Environmental cues and extended estuarine residence in seaward migrating eels (Anguilla australis)

Abstract

Summary Spawning migration by freshwater eels to their marine spawning grounds is widely considered to be direct and rapid; however, emerging evidence suggests that eel migratory behaviour is more complex than previously thought, with potential implications for eel conservation and management. Over a 5‐year period, we tagged 97 yellow‐phase short‐finned eels Anguilla australis with acoustic transmitters in the freshwater reaches of a south‐eastern Australian river to: (i) examine environmental correlates associated with seaward migration; (ii) test the hypothesis that migration is rapid and direct once initiated and (iii) assess individual variation in behaviour associated with seaward migration. Twenty‐three of the tagged eels migrated from fresh water into the estuary, whilst the remainder stayed within fresh water. Movement was detected primarily at night and eels entered the estuary throughout the year, with an increase in frequency over summer and following high river flows. Time in the estuary ranged from 1 to 305 days (median: 77 days). Movement into the sea was influenced primarily by the lunar phase, and to a lesser degree by water temperature, and occurred from late summer to early autumn. The extended residence and complex movements of migrating eels in the estuary suggest that they are considerably more vulnerable to exploitation than would be predicted by the generalised eel migration model of direct movement out to sea.

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

“…The effect of environmental conditions on the periodicity of silver‐eel migrations has been extensively researched. The list of influential factors includes lunar period (Burnet, ; Todd, 1981 a ; Haraldstad et al, ; Euston et al, ), water temperature (Vøllestad et al, ; Boubée et al, ; Reckordt et al, ), rainfall and increased flow (Todd, 1981 a ; Vøllestad et al, ; Boubée et al, ; Behrmann‐Godel & Eckmann, ; Haro et al, ; Bruijs & Durif, ; Crook et al, ; Marohn et al, ), turbidity (Durif et al, ), wind (Reckordt et al, ) and the passage of depressions (Burnet, ). Of course, many of these factors are inter‐related and hence isolating causative ones is often difficult.…”

“…All receivers showed patterns of distinct diel activity, although this was less obvious once A. dieffenbachii had entered the outlet. Eels are negatively phototrophic and night‐time activity of silver eels is widely reported (Lowe, ; Boubée et al ., ; Tesch, ; McCarthy et al, ; Crook et al, ). Silver A. dieffenbachii arriving at the outlet of Lake Manapouri showed various and complex patterns of movement (NIWA, unpubl.…”

“…For both main species of New Zealand freshwater eels, the shortfinned eel Anguilla australis Richardson 1841 and A. dieffenbachii, there are some studies of the timing of movement of silver eels (Burnet, ; Todd, 1981 a ; Sloane, ; Crook et al, ) and their behaviour at hydro dams (Boubée et al, ; Watene et al, ; Brown et al, ). Entry of A. dieffenbachii silver eels to the sea occurs within restricted periods of 2–3 months (Todd, 1981 a ) with males (March to April) preceding females (April to June).…”

Freshwater and coastal migration patterns in the silver‐stage eel Anguilla anguilla

“…anguilla maturation may be more flexible than originally thought (Svedäng & Wickström, ) and that individuals may have the ability to interrupt migration and begin feeding again. Crook et al () demonstrated extended estuarine residence time for Anguilla australis Richardson 1841, highlighting the possibility of more complex migration behaviour instead of the rapid and direct seaward migration originally assumed. Stein et al () also highlighted the possibility that A .…”

Migration of eels tagged in the Baltic Sea and Lake Mälaren—in the context of the stocking question

“…An initial delay among migrating eels has also been identified and discussed earlier (Crook et al, 2014;Westerberg, Sjöberg, Lagenfelt, Aarestrup, & Righton, 2014). One complicating matter is that eel having spent all their lives at the coast-which means they most likely are of natural origin-also chose to overwinter (Väddö in 2008).…”