Water velocity shapes juvenile salmonids

Phenotypic plasticity in morphology is often considered adaptive. Stream-living fish encounter considerable spatial and temporal environmental variation in their native habitats, and the ability to adapt to this variation is of utmost importance. We studied experimentally whether water velocity affects the body shape of juvenile Atlantic salmon (Salmo salar m. sebago Girard) and brown trout (Salmo trutta m. lacustris L.). The fish were reared in slow and fast water flow, and their morphology was studied by measuring a number of morphometric characters. We studied which characters differed between the environments in each species, and found that water velocity caused morphological differentiation in both salmon and brown trout. The differences occurred especially in body height as well as in fin sizes, characters that are very likely to be of functional importance for life in the stream environment. Salmon in fast flow became more robust, whereas brown trout in fast flow became slightly more streamlined. The observed variation in body morphology of salmon and brown trout indicates phenotypic plasticity, but the species differed in their response to environmental variation, which may be due to different energetics and cost reduction strategies. Morphological differentiation caused by water flow occurred very rapidly, within 1-month exposure to the different water flows. This revised version was published online in November 2006 with corrections to the Cover Date.     

Segregation of infectious pancreatic necrosis resistance QTL in the early life cycle of Atlantic Salmon (Salmo salar)

In a previous study, three significant quantitative trait loci (QTL) associated with resistance to Infectious Pancreatic Necrosis (IPN) disease were identified by analysing challenge data from one sub-population of Landcatch Atlantic salmon (Salmo salar) smolt. While these QTL were shown to affect the resistance in seawater, their effect in freshwater was unknown. This study investigates the effect of these QTL on IPN resistance in salmon fry in freshwater. Twenty families with intermediate levels of IPN mortality were analysed from a freshwater challenge trial undertaken on a different sup-population of LNS salmon to that studied previously. Only the QTL from linkage group 21 (LG21) appeared to have a significant and large effect on resistance in freshwater; the same QTL was found to have the largest effect in seawater in the previous study. Variance component analysis showed a high heritability for the QTL: 0.45 ± 0.07 on the liability scale and 0.25 ± 0.05 on the observed scale. In a family where both parents were segregating for the QTL, there was a 0% vs. 100% mortality in homozygous offspring for resistant and susceptible QTL alleles. The finding that the same QTL has major effect in both freshwater and seawater has important practical implications, as this will allow the improvement of resistance in both phases through marker assisted selection by targeting this QTL. Moreover, the segregation of the LG21 QTL in a different sub-population gives further evidence of its association with IPN-resistance.     

Allozyme variation in Atlantic salmon from the British Isles: associations with geography and the environment

Data on geographical variation in allele frequencies at enzyme coding loci in Atlantic salmon from the British Isles were collated from published and unpublished sources. Statistically significant differences in allele frequencies were found among samples both within and among river systems, suggesting that the Atlantic salmon in the British Isles is not a panmictic population and that even within major river systems it cannot be treated as a single genetic stock for fisheries management purposes. Although there was some evidence of regional differences in the frequency of some rare alleles, most single‐locus variation did not show strong geographic patterns, with the exception of the AAT‐4 * locus at which allele frequencies had a significant latitudinal cline. There was some evidence for the existence of genetically‐distinct celtic and boreal races of Atlantic salmon in the British Isles as previously has been suggested. Multiple regression analyses revealed associations between genetic variation and local environmental conditions ( i.e . between variation at MEP‐2 * and both temperature and local river gradient), providing additional evidence for adaptive population divergence in the species.     

Detection of QTL affecting harvest traits in a commercial Atlantic salmon population

Genetic variation in performance and quality traits measured at harvest has previously been demonstrated in Atlantic salmon aquaculture populations. To map major loci underlying this variation, we utilized data from 10 families from a commercial breeding programme. Significant QTL were detected affecting harvest weight and length traits on linkage group 1, and affecting waste weight on linkage group 5. In total, 11 of the 29 linkage groups examined showed at least suggestive evidence for a QTL. These data suggest that major loci affecting economically important harvest characteristics are segregating in commercial salmon populations.     

Does river of origin influence the early marine migratory performance of Salmo salar?

