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New knowlegde for improved photoperiod protocols for 1) salmon, 2) sea bass and 3) rainbow trout

A range of long-term experiments was conducted with different photoperiod treatments to affect puberty in Atlantic salmon, sea bass and rainbow trout. These experiments have been conducted in tanks for sea bass and trout, and in the case of salmon both in seawater tanks and sea cages of various sizes.

In sea bass various photoperiod regimes were tested at different time-points in the lifecycle, in fish of differing size, and combined in one study with long-term sex steroid treatments. The sea bass trials show that continuous light treatment can be employed to delay or arrest early puberty in this species, in particular the precocious maturation in males at one-years of age. Long-term treatment with 11-ketotestosterone implants was able to cancel the inhibitory effect of LL on the precocious maturation in sea bass. An accelerating photoperiod regime was found to reduce the proportion of sexually mature rainbow trout, in line with previous studies in this species.

The effects of photoperiod treatments have been closely examined as regards cell proliferation and apoptosis in testis in males and oogenesis and atresia in ovaries in females. Testis incubations in vitro has examined the responsiveness to gonadotropins at different stages of pubertal development in salmon, as well as following light treatments that either arrest or accelerate spermatogenesis.

The efficiency of the light/photoperiod treatments has also been monitored by diel plasma melatonin profiles in salmon sampled in the cages, by assessment of sex steroid levels, gonad histology, growth performance and the final proportion of sexually maturing fish. In sea bass also LH levels in pituitary and plasma have been monitored following the photoperiod treatments along with expression profiles of pituitary mRNA levels of GTHa, FSHb and LHb.

Continuous light treatment (LL) in sea cages from mid-winter onwards was found to reduce the proportion of sexually maturing salmon in line with previous experience. However, this reduction was less pronounced than expected, and varied between the experiments with different age groups, sibling groups and feeding history. Combination of restricted feeding during the autumn followed by LL treatment from January resulted in slightly fewer maturing females compared to LL treatment alone, but the restricted feeding had much less effect on age at puberty than expected in spite that the reduced feeding from September to January lead to a dramatic reduction of growth rate, condition factor and adiposity in fillet and viscera by January. Examination of 3 different sibling groups in this set-up revealed genetic differences in the response to both light treatment and reduced feeding, which to some extent may explain some of the variable results in the arresting effect of LL treatment observed in the different trials.

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