Author: hilduryr@matis.is

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Davíð Gíslason

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davidg@matis.is

Resource polymorphism has been proposed as an important phase of diversification and speciation in vertebrates. Studies of fish in young lakes of the Northern Hemisphere indicate variably advanced cases of adaptive trophic diversification. We have previously proposed a scheme describing this variation in terms of a gradient of resource-based polymorphic traits, emphasizing flexible behavior in early phases and morphological divergence in more advanced phases. Here, we present data on Arctic char (Salvelinus alpinus) in Icelandic lakes exhibiting a variable degree of phenotypic and genotypic segregation. We show that (i) the morphs are at different levels of phenotypic segregation and reproductive isolation and in one case completely reproductively isolated, (ii) morphs within lakes appear to be of intralacustrine origin, and (iii) the morphological and genetic divergence of morphs is correlated, suggesting a role for trophic adaptation as a driving force in morph segregation.

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Davíð Gíslason

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davidg@matis.is

The biology of Brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) was studied in four Faroese lakes in late July – early August 2000. Relative density and condition of Brown trout were lowest in Leynavatn and highest in Saksunarvatn. Arctic charr only occurred in Leynavatn, where it outnumbered brown trout by a factor of 2.7. For all fish populations, growth rates were slow, fish were small and maximum age of fish was low. The oldest fish in Toftavatn was 5+yr and the oldest fish in the study, in Saksunarvatn, was 9+yr. Asymptotic length was approx. 21 cm for arctic charr in Leynavatn and approx. 25 cm for Brown trout in both Leynavatn and Toftavatn, but for Brown trout in Eystara Mjáavatn and Saksunarvatn, no asymptote was observed. Small but significant differences in morphology were observed among brown trout populations with corresponding differences in diet. Brown trout in Toftavatn had a more forked caudal fin and the diet was more pelagic/epibenthic than in the other lakes. In Leynavatn, Brown trout fed more on benthic diet and had a more curved snout than Brown trout in the other lakes, which may be signs of character displacement resulting from interspecific competition with Arctic charr. The Arctic charr in Leynavatn did not show signs of morphological or ecological polymorphism and their diet was both of benthic and planktonic origin. Genetic analyzes of Arctic charr showed minor variation at most loci, high homozygosity and genotypic disequilibrium between three loci, indicating extensive inbreeding and random genetic drift in a small, isolated population. It is suggested that the restriction of Arctic charr to one natural lake in the Faroe Islands is primarily due to unfavorable water temperatures. Evidence suggests that the slow growth, small size and short lifespan of Arctic charr and Brown trout in Leynavatn may be due to interspecific competition between the two species along with relatively poor food conditions in the lake.

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Davíð Gíslason

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davidg@matis.is

To examine the population genetic structure of lake-resident Arctic charr, Salvelinus alpinus from northwest Europe on multiple spatial scales, 2367 individuals from 43 lakes located in three geographical regions (Iceland, the British Isles and Scandinavia) were genotyped at six microsatellite loci. On a large scale, data provided little evidence to support clustering of populations according to geographical region. Hierarchical analysis of molecular variance indicated that, although statistically significant, only 2.17% of the variance in allelic frequencies was partitioned at the among-region level. Within regions, high levels of genetic differentiation were typically found between lakes regardless of the geographical distance separating them. These results are consistent with the hypothesis of rapid postglacial recolonization of all of northwest Europe from a single charr lineage, with subsequent restriction of gene flow. On a smaller scale, there was evidence for close genetic relationships among lakes from within common drainage basins in Scotland. Thus, interlake genetic structure reflects localized patterns of recent (or contemporary) gene flow superimposed onto a larger scale structure that is largely a result of historical processes. There was also evidence for widespread genetic structuring at the within-lake level, with sympatric populations detected in 10 lakes, and multilocus heterozygote deficits found in 23 lakes. This evidence of the Wahlund effect was found in all lakes known to contain discrete phenotypic morphs, as well as many others, suggesting that morphs may often represent separate breeding populations, and also that the phenomenon of polymorphism in this species may be more widespread than is currently realized.

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Davíð Gíslason

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davidg@matis.is

Harvest can change phenotypic traits of populations through immediate demographic consequences, evolutionary responses to harvest selection, or developmental responses by individuals. This study investigated the plastic phenotypic effects of harvest on size and age at maturity in a commercially exploited freshwater fish. We tested an individual growth and life history plasticity model using lagged correlations incorporating how harvesting of ages 2 and older fish influenced the abundance of juvenile fish, resource availability, individual growth rates, and carry-over responses in age and size at maturity. Our test used cohort data for Lake Erie yellow perch (Perca flavescens). Age and size at maturity fluctuated widely and rapidly across 23 cohorts between 1991 and 2013, suggesting phenotypic plasticity contributed strongly to maturation dynamics. The changes in maturity could not be explained by responses to harvest, as expected under the plasticity model. In Lake Erie, age and size at maturity in yellow perch appear to be responding to other drivers, such as harvest-induced dynamics of other fish stocks or ecosystem changes that are independent of harvest.

