Male dominance in the New Zealand longfin eel population of a New Zealand river: probable causes and implications for management

Abstract.—The endemic New Zealand longfin eel Anguilla dieffenbachi (hereafter, longfin eel), is overfished, and in southern South Island, New Zealand, rivers have recently become predom-inated by males. This study examined length and age at sexual differentiation in male eels in the Aparima River catchment (area, 1,375 km2; mean flow, 20 m3 · s21) and the sex ratio and distribution of eels throughout the catchment. Longfin eels differentiated into males mostly at lengths from 300 to 460 mm and ages from 10 to 251 years. Females were rare: Of 738 eels examined for sexual differentiation, 466 were males and 5 were females, and a few others, not examined, were large enough to be female. These counts suggest a male: female ratio among differentiated longfin eels of 68:1. Of 31 differentiated shortfin eels A. australis, less common in the Aparima River, 26 were females. Male longfin eels were distributed throughout the main stem and tributaries; un-differentiated eels were more prevalent in lower and middle reaches and in the main stem than i