour and fishing activity that confounds any analysis of population movements. Tagging data can be adjusted for spatial variations in fishing effort, where this is known, but movements of fish into un-fished or un-fishable areas, or changes in fish behaviour which alter availability or catchability, cannot easily be accounted for. It is only by understanding the movements and behaviour of individuals over short (hours and days) medium (days and weeks) and long (seasons and years) time scales, that we can reveal the mechanisms fish use to move about. Understanding these mechanisms allows us to be predictive, rather than simply descriptive. Large-scale fish mark-recapture programmes are therefore becoming more popular as fishery managers realise the importance of including spatial structure in assessment and management models.

Two recent EU-funded projects on plaice (EU FAIR Programme, PL96-2079) and cod (CODYSSEY QLRT – 2001 – 00813) have shown how information from electronic tags can be integrated with large mark-recapture tagging programmes to gain new insights and add value to historic tagging data. Highlights have been the demonstration of unexpected population sub- structuring in plaice, and the realisation that cod behaviour is very variable in response to regional environments. Success does not come without planning and management; of staff, of data and of expectations. We share our experiences from the last 10 years of electronic tagging, and review those of others, to provide an up-to-date analysis of what makes a good tagging programme, and how to get the most from it.

METHODS

Planning a tagging programme

Simply wanting to know where fish move to is not, in itself, a good enough reason to embark upon a large mark-recapture programme. Tailoring a tagging programme too closely to existing management regimes and assumptions, however, would be a missed opportunity to collect other useful biological and ecological information. The objectives of a tagging project should therefore be a careful compromise between curiosity-driven research and data collection that will deliver information relevant to fisheries management. Tagging programmes are also multidisciplinary activities, and researchers engaged in them need to be adept in a number of different fields, from publicity to data management.

Designing fish tagging programmes. Metcalfe et al.

The information that can be applied to fisheries management will vary depending on regional legislation and practice, but the core requirement will be knowledge of the areas occupied by the target stock and the seasonal movements of individuals between them. There are many objectives that can be nested within this core requirement depending on the specific nature of regional management measures. For instance the locations of key habitats such as spawning areas and feeding grounds, the migration routes between them, and patterns of vertical distribution. This kind of information may help to define areas that should be closed seasonally, or fishing gears that should be subject to limitations or exemption (e.g. Thornback rays, Hunter et al. 2004a).

Investigating the scope for tagging

Having identified the objectives of the tagging programme, it is necessary to understand as much as possible about the target stock, in addition to its environment and fishery, before any further plans can be made. Key to the success of any tagging programme is good recovery of the tag (essential for studies using archival tags), the tagged fish and associated recapture data. For example, it is important to have an idea about the size the "stock", the extent of its geographical distribution, and what tag recovery rates are likely to be. These factors are important in estimating how many fish need to be tagged, where and when, in order to achieve a statistically robust result. In marine fisheries, the area of encounter is potentially vast but can be reduced significantly with backup information from catch data or earlier tagging studies. For electronic tagging programmes, pre-tagging surveys with conventional tags should be carried out to provide a rough estimate of where the electronic tags will be recovered and what the target fisheries are likely to be. It is also important to understand who is likely to catch the fish; can they be used for tagging, or is experimental fishing necessary to get sufficient fish tagged in the appropriate location(s).

Decide upon your tag type

Since the late 1960s electronic tags that transmit radio (for use in fresh water) or acoustic (for use at sea) signals have increasingly been used to track the movements of individual free-ranging fish for limited periods. Such work has yielded substantial advances in our understanding of how some species of fish migrate (e.g. plaice, above). But this technique is limited because, in most applications, only one fish can be followed at a time, each fish can only be followed for a short period (often only a few days), and sea-going work aboard research vessels is expensive. More recently, substantial advances in microelectonic technology have permitted the successful development of electronic “data storage” or “archival” tags that are small enough to be attached to fish (Figure 1). These devices record and store environmental and behavioural data and, because there is no need for human observers to follow the fish, now make it possible to monitor the behaviour and movements of many fish simultaneously over entire migrations (Metcalfe and Arnold 1997, 1998). A variety of such devices (Figure 1) is now being used to study the movements of species as diverse as plaice (Metcalfe & Arnold, 1997), salmon Designing fish tagging programmes. Metcalfe et al.

(Walker et al. 2000), tuna (Gunn 1994, Gunn et al. 1994, Block et al. 1998, Block et al. 2005), Thornback rays (Hunter et al. 2005) and others.