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Davíð Gíslason

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davidg@matis.is

Overexploitation and collapse of major fisheries raises important concerns about the effects of harvest on fish populations. We tested for ecological and evolutionary mechanisms by which harvest could affect exploited fish populations in Lake Erie over the last four decades, over most of which intensive fisheries management was implemented. We did not detect evidence of long-term negative effects of harvest on yellow perch (Perca flavescens), walleye (Sander vitreous), white perch (Morone americana), or white bass (Morone chrysops) populations, either through recruitment success or through alteration of maturation schedules. Current fisheries management in Lake Erie has been relatively successful with respect to minimizing negative harvest effects, such that the dynamics of exploited fish populations in Lake Erie were more strongly affected by environment than harvest. Our study adds to the evidence that effective fisheries management is capable of rebuilding depleted fisheries and (or) maintaining healthy fisheries. Nevertheless, fisheries management needs to move beyond the ecological dimension to incorporate economic, social, and institutional aspects for society to be better assured of the sustainability of fisheries in rapidly changing ecosystems.

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Davíð Gíslason

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davidg@matis.is

Harvested marine fish stocks often show a rapid and substantial decline in age and size at maturity. Such changes can arise from multiple processes including fisheries‐induced evolution, phenotypic plasticity, and responses to environmental factors other than harvest. The relative importance of these processes could differ systematically between marine and freshwater systems. We tested for temporal shifts in the mean and within‐cohort variability of age‐ and size‐based maturation probabilities of female yellow perch (Perca flavescens Mitchill) from four management units (MUs) in Lake Erie. Lake Erie yellow perch have been commercially harvested for more than a century and age and size at maturation have varied since sampling began in the 1980s. Our analysis compared probabilistic maturation reaction norms (PMRNs) for cohorts when abundance was lower and harvest higher (1993–1998) to cohorts when abundance was higher and harvest lower (2005–2010). PMRNs have been used in previous studies to detect signs of evolutionary change in response to harvest. Maturation size threshold increased between the early and late cohorts and the increases were statistically significant for the youngest age in the western MU1 and for older ages in the eastern MU3. Maturation envelope widths, a measure of the variability in maturation among individuals in a cohort, also increased between early and late cohorts in the western MUs where harvest was highest. The highest rates of change in size at maturation for a given age were as large or larger than rates reported for harvested marine fish where declines in age and size at maturation have been observed. Contrary to the general observation of earlier maturation evolving in harvested stocks, female yellow perch in Lake Erie may be rapidly evolving delayed maturation since harvest was relaxed in the late 1990s, providing a rare example of possible evolutionary recovery. This article is protected by copyright. All rights reserved.

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Davíð Gíslason

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davidg@matis.is

Biodiversity in the oceans has dramatically declined since the beginning of the industrial era, with accelerated loss of marine biodiversity impairing the ocean's capacity to maintain vital ecosystem services. A few organisms epitomize the damaging and long-lasting effects of anthropogenic exploitation: Some whale species, for instance, were brought to the brink of extinction, with their population sizes reduced to such low levels that may have caused a significant disruption to their reproductive dynamics and facilitated hybridization events. The incidence of hybridization is nevertheless believed to be rare, and very little information exists on its directionality. Here, using genetic markers, we show that all but one whale hybrid sample collected in Icelandic waters originated from the successful mating of male fin whale and female blue whale, thus suggesting unidirectional hybridization. We also demonstrate for the first time the existence of a second-generation adult (male) hybrid resulting from a backcross between a female hybrid and a pure male fin whale. The incidence of hybridization events between fin and blue whales is likely underestimated and the observed unidirectional hybridization (for F1 and F2 hybrids) is likely to induce a reproductive loss in blue whales, which may represent an additional challenge to its recovery in the Atlantic Ocean compared to other rorquals.

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Davíð Gíslason

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davidg@matis.is

Decreases in size at maturation in harvested fish populations can reduce productivity and resilience. Delineating the causes for these changes in maturation is challenging. We assessed harvest and large-scale ecosystem variability as causes for changes in maturation in four Lake Erie fishes. Regulated harvests of yellow perch (Perca flavescens) and walleye (Sander vitreous) are greater than unregulated harvests of white perch (Morone americana) and white bass (Morone chrysops). Our assessment considered cohort data from 1991 to 2012 for each species. We used a conceptual model of harvest-induced plasticity to show that changes in female length at 50% maturity (L ₅₀) were unrelated to harvest intensity in all species. We then demonstrated that changes in females L ₅₀ among cohorts were synchronous across species. Post hoc analysis of variables capturing year-to-year variation in climatic and lake conditions suggested L ₅₀ was larger when water levels were near the norm for the study period and smaller at low and high levels. We conclude that changes in L ₅₀ were most strongly related to ecosystem changes unrelated to harvest intensity.

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Davíð Gíslason

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davidg@matis.is

Understanding the genetic differentiation among populations of most marine fish requires investigating the differences among spawning grounds. However, this can be challenging as spawning grounds for some species are not well known, or spawning fish are difficult to collect. An alternative is to collect juvenile fish in nursery habitats closely associated with potential spawning grounds. Greenland halibut is a deep-dwelling, commercially important species with at least two identified major offshore spawning grounds in the North Atlantic and weak genetic differentiation across the Atlantic. In this study, we sampled juveniles from three sites representing the Davis Strait spawning area in the northwest Atlantic and one site in the northeast Atlantic representing the primary spawning area along the western slope of the Barents Sea. We applied genotype by sequencing and discovered 90 genetic markers that could be used to assess genetic differentiation among the four sites. The northeast and northwest Atlantic showed major genetic differentiation, supporting the existence of the two primary spawning clusters. Additionally, we found genetic differentiation between the three northwest Atlantic samples implying the existence of more than one spawning area in the northwest.

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