AGE VALIDATION AND MOVEMENT OF FRESH-WATER EELS (ANGUILLA-DIEFFENBACHII AND A-AUSTRALIS) IN A NEW-ZEALAND PASTORAL STREAM

Forty-seven eels were captured tagged, and released into a 420 m lowla nd section of the pastoral Ahirau Stream, North Island, New Zealand. E els were marked with jet inoculations of alcian-blue dye, and plastic tube tags were surgically inserted into the gut cavity. Oxytetracyclin e hydrochloride was injected intraperitoneally to provide markers on o toliths. Over 3 years, 25 tagged eels were recaptured in the same stre am 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 t he first 2 years retained dye marks. Annual length increments, determi ned from measurement of tagged individuals, were 65 +/- 15 mm (mean +/ - 95% confidence limits) for longfinned eels, and 29 +/- 18 nun for sh ortfinned eels. Translucent zones counted after single fluorescent rin gs corresponded to die number of years eels had been at liberty. The o ccurrence of the OTC fluorecence within the translucent zone confirms that these zones are formed in winter in both longfinned and shortfinn ed 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.