Environmental influences on growth, maturation and smoltification in Atlantic salmon parr, Salmo salar

Abstract

The maturation of Atlantic salmon (Safmo safar) parr, and its effects on growth and smoltification, causes significant economic losses to commercial aquaculture. The current thesis investigates the role of environmental factors on freshwater development, with the aim of providing information which would help reduce the currently observed levels of maturation in farmed salmon. The effects of short day "winter" photoperiods were investigated by exposing three replicated groups of fish to an 8 week "winter" photoperiod (LDlO:14) commencing in May, August or September, in an otherwise continuous light (LD24:0) regime. A further group was held on LD24:0 throughout. 200 to 300 individuals were PIT tagged in each group in order to follow the growth of fish undergoing different developmental strategies, with the retrospective analysis of such development also possible. The highest incidence of maturation (>20%) was observed in the May winter photoperiod group, with low levels recorded in the August and September fish «4%), suggesting that maturation may be influenced during a "critical" period in early development. Maturation levels were intermediate «9%) in the continuous light group indicating that seasonally-changing photoperiodic cues are not necessarily required for gonadal development. The size of mature fish was initially the same as both immature parr and smolts, although the growth of mature individuals subsequently declined, and at the conclusion of the experiment they were significantly smaller. The August photoperiod resulted in the highest incidence of smoltification, with all other treatments resulting in low levels. In a second experiment, PIT tagged fish were reared under an 8 or 12 week ''winter'' photoperiod (LD 10: 14) starting in Mayor June, in an otherwise continuous light (LD24:0) regime. The highest incidence of maturation (> 11 %) was found in the 12 week May fish, with intennediate levels in the 8 week May and 8 week June groups «8%). Low levels were found within the 12 week June group «0.6%) and it is suggested that a critical period when maturation is influenced may occur during a specific, short period in early development. Throughout the experiments, mature individuals maintained the same size as their immature siblings. The 12 week June photoperiod appeared to result in the highest level of smoltification, although those exposed to the 12 week May photoperiod showed the greatest seawater survival. In both photoperiod experiments, fish showing some signs of smoltification were also found to be undergoing gonadal development, indicating that maturation and smoltification are not completely mutually exclusive processes. Possible nutritional effects were considered using different dietary lipid inclusions (either 12.5% or 25%) and variable rations of feed (either full, 2/3 or 113 rations). Different dietary lipid inclusions had no effect on growth, although the whole body fat content of individuals was affected, with a switch in dietary fat content during development resulting in a rapid change in body composition. Fish size increased with ration and, although at the lowest ration of feed whole body fat levels were reduced, they were maintained at a set level under the high and intennediate rations, implying a lipostatic control of growth. Maturation levels were low throughout the nutrition experiments, suggesting that genetic influences may have been important. Dietary lipid level had a negligible effect on smoltification, although increases in ration resulted in a greater incidence of smoltification. Using a 0+ photoperiod regime (i.e. LD24:0 applied from March until December, with the exception of an 8 week period of LD17:7 applied from August), smolting individuals showed a reduction in smolt status when compared to those developed under a natural photoperiod. It is suggested that such regimes restrict the mobilisation of long-tenn energy stores, with the subsequent development of seawater tolerance affected. However, it was noted that the 0+ regime had increased the incidence of smolts. In summary, it has been shown that environmental factors such as photoperiod, nutrition and temperature can play an important role in the developmental strategies taken by juvenile Atlantic salmon. Such factors are likely to greatly influence the attainment of size and/or nutritional thresholds necessary for various developmental strategies, in particular if such thresholds occur during seasonally-sensitive "critical" periods when development can be influenced. Furthermore, the life history strategy undertaken by an individual may be affected by endogenous rhythms, cued by seasonally-changing environmental factors. However, there are clear indications that the underlying genetic control of maturation may also be of importance.