Daniel J. Hasselman

Human disturbance causes the formation of a hybrid swarm between two naturally sympatric fish species

Most evidence for hybrid swarm formation stemming from anthropogenic habitat disturbance comes from the breakdown of reproductive isolation between incipient species, or introgression between allopatric species following secondary contact. Human impacts on hybridization between divergent species that naturally occur in sympatry have received considerably less attention. Theory predicts that reinforcement should act to preserve reproductive isolation under such circumstances, potentially making reproductive barriers resistant to human habitat alteration. Using 15 microsatellites, we examined hybridization between sympatric populations of alewife (Alosa pseudoharengus) and blueback herring (A. aestivalis) to test whether the frequency of hybridization and pattern of introgression have been impacted by the construction of a dam that isolated formerly anadromous populations of both species in a landlocked freshwater reservoir. The frequency of hybridization and pattern of introgression differed markedly between anadromous and landlocked populations. The rangewide frequency of hybridization among anadromous populations was generally 0–8%, whereas all landlocked individuals were hybrids. Although neutral introgression was observed among anadromous hybrids, directional introgression leading to increased prevalence of alewife genotypes was detected among landlocked hybrids. We demonstrate that habitat alteration can lead to hybrid swarm formation between divergent species that naturally occur sympatrically, and provide empirical evidence that reinforcement does not always sustain reproductive isolation under such circumstances.

Combining genetic and demographic information to prioritize conservation efforts for anadromous alewife and blueback herring

A major challenge in conservation biology is the need to broadly prioritize conservation efforts when demographic data are limited. One method to address this challenge is to use population genetic data to define groups of populations linked by migration and then use demographic information from monitored populations to draw inferences about the status of unmonitored populations within those groups. We applied this method to anadromous alewife (Alosa pseudoharengus) and blueback herring (Alosa aestivalis), species for which long‐term demographic data are limited. Recent decades have seen dramatic declines in these species, which are an important ecological component of coastal ecosystems and once represented an important fishery resource. Results show that most populations comprise genetically distinguishable units, which are nested geographically within genetically distinct clusters or stocks. We identified three distinct stocks in alewife and four stocks in blueback herring. Analysis of available time series data for spawning adult abundance and body size indicate declines across the US ranges of both species, with the most severe declines having occurred for populations belonging to the Southern New England and the Mid‐Atlantic Stocks. While all alewife and blueback herring populations deserve conservation attention, those belonging to these genetic stocks warrant the highest conservation prioritization.

The Rapid Establishment, Dispersal, and Increased Abundance of Invasive American Shad in the Pacific Northwest

Abstract American shad (Alosa sapidissima) were repeatedly introduced into two Pacific coastal rivers in the late 1800s. They rapidly established, dispersed, and expanded their range over 5,000 km of coastline from Mexico to Russia. American shad also increased dramatically in abundance and now comprise the largest spawning run of anadromous fish in the Columbia River (>4 million fish annually). Despite constituting a remarkable biological invasion, invasive shad have not been the subject of much scientific investigation and remain relatively obscure in the Pacific Northwest. In this first of two complementary articles, we attempt to raise the profile of invasive American shad among fisheries professionals and generate scientific interest in this biological invasion. We provide an overview of the history of their introduction and discuss the mechanisms that may have contributed to the species ‘establishment, rapid dispersal along the Pacific coast, and its increased abundance in the Columbia River. In the...

American Shad of the Pacific Coast: A Harmful Invasive Species or Benign Introduction?

Abstract Research on the ecological effects of aquatic invasive species in North America has largely focused on the Laurentian Great Lakes, with less attention directed to their influence in coastal ecosystems. American shad (Alosa sapidissima) were introduced to the Pacific coast in the late 1800s, rapidly established, dispersed, and became prolific. Despite constituting a remarkable biological invasion, shad in their introduced range have not been the subject of much scientific investigation. The species persists in relative obscurity in the Pacific Northwest and remains ecological overlooked and evolutionarily underappreciated. In this second of two complementary articles, we consider the potential ecological effects of invasive American shad on Pacific coastal ecosystems and the possible consequences for the persistence of native salmonids. We also reflect on the intrinsic value that this invasion provides to studies of ecology and evolution and highlight several areas of research that require immedia...

