Inhibition of sexual maturation in tank reared haddock (Melanogrammus aeglefinus) through the use of constant light photoperiods

The haddock (Melanogrammus aeglefinus) is believed to be a potential candidate for aquaculture in the Atlantic coastal countries including the UK, Norway and Canada. However, under culture conditions, haddock will sexually mature prior to the attainment of a suitable harvest weight. Therefore, a long term tank based experiment was performed where three populations of haddock (hatched spring 2002, approximately 150 fish per population) were exposed to either a simulated natural photoperiod (SNP) or SNP until January or July 2003 and thereafter continuous illumination (Jan LL or July LL respectively) with individual growth rate and maturation status being recorded until July 2004 (27 months post-hatch). While the SNP treated population matured in the spring of 2004 (two years post-hatch) with 88% of the population being observed to release gametes, no mature individuals were observed in either of the LL treated populations and furthermore there was no evidence of gonadal development or elevation in sex steroids (testosterone, estradiol-17β or calcium). The application of LL appeared to directly stimulate the growth rate of haddock (e.g. 14 to 27% increase in weight thermal growth coefficient) and improve food conversion rates. At the end of the trial there was an approximate 50% increase in wet weight (SNP: 647 ± 53 g, Jan LL: 982 ± 34 g and July LL 985 ± 33 g), however no significant difference in weight in relation to the length of LL exposure was observed. Plasma melatonin analysis revealed that the natural diel rhythm had been inhibited in both LL treated populations. Along with highlighting the similarity in reproductive entrainment between haddock and other gadoids, these results demonstrate how photoperiod manipulation could be used as a management strategy to improve growth performance in farmed haddock stocks.