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.