Genomic patterns of diversity and divergence of two introduced salmonid species in Patagonia, South America

Invasive species have become widespread in aquatic environments throughout the world, yet there are few studies that have examined genomic variation of multiple introduced species in newly colonized environments. In this study, we contrast genomic variation in two salmonid species (anadromous Chinook Salmon, Oncorhynchus tshawytscha, 11,579 SNPs and resident Brook Charr Salvelinus fontinalis, 13,522 SNPs) with differing invasion success after introduction to new environments in South America relative to populations from their native range in North America. Estimates of genetic diversity were not significantly different between introduced and source populations for either species, indicative of propagule pressure that has been shown to maintain diversity in founding populations relative to their native range. Introduced populations also demonstrated higher connectivity and gene flow than those in their native range. Evidence for candidate loci under divergent selection was observed, but was limited to specific introduced populations and was not widely evident. Patterns of genomic variation were consistent with general dispersal potential of each species and therefore also the notion that life history variation may contribute to both invasion success and subsequent genetic structure of these two salmonids in Patagonia.

The Role of Impoundments, Temperature, and Discharge on Colonization of the Columbia River Basin, USA, by Nonindigenous American Shad

Abstract Ecologists have become increasingly aware of the combined effects of habitat disturbance and climate change on the establishment and proliferation of invasive species. Long-term data on the population of the invasive American Shad Alosa sapidissima in the U.S. portion of the Columbia River basin provide an opportunity to examine how habitat disturbances affect the abundance and spatial distribution of an invasive species in a heavily modified environment. After the establishment of American Shad in the Columbia River in the late 1800s, the drainage was transformed from its natural lotic state to a series of reservoirs, with concomitant changes to discharge and temperature regimes, which are confounded by climate change. As the Columbia River was dammed, American Shad extended its range and increased in abundance. A large and rapid increase in spawning population abundance (recruits per spawner = 63) followed completion of The Dalles Dam in 1957, which inundated Celilo Falls, a natural barrier to ...

Discovery and characterization of single nucleotide polymorphisms in two anadromous alosine fishes of conservation concern

Abstract Freshwater habitat alteration and marine fisheries can affect anadromous fish species, and populations fluctuating in size elicit conservation concern and coordinated management. We describe the development and characterization of two sets of 96 single nucleotide polymorphism (SNP) assays for two species of anadromous alosine fishes, alewife and blueback herring (collectively known as river herring), that are native to the Atlantic coast of North America. We used data from high‐throughput DNA sequencing to discover SNPs and then developed molecular genetic assays for genotyping sets of 96 individual loci in each species. The two sets of assays were validated with multiple populations that encompass both the geographic range and the known regional genetic stocks of both species. The SNP panels developed herein accurately resolved the genetic stock structure for alewife and blueback herring that was previously identified using microsatellites and assigned individuals to regional stock of origin with high accuracy. These genetic markers, which generate data that are easily shared and combined, will greatly facilitate ongoing conservation and management of river herring including genetic assignment of marine caught individuals to stock of origin.

Formation of population genetic structure following the introduction and establishment of non-native American shad (Alosa sapidissima) along the Pacific Coast of North America

Biological invasions provide opportunities to examine contemporary evolutionary processes in novel environments. American shad, an anadromous fish native to the Atlantic Coast of North America, was introduced to California in 1871 and established spawning populations along the Pacific Coast that may provide insights into the dynamics of dispersal, colonization, and the establishment of philopatry. Using 13 neutral microsatellite loci we genotyped anadromous, freshwater resident and landlocked American shad from 14 locations along the US Pacific Coast to resolve population genetic structure. We observed significant differences in multilocus allele frequency distributions in nearly all (61/66; 92%) pairwise comparisons of non-native anadromous, freshwater resident and landlocked populations, and detected significant genetic differentiation for most (55/66; 83%) of these comparisons. Genetic divergence between landlocked and anadromous populations is due to genetic drift in isolation because of a physical migration barrier. However, some reproductive isolating mechanism maintains genetic differentiation between sympatric populations in the Columbia River exhibiting alternative life history strategies (i.e. anadromous vs. ‘freshwater-type’). Non-native populations possessed genetic variants that were not observed in the species’ native range and were strongly differentiated from Atlantic Coast populations (

Discrimination of the Endangered Atlantic Whitefish from Lake Whitefish and Round Whitefish by Use of External Characters

{\text{G}}^{{\prime }}_{\text{ST}}