Summer upstream migration of juvenile freshwater eels in New Zealand

Abstract

Abstract During summer, upstream migrations of juvenile freshwater eels (elvers) of both species (Anguflla australis schmidtii Phillipps, the short‐finned eel, and A. dieffenbachii Gray, the long‐finned eel) are observed at hydro‐electric dams and waterfalls. The migration was sampled at Karapiro Dam on the Waikato River, 1970, 1971, and 1974, and samples from a further 11 areas throughout the country were analysed. Elvers, active at night or during overcast and damp days, surmount steep obstacles by climbing; they adhere to damp surfaces by friction and surface tension, obtaining maximum resistance by undulating their bodies and keeping them closely adpressed to the substrate. The maximum size for vertical climbing is 12 cm. As elvers were sampled at sites where their migration was interrupted, no distinct schools were observed, but the elvers were strongly social and thigmotactic. Migrations are made up of elvers of similar size of one or both species. In large river systems, elvers apparently migrate successively further upstream each year for several, years. North Island samples contained elvers up to 3 y of age, but in the South Island elvers up to 7 y old were recorded; aging was by burnt otoliths. Migration is perhaps initiated by increases in water temperature, day length, and sociability; increased water flow may also coincide with ‘runs’. Electric fishing of streams shows that the two species have different habitat preferences, and these are reflected in the proportion of elvers caught at different sites: longfins prefer swift stony rivers, shortfins silty backwaters and lakes, but both are often found together. Longfin elvers have thick dermal skin layers, and experiments in air of constant temperature and humidity showed that they cannot survive prolonged exposure requiring cutaneous respiration as well as shortfins; longfins may also have higher oxygen requirements.