The early marine migratory behaviour of two populations of hatchery-reared Atlantic salmon Salmo salar was compared in a common-garden experiment. Post-smolts from a river in a long fjord (Laerdal River, 144 km from the open coastline, n = 79) and a short fjord (Flekke River, 20 km from the open coastline, n = 80) in western Norway were tagged with acoustic transmitters and released during the spring of 2005 and 2006 in the inner part of the Hardangerfjord system (Opo River mouth, 179 km from the open coastline). The migratory behaviour of the tagged fish was monitored by acoustic listening stations in the fjord system up to 167 km from the release site. The Laerdal fish began migrating before the Flekke fish and had higher progression rates in the middle part of the fjord system. A greater number of Laerdal fish was detected along the most direct migratory route and in the outermost part of the Hardangerfjord system, which is indicative of a higher survival. The results from this study demonstrate differences in early marine migratory behaviour between S. salar from two different stocks and suggest that the distance a S. salar population travels to reach the open coastline may influence its early marine migratory behaviour and performance. The selective pressures of marine predation and arrival time at feeding areas in the ocean may be stronger for stocks with a longer inshore migration, creating more efficient migrants over time.     

Patterns of variability at the major histocompatibility class II alpha locus in Atlantic salmon contrast with those at the class I locus

In order to investigate the mechanisms creating and maintaining variability at the major histocompatibility (MH) class II alpha (DAA) locus we examined patterns of polymorphism in two isolated Atlantic salmon populations which share a common post-glacial origin. As expected from their common origin, but contrary to the observation at the MH class I locus, these populations shared the majority of DAA alleles: out of 17 sequences observed, 11 were common to both populations. Recombination seems to play a more important role in the origin of new alleles at the class II alpha locus than at the class I locus. A greater than expected proportion of sites inferred to be positively selected (potentially peptide binding residues, PBRs) were found to be involved in recombination events, suggesting a mechanism for increasing MH variability through an interaction between recombination and natural selection. Thus it appears that although selection and recombination are important mechanisms for the evolution of both class II alpha and class I loci in the Atlantic salmon, the pattern of variability differs markedly between these classes of MH loci.     

Residence time of the sea louse, Lepeophtheirus salmonis K., on Atlantic salmon, Salmo salar L., after immersion in fresh water

Returning adult salmon caught at the mouth of the River Dee, Aberdeenshire, were transferred to tanks in the laboratory. For fish placed in fresh water, sea lice remained attached for up to 6 days, though most lice were lost in the first 48 hours. Few lice were lost from salmon maintained in sea water. The experiments were conducted in water within a temperature range of 12·8 to 16° C, equivalent to summer river temperatures in the Aberdeenshire Dee.     

COMPARING STRENGTHS OF DIRECTIONAL SELECTION: HOW STRONG IS STRONG?

The fundamental equation in evolutionary quantitative genetics, the Lande equation, describes the response to directional selection as a product of the additive genetic variance and the selection gradient of trait value on relative fitness. Comparisons of both genetic variances and selection gradients across traits or populations require standardization, as both are scale dependent. The Lande equation can be standardized in two ways. Standardizing by the variance of the selected trait yields the response in units of standard deviation as the product of the heritability and the variance-standardized selection gradient. This standardization conflates selection and variation because the phenotypic variance is a function of the genetic variance. Alternatively, one can standardize the Lande equation using the trait mean, yielding the proportional response to selection as the product of the squared coefficient of additive genetic variance and the mean-standardized selection gradient. Mean-standardized selection gradients are particularly useful for summarizing the strength of selection because the mean-standardized gradient for fitness itself is one, a convenient benchmark for strong selection. We review published estimates of directional selection in natural populations using mean-standardized selection gradients. Only 38 published studies provided all the necessary information for calculation of mean-standardized gradients. The median absolute value of multivariate mean-standardized gradients shows that selection is on average 54% as strong as selection on fitness. Correcting for the upward bias introduced by taking absolute values lowers the median to 31%, still very strong selection. Such large estimates clearly cannot be representative of selection on all traits. Some possible sources of overestimation of the strength of selection include confounding environmental and genotypic effects on fitness, the use of fitness components as proxies for fitness, and biases in publication or choice of traits to study.