Characteristics of common smelt,Retropinna retropinna(Richardson), of the Waikato River system

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Abstract
Populations of lacustrine and diadromous common smelt (Retropinna retropinna), in the Waikato River system, North Island, New Zealand, have different average numbers of first arch gill rakers and vertebrae: c. 28 versus c. 21, and c. 52 versus c. 60, respectively. A third group, inhabiting the 7 hydro‐electric reservoirs of the Waikato system, average both low gill raker (c. 21) and low vertebrae numbers (c.52). Reservoir smelt have approximately 2 fewer first arch gill rakers than Lake Taupo fish. Most diadromous smelt mature in late summer and autumn, whereas reservoir fish mature in spring and early summer. Reservoir and Lake Taupo smelt have smaller eggs and larvae than diadromous smelt. The larger diadromous larvae have larger yolk sacs than Lake Taupo larvae; this is probably an adaptation associated with a direct larval migration to sea. Reservoir and lacustrine post‐larvae were found in the lower Waikato River but diadromous post‐larvae were confined to the estuary and sea. Reservoir populations probably originated from fish displaced down stream from Lake Taupo.