Concerted Action "Improvements of Tagging Methods for Stock Assessment and Research in Fisheries" (CATAG)

FAIR. CT.96.1394, FINAL REPORT May 1999 (DRAFT)


 There are several subjects in tagging that are independent of tag type, species and area. These include strategic planning and decisions made prior to tagging, proper sampling of fish for tagging, treatment of fish during the tagging process, and finally, efficient means of obtaining consistent high quality recapture information. These are all crucial aspects for the success of a tagging programme that tend to be overlooked or may be considered too late to be dealt with in an appropriate way.

3.1 Strategic planning of tagging programmes 

There are some general aspects that should be considered when planning a tagging operation. A careful assessment of the objectives, relating them to a cost/benefit analysis is useful before deciding on the most appropriate tags and tagging methods. Chapters 4 and 5 have good descriptions of various tags and the pros and cons of their use. The analysis should include the entire process from deciding on the hypothesis to be tested, to the evaluation and presentation of the results. In this process the number of fish to be tagged in relation to the expected number of recoveries must be considered, as well as the number of recoveries needed for the statistical analyses planned for the data. In this report chapter 8 is concerned with plans of experiments, data handling and modelling. The planning process ought also to include some consideration of other experiments performed with the tag of type that has been selected and the species of fish it is proposed to use. The CATAG web-site ( has online various examples of tags, tagging methods and examples of experiments being carried out in various places.

Legislation governing tagging practice is not something that many think about when planning a tagging experiment. It is important though to look at general legislation which concerns tagging because this may save problems at a later stage when the experiment is in full swing or when it is published. This report has a special section devoted to the legislative control of tagging in various European countries (chapter 6).

In planning tagging procedures it should be appreciated that the handling time for each fish needs to be short. This is necessary for better fish survival, and also for the economy of the project. If a method is used for the first time it is very important to practice or rehearse the tagging procedures, to minimise handling time. One can practise on dead fish, or follow the effects of tagging on fish held in captivity. Many tagging methods have been tested by controlled survival and tag retention experiments. If it is not possible to get such information from the literature some experiments of this sort should be planned. Chapter 7 of this report is written for those who want to know about possible health and behaviour changes associated with tagging, best surgical procedures, most appropriate anaesthetics and disinfectants.

In planning for each tagging or marking programme one should check if the local tag recovery and refunding system applies to the recaptured tags derived from the programme, or if some special arrangements have to be made. Section 3.3 deals with the recovery of tags and further information is provided in Chapter 5 (section 5.6).

3.2 Treatment of fish during capture, tagging and release

Here we are concerned with the wellbeing of the fish during the time it is in the fishing gear, hauled aboard a vessel, maintained within a holding tank, tagged or marked and then released.

3.2.1. Capture of fish for tagging

The most important consideration during capture is the survival of the fish to be tagged or marked. Different species of fish vary a great deal in how vulnerable they are when handled. Some, like plaice, can endure much handling without problems. Others can hardly be touched without their life expectancy being greatly reduced. Fish for tagging can be obtained by any conventional capture methods, but the suitability of the catching method may vary and one should survey the best methods available for catching the fish. However, planning of tagging experiments is often constrained by the requirement that tagging must be carried out at certain locations - where only a limited selection of fishing techniques may be available.

When fish are using much energy during ‘flight or fright’ reactions, lactic acid is produced by glycolysis in the muscles. An excess of this can build up in the blood if the fish does not have time to recover (Wendt and Saunders 1973). It should be realised that the process of fatigue in fish continues while resting after handling, i.e. the increase of lactic acid in the blood continues after the fish has been stressed (Wendt and Saunders 1973). The fatigue process is also temperature-dependent. Build up and recovery are slower at low water temperatures and the total increase of lactic acid is less than at higher water temperatures where lactic acid can reach lethal levels (Wendt 1967). This indicates that resting fish should be kept relatively cool.

Commercial fishing methods are normally designed to optimise harvest and not to keep fish alive and healthy. This means that individual fish may have been stressed for long periods in the gear and thus become less fit for tagging. Soaking time for stationary fishing gears should therefore be limited, while towing times for active fishing gears, such as trawls, should be reduced. Seasonal variation in vulnerability to stress and/or damage during the tagging process can lead to incompatibility between experiments and obviate comparison. This factor should be taken into account during planning.

