Thermal preferences in upstream migrating glass-eels of Anguilla anguilla (L.)

The effect of temperature on the migratory behaviour of European glass-eels, Anguilla anguilla, during the first stages of their ascent into inland waters has been investigated. Very little research has documented the thermal preferences of glass-eels, most being directed instead to the study of their thermal resistance and survival. The experimental method adopted aimed at acquiring information on short- and long-term thermal choices and sought to provide evidence of the influence of preceding experiences on choices made during the tests. The results obtained in short-term choices (acute preferendum tests) show a clear preference for flows of water at temperature equal to acclimation temperature rather than for flows at temperatures lower or higher by 3° C. It is sufficient for the glass-eels to remain for about 1 h at a different temperature (acclimation temperature ± 3–4°C) for their subsequent choices to be influenced. When choosing between flows warmer or colder than acclimation water, glass-eels prefer the lower temperature. When the temperature presented is very low (about 3° C) the preference is inverted. There is evidence that the temperature gradient represents one of the main orienting cues. In long-term choices (final preference tests) the experimental apparatus allowed prolonged observation (from 24 to 52 h) of the choices between three shelters, the only difference between these being their internal temperatures. In this situation the glass-eels show a clear preference for temperatures higher than that of the environment. For both intervals of environmental temperature considered (11.0–12.6°C and 14.3–17.3° C) a linear correlation exists between temperature difference and number of choices made for the shelters at higher temperatures. The number of glass-eels choosing a higher temperature is greater in the case of lower environmental temperature. In acute preference tests the thermal stimulation is added to a rheotactic response. The glass-eels find themselves in an analogous situation to that during the ascent. On the other hand, in the second experimental situation the choices are an expression of a pure thermal preference.