Upstream migration of glass‐eels(Anguillaspp.) in the Waikato River

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Abstract
After initial invasion of fresh water from the sea, upstream migration of glass‐eels is delayed in estuarine or tidal areas while the eels undergo certain physical and behavioural transitions. The morphology and periodicity of movement of upstream‐migrating glass‐eels of Anguilla australis Richardson and A. dieffenbachii Gray were examined from samples collected during August‐November 1970 and 1972–1974. These samples were taken from commercially netted catches made on the Waikato River, New Zealand. A seasonal decline in condition was found in migrating glass‐eels, and changes in body proportions and stomach development were correlated with pigmentation and the onset of growth. Movement is thought to occur preferentially on the flood tide, although migrations are frequently recorded over both tide phases. A bimonthly catch periodicity is related to the spring tides of full and new moons; delay in peak catches reflects the distance upstream of the catching stations. River height, light, and water temperature were not found to influence either the periodicity or the magnitude of migration.