Effect of temperature and salinity on glass eel (Anguilla anguilla) pigmentation development

The evolution of pigment stages was monitored on six glass eel samples collected in the Vilaine estuary, and kept in cages in freshwater, and in the estuary. In each location one sample was kept for reference to assess the effect of handling procedure on pigment development. It was pulled out the water only to monitor and retrieve mortality. The other sample was anaesthetised and analysed for pigment developmental stages three times a week. The analysis of 9,920 glass eels showed first that anaesthesia and reading of pigment stages did not modify the pigmentation process. The second result, in accordance with other published results, was to show that temperature is the main factor controlling pigmentation dynamics. In freshwater, the duration to reach from 88-99% of young stages VB and VIAO to 86-99% of old stages VIA2 VIA3 and VIA4 varied from 6 days at 18degreesC to 45 days at 8degreesC. We finally confirmed that the salinity acts as a secondary factor slowing down the pigmentation. The samples kept in the estuary were less pigmented than the samples kept in freshwater, except during flood periods when there was a differential mortality of young stages in the estuary. This result was in accordance with most of published and unpublished results re-analysed in the paper.