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

The maturation of Atlantic salmon (Safmo safar) parr, and its effects on growth and \nsmoltification, causes significant economic losses to commercial aquaculture. The \ncurrent thesis investigates the role of environmental factors on freshwater \ndevelopment, with the aim of providing information which would help reduce the \ncurrently observed levels of maturation in farmed salmon. \nThe effects of short day "winter" photoperiods were investigated by exposing three \nreplicated groups of fish to an 8 week "winter" photoperiod (LDlO:14) commencing \nin May, August or September, in an otherwise continuous light (LD24:0) regime. A \nfurther group was held on LD24:0 throughout. 200 to 300 individuals were PIT \ntagged in each group in order to follow the growth of fish undergoing different \ndevelopmental strategies, with the retrospective analysis of such development also \npossible. The highest incidence of maturation (>20%) was observed in the May winter \nphotoperiod group, with low levels recorded in the August and September fish «4%), \nsuggesting that maturation may be influenced during a "critical" period in early \ndevelopment. Maturation levels were intermediate «9%) in the continuous light \ngroup indicating that seasonally-changing photoperiodic cues are not necessarily \nrequired for gonadal development. The size of mature fish was initially the same as \nboth immature parr and smolts, although the growth of mature individuals \nsubsequently declined, and at the conclusion of the experiment they were significantly \nsmaller. The August photoperiod resulted in the highest incidence of smoltification, \nwith all other treatments resulting in low levels. \nIn a second experiment, PIT tagged fish were reared under an 8 or 12 week ''winter'' \nphotoperiod (LD 10: 14) starting in Mayor June, in an otherwise continuous light \n(LD24:0) regime. The highest incidence of maturation (> 11 %) was found in the 12 \nweek May fish, with intennediate levels in the 8 week May and 8 week June groups \n«8%). Low levels were found within the 12 week June group «0.6%) and it is \nsuggested that a critical period when maturation is influenced may occur during a \nspecific, short period in early development. Throughout the experiments, mature \nindividuals maintained the same size as their immature siblings. The 12 week June \nphotoperiod appeared to result in the highest level of smoltification, although those \nexposed to the 12 week May photoperiod showed the greatest seawater survival. \nIn both photoperiod experiments, fish showing some signs of smoltification were also \nfound to be undergoing gonadal development, indicating that maturation and \nsmoltification are not completely mutually exclusive processes. \nPossible nutritional effects were considered using different dietary lipid inclusions \n(either 12.5% or 25%) and variable rations of feed (either full, 2/3 or 113 rations). \nDifferent dietary lipid inclusions had no effect on growth, although the whole body fat \ncontent of individuals was affected, with a switch in dietary fat content during \ndevelopment resulting in a rapid change in body composition. Fish size increased with \nration and, although at the lowest ration of feed whole body fat levels were reduced, \nthey were maintained at a set level under the high and intennediate rations, implying a \nlipostatic control of growth. Maturation levels were low throughout the nutrition \nexperiments, suggesting that genetic influences may have been important. Dietary \nlipid level had a negligible effect on smoltification, although increases in ration \nresulted in a greater incidence of smoltification. Using a 0+ photoperiod regime (i.e. \nLD24:0 applied from March until December, with the exception of an 8 week period \nof LD17:7 applied from August), smolting individuals showed a reduction in smolt \nstatus when compared to those developed under a natural photoperiod. It is suggested \nthat such regimes restrict the mobilisation of long-tenn energy stores, with the \nsubsequent development of seawater tolerance affected. However, it was noted that \nthe 0+ regime had increased the incidence of smolts. \nIn summary, it has been shown that environmental factors such as photoperiod, \nnutrition and temperature can play an important role in the developmental strategies \ntaken by juvenile Atlantic salmon. Such factors are likely to greatly influence the \nattainment of size and/or nutritional thresholds necessary for various developmental \nstrategies, in particular if such thresholds occur during seasonally-sensitive "critical" \nperiods when development can be influenced. Furthermore, the life history strategy \nundertaken by an individual may be affected by endogenous rhythms, cued by \nseasonally-changing environmental factors. However, there are clear indications that \nthe underlying genetic control of maturation may also be of importance.