Challenges for Diadromous Fishes in a Dynamic Global Environment

Abstract

<em>Abstract</em>.- Juvenile inanga <em>Galaxias maculatus </em>are the main species in the New Zealand whitebait fishery. Whitebait were sampled throughout the spring fishing season (August-November 2002) as they entered the mouths of two New Zealand rivers, one in the North Island and the other more than 500 km south in the South Island. Fish length, date of arrival at the river mouth, and the number of daily growth rings in otoliths were used to determine age, hatch date, and mean growth rate at sea. Backdating indicated that whitebait hatched between mid-March and late July 2002, spent 104-168 d at sea, and experienced overall growth rates ranging from 0.24 to 0.41 mm/d. Mean length on return to the river mouth ranged from 45 to 55 mm total length and varied among months as well as between rivers. The duration of the oceanic rearing period (in days) was inversely related to growth rate, implying that the time of return to freshwater is endogenously determined. However, average size at river entry was larger for the South Island than for the North Island fish because, on average, the South Island fish remained at sea for a further 10 d. A longer journey from marine feeding grounds back to the coast rather than a delay at the river mouth before entry would account for this difference, but colder oceanic waters in the south may also play a role.