Combined effects of
photoperiod and salinity on growth, survival, and osmoregulatory ability of
larval southern flounder Paralichthys lethostigma
C.Th. Moustakas, W.O. Watanabe, K.A. Copeland-2004
Aquaculture, 229(1-4): 159-179
Abstract:
The southern flounder, Paralichthys lethostigma,
is an important commercial and recreational marine flatfish that inhabits
estuaries and shelf waters in the south Atlantic, from North Carolina
through the Gulf coasts, with the exception of south Florida. Because
juvenile and adult fish are highly euryhaline, it is a prime candidate for
aquaculture. Methods for captive spawning of southern flounder are well
developed; however, information on optimal culture requirements of the early
larval stages is required for reliable mass production of juveniles.
To determine the optimal photoperiod and salinity
conditions for culture from hatching to day 15 post-hatching (d15ph),
embryos were stocked into black 15-l tanks (75 l-1) under four
photoperiods (24L:0D, 18L:6D, 12L:12D, and 6L:18D) and two salinities (25
and 34 ppt) in a 4×2 factorial design. Temperature was 18 °C, light
intensity was 150 lx, and aeration was 50 ml min-1. Significant (P<0.05)
effects of photoperiod and salinity on growth (notochord length, wet and dry
weights) were obtained. Growth increased with increasing photoperiod and
salinity and was significantly greater at 24L and 18L than at 12L or 6L, and
at 34 than at 25 ppt. On d11ph and d15ph, significant interactive effects
between photoperiod and salinity on growth (wet and dry weights) were also
evident. Growth of larvae reared at 25 ppt increased with increasing
photoperiod to a maximum at 24L, while growth of larvae at 34 ppt reached a
plateau at 18L. While there were no significant photoperiod effects on these
parameters, larval survival, body water percentage, and larval osmolality on
d15ph were significantly higher at 34 than at 25 ppt (41% vs. 16% survival;
322 vs. 288 mosM kg-1; and 84% vs. 76% water, respectively),
suggesting stress and nonadaptation to 25 ppt, a salinity more nearly
isoosmotic than full-strength seawater. Since larvae from both salinity
treatments were neutrally or positively buoyant at 34 ppt, but negatively
buoyant at 25 ppt, larvae reared at 25 ppt probably allocated energy to
maintain vertical positioning, compromising growth and survival.
The results demonstrate that growth and survival of
early-stage southern flounder larvae are maximized under long photoperiods
of 18–24L and in full-strength seawater. Longer photoperiods probably
extend the time larvae have for feeding, while full-strength seawater
salinity optimizes buoyancy and vertical positioning, conserving energy. The
results show that early larval stage southern flounder larvae are not
entirely euryhaline, which involves not only the ability to osmoregulate,
but to conserve energy under reduced buoyancy. This is consistent with
suboptimal vs. maximal growth of larvae reared at 25 and 34 ppt,
respectively, under 18L (i.e., photoperiod×salinity interaction). This is
also consistent with other reports that tolerance to lower salinities in
these euryhaline flatfish increases post-metamorphosis when transition from
a pelagic to benthic existence alleviates the need to counteract reduced
buoyancy.
(The University of North Carolina at Wilmington,
Center for Marine Science, 7205 Wrightsville Ave., Wilmington, NC 28403,
USA, e-mail: moustakas@hotmail.com)