Factors influencing juvenile eel (Anguilla spp.) survival in lowland New Zealand streams

Abstract

Factors influencing the survival rates of juvenile longfin (Anguilla dieffenbachii) and shortfin eels (A. australis) (<400 mm total length) were investigated in three lowland New Zealand streams to manage and conserve eel stocks and fisheries. A survival model which included terms for recruitment, growth, up‐stream movement, and vulnerability to sampling was fitted to 3 years of field data using a maximum likelihood approach. The proportion of 0+ glass eels of either species that survived to 400 mm in length was highly variable and ranged from 0.02% to 20% in different streams. Juvenile longfins generally had higher annual survival rates (0.78–0.96) than shortfins (0.43–0.90). Small eels (<150 mm) experienced low and variable survival rates (0.41–0.99) and although survival rates increased (0.80–1.00) for intermediate sized fish (150–299 mm) they then declined (0.23–0.85) for large juveniles (300–399 mm). The decline in survival rates for large juvenile eels supported a habitat bottleneck hypothesis and was possibly caused by competition from adult eels for limited areas of suitable cover (bank cover and debris clusters), although competition for food may also have been important.

Keywords:

• habitat bottleneck hypothesis
• maximum likelihood survival model
• population dynamics
• growth
• recruitment
• migration