The effect of modifications in photoperiod on spawning time, ovarian development and egg quality in the sea bass ( Dicentrarchus labrax L.)

Abstract

Under simulated natural conditions female sea bass ( Dicentrarchus labrax) in eastern Spain (latitude 40° N and longitude 0°) spawned over a 6–8-week period in February and March, with each female spawning on more than one occasion. Generally, male fish began spermiation up to 2 months before, and continued for at least 1 month after the spawning period of the female fish. Exposure of fish to 1 month of long days (LD 15 9 ) from either 2nd May (Group C), 3rd June (Group D) or 3rd July (Group E) in an otherwise constant short day (LD 9 15 ) photoperiod regime, speeded up the rates of maturation, thus increasing the proportions of oocytes entering exogenous vitellogenesis during October and November, and also brought forward the timings of ovulation and spawning. In contrast, constant long days from 2nd May (Group B) delayed maturation and spawning time by 2–3 months. Fish maintained under constant short days throughout the experiment (Group A) spawned up to 6 weeks in advance of the control fish, suggesting that endogenous timing mechanisms may operate in this species. Spawning occurred naturally in all the experimental and control fish without the necessity for induction with pituitary or hypothalamic hormones. For the control fish, egg quality (defined as the proportion of eggs spawned which were floating and viable), hatching rate and survival to first feeding (both expressed as percentages of the numbers of floating or good quality eggs) averaged 78.3±7.7% (mean±s.e), 84.8±3.3% and 54.6±20% respectively. With the exception of the eggs and fry from Group B, these measures of quality and survival appeared unaffected by the photoperiod treatments. The eggs and fry produced by the fish maintained under long days (Group B) towards the end of their spawning period (i.e. in May), showed much more variable egg quality and survivals through hatch and first-feeding, possibly because of the higher sea water temperatures which prevailed at the time of their delayed spawning. Fecundities expressed as numbers of eggs per kg of post-spawning fish weight ranged from 247–305 000/kg for Groups A, C, D and E and the controls, but were significantly reduced in Group B fish. Egg sizes, which ranged from 1.147–1.176 mm in diameter, were unaffected by the photoperiodic manipulations of spawning time. It is concluded that artificial control of daylength can improve the supply of eggs and fry for commercial on-growing without any adverse effects on the fecundity of the broodstock or the quality and survival of the eggs and fry.