Although most data storage tags currently measure only simple environmental variables such as pressure (depth), temperature (internal and external) and ambient daylight, the data can nonetheless be used to derive detailed information about the movements of fish. On the European continental shelf, tidal information (times of high- and low-water and tidal range) derived from pressure measurements can be used to locate fish whenever they remain stationary on the seabed for a full tidal cycle or more (Figure 2; Metcalfe & Arnold 1997, 1998; Hunter et al. 2003a). In the open ocean, records of ambient daylight can be used to derive latitude (from day length) and longitude (from the time of local noon) (Hill 1994, Gunn et al. 1994, Metcalfe, 2001). The development of further onboard sensors that can monitor more complex variables such as compass heading, swimming speed or feeding activity will do much to increase our understanding of migration of many more species of fish.

Despite such technical advances, the use of data storage tags with many species remains limited because the prospect of the fish being caught and the tags returned is very low. To avoid the need to rely on a commercial fishery, and increase the probability of data recovery, a major area of development has been the “pop-up” tag. These tags are attached externally and have a release mechanism which causes the tag to detach from the fish at a predetermined time and “pop-up” to the sea surface where the data can be recovered by airborne radio or satellite (Nelson 1978, Hunter et al. 1986). Such devices are now commercially available, and are being deployed on large pelagic species such as tuna (Block et al. 1998, Lutcavage et al. 1999, Block et al. 2005). Although, data transmission capabilities are currently very limited, further developments in this field give the prospect of much improved data recovery rates in the future, while further miniaturisation will allow the technology to be applied to small species.

Implementing a tagging programme

Fish capture, handling and attachment methods

It is important to choose capture and tagging methods that will result in the collection of reliable and useful data. The legislation around animal welfare and animal experimentation, and the increasing sophistication of small electronic devices, has rightly changed the face of tag attachment methods. The methods used to capture fish are diverse and the method chosen for use in tagging experiments must minimise the stress and damage that any captured fish will experience. Similarly, once caught, fish must be handled gently. They should be tagged and returned to the water as quickly as possible and not dropped on the deck or allowed to strike the side of the boat or the bulkhead. When picked up they should be held horizontally and the gills should not be touched with the fingers. Only fish in good condition should be tagged and released. This is not only important from a fish welfare point of view, but also because electronic tags (where used) are expensive so long- Designing fish tagging programmes. Metcalfe et al. term survival of the fish is critically important. In field experiments, the ideal conditions for handling fish cannot always be met. Setting up facilities for anaesthesia and recovery may be difficult because of spatial restrictions or poor weather at sea. The experimenter must then evaluate the relative difficulties of applying anaesthesia against possible trauma and damage caused by handling unanaesthetised fish, although legal considerations may be paramount. When tags can be attached rapidly and non-intrusively, anaesthesia has often been replaced by simpler methods of keeping the fish quiet during tagging such as blindfolding. For a full review of tagging methods and marks, readers are directed to the report for the report of Concerted Action (FAIR CT.96.1394) Tagging methods for and Research in Fisheries (Thorsteinsson, 2002).

Programming tags to sample data

Where electronic tags are to be used, thought must be given to how the sensors, battery and memory life can be used to best effect. The potential for data collection is not always fully exploited in many archival tagging programmes because tagged fish are often caught soon after release and much of the tag power and memory is unused. For example, Godo & Michalsen (2000) programmed tags on NE Arctic cod to record pressure and temperature data in a cycle of every two hours for six days, and every 24 hours for one day. Righton et al. (2001) and Righton & Metcalfe (2002) programmed tags on North Sea cod to record data every ten-minutes for the duration of a tag’s life. While, these tagging studies to date have largely achieved the goals set at their inception, relatively few a priori or postpriori analyses have been undertaken to determine the most useful data-logging regimes, or the limitations of the logging regime used. Instead of using a regular data sampling regime that maximises the potential deployment time of the tag, a sampling regime that samples more frequently at the beginning of the deployment than it does at the end should be considered (Figure 3). Alternatively, tags that offer time extension or telescoping logs should be used.

Measuring environmental variables requires data to be recorded frequently enough to minimise sampling error, but not so frequently that tag memory is used up too rapidly. Measurements of rapidly changing variables such as movement rate requires frequent sampling, whereas infrequent sampling can be used to measure environmental preferences such as maximum depth or water temperature. Furthermore, some analytical methods, such as activity or periodicity analysis, are often best conducted with data recorded at high frequency because the statistical power of such analyses is compromised by low frequency data. Examples of analyses requiring a rapid sampling rate include estimation of tilt angle, accurate estimation of the depth of the thermocline and estimation of longitude and latitude from light intensity data.

