Liam J. Zarri

Species invasions spur costly and labor-intensive control efforts, yet even local eradication is seldom achieved. When control measures are initially effective, they may drive evolutionary adaptation that prevents full eradication, as has been documented for some chemical and biocontrol approaches. Although the intensity, directionality, and persistence of selection required to increase the frequency of resistant genotypes in complex natural ecosystems remains an open question, theory predicts that high mortality can cause life-history evolution even in the absence of a strong selective agent. Here, we use annually collected ecological and genetic data to show that rapid evolution of introduced smallmouth bass has undermined a 20-y manual suppression effort in a mid-sized lake. Despite nearly doubling annual mortality, our intensive control program produced a larger bass population dominated by young and early-maturing fish. These shifts were accompanied by large allele frequency changes in three genomic regions associated with earlier maturation and increased somatic growth. Our findings bear out the theoretical prediction that high mortality can drive evolutionary adaptation in target species. Controlling species invasions are worldwide practices that typically remove a substantial proportion of a population during each of many successive generations, hence life history adaptation may be commonplace. Such evolutionary responses could be salient in explaining the widespread failure of invasion control efforts. Genetic and phenotypic monitoring to detect cryptic adaptation and preemptive design of invader eradication programs to deliberately disrupt directional selection for resistance could improve invasion control outcomes.

Understanding the full impacts of introduced and invasive species suppression on ecological communities requires long‐term suppression experiments where native species with different life histories can reach new population equilibria. Responses documented in shorter term studies are likely to be transient. This study evaluated the abundance and size structure of 11 species of littoral fishes across 20 years in response to annual, lake‐wide removal of smallmouth bass, a non‐native predator that competes with and consumes native species. Many changes in relative abundance of species were evident from comparing community composition before, immediately after, and 20 years into the invader suppression experiment. Long‐term dynamics were classified as showing abrupt change, gradual change, or no discernible change, based on a change point analysis. Relative abundance of most fish community members increased in the year following initiation of bass removal but became more varied thereafter. After initial increases, most species declined in abundance, with some declining to near or below their pre‐experiment levels and others remaining above pre‐removal levels. Many species showed an initial decrease in body size following bass removal, indicating increased abundance of young‐of‐year, but long‐term patterns varied among species. Ongoing fluctuations of populations throughout this study indicate that the direction, tempo, and stability of population responses to removing a dominant invader are often transient. Changes in population abundance and body size across our 20‐year experiment were not correlated across species, suggesting that fish communities require many generations to achieve a new stable state following the loss or suppression of a dominant predator. Synthesis and applications. Fish community restoration efforts are rarely followed by long‐term evaluations, hence current understanding of responses to removal of introduced and invasive species may be largely based on transient dynamics. Instead, both population abundance and size structure should be documented for at least a few generations of the longest‐lived species. Insights into management outcomes of suppressing or eliminating introduced and invasive predators require accounting for demographic time lags as species interactions become restructured over long periods of time.

Dams and other anthropogenic barriers have caused global ecological and hydrological upheaval in the blink of the geological eye. In the present article, we synthesize 307 studies in a systematic review of contemporary evolution following reduced connectivity and habitat alteration on freshwater fishes. Genetic diversity loss was more commonly observed for small populations impounded in small habitat patches for many generations behind low-passability barriers. Studies show that impoundments can cause rapid adaptive evolution in migration timing, behavior, life history, temperature tolerance, and morphology, as well as reduce phenotypic variance, which can alter adaptive potential and ecological roles. Fish passage structures can restore migratory populations but also create artificial selection pressures on body size and migration. The accelerating pace of dam removals and the paucity of data for fishes other than salmonids, other vertebrates, invertebrates, and