The sustainable management of the New Zealand longfin eel : a bioeconomic analysis : a thesis presented in partial fulfilment of the requirements for the degree of Master of Applied Economics at Massey University
Annual recruitment of the New Zealand longfin eel (Anguilla dieffenbachii) has declined by around 75 percent since heavy levels of commercial fishing began in the early 1970s. Longfin eels live in freshwater for many years, sometimes over one hundred, before reaching sexual maturity and migrating to oceanic spawning grounds. Longfin eels are semelparous, in that they die after making only a single reproductive contribution following migration. Late maturation and semelparity render longfin populations extremely sensitive to recruitment overfishing. Consequently, poorly defined property rights and fragmented regulation have permitted multiple user groups, but primarily the commercial fishery, to reduce these stocks to the point of near-collapse. In this research, a deterministic multiple-cohort bioeconomic model is developed and applied to a longfin eel population to investigate sustainable management strategies for the fishery, subject to its biological and economic characteristics. The optimisation framework incorporates density-dependent growth and spawner-recruitment relationships and a delay-difference equation to express the significant lag between the sexual maturity of adults and the vulnerability of corresponding young to the fishery. The model also permits the investigation of alternative weight restrictions and a price that varies with age/size. The model demonstrates the insufficiency of using past harvests to calculate sustainable catch, as done recently for the South Island fishery. The model results also indicate the need for a minimum weight restriction higher than that maintained under the existing regulatory system. The importance of no maximum weight restriction is also identified. Additionally, the model results indicate that there is a significant inverse relationship between the level of exploitation and the annual breeding population, since no harvested eel has ever spawned. The sensitivity of longfin eel populations to recruitment overfishing is greater in reality due to uncertainty, competition among harvesters, price and harvest incentives, and this specie's biology. These factors suggest that the use of any harvest-based regulatory system without significant investment in area closure will fail to protect longfin eel stocks through the recovery and maintenance of spawning biomass. The analysis identifies the need for an integrated management strategy, incorporating area closures, for rebuilding and maintaining spawning biomass, and the use of ITQ management in open fisheries to aid the allocation of fishing rights among users. Efficient management of these open areas requires a higher minimum weight limit than under the current management system, and no maximum weight restriction. The calculation of sustainable harvest levels remains problematic due to poor information; however, active adaptive management may be used to work towards their identification. This approach might be aided by density-dependent growth, which would assist the recovery of populations if sustainable harvest were overestimated. Additionally, spawners from closed populations would help to safeguard against recruitment overfishing during the investigation of sustainable exploitation rates. This integrated policy represents a biologically sound and economically relevant management strategy that has the potential to sustain longfin populations and their harvest indefinitely.