Diel and seasonal movements of silver eels, Anguilla dieffenbachii, emigrating from a lake subject to hydro-electric control

To investigate the behaviour and fate of silver female longfin eels Anguilla dieffenbachii in relation to flow variations from New Zealand's largest (700 MW) hydro-electric station, 210 A. dieffenbachii were tagged with acoustic transmitters during summer and autumn over 6 years from 2005 to 2010. A shoreline array of receivers (2005-2008) was supplemented by positioning systems near the power station and natural outlet (2009-2010) that enabled continuous tracking of A. dieffenbachii within these areas. Tagged A. dieffenbachii (total length 785-1372 mm) emigrated from the lake over an 8 month period, either in the same season they were tagged (75%) or the following spring after over-wintering for up to 7 months (25%). Most A. dieffenbachii (67%) emigrated via the natural outlet. Swimming activity was related to water temperature and was greatest during the summer months. Activity was strongly diel, although less so at the outlet where A. dieffenbachii migrating at depths greater than 20 m often did so during daylight. To analyse searching behaviour, sustained periods of continuous swimming activity (termed forays) were examined in the vicinity of the outlet. Most successful forays (where the A. dieffenbachii exited the lake via the lower Waiau River) were associated with outflows; such outflows were sometimes <10 m(3) s(-1) although most successful forays took place at flows >50 m(3) s(.)(-1) The timing of successful forays was related to outflow, the day of year and whether the A. dieffenbachii had delayed migration or not, but not lunar phase or rainfall. Passage time from the lake to the lower Waiau River 81 km downstream decreased from an average of 58 days early in the season to <1 day in late autumn. The wide range of behaviours and responses displayed by female silver A. dieffenbachii were interpreted as an inherent form of bet-hedging to spread the risks associated with silvering and seaward migration in this slow-growing species.