Size‐related differences in diel activity of two species of juvenile eel (Anguilla) in a laboratory stream

Abstract

Abstract – The diel activity of three size groups (small=<100 mm; medium=100–199 mm; large=200–299 mm total length) of juvenile shortfinned (“shortfin”) eels (Anguilla australis) and longfinned (“longfin”) eels (A. dieffenbachii) was tested in a laboratory flow tank over a 48‐h period during summer. All size groups of both species were nocturnally active, with the eels hiding in the substratum during the day and coming out on top of the cobbles from dusk to dawn, to feed. During the foraging period, the numbers and activity of all sizes of longfins visible were greater than those seen of shortfins, with the differences being more pronounced for small and medium eels. The activity of all eels consisted mostly of foraging by crawling, searching and probing for prey among the cobbles. Rate of activity increased with size of eel for both species. Small eels of either species did more swimming than eels of the larger sizes, whereas large eels were observed more frequently with only their head out of the substrate than were the smaller individuals. Feeding of small eels within the interstitial spaces of the streambed may explain their significantly lower activity on top of the substrate at night. The significantly lower rate of activity recorded for shortfins than longfins of all sizes may be due partly to their ability to feed within the interstices of the stream bed, and (or) longer time to recover from handling and habituate to the test environment.Note