Fish Population Dynamic and Stock Assessment

2.3.1. Introduction

Age class structure in fisheries is a part of population assessment. Age can be determined by counting growth rings in fish scales, otoliths, cross-sections of fin spines for species with thick spines such as triggerfish, or teeth for a few species. Each method has its merits and drawbacks. Fish scales are easiest to obtain, but may be unreliable if scales have fallen off of the fish and new ones grown in their places. Fin spines may be unreliable for the same reason, and most fish do not have spines of sufficient thickness for clear rings to be visible. Otoliths will have stayed with the fish throughout its life history, but obtaining them requires killing the fish. Also, otoliths often require more preparation before ageing can occur.

The ability to perform age determinations based on the examination of hard anatomical parts is of fundamental importance in fisheries research. Certain structures in finfish and invertebrates taken from temperate waters show alternating patterns caused by seasonal changes in growth rates; a phenomenon similar to trees for which an age may be determined by counting annual rings in a cross section of the trunk. Validation of a regular periodicity in these marks permits assigning a time scale and determination of age. The successful application of techniques to enhance the detection of age marks in biological specimens is of vital importance in estimating growth and mortality rates, population age structure, and other parameters needed for understanding the dynamics of fishery resources and their response to natural phenomena and exploitation.

A wide variety of age determination techniques have been developed for finfish and invertebrates which depend on the detection of contrasting bands in body parts such as scales, otoliths, fin rays, spines, and vertebrae of fish, as well as external and internal structures of mollusc valves.

Determination of age and growth of a tropical species is more cumbersome compared to temperate species which exhibits seasonal spawning. Since the tropical system is more dynamic, further fluctuation in the environmental factors are common, the age of species gets affected. The food availability when abundance, when the physiological factors of fish are also at optimum, the growth of a fish is fast and as a result the attainment of maturity is early and more cohorts are released, in contrast to less food supply.

When more recruits are released particularly for a prolific breeder, assessing of age of fish by Length Frequency Data (LFD) data will be erroneous by usage of conventional Peterson’s method.

To ascertain the age of fish using LFD data, the most commonly used methods are

i) Petersen’s is method

ii) Modal class progression analysis

iii) The integrated method of Pauly (Combination of first two methods)