A strategy to detect fish discarding by combining onboard and onshore sampling

Abstract

Discarding of small fish is considered to be an important conservation problem and has become illegal in some fisheries. We present a cost-efficient strategy to help enforce regulations against discarding. A discarding indicator is defined using the change in slope between two reference points on the empirical length-frequency density of the catch. This discarding indicator is then used according to the external distribution concept: the sampling distribution of the discarding indicator, when no discarding occurred, is obtained directly from samples taken by onboard observers; the value of the discarding indicator observed by onshore observers from a boat not covered by onboard observers is then compared with this sampling distribution. This procedure offers a nonparametric test for discarding. Application of the strategy is illustrated using data from the 1991 Atlantic cod (Gadus morhua) fishery in the southern Gulf of St. Lawrence. We describe several enforcement frameworks within which the method can be applied. The cost efficiency of the strategy comes from shifting resources from high-cost onboard observation to lower cost onshore observation.