And the Last Shall Be First: Heterochrony and Compensatory Marine Growth in Sea Trout (Salmo trutta)

Abstract

Early juvenile growth is a good indicator of growth later in life in many species because larger than average juveniles tend to have a competitive advantage. However, for migratory species the relationship between juvenile and adult growth remains obscure. We used scale analysis to reconstruct growth trajectories of migratory sea trout (Salmo trutta) from six neighbouring populations, and compared the size individuals attained in freshwater (before migration) with their subsequent growth at sea (after migration). We also calculated the coefficient of variation (CV) to examine how much body size varied across populations and life stages. Specifically, we tested the hypothesis that the CV on body size would differ between freshwater and marine environment, perhaps reflecting different trade-offs during ontogeny. Neighbouring sea trout populations differed significantly in time spent at sea and in age-adjusted size of returning adults, but not on size of seaward migration, which was surprisingly uniform and may be indicative of strong selection pressures. The CV on body size decreased significantly over time and was highest during the first 8 months of life (when juvenile mortality is highest) and lowest during the marine phase. Size attained in freshwater was negatively related to growth during the first marine growing season, suggesting the existence of compensatory growth, whereby individuals that grow poorly in freshwater are able to catch up later at sea. Analysis of 61 datasets indicates that negative or no associations between pre- and post-migratory growth are common amongst migratory salmonids. We suggest that despite a widespread selective advantage of large body size in freshwater, freshwater growth is a poor predictor of final body size amongst migratory fish because selection may favour growth heterochrony during transitions to a novel environment, and marine compensatory growth may negate any initial size advantage acquired in freshwater.

Mixed-Effects Modelling of Scale Growth Profiles Predicts the Occurrence of Early and Late Fish Migrants

“…To ascertain the precision of the scale analysis, we estimated the repeatability of the point of entry into the sea and of the end of the first freshwater growing season by measuring the scales of 30 individuals twice in a double blind fashion and calculating the intra-class correlation coefficient (α-Cronbach) as per [21]. The Pearson correlation coefficient was used to evaluate the strength of the association between scale radius and body size of fish in each river.…”

“…The Bayesian Information Criteria (BIC) was used for model selection of fixed effects, mixed effects and autocorrelation structure of errors terms [48]. We employed the coefficient of variation (CV) to quantify individual variation in inter-circuli spacing at each circulus [21].…”

“…Scales were collected from anaesthetized fish (clove oil) using a small fish knife and according to current Spanish Regulations, as described in a previous study [21]. After the fish had fully recovered from the anaesthesia (approx.…”

“…Anadromous salmonids are good models to examine the fitness consequences of individual variation in growth because resident and migratory individuals commonly coexist and alternative reproductive tactics are often size-dependent [18], [19] reflecting considerable phenotypic plasticity [20], [21]. Thus, the choice to remain in freshwater or to migrate to sea is often determined by size and/or growth thresholds [22] and these may in turn depend on metabolic efficiency [23].…”

Environmental influences on life history strategies in partially anadromous brown trout (Salmo trutta, Salmonidae)

“…Sea trout smolts have very low energy density at the time of migration (ca. 1.5 g/100 g wet mass), which may contribute to the compensatory growth exhibited by sea trout during the first weeks at sea (Marco-Rius, Caballero, Moran, & Leaniz, 2012). This is due to a low lipid density (ca.…”