It is only during the 20th century that fish tagging has been practised extensively for investigating the movements of fish, for providing estimates of survival rate and in some instances for estimating population size. Tagging has also provided a means of measuring the growth of individual fish directly in their natural environment, with the possible limitation that the growth of a tagged fish may not necessarily be the same as that of an untagged fish. The Great White Shark (Carcharodon carcharias) with its total length range of 140 to 640cm, seems to preclude any risk of growth rate disturbance.

The kind of tag used depends on the nature of the study. For many of the commercially important marine fish species, it is necessary to rely on commercial fishermen for returning tags. The tag must be attached externally to the fish and it is important that it should be as large and conspicious as possible without actually interfering with swimming. Bright attractive colours are important, and orange and yellow have been found to be particularly successful. But the larger the tag, the faster parasites and algae will cover it...

There are two important stages to a tagging operation. The first concerns the method of capture or attracting of the targeted fish, and the second the way it is handled for the tagging to take place. For many marine species the most successful methods of capture appear to be lining, long-lining and purse-seining. At the Bimini Biological Field Station (University of Miami) directed by Samuel H.Gruber, the targeted Lemon Sharks (Negaprion brevirostris) are captured using a set of long-lines. The shark is then attached along side the boat using the hook line and a tail rope, and by turning the shark on its back, it sinks in some sort of letargy, allowing the shark to be tagged and equipped with an ultrasonic transmitter.

Moreover, according to the condition of the fish on capture, there may or may not be a handling problem during the tagging operation itself. Such involvement is of course not acceptable working with a White Shark, being a protected species in South Africa.

An important aspect of the tagging program is the means for recovery, particularly when the investigators themselves are not the main recovery agent. An adequate advertising system or even a reward system might have to be set up to improve the returns of resighting data. The incomplete reporting of tags is something that must also be taken into account when analysing this kind of tag and resight study. With the Great White Shark situation this problem is even accentuated with being a protected species, and in the eventual case of accidental catch or poaching it is most probable that it won’t be reported.

Most of these problems are solved or non existant when using Photographic Identification instead of physical tags.

A number of tag-recapture or -resight models have been considered and analysed at various levels of complexity. The most general situation is one in which individuals are tagged on more than one occasion, and in which samples are taken on more than one occasion and examined for the presence of tagged individuals. This is known as the multiple tagging and recapture situation: the Jolly-Seber method. Most animal populations are constantly changing in size because of births, deaths, immigration and emmigration: open populations.

The Jolly-Seber method is designed for open populations. Random sampling is the crucial assumption along with the following statements:

  1. Every individual has the same probability (at) of being caught in the t th sample, whether it is marked or unmarked;
  2. Every marked individual has the same probability (ft) of surviving from the tth to the (t+1)st sample;
  3. Individuals do not lose their marks, and marks are not overlooked at capture;
  4. Sampling time is negligible in relation to intervals between samples.

The most general solution is a stochastic model proposed by both Jolly (1965) and Seber (1965). The recaptures are grouped according to the last time an individual was released, tagged or resighted and the numbers released are classified according to the total number of tagged individuals released on each occasion (that is the newly tagged plus previously tagged). The important point with this model is to be able to answer for each marked animal in a sample: « When was this marked individual last captured? ».

The time interval between samples need not be constant, and any number of samples can be accommodated so that series of data extending over many years can be used in this method. All animals in the first sample must be unmarked, by definition. For the second and all subsequent samples the total catch can be subdivided into two fractions - marked animals and unmarked animals. For marked individuals we ask one important question: « When was this individual last captured? ».

The manner in which classical fish population dynamics developed has resulted from two series of fortuitous accidents. Firstly, when the large-scale exploitation of fish started, the main species caught were teleosts. This group of fish was therefore the first to show signs of overfishing, and consequently fisheries research started on teleosts. Secondly, the majority of teleosts have either large scales or large, calcareous otoliths, or both, which respond to variations in environmental factors by laying down rings in their growth process. The most marked environmental fluctuation in temperate waters is the seasonal change between summer and winter, and consequently the dominant rings on the scales and otoliths are annual. Teleost fish can therefore be aged. Without this possibility the fundators of the classical theories of population dynamics could not have developed them in the form they have.

There are three fundamental differences between teleosts and elasmobranchs which have affected the population dynamics of the latter. Firstly, elasmobranch cannot be aged easily since their skeleton is essentially made of cartilage (not bone like in Teleost fishes) and presents only some deposits of minerals (apatite) in the parts that need to be strenghtened (jaw and vertebrae). Secondly, stock and recruitment are closely related which has been assumed to be negligible for the teleosts. Thirdly, there are no major (and sustainable) fisheries directed specifically for elasmobranchs. Some of the classical theories of fish population dynamics are inapplicable to elasmobranchs.

Elasmobranchs do not posses large calcareous otoliths and their placoid scales are very small. It as been suggested to use the centra of the vertebrae which carry rings similar to those found on scales and otoliths. However, it prooved difficult to show conclusively that the rings observed were either annual or had any other pattern related to a factor whose timing could be established. This prooved a stumbling block for the use of the centra. This situation contrasts strongly with that for northern temperate marine teleost species for which it is accepted, almost without question, that clearly marked, distinct zones on otoliths are annual. It has been proved conclusively, by the injection of tetracycline into tagged thornback ray, Raja clavata, that the rings which occur on the centra of this species are annual, at least for the population which inhabits the southern North Sea (Holden and Vince, 1973). Tetracycline is a chemical which is readily absorbed by vertebrates. In teleosts, which posses acellular bone, it is deposited at sites where active calcification is occuring. In elsamobranchs, which possess cellular, partially calcified porous cartilage, it is deposited in all the calcified tissues present at the time of injection. As it fluoresces in ultraviolet light these tissues can easily be identified and subsequent growth recognized. But of course this procedure involves the animal to be killed at the second capture, and again no study of this kind exists for the Great White Shark.