Photoperiodic control of smoltification and aspects of broodstock management in Atlantic salmon, Salmo salar

Abstract

At present, the restricted temporal availability of Atlantic salmon smolts leads to a seasonal supply of marketable salmon and as a consequence an unstable price. The development of smoltification appears to be controlled by the .seasonal changes in daylength. The primary aim of this thesis, in collaboration with commercial smolt producers and on- growers, was to achieve a “year-round” supply of Atlantic salmon using photoperiod techniques. Potential SI (l-l-) and S2 (2+) Atlantic salmon smolts were reared in freshwater under compressed and extended seasonally-changing photoperiods and under ‘square wave’ (direct change from short to long) light regimes. Pre-smolts underwent body silvering and fin darkening, a reduction in condition factor and developed strong hypo-osmoregulatory ability in respon.se to increasing daylengths. In this study, the completion of smoltification was advanced by a maximum of 3- and 7- months among potential S 1 and potential S2 smolts respectively, and was delayed by 1-month among potential S2 smolts. The results were consistent with the hypothesis that changes in daylength entrain an endogenous circannual timing mechanism which controls the development of smoltification. Results were also consistent with the hypothesis that a ‘decision to smolt’ is made under the influence of a decreasing daylength in the year preceding seaward migration. Furthermore, It was apparent that this decision period may extend into the winter and that recruitment into the smolting fraction is probably terminated by the increasing daylength after the winter solstice. Parr reared under a compressed photoperiod with low intensity night-time illumination failed to complete smoltification at the same time as tho.se without night-time illumination. Serum melatonin was elevated throughout the period of the low intensity pha.se among fish maintained on the dual light intensity regime, although peak levels were attenuated compared to fish reared under light-dark cycles in other published studies. Temporally advanced SI smolts gained a growth advantage from an early transfer to seawater, and potential S2 smolts transferred to .seawater between September and December grew well over the winter. Mixed .sex S1 smolts transferred to seawater early did not mature as post-smolts, but advanced S2s did show post-smolt maturation and an elevated incidence of maturation after 1-sea winter. None of the all-female and triploid all-female stocks transferred to seawater in November and December showed any signs of maturation the following year, after 1-sea winter. Another important aspect of salmon culture includes the supply of eggs, however little attention has been given to the suitability or superiority of various broodstocks currently maintained by the industry for this task. This thesis provides an assessment of various aspects of the reproductive performance among a number of farmed Atlantic salmon stocks with the aim of identifying important criteria which may be used in future stock selection. Total egg number (fecundity), egg size and total egg volume data were collected from individual 2 and 3-sea winter (SW) spawning females of four farmed salmon stocks. Relationships between these parameters and post-strip fish weight were analysed using regression and analysis of covariance (ANCOVA) techniques to evaluate differences in reproductive performance between the broodstock groups. Egg size was found to be poorly related to fish weight, with coefficients of determination (H) of between 0% and 19%. The regression of all data combined provided a value of only 18.2%. Total fecundity increased with increasing post-strip weight in 2-SW and 3-SW females in all four stocks and H values ranged from 12% to 62%. Total egg volume provided the best measure of reproductive performance. All regressions of this parameter on weight were positive and highly significant (f*<0.001) with individual values ranging from 31% to 77%. With data for all stocks combined, ANCOVA showed the rate of increase of both fecundity and total egg volume, with increasing fish weight, to be significantly greater in 3-SW than in 2-SW females. Significant slope differences were also found between ages within stocks, and between different stocks of the same age. Within groups of common slope, after adjustment to a common log weight, significant differences were found between elevations of both fecundity and total egg volume regressions. Within 2-SW groups, differences in fecundity on weight and total egg volume of up to 9% and 20% were found respectively; within 3-SW groups, differences of up to 35% and 15% were found respectively.