MULLEN: FISH THROUGH VARIABLE DIFFUSIVITY 



plicltly within the model, has the same effect as 

 increasing catchability or, perhaps through im- 

 proved handling and distribution of the fish, rais- 

 ing the price. Whatever the reason, increasing 

 power allows fish to be profitably caught when there 

 are fewer of them. This enables fishing to take place 

 at locations other than the hot spot, but the change 

 reduces the catch at the hot spot even though ef- 

 fort there increases. There is the familiar reason of 

 local overexploitation, but also the reduction in fish 

 elsewhere decreases the flow of fish to the hot spot. 

 If the hot spot contains an island with a government 

 that can create local restrictions on effort, the local 

 catch rate could be held at the original level only 

 by drastically reducing the effort to far less than 

 that of the original local fishery. This might prove 

 a difficult choice for a small island dependent upon 

 fishing. 



CONCLUSION 



A constant value for a'^ creates an unrealistic, 

 almost homogeneous distribution offish. A variable 

 a- allows fish to spend more time in good habitats. 

 They can vary a- by changing direction more or 

 less frequently. Preliminary observations indicate 

 that a- does vary as this notion predicts. 



Fishing, by removal of competing individuals, 

 increases the potential production of a surviving in- 

 dividual; thus the habitat appears enhanced and 

 migrants are more likely to stay. In this way fishing 

 has an impact far beyond its location. The catch rate 

 close to an island is partially sustained by tapping 

 the resources from the contiguous region. An effect 

 of a high-seas fleet is to reduce the gradient of 

 habitat saturation from what would occur if there 

 were only fishing close to islands; thus the relation- 

 ship between a high-seas fleet and artisanal inshore 

 fishery is clearly competitive. 



ACKNOWLEDGMENTS 



Pat Tomlinson provided the parameters for the 

 population dynamics, and Bill Bayliff and Pierre 

 Kleiber lent their insights from tagging experiments 

 in the eastern and western Pacific. Many people 

 reviewed various versions of the manuscript in- 

 cluding Bill Bayliff, John Beddington, Michael 

 Bergh, Colin Clark, Rick Deriso, John Gulland, Alec 

 MacCall, and Karen Smith; it was improved by each 

 of them. Thanks to Jim Joseph, the Director of the 

 Commission, who suggested that I work on move- 

 ment in the first place and lent his support and 

 encouragement. 



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