The ascent of Anguilla anguillu (L.) which shows a variable pattern, has prompted much research that is not related to environmental conditions in a simple manner. Physico-chemical parameters of the water can be used by glass-eels as orienting cues during upstream migration. Temperature, salinity and odour linked with marine and river waters are three of the most important cues. Laboratory experiments were carried out by testing the responses of groups of 20 glass-eels, acclimated to either salt or fresh water at 11" C , towards water flows differing in temperature (8,11 and 14" C), salinity (fresh water and 33% salt water) and odour (natural surface, marine or odourlesstapwater). Preferencesweretestedvaryinganyoneortwoofthethreefactorsatatime,or presenting the three factors simultaneously, in all possible combinations. The three factors tested singly reveal that thermal and salinity preferences recorded using odorous waters were analogous to those already observed with odourless water. Freshwater flows are preferred to salt-water. Temperatures below the acclimation temperature are preferred to higher temperatures. Natural odorous waters are preferred to odourless water at the same salinity. Data from 2900 choices recorded in 163 tests demonstrate that salinity is the most important factor guiding flow choice. Preference for fresh water is affected more by temperature than by odour. The latter, in turn, influences more thermal than salinity choices. Odour attractiveness mainly acts by reinforcing preferred stimuli or offsetting unpleasant ones. Preference differences were found between glasseels acclimated to fresh and salt water. Preference for fresh water strongly affects the choices of the former group, blunting the effects of thermal and odorous stimuli. Salt-water reared glass-eels, less conditioned by salinity, seem to be more sensitive to thermal and odorous attractions.

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Odours are one of the most important cues in anadromous migration in fish, but to date the substances responsible for freshwater attractiveness are not well‐known. This is the first report on the behavioural response of a long‐range migrating fish, the eel, induced by geosmin, an odorant produced by actinomycetes, which is widely distributed in freshwater. The high sensitivity of glass eels towards very diluted solutions of geosmin, the avoidance elicited by the unnatural combination of marine salinity and earth smell, and evidence of a strong attraction as the level of salinity is reduced, indicate geosmin to be identified as an important inland water marker involved in the orientation of glass eels towards freshwater.

During their upstream migration European glass-eels, Anguillu unguiflu (L.), encounter a series of varying environmental situations. The migration requires a sequence of physiological a d a p tations determined by the different chemico-physical conditions they meet. Temperature and salinity are two of the most important factors. It is reasonable that glass-eels may utilize them as cues to orientation. Laboratory experiments were designed to elucidate the thermal and salinity preferences of glass-eels. These were assessed by examining the choices of specimens caught either at sea and then kept in salt water (33%). or in the Arno river and then reared in fresh water. Water flows, triggering the rheotactic reaction, prompted glass-eels to choose between two different salinities and/or temperatures. The results confirm the preference of glass-eels for flows whose temperature does not differ from that of acclimation. Specimens tested towards two water flows, both at different temperatures from that of acclimation, preferred the colder. Fresh water was usually preferred to salt water, this preference being not so marked in the case of the glass-eels caught at sea and thus not yet adapted to fresh water. Clear-cut choices were recorded when one of the tested flows presented both the preferred temperature and preferred salinity. When only one of the two parameters reproduced the preferred situation, the choices were differently affected by temperature and salinity at different values of temperature. When the temperature of both flows was below 1 1-12" C. glass-eels preferred fresh water; at higher temperatures the colder of the two flows was preferred, even if salty.

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