Fish Tagging: What You Need to Know

by Scotty Kyle

Some people fish to eat, and others fish for fun; some keep that catch, and others release what they have caught. And a few anglers, before release, attach a small bit of plastic to the fish. If you have seen this, or have caught a tagged fish, you might be wondering why these people take such trouble and possibly inflict additional pain to the poor creature. Let me tell you about fish tagging, and why it matters. 

Fish tagging is simply the insertion of a uniquely numbered, plastic tag into the fish’s body before releasing it to swim away. The tag can be a “spaghetti” marker inscribed with a number, and usually an address to which information on a recovered fish can be sent, or it could now be an electronic device that stores or transmits information on the fish’s movements. The correct name for this process is “mark and recapture” and information is only obtained from the “spaghetti” tag if the tagged fish is recaught and the information on the tag is reported to the authorities. 

We all know of tragic and disastrous collapses of fish stocks all round the world in recent decades and many of us have read information such as “90 % of all the large fish in the oceans have already been caught”. How do we know about the collapses and how can we better manage our fish stocks? The answer is that fisheries scientists use various “tools” to give insights into fish populations, stocks, movement and growth rates and tagging fish is an important tool. 

North America has excellent examples of good fisheries management – so how have they achieved this? There are several components to the answer and the first is to establish exactly what is going on on the ground. Monitoring catches is essential, but it only shows what is happening and what has already happened. It cannot give any insight into what is happening to the fish stocks under the water.  In its simplest explanation, fish tagging can link the catches that we can see and count, to the population that we cannot see or estimate.

A blue emperor – Lethrinus nebulosus – tagged on the east Coast of South Africa.

The principle is that if you catch, tag and release 100 fish in a stocked, closed pond and then every fish that you catch subsequently has a tag, it is likely that there were approximately 100 fish in the pond. If half of the fish recaught had tags, then there were probably approximately 200 fish in the pond and so on. Similarly, if you catch a tagged fish one year later and it has grown by 10 centimeters then one is able to estimate annual growth rates. If the pond is linked to another nearby and a tagged fish is caught in the linked pond, then one can say with confidence that there is some movement of fish between the ponds. We can thus obtain insights into fish populations, growth and movements. These parameters are critically important if we want to manage the fish in the pond wisely and sustainably.  With this information, if we are in control of the fishers, we can implement rules to optimize the fishing and the fish population and thus avoid overfishing and stock collapse. 

There are many good examples of fish stocks declining, suitable management being put in place and subsequent stock recoveries. Sadly, most of them are recreational freshwater fisheries as marine commercial fisheries tend to be much more complex. In many instances fish stocks are shared between several countries, different fishing methods and often there are “traditional” plus “commercial” fisheries competing for the same stock. 

In South Africa, where I live, all marine fish tagging is orchestrated through the Oceanographic Research Institute which is associated with Ushaka Aquarium in Durban. They issue all the tags and their contact details are on the tags, ensuring that all recaptured fish information should be sent to them. They have been managing the scheme for several decades and tens of thousands of fish have been tagged, resulting in amazing and useful information being obtained. Initially, anyone who requested a kit was given a tagging kit and could tag fish, but problems soon arose. Untrained people tended to tag fish that they did not want to eat or keep, or fish that were, simply, too small to be worth keeping. Unsuitable fish species and sizes were frequently tagged and the tags were commonly inserted in the wrong place on the body of the often injured and exhausted fish. These fish were then released back into the ocean where many of them died soon afterwards. 

The answer was to require a significant cash deposit before anyone was given a kit and also train everyone before they could begin tagging. The fish species to be tagged is very important. Some species, such as most mullets, very often die after tagging and so tagging “delicate” fish, with a high tag mortality, is counter-productive. Others, such as eels, take tags well, but very few are ever recaught. Fish taggers are now encouraged to target reasonably large members of “robust” species that are caught in good numbers by anglers to obtain useful recovery rates. The training includes this information as well as ways to reduce trauma to the fish that is to be tagged. 

Oceanographic Research Institute tag kit.
Oceanographic Research Institute tag kit.

Initially, people would play the fish gently so as not to injure it, but now it is generally thought that, within reason, the fish should be brought to the beach or boat as soon as possible to reduce fatigue. The information required of a fish being tagged is simply its correct species, length, location, date of capture and tag number. Weighing a fish usually causes extra stress and so this is no longer required and the total length is adequate. Fish should be handled gently, as little as possible, with wet hands or a damp cloth and be tagged and released rapidly.   

