Since 2006, the Pacific Tuna Tagging Programme, endorsed by the Western and Central Pacific Fisheries Commission and implemented by the Pacific Community (SPC), has been organising fish tagging events annually. On this year’s pole-and-line tagging cruise through Solomon Islands, Papua New Guinea and Federated States of Micronesia, tunas labelled with conventional white tags also received an injection of strontium chloride to validate the deposition rate of the increment formations (often called growth rings) that are observed and counted in fish otoliths as a way to estimate fish age and growth.
Otoliths are small ‘ear stones’, calcium carbonate structures located on either side of the head. They allow fish to find their balance and perceive linear acceleration, both horizontally and vertically. As otoliths grow, they incorporate chemical ‘markers’ from the water (such as calcium, strontium, and other elements and stable isotopes). The concentrations of these markers reflect both the environment the fish swims through, and intrinsic processes such as physiology and metabolism. Once a marker is incorporated into a growth ring, it remains there permanently, providing a time-stamped chemical record of the fish’s experience. By counting the growth rings on otoliths, scientists can estimate the age of a fish; however, the periodicity of ring formation needs to be validated. The external application of chemical markers during tagging events has proved to be a useful method in this regard.
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