Dynamic discarding decisions: Foraging theory for high-grading in a trawl fishery

The decision by fishermen to discard or retain fish of low value to make room for more valuable fish in the hold of a boat (high-grading) is similar to diet choice problems faced by natural foragers. In our study, we apply the rationale of diet choice theory to high-grading behavior in the Oregon trawl fishery by treating fishermen as foragers who must decide how much of each net's haul to "ingest" before searching for more prey. We derive a state-dependent, temporal model of discarding behavior within a fishing trip. This optimization considers the availability of differently valued fish, trip quotas set by the regulatory agency, and the risk of premature trip termination due to loss of gear or injury. The results indicate that those parameters affect discarding behavior through their effect on the probability of exceeding the allowable catch, which we consider analogous to gut capacity. High-grading (partial prey consumption) occurred throughout many simulated trips. The predictions were consistent with the trends in discarding observed in the Oregon trawl fleet. Behavioral models such as ours can be useful to fishery managers by providing a means to explore the potential responses of fishermen to new regulations before they are implemented.