Laboratory studies on associations between disease resistance and susceptibility and major histocompatibility (MH) genes in Atlantic salmon Salmo salar have shown the importance of immunogenetics in understanding the capacity of populations to fight specific diseases. However, the occurrence and virulence of pathogens may vary spatially and temporally in the wild, making it more complicated to predict the overall effect that MH genes exert on fitness of natural populations and over several life-history stages. Here we show that MH variability is a significant determinant of salmon survival in fresh water, by comparing observed and expected genotype frequencies at MH and control microsatellite loci at parr and migrant stages in the wild. We found that additive allelic effects at immunogenetic loci were more likely to determine survival than dominance deviation, and that selection on certain MH alleles varied with life stage, possibly owing to varying pathogen prevalence and/or virulence over time. Our results highlight the importance of preserving genetic diversity (particularly at MH loci) in wild populations, so that they have the best chance of adapting to new and increased disease challenges as a result of projected climate warming and increasing aquaculture.
Understanding the extent, scale and genetic basis of local adaptation (LA) is important for conservation and management. Its relevance in salmonids at microgeographic scales, where dispersal (and hence potential gene flow) can be substantial, has however been questioned. Here, we compare the fitness of communally reared offspring of local and foreign Atlantic salmon Salmo salar from adjacent Irish rivers and reciprocal F1 hybrid crosses between them, in the wild ‘home’ environment of the local population. Experimental groups did not differ in wild smolt output but a catastrophic flood event may have limited our ability to detect freshwater performance differences, which were evident in a previous study. Foreign parr exhibited higher, and hybrids intermediate, emigration rates from the natal stream relative to local parr, consistent with genetically based behavioural differences. Adult return rates were lower for the foreign compared to the local group. Overall lifetime success of foreigners and hybrids relative to locals was estimated at 31% and 40% (mean of both hybrid groups), respectively. The results imply a genetic basis to fitness differences among populations separated by only 50 km, driven largely by variation in smolt to adult return rates. Hence even if supplementary stocking programs obtain broodstock from neighbouring rivers, the risk of extrinsic outbreeding depression may be high.
Environmental and habitat change can have profound and complex impacts on fish. We examined an unexploited population of European eel (Anguilla anguilla) from a West of Ireland catchment. The population is long-lived and slow-growing compared to many other European eel populations. Von Bertalanffy growth curves showed decadal changes, with a trend towards larger K, and t0 values in both males and females and a smaller L∞ in females. A growth biochronology spanning seven decades (1950s–2010s) was constructed using otolith annual increment measurements. We found evidence of high variability in growth over the course of the time series. A decrease in growth occurred after the early 2000s, potentially driven by habitat and climatic changes. Growth was negatively correlated with early spring and winter temperatures, providing strong evidence that the length of the growing season impacts eel growth. Growth was also positively correlated with summer temperatures and the number of days that exceeded 16˚C (GSL16˚C). The response to temperature was age-dependent; at age one the positive relationship with GSL16˚C was most pronounced and the negative relationship with winter temperatures was not evident. This study demonstrates the impact of climate change and highlights the complexities of eel growth strategies in a changing environment.
Bacteria from the Vibrionaceae family have been implicated in mass mortalities of farmed Pacific oysters (Crassostrea gigas) in multiple countries, leading to substantial impairment for growth in the sector. In Ireland there has been concern thatVibriohave been involved in serious summer outbreaks. There is evidence thatVibrio aestuarianusis increasingly becoming the main pathogen of concern for the Pacific Oyster industry in Ireland. While bacteria belonging to theVibrio splendidusclade are also detected frequently in mortality episodes, their role in the outbreaks of summer mortality are not well understood. To identify and characterise strains involved in these outbreaks, 43Vibrioisolates were recovered from Pacific oyster summer mass mortality episodes in Ireland from 2008-2015 and these were whole genome sequenced. Among these, 25 were found to beV. aestuarianus(implicated in disease) and 18V. splendidus sensu lato(role in disease undetermined). Two distinct clades ofV. aestuarianus— Clade A and Clade B — were found that had previously been described as circulating within French oyster culture. The high degree of similarity between the Irish and FrenchV. aestuarianusisolates points to translocation of the pathogen between Europe's two major oyster producing countries, probably via trade in spat and other age classes.V. splendidusisolates were more diverse, but the data reveal a single clone of this species that has spread across oyster farms in Ireland. This underscores thatVibriocould be transmitted readily across oyster farms. The presence ofV. aestuarianusClades A and B in not only France but also Ireland adds weight to the growing concern that this pathogen is spreading and impacting Pacific oyster production within Europe.
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