International eel symposium, 2003 - Quebec August 11, 2003

Sex Ratio of Longfin Eels in New Zealand Rivers: Effects of Commercial Fishing

McCleave, J.D.* School of Marine Sciences, University of Maine, 5741 Libby Hall, Orono, Maine 04469-5741 USA, mccleave@ maine.edu
Jellyman, D.J. National Institute of Water and Atmospheric Research, P.O. Box 8602, Christchurch, New Zealand.

Presenter email address: mccleave AT maine.edu

Abstract Text: Populations of the endemic longfin eel Anguilla dieffenbachii have become male dominated in recent decades in rivers of the southern South Island of New Zealand. Recently in the Aparima River, there was a male:female ratio of about 68:1 among eels that were sexually differentiated. Differentiation into males occurred mostly at lengths from 300-460 mm and ages from 10-25+ years. This situation contrasts with strong dominance by females in those rivers in the middle of last century. Intense commercial fishing developed in the Aparima and other southern rivers in the last 25 years. The change in sex ratio is attributable partly to selective harvest of females, but is likely due partly to changes in the structure of the population from fishing such that differentiation into males is favored. A speculation is that reduced numbers of large, piscivorous females in a population may increase the density of, or social interaction among, undifferentiated eels. Increased population density is thought to favor differentiation into males. Longevity, delayed sexual maturity, semelparity, and endemism with restricted geographical range make the longfin eel particularly vulnerable to overfishing. Social factors within the eel population, unexpectedly shifting the sex ratio away from egg-bearing females, exacerbates the problem.