OpinionsEnvironment ConnectionKnowing the age of fishes

Knowing the age of fishes


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We often ask questions about the age of things, plants, animals or people just as a matter of curiosity. But such questions have deep implications, and are legitimate questions to ask. Scientists especially make it a point to know ages of things. Thus, so much research has been done on questions of age in relation to many variables in nature.

Recently, a graduate student at Silliman University presented a masters’ thesis dealing partly on the age of a fusilier species (Pterocaesio pisang), a species of fish widely used as food in the Philippines. His findings throw much light into questions such as why this species appears to be resilient to intense fishing, that is, why it is able to resist local extinction under high rates of exploitation in the southeastern Negros Island and elsewhere. He found that the reason lies in the rapid growth to sexual maturity within less than a year. The species population has a fast turnover cycle of three to four years. Its life span is generally just three years and rarely four years, growing most rapidly during the first year.

This is in contrast to other fish species found on coral reefs, namely, groupers, snappers, parrotfishes and surgeonfishes. Surgeonfishes are especially long-lived, 30 to 40 years. These fishes are likely to become locally extinct under high rates of exploitation in contrast to the fusilier discussed above. These facts should serve as guide for the managers of fishery resources.

However, our research finding at Sumilon Island indicates that under extremely high rates of fishing mortality, the fusilier can also suffer depletion that could lead to eventual local extinction. But why it persisted in the Sumilon area, being the most abundant species caught by fishers, was most probably because there was a place where it could seek refuge from fishing on the island. The place was the Sumilon no-take reserve, which was fully protected for 10 years, 1974 to 1984.

The student’s study sites by coincidence were also in the areas with no-take marine reserves. The existence of no-take marine reserves seems to be a favorable factor for the maintenance of the populations of the fusilier. They (marine reserves) provided the refuge from fishing, where they (the fusiliers) can grow to larger sizes. Large-sized fish produce more eggs than small-sized fish, increasing the fecundity or reproductive potential of females. One can infer the importance of fully protected marine reserves as a fishery management tool.

The other notable aspect of the above study is the use of otoliths, the calcium carbonate structures found in the semicircular canals, which are responsible for the stability and balance of the fish. The canals are located near the brain. The layers of the otoliths are laid down at specific time intervals during the lifetime of the fish. In this fusilier species, a layer is laid down each day, as determined empirically by raising the fish in the laboratory. An otolith is sectioned, stained and examined under a microcope, and the number of layers counted as the basis for determining the age of a fish. The otolith layers are analogous to the tree rings also used for determining the ages of trees.

The use of otoliths is recommended for studies on other species of fish as guide to fishery management.

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