Age validation and movement of freshwater eels ( <i>Anguilla dieffenbachii</i> and <i>A. australis</i> ) in a New Zealand pastoral stream

Abstract

Forty‐seven eels were captured, tagged, and released into a 420 m lowland section of the pastoral Ahirau Stream, North Island, New Zealand. Eels were marked with jet inoculations of alcian‐blue dye, and plastic tube tags were surgically inserted into the gut cavity. Oxytetracycline hydrochloride was injected intraperitoneally to provide markers on otoliths. Over 3 years, 25 tagged eels were recaptured in the same stream reach, 16 of these within 20 m of their original sites of capture. All recaptured eels retained internal tags which had been incorporated into intestinal mesentary membrane, and 57% of eels recaptured over the first 2 years retained dye marks. Annual length increments, determined from measurement of tagged individuals, were 65 ± 15 mm (mean ± 95% confidence limits) for longfinned eels, and 29±18 mm for shortfinned eels. Translucent zones counted after single fluorescent rings corresponded to the number of years eels had been at liberty. The occurrence of the OTC fluorecence within the translucent zone confirms that these zones are formed in winter in both longfinned and shortfinned eels. Comparison of rings on unburnt and burnt otoliths from tagged eels validated the break‐and‐burn technique of age determination for both species of New Zealand eel.