Morphometric changes, gonad histology, and fecundity estimates in migrating New Zealand freshwater eels (Anguillaspp.)

Abstract

During transition from the non‐migrant feeding stage to the migrant stage in Anguilla australis and A. dieffenbachii the eyes increase in area by a factor of 2 in males and of 1.5–1.6 in females. A. dieffenbachii is more sexually advanced at the beginning of the spawning migration. In males the gonadosomatic ratio (GSR) was 0.5–2.7 and histological sections showed spermatocytes as the most common stage of spermatogenesis. For A. australis the GSR was 0.03–0.70, and histologically spermatogonia were predominant. In A. dieffenbachii females the mean GSR was 8.12, mean egg diameter was 0.33 mm, and histological sections showed that most eggs were at the stage of yolk deposition. In contrast, the mean GSR of A. australis females was 3.5, mean egg diameter was 0.22 mm, and few eggs were at the yolked stage. Egg counts showed that fecundity increased with total length. Fecundity estimates for 23 A. dieffenbachii (total length 711–1452 mm) varied from 1.05 million to 20.83 million and for 26 A. australis (total length 516–933 mm) from 0.46 million to 3.06 million.