The information obtained from a tagged fish recovery is its growth and movement since it was first tagged. Information on all the tagged fish recovered per year will often give information on the proportion of fish in a population that have been caught that year. There are problems associated with all this information that distort any extrapolations or estimates, but scientists have methods to minimize and quantify these. 

It is clear that fish tagging can provide critical fisheries information that can help to improve fisheries management. On the “con” side, the method requires quite a lot of fish to be captured and have tags inserted into them. For scientific purposes it is often best to organize anglers into an “event” to catch adequate numbers in a locality and then tag the fish caught. This way everything is controlled and monitored, with many fish being caught. Another option is to use trained and equipped recreational anglers to tag fish during their regular fishing outings. The training ensures that large enough fish of the target species are well caught, carefully tagged and rapidly released. Things like taking photos, requiring extra time and inappropriate handling, are discouraged. This second method offers the public an opportunity to play an interesting and important part in the management of the fish stocks they utilize. 

Both the above approaches efficiently use the fish that are caught, but the tagging will injure or damage the fish beyond the trauma of normal capture. There are, however, other measures that fish taggers, and anyone else, can use to reduce fish morbidity and mortality. Most rod and line tagging is carried out with mild steel hooks and not stainless steel ones. Hooks are usually barbless or have their barbs crushed to facilitate unhooking. Circle hooks often result in fish being hooked in the lips and not further into the mouth or throat and gaffs may not be used to land fish. All this will increase the fish’s chances of surviving capture and tagging, but what are their chances?

a trevally caught and tagged by an angler
South African Recreational marine tagging, bluefin trevally (Caranx melampygus). Tag is below the fish’s dorsal fin. 

In 2007 our one son, who works for Ushaka Aquarium, drove 400 kilometers north to Kosi Bay, where we lived, to catch 30 rock salmon. We caught the fish carefully on rod and line and they were placed in a marine tank on the back of the vehicle and taken back down to the aquarium. Most of the fish caught had the hooks easily removed, but, if they were firmly hooked or the hook had been swallowed, the line was simply cut leaving it in situ.  Some of the fish were tagged, but not all. No fish died in transit and all were released into the public aquarium after suitable quarantine. No fish died in the first month, or year or within five years. We visited our son recently and he showed us several of the rock salmon, some with tags still in, after over 15 years in captivity. This showed conclusively that if suitable fish are carefully caught, handled and released (tagged or not) they can survive and grow. 

People worry that the fish are “hurt” or traumatized by capture or tagging, but the facts are that sometimes, after tagging, some fish seem to very quickly return to normal habits. The following are a couple of examples out of many, of recapturing fish soon after they were tagged.  I was tagging river bream (Acanthopagrus vagus) in an estuarine lake, spinning with a silver Mepp No. 2, when I recaptured a fish I had tagged less than an hour before. Another evening I tagged a greenspot trevally (Caranx melanpygus) in the open ocean and recaught it on the same plastic bait the next morning 300 meters down the shore. The Oceanographic Research Institute has run a fish tagging scheme in the Indian Ocean for over 20 years and, during this time, one individual fish has been caught and released five times. We cannot comment on things like “pain” in fish, but clearly some fish return to normal activities soon after release.

Most of us fish for fun and, while some people see this as intrinsically wrong, we have come to terms with this – just as we have accepted abattoirs, feedlots and paying rates and taxes for things we may not actually receive or agree with. People are entitled to their perceptions and views; imposing them on others is another matter. I fully agree that every effort must be made to minimize injury, fatigue or “pain” to fish, and that unnecessary hurting of fish, or other organisms, is simply wrong. But the above examples make it clear that fish tagging can produce essential information that can lead to improved fisheries management. It is also clear that, with care and insight, fish mortality and injury from capture and tagging, can be minimized. 

Going forward, fish tagging will be continuously refined and improved upon to optimize the quality of information generated and reduce the impacts on the fish captured. Most recreational fish tagging, however, will only work with the full understanding and cooperation of the anglers and public. History has shown that, without suitable strict management, almost any fishery can and probably will collapse, but, with suitable management, sustainable fishing levels can be achieved and maintained. 

Learn more about how fish tagging can be used to draw sustainable fishing regulations and make scientific discoveries.

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