Should I stay or should I go: developmental, physiological, behavioural and morphological differences between offspring from alternative life histories

In many taxa there is considerable intraspecific variation in life-history strategies from within a single population, reflecting alternative routes through which organisms can achieve successful reproduction. Brown trout (Salmo trutta) and Atlantic salmon (Salmo salar) show some of the greatest within-population variability in life history strategies amongst vertebrates, with some individuals of a population migrating while other individuals remain resident (the phenomenon of ‘partial migration’). Furthermore within each migratory ecotype, multiple discrete male and female life histories (e.g. time spent in freshwater and saltwater) co-exist and interbreed on many spawning grounds, although the effect of the various combinations of life histories on offspring traits remains unknown. These issues are covered in the General Introduction (chapter 1), which introduces the rationale for the experimental work which forms the core of this thesis. In the first of these empirical investigations, full-sibling groups of offspring from freshwater-resident and anadromous brown trout and half-sibling groups of Atlantic salmon were reared in the laboratory under common garden conditions to examine potential differences in their early development, physiology, behaviour and phenotypic flexibility. I found that freshwater-resident parents produced eggs that were slower to hatch than did anadromous parents, but their offspring were quicker to absorb their yolk and reach the stage of exogenous feeding. Their offspring also had a higher conversion efficiency from the egg stage to the start of exogenous feeding (so were larger by the start of the fry stage) than did offspring from anadromous parents (chapter 2). Using Atlantic salmon I showed that mothers that had spent longer at sea (two versus one year) produced offspring which were heavier, longer and in better condition at the time of first feeding. However, these relationships disappeared shortly after fry had begun feeding exogenously. At this stage, the juvenile rearing environment (i.e. time spent in fresh water) of the mother was a better predictor of offspring traits, with mothers that were faster to develop in fresh water (migrating to sea after two rather than three years of age) producing offspring that had higher maximal metabolic rates and aerobic scopes, and that grew faster. Faster developing fathers tended to produce offspring that grew faster, were in better body condition and had higher maximal metabolic rates (chapter 3). I also found that the parental type in brown trout had a significant effect on dominance status in territorial interactions, with offspring of anadromous fish being dominant over size-matched offspring of freshwater-residents, but only when both had been raised under intermediate levels of food availability (chapter 4). Lastly I found that offspring of anadromous trout were less able to maintain their growth rate when reared on a Daphnia diet than were those of freshwater-residents, and showed a correspondingly greater increase in growth following a switch in diet from Daphnia to Chironomid larvae. Offspring of anadromous parents also showed less morphological flexibility in response to diet than did the offspring of freshwater-residents (chapter 5). The results of these studies are summarised in the General Discussion (chapter 6), which suggests that parental effects (either direct or environmentally-mediated through differences in rearing environments) may be an important mechanism underlying the expression of threshold traits and play a significant role in the perpetuation of non-breeding partial migration and life history strategies within populations