However, for females and males, there was no significant DL effect on plasma sexual steroids and thyroid hormones levels. Likewise, no DL effect on N and P retention and loss was found. From
this study, it appears that manipulating DL is a promising tool for improving rearing performance of Eurasian perch. q 2000 Elsevier Science B.V. All rights reserved.
Keywords: Eurasian perch; Perca fluÕiatilis; Daylength; Growth; Weight variability; Gonad development; Food efficiency
1. Introduction
Ž .
The Eurasian perch Perca fluÕiatilis L. has been described recently as a new
Ž .
candidate for intensive freshwater aquaculture Kestemont and Dabrowski, 1996 .
However, this species shows large variations in growth between fish of the same cohort Ž
. Ž
Melard et al., 1995 and significant mortality rates due to cannibalism Kestemont et
´
. Ž
. al., 1995 . These led to economic losses in aquaculture Smith and Reay, 1991 . Growth
Ž heterogeneity is considered to be caused by initial size heterogeneity Loadman et al.,
. 1986; Katavic et al., 1989; Van Damme et al., 1989; Folkvord, 1991; Brabrand, 1995 ,
Ž .
Ž density Baardvik and Jobling, 1990; Melard et al., 1996 , social interactions Abbott et
´
. Ž
al., 1985; McCarthy et al., 1992; Jobling et al., 1993 or genetic factors Bry and Gillet, .
1980; Kindschi and MacConnell, 1989 . It was recently reported for Eurasian perch that Ž
. Ž
. rearing conditions Fontaine et al., 1996a and temperature Melard et al., 1995, 1996
´
can also affect variation in growth. Considering that Eurasian perch show a typical diurnal feeding activity when fed on
Ž .
demand with self-feeders Anthouard and Fontaine, 1998 , one might hypothesise that daylength will influence its growth and heterogeneity, as it has already been shown in
Ž . Ž
other perciform species such as juvenile sunfish Lepomis cyanellus
Gross et al., .
1965 . Other effects of daylength may relate to growth heterogeneity and cannibalism, but
whether this latter parameter is a cause or a consequence of the former is not known. Ž
. Indeed, daylength can also act on gonad development De Vlaming, 1972 , which in turn
Ž may induce changes in body and muscle composition Norton and Macfarlane, 1995;
. Jørgensen et al., 1997 . In addition, daylength may interfere with feeding activity rhythm
Ž .
Ž and feed intake Barlow et al., 1995 and, in turn, may affect nutrient retention Boujard
. Ž
. and Leatherland, 1992; Boujard and Luquet 1996 and loss Thorpe and Cho, 1995 .
Consequently, the purpose of our study was to investigate the influence of three daylengths on growth and growth heterogeneity in Eurasian perch juveniles reared in
recirculation systems, with particular attention to physiological variables, such as feed efficiency, plasma thyroid and sexual steroid levels, and N and P balance.
2. Materials and methods
