Importance of stream versus early lake rearing for rainbow trout fry in Lake Alexandrina, South Island, New Zealand, determined from otolith daily growth patterns

Abstract

Otolith daily growth increment patterns were used to discriminate between early life‐history patterns of 0+ rainbow trout and to determine the contribution of each to adult recruitment. Inlet stream residents had narrow increments (2.13–3.18 μm) and Outlet Creek residents generally had wide increments (3.63–4.26 μm) across the entire otolith radius. The dominant otolith growth pattern in lake juveniles comprised narrow increments before emergence (3.23–3.66 μm) with wide increments (4.34 μm) soon after emergence. This “early lake migrant” pattern was consistent with incubation in cool inlet streams followed by emigration to the warmer lake upon emergence. Discriminant function analysis (DFA) using mean increment widths at 50, 150, and 200 |im from the primordia discriminated between these patterns (78% overall classification success; 96.5% for inlet stream residents, 60.0% for Outlet Creek residents, 77.0% for early lake migrants). Estimated proportions (from DFA) of the early life‐history types in the adult population were 28% inlet stream residents, 17% Outlet Creek residents, and 55% early lake migrants. Assuming daily formation of growth increments, mean fork length and mean number of days after emergence that early lake migrants entered the lake was 25.8 (± 1.1) mm and 1.4 (+ 6.4) days, respectively. Inlet stream residents appeared to emerge earlier in the spring and have higher survival than early lake migrants.

Keywords:

• Lake Alexandrina
• rainbow trout
• Oncorhynchus mykiss
• otoliths
• daily growth increments
• otolith microstructure
• migratory strategies
• stream residence
• early life‐history