Fine-scale fishing strategies of factory trawlers in a midwater trawl fishery for Pacific hake (Merluccius productus)

A Markov decision process model is developed to investigate the optimal scheduling of haul setting and retrievals on a factory trawler in the Pacific hake (Merluccius productus) fishery. The model was used to investigate changes in fishing behavior during a temporary ban on night fishing in 1992. The cycle of setting the net, fishing, and retrieving the net is modeled with different costs for each activity. The optimal controls generally consist of a bin threshold that signals the vessel to start fishing and a catch threshold that signals the vessel to stop fishing. A range of simple "rule of thumb" strategies generated nearly as much net revenue as the optimal control, indicating that the reward surface is flat in the region of the optimal control. A model with diel variation in catch rates caused the vessel to adjust bin and catch thresholds through the day to stockpile fish during the daylight hours and then to cut back on fishing at night when the catch rate is lower. With a ban on night fishing, vessels accumulated fish to a greater extent during the day, but daily net revenue did not decline significantly. Operational models of fishing have important applications in evaluating the consequences of management actions on the fishing industry. Regulations that ignore the constraints and trade-offs under which fishing vessels operate may fail to achieve their intended purposes, or may have unforseen adverse consequences.