Press, publicity and reward schemes

Tagging experiments are usually costly exercises requiring vessel time, experienced staff and, if using electronic tags, expensive devices. Tagging Designing fish tagging programmes. Metcalfe et al. data is valuable, particularly if it involves the used of electronic tags since the data recorded by even a single archival tag can be significant. It is therefore paramount the appropriate resources are deployed to encourage fishers to return tags together with accurate tag recapture details and, when appropriate, the fish carcass. In some cases, particularly where tagging has been opportunistic, programmes can fail to achieve their full potential because tags returns are poor as a result of insufficient publicity and rewards.

The number of tags recovered will improve considerably with good publicity and reward systems. Initially, the objectives, tag type, secondary tag type (where used) and the rewards (if any) should be clearly advertised. Prospective individuals who are likely to recover tags or be aware of recovered tags (fishermen, fish processors, sport anglers etc) should be informed by press, posters or presentations that tags of different types may be present in the fish they handle. It is important to emphasise the scientific value of the tagging programme, and the value of the data recovered from electronic tags (where used) as well as the overall benefits of the data for protecting and possibly enhancing stock assessment and management. In particular, there should therefore be a good incentive to return tags, particularly if tag recovery is dependent on commercial fishermen or processors.

As technology advances, electronic tags are becoming smaller and internal placement rather that external attachment is increasingly possible. While internal placement (usually in the peritoneum) is better for fish welfare and may improve tag retention (but see Moore et al. 1990 and references cited therein) internal placement, particularly of small tags, may lead to reduced recovery as a result of the tags not being detected by fishers or fish processors (Righton et al. 2006a). If internal placement is to be used, appropriate external tags that indicates the presence of an internal tag, together with suitable publicity, is paramount.

RESULTS & DISCUSSION

Stock movements & distribution

The first step of any fish tagging programme is to assess where individuals have moved to. Jones (1976) provides a summary of the statistics that can be used to describe movement and dispersion using simple mark-recapture data. However, to avoid difficulties with interpretation of the results, it is crucial to select data appropriately before applying the statistics. Patterns of space use that differ between seasons or sub-stocks may then emerge, for example in cod in the North Sea (Figure 4; Righton et al. 2006b). Similarly, the plaice sub-populations in the North Sea have been identified from previous mark-recapture data (de Veen, 1978; Cushing, 1990), with results indicating that there are discrete plaice sub-populations that aggregate during the winter spawning then disperse during the summer over distinct but overlapping feeding grounds.

The advent of electronic tags has permitted more detailed analysis of the movements of individuals because the data that are collected permit the Designing fish tagging programmes. Metcalfe et al. geolocation of individuals at frequent intervals (Hunter et al. 2003a; 2003b). In a sense, such geolocations can be viewed as multiple recapture positions, and used to plot the change in spatial distribution of populations over time (Figure 5). Since the early 1990s extensive studies of the movements of plaice equipped with electronic data storage tags have significantly advanced our understanding of the distributions and movements of plaice in the North Sea. In contrast to earlier analyses of mark-recapture data, detailed analysis of the data from these DSTs indicates that the adult plaice population in the central and southern North Sea forms three geographically discreet feeding aggregations during the summer, that disperse over the southern North Sea and eastern English Channel to spawn in the winter (Hunter et al. 2004b).

Individual migrations

The detailed movements of individuals can be just as revealing and provide insights into migratory mechanisms. For example, plaice in the central North Sea exhibit clear repeat migrations between feeding and spawning grounds that are remarkably consistent in direction and timing (Figure 6; Hunter et al. 2004b). Detailed results like this can help when interpreting data from mark- recapture studies. For example, the distance from (spring) release positions of cod in the North Sea increases to a peak after six months, before decreasing again (Figure 7). This pattern is repeated in the following 12 months, and is indicative of a significant proportion of the population undertaking repeat migrations (Metcalfe et al. 2005). This is corroborated, at least for some individuals, by findings from electronic tagging studies (Righton et al. 2006).

Vertical movements and activity

Electronic tagging studies also reveal significant detail of the behaviour and activity of individuals. The use of selective tidal stream transport by plaice to aid their pre- and post spawning migrations in the southern North Sea was revealed by extensive acoustic tracking (Greer Walker et al. 1978; Metcalfe et al. 1992) and data storage tagging studies (Metcalfe and Arnold, 1997; 1998). Also, simple indices of vertical activity can be used to compare the behaviour of individuals in different areas. For example, Righton et al. (2001) used a depth change threshold to classify activity of cod in the North and Irish Seas (Figure 8) and showed that seasonal patterns of activity differed significantly between these two habitats. The results can be used to inform more analytical approaches, such as periodogram analysis (Figure 9; Hunter et al. 2004c). Such analyses can reveal the extent to which vertical movements are related to environmental and biological cues, and give insights into the availability of fish to fishing activity (Figure 10).

CONCLUSIONS

As world fisheries continue to be heavily exploited, with drastic reductions in catches, or even closures of entire fisheries (e.g. Newfoundland cod in the 1990s) being necessary to conserve stocks, there is an increasing need for rational management that takes more account of fundamental biology. This Designing fish tagging programmes. Metcalfe et al. applies not only to traditionally exploited species like cod and tuna, but also to newly developing commercial fisheries, like those for deep-water species such as orange roughy and round-nosed grenadier. For most of these species we know little of their migratory behaviour, or of the environmental factors that affect it.

We have used the example of plaice and cod in European waters to show how mark-recapture experiments using conventional tags, and tagging studies using various types of electronic tag, can provide information about the behaviour and geographical movements of exploited fish populations that is relevant to assessment and management. Fortunately, management agencies are becoming increasingly aware of the need to understand fish migration, not just because it is interesting, but because it is fundamental to many basic elements that underpin fisheries management. The challenge for us now is to ensure that this new knowledge is built into future assessment and management methodologies, and that the outputs are taken through into management advice (Schnute & Richards, 2001).

Improved uptake of new biological knowledge into assessment and management methodologies will hopefully lead to improved support for fish tagging studies. But this will be of little value without the necessary tools, so continuing to improve methodologies will be important too. As technology develops, smaller, cheaper, more sophisticated electronic tags will become available. Such devices, combined with other techniques used to study population movement, such as genetics and otolith microchemistry, can only improve our understanding of how and why fish migrate, where they go and, ultimately, what environmental factors determine their behaviour, movements and distribution.

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FIGURES

Figure 1. Example of conventional marker tags, data storage tag design and the variety of data storage tags now available on the commercial market. Designing fish tagging programmes. Metcalfe et al.

Figure 2. Reconstruction of the migration of a plaice tagged with an data storage tag. The Proudman Oceanographic Laboratory’s numerical storm surge model was used to identify areas of similar tidal range (red areas) and similar times of high or low water (yellow areas) recorded by the tag (insets at top right of each panel).

Figure 3. Comparison of the efficiency of data collection by data storage tags. The bar chart shows the number of tags returned each month after the release of a large number of tagged cod. The symbols show the proportion of tag memory used if the frequency of data collection is uniform (red) or rapid to start but declining over time (blue) Designing fish tagging programmes. Metcalfe et al.

Figure 4. An estimate of the extent of the geographic range of the cod population by quarter in the Southern Bight. Shading shows the 95% kernel density probability function (KPDF) contour of tag recaptures. Points show locations of recaptures. Designing fish tagging programmes. Metcalfe et al.

Figure 5. Monthly composite plots of geolocations for 144 electronically tagged plaice. During the summer months, the horizontal movement of plaice was minimal, therefore these positions were plotted together. Three geographically distinct feeding aggregations were identified on the Indefatigable Banks to the west (blue) on the Amrum Ground to the east (green) and on Ekofisk Field to the North (red), which dispersed onto mixed spawning areas during the winter. (from Hunter et al .2004b) Designing fish tagging programmes. Metcalfe et al.

Figure 6. Ground track reconstructions for three plaice tagged with data storage tags that recorded data for over a year. Solid lines show the pre-spawning migration, broken lines the post spawning migration. Black lines show migrations in 1997 - 1998, blue lines show migrations in 1998-1999. Grey lines show depth contours. (From Hunter et al . 2003b)

Figure 7. Mean distance between release and recapture positions by week of the year after tagging in the Southern Bight. Only the first two years of data are shown due to diminished sample size beyond 104 weeks. The line shows the best fitting curve according to the model: distance=α sin (t +β), where t is time in weeks from release. Designing fish tagging programmes. Metcalfe et al.

Figure 8. Activity of cod between April and November 1999. Active and inactive states of each individual were determined from the depth (measured every 10 min) record of its tag. When an individual was ‘inactive’ on the seabed, its tag recorded only the smooth changes in pressure resulting from the rise and fall of the tide. Individuals were classed as active when vertical movements were more rapid or irregular than could be accounted for by tide alone. For each hour of the day, summed hourly activity held a value between zero (white, inactive) and six (black, most active) (From Righton et al . 2001)

Figure 9. Double plot actogram and periodogram analysis illustrating how the actogram can be used to identify periods of time when an individual is exhibiting period behaviours. (From Hunter et al . 2004c) Designing fish tagging programmes. Metcalfe et al.

Figure 10. Monthly plots illustrating the proportion of time plaice spent swimming in midwater per ICES rectangle December to May. Size of the circles is proportional to the average number of hours spent in midwater per ICES rectangle. Largest circles equal 15 hours. (from Hunter et al ., 2004c)