The profitability of stocking can be calculated if the costs and benefits are known or easily obtainable. The costs should include spawn collection from natural stocks or ova production in fish farms, incubation of eggs, rearing of larvae and juveniles, transportation and stocking. Capital costs should also be considered in full.
The value of the catch is obtained by multiplying the yield from stocking by the average price of fish. It is recommended that in commercial fisheries fishing costs should be considered; in Finland and Poland these costs are estimated at about 30% of the catch value. In subsistence and recreational fishing, fishing costs can be ignored. It is obvious that if the probability of catching the stocked fish increases, the demand for licences increases as well. Therefore at least some measure of yield from stocking is also needed in cases where the benefits are measured as the increase in income from licence fees.
The most simple index of profitability of stocking is the economic cost/benefit ratio. The cost/benefit ratio can be lower if profitability is not the sole objective of stocking. It should also be borne in mind that some of the benefits of stocking are difficult or even impossible to determine in financial terms. This is true of all cases in which the stocking results are social or ecological benefits which a priori cannot be expressed in monetary units.
The profitability of stocking is affected by the size (developmental stage) of the coregonids released. It is known that the yield from juveniles is higher than from yolk-sac larvae (Figure 2a), although in the juvenile group, the size has only a slight effect on the results. The yield from stocking with yolk-sac larvae is more difficult to predict and highly variable (due to stochastic environmental factors) while that with juveniles can be adjusted to a high degree (density-dependent survival and growth).
Production costs of stocked coregonids increase as a function of fish size (Figure 2b), but stocking costs increase less rapidly and not linearly due to the fact that fewer fish are needed to obtain the same amount of additional catch from stocking when the size of released fish increases. The uncertainty of the yield from stocking and hence economical uncertainty also decreases as a function of fish size (Figure 2c). There is a transition size after which the yield from stocking becomes more predictable.
The aim should be to use the size of stocking material which optimises the ratio between the yield and the stocking costs whilst minimising the economic and social uncertainties. In no case has the size which gives the greatest profitability been finally determined, but logically it must lie somewhere in the larval or early post larval stages (Fig. 2-D).
Figure 2. Conceptual model indicating optimum size of whitefish from economical point of view. The shaded area shows the optimum size. A) Range of yields from stocking with whitefish under controlled and uncontrolled management. B) Production costs (Production), a function of fish size and stocking costs/unit of additional catch (Stocking = No. released × (production costs + transportation and other stocking costs)). C) Uncertainty of the yield from stocking in relation to size of fish released. D) Profitability of stocking in relation to the size of whitefish released.
