Modelling Vessel Activity in the Scottish Pelagic Fleet: The Relationship between Capacity and Effort and the Evaluation of a Range of Possible Management Plans

Abstract

There is increasing interest both in Europe and further afield in moving beyond traditional TAC based approaches to fishery management, towards effort and/or capacity based management regimes. This paper presents the results of a case study on effort and capacity metrics in the Scottish pelagic fleet. Data from 2003 to 2006 were used to model the activities of this fleet to create a series of linked process models that simulate the trip by trip activity of vessels exploiting the mackerel, herring and blue whiting fisheries in the North Sea and Western Atlantic. Scaling the predicted port departure probabilities for the 2003-2006 period allows changes in effort within the fleet to be simulated. This suggests that fleet capacity would be 260,000 tonnes for mackerel with mean days at sea per vessel of 119 days, in comparison with the mean over the 2003-2006 period of 126,900 tonnes which corresponds to 58 days at sea per vessel. Likewise the capacity of the fleet in the herring fishery would be 151,700 tonnes with a mean of 53 days at sea per vessel, in comparison to 57,310 tonnes and 20 days at sea per vessel for the 2003-2006 period. Simulations also suggest that days at sea limitations are likely to be a more effective management measure to regulate landings than closed seasons and that, at least for the mackerel fishery, vessel reduction schemes would result in a proportionate decrease in landings that would not vary greatly regardless of which vessels were removed from the fleet. Trends in vessel specifications since 1980 are likely to have increased the capacity of the fleet within the mackerel fishery by approximately 46%.