Fish Population Dynamic and Stock Assessment

9.2.3 Catchability coefficient

For a fishery manager to estimate the amount of fishing effort, it is not sufficient to count the amount of gear. One must count or estimate the amount of gear-days (or other measure) for each type of gear and derive empirical correction factors to express all the fishing effort in terms of one (or a few) standardized fishing gears. The task is indeed formidable. Careful gear comparisons are an important part of fishery management.

The fishing mortality, F, is almost assumed to be proportional to fishing effort. Mathematically, this is expressed as

qf = F

Where f is the amount of fishing effort (e.g in boat-days) and q is the proportionality coefficient, frequently referred to as the catchability coefficient. The more the efficient the gear is, the higher the value of ‘q’ because ‘q’, is the measure of the ability of the gears to catch the fish.

In the estimation of mortality parameters in surplus production models and in prediction models, the catch and effort statistics are taken as an input data. According to Rothschid (1970), the current estimation of the quanity of fishing effort is not only invaluable in following changes in abundance through CPUE index, but fishing effort is also used in (i) the relation between total catch and effort (ii) the inter-relation between yield per recruit and size of the fish caught (particularly the smallest fish caught) and fishing effort (iii) the measurement of the relation between stock and recruitment.