Interaction between longline and purse seine in the south-west Pacific tuna fishery

While long line effort has remained relatively constant, there has been a marked increase in purse seine effort in the south-west: Pacific from 1979 to 1986.Purse seine catches skipjack and young yellowfin, whereas longiine takes older yellowfin and a variety of other large tuna and billfish.The aim of this study was to quantify the impact of purse seine on longiine.Standard linear models of purse seine catch per unit effort were found to detect no decline in stock abundance.A theory was then developed to predict the effect of purse seine on the spatial structure of the stock and the dynamics of stock movements.A detailed model of longiine catch rates is presented linking catches to school size and density, and bait loss.The model failed, to fully explain observed catches suggesting that the tuna spatial distribution is more complex.Deterministic models suggest that there will be some interaction.Generalised linear models are u s e d to d e t e c t a s i g n i f i c a n t decline in longiine catch rates w i t h i n c r e a s i n g p u r s e seine activity, after removing other effects recorded in the data set.However there is no direct evidence that it is due to purse seine activities.If a simple age structure is assumed and purse seine and longiine are exploiting the same homogeneous stock, the problem can be approached using optimal control theory.In such a system with fixed gear selectivity and realistic parameter values it is found that the most efficient gear should be allocated sole access to the resource.The economic implications of the results are discussed from the point of view of both the vessels and the management of the fishery.The interim recommendation is to maintain a mixed gear fishery using the two separate markets.Finally recommendations are made as to the collection and analysis of data.Forum Fisheries Agency, without whom this study could never have been carried out., I hope it proves useful to them.Thanks are also due to the inhabitants of Eardley, Winston and Fullbrook for moral support, as well as to my parents who h a v e n 't seen me much recently.F i n a l l y t h ank y o u to G u d r u nGaudian, who has been very important in the production of this work and to whom it is dedicated.