Experimental analysis of the thermal and salinity preferences of glass‐eels, Anguilla anguilla (L.), before and during the upstream migration

Abstract

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.

Role of glass eel salinity preference in the control of habitat selection and growth plasticity in Anguilla anguilla

“…Our first salinity preference test showed that, on arrival from the sea, most active glass eels preferred FW to SW, in accordance with the previous results of Tosi et al (1988Tosi et al ( , 1989Tosi et al ( , 1990. However, through the second salinity preference test, we further demonstrated that a highly significant link exists between salinity preference and locomotor activity.…”

“…The behavioral test apparatus was similar to that of Tosi et al (1988), modified to include 3 tanks providing a triplicate for each experiment (Fig. 1).…”

“…This suggests that glass eels may exhibit salinity tolerance variations in terms of osmoregulatory ability. Accordingly, experimental behavioral studies indicate that, although the majority of Anguilla anguilla glass eels are FW-seeking, the proportion of SW-seeking fish may still reach 30 to 50% (Tosi et al 1988(Tosi et al , 1989(Tosi et al , 1990. Moreover, some individuals may reverse their salinity preference from FW towards SW over an acclimatization period in brackish water (E. Edeline et al unpubl.…”

Proximate and Ultimate Control of Eel Continental Dispersal

“…Indeed, despite exceptional osmotic tolerance, glass eels arriving from the sea need a delay period before voluntary entry into FW (Petit andVilter 1944, Deelder 1958), indicating that salinity preference may reflect subtle processes that are not revealed by osmoregulation studies. Salinity preference tests show that, at arrival from the sea, the proportion of FW-seeking glass eels varies between 50 and 70 % (Tosi et al 1988, Tosi et al 1989, Tosi et al 1990), suggesting inter-individual variation in salinity preference that could possibly influence migration and/or habitat selection.…”

Quantification of the maximal swimming performance of Australasian glass eels,Anguilla australisandAnguilla reinhardtii, using a hydraulic flume swimming chamber

“…Glass eels, unpigmented juvenile anguillid eels, exhibit behavioural responses to a range of environmental stimuli which assist them to locate regions of fresh water (Tesch 1977;Jellyman & Ryan 1983;Tosi et al 1988;Yuh-ling & Houng-Yung 1991;Sola 1995). To facilitate their upstream migration, glass eels selectively make use of a combination of swimming speeds (McCleave 1980) and tidal phases (Creutzberg 1961;Weihs 1978;Wippelhauser & McCleave 1987).…”