Searching behaviour and catch of horse mackerel (Trachurus murphyi) by industrial purse-seiners off south-central Chile

Abstract

During 11 voyages on two purse-seiners off south central Chile, the size of catch and time spent searching per vessel was related to the behaviour of the jack mackerel (Trachurus murphyi) schools. Radar data were used to define groups of fishing vessels. Time spent at sea by purse-seiners was employed either cruising or searching. Cruising ships travelled in a straight line and cruising was associated with deeper (65 m on average) and larger schools. When schools became more numerous and shallow, skippers began searching, during which they slowed down and changed course frequently. Searching tended to be more persistent outside fishing groups. A search could end or not with a set of the net. The net was set when schools were at a mean of 37 m deep compared with 56 m when no set was made. Abandoned search was more likely outside a fishing group and in groups with few vessels. Setting the net and hauling it again had a high opportunity cost, absorbing on average 1.8 h. During the search period, and before the net was set, the jack mackerel schools tended to coalesce and move deeper in the water column. Some sets failed to catch fish and this usually occurred when the schools were deeper than during a successful set. Unsuccessful sets were also associated with larger groups of fish. Catch per unit search time was highest in the centre of a fishing group where searching was less. The highest amount of time spent searching was in the inner margin of the group. Searching then fell away as vessels moved away from the core of the collection of vessels. The implications are discussed of the grouping behaviour for understanding the relationship between CPUE and fish abundance. The relationships described in the paper are summarised into a set of rules that the fishermen might follow. It is suggested that success in the fishery is mostly a result of information acquisition. To regulate effort it might be effective to put a tax on the technology used to gather information.