Fish Population Dynamic and Stock Assessment

4.2.1. Introduction

Estimating natural mortality is one of the most difficult and critical elements of stock assessment. Natural mortality is denoted as ‘M’. It will be high in larval and in early juvenile stages of fish. However, once the fish attain a certain age or size, natural mortality rate becomes much lower and much steadier and is generally considered to be constant. The von Bertalanffy’s growth parameters, ‘K’ and ‘L $$\infty$$ ’ are closely linked to M. As a rough generalization, fish species with a high K value have a high M value and fish species with low M will have low ‘K’ value. The ratio of ‘M/K’ value will be mostly in the range of 1.5 to 2.5 (Beverton and Holt, 1959). Natural mortality is linked to L $$\infty$$ or the maximum weight of the species W $$\infty$$ , as the large fish have fewer predators than small fish. According to Rikhter and Efanov (1976), fish with high natural mortality mature early in life and compensates the high ‘M’ by starting to reproduce earlier. Natural mortality also has close relationship with the ratio of gonad weight to somatic weight in fishes. Thus, fish with high M may compensate by producing more eggs. As most biological processes go faster at higher temperature, M is also related to environmental temperature.