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Abstract
The New Zealand Ministry of Fisheries constrains the incidental capture of Hooker’s
sea lions in trawl nets of the southern squid fishery by closing the season once an upper
limit on sea lion deaths is reached. The regulatory measure is in fact a limit on effort
because the number of sea lion deaths is calculated from an estimated mortality rate
per standard unit of effort measured in tows. During recent years, vessels have been
observed to increase the median time per tow, suggesting that the industry is expanding
the capacity of an unregulated input in response. This paper formalises the current
situation analytically by constructing a bioeconomic model that captures the idiosyncrasies
of the squid fishery and the imposed regulation. Reducing the regulatory constraint
to an isoperimetric problem can show how the current management regime
may skew incentives leading to the observed increase in tow time. An extension to the
current regulatory framework by introducing a spatial dimension to the estimated sea
lion mortality rate may lead to more efficient behaviour. Despite retaining an upper
limit on sea lion deaths, the profit-maximising squid industry is given the incentive to
increase effort in areas of high squid density relative to sea lion density.