It should also be appreciated that trawls and other active fishing gears can cause considerable damage to fish. This can stem from the spines of fish or invertebrates such as sea urchins, entrained rocks or sharp garbage items, or may stem from the fishing gear itself (Jakobsson, 1970; Jones 1979).

Fish with closed swimbladders (physoclistous species) are very vulnerable to pressure changes (e.g. Harden Jones & Scholes, 1985) when hauled too quickly by fishing gear from depth to the surface. The physiology of the swimbladder of physicist species is dealt with in some detail in chapter 5, which also provides more information on capture and handling fish (section 5.3).

3.2.2. Treatment before, during and after tagging

The following factors need to be considered:

The vulnerability of fish during the process from capture to release of tagged fish makes it crucial that the whole sequence is kept as short as possible, since prolonged handling may harm the fish more than the tagging itself.

3.3 Quality and consistency of recapture information

Even if performed technically as well as possible, no tagging experiments could be regarded as successful unless accompanied by good reporting rates of recaptured tagged fish. Most fisheries research institutes have well established offices or systems to receive recaptured tags or information about tags and marks. If a tagging experiment or monitoring programme is dependent on tags and information being returned to the laboratory controlling releases, it is necessary to have such a system in place.

The work of receiving tags and information on tags and marks should be organised in such a way that all incoming tags are responded to immediately and all information filed. Lack of immediate interest by an institute will discourage fishermen from returning tags. It should be made very clear to all those who come across tags or marks what they should do with them - where to send the tags and what information to give. It is very useful to produce envelopes with the return address of the main fisheries institute in the neighbourhood on the front, and, on the back, a check list of all the information the institute wants to accompany the recaptured tag. It is important that as many people as possible should know that all tags and marks need to be returned and/or reported to the nearest fisheries institute. Refunds and information on the release of the tag (e.g. where and when released; what increments of growth have occurred since release) will in most cases make people motivated to return tags, as will some information concerning the experiment, its objectives and possible benefits to the fishing community.

The number of tags reported could be increased by several directed actions. In these actions it is important to emphasise the high scientific value of the reported tag and the overall benefit to industry and consumers of better knowledge. Protection and enhancement of stocks for better catches in future should also be highlighted. When dealing with Data Storage Tags, not only must the need to return the tag be shown, but it must also be clearly emphasised that such tags carry data of great importance for scientific research.

Publicising the need for reporting recovered tags include the following: advertisements in trade and local/national newspapers; setting up posters where the presence of tagged fish occurring in a certain area is highlighted; presentation of results in fisheries papers combined with statements of the need to get more data by obtaining more tag returns; and, where appropriate, personal contacts with fishermen in particular areas.

In some cases, when large shoals of fish have to be scrutinised for tags, direct co-operation between the institution originally tagging the fish, and the fishing industry itself may be necessary. If possible, the whole catch should be scrutinised for tags, but if this is not feasible for practical reasons, a proportion big enough to make it probable that sufficient tags will be found should be sampled.

Co-operation between institutions within the country as well as international agreements of exchange of reported tags should be established. To further underline the mutual benefit of such exchanges, offers of some exchange of data will be beneficial to the evaluation of the tagging results.

Besides information on tagging programmes and data exchange, the fishermen reporting tags need more incentive to send in each tag. An adequate reward scheme can take several forms. Simplest is a direct payment per tag reported, but alternatives include receiving souvenirs or the chance to take part in a later draw or lottery. This must be combined with effective publicity, and the encouraging information described above.

Examples of actions and particular arrangements, announcements and rewards to improve return rates of tags are numerous and some can be viewed on the CATAG web-site ( ). The web-site also features an example of how it can be used to aid searches for the origin of release for a tag that is found. The Internet is used and viewed by more and more people. Web-site information can be increasingly useful for the retrieval of tags both internationally and internationally. It will not be too long before a captain of a fishing vessel will have access to the WWW in the wheelhouse and can use a web-site to trace tags that his crew have found in the catch.

3.5. References