Eels at the Edge: Science, Status, and Conservation Concerns

Forty Years On—the Impact of Commercial Fishing on Stocks of New Zealand Freshwater Eels

Don J. Jellyman


Abstract.—The two main species of freshwater eels in New Zealand, the shortfin Anguilla australis and the endemic longfinned eel A. dieffenbachii, are extensively commercially exploited and also support important customary fisheries. Since there are no commercial glass eel fisheries in New Zealand, other indices must be used to indicate changes in recruitment over time. While there is some anecdotal evidence of reductions in glass eel recruitment, there is evidence of poorly represented cohorts of longfins within some populations, and modeling of these data indicate a substantial reduction in recruitment over the past two decades. Growth of both species is typically slow at 2–3 cm per year, meaning that both species are susceptible to commercial capture for many years until spawning escapement. Extensive commercial fishing has resulted in more substantial changes in length-frequency distributions of longfins than in shortfins; likewise, regional reductions in catch per unit effort are more significant for longfins. Theoretical models of silver eel escapement indicate that longfin females are especially susceptible to overexploitation. Shortfins would have been more impacted than longfins by loss of wetlands, but the impact of hydro stations on upstream access for juvenile eels and downstream access for silver eels would have been more severe for longfins. Overall, there is no clear evidence that the status of shortfin eel stocks has been seriously compromised by the extensive commercial eel fishery, but there is increasing evidence that longfins are unable to sustain present levels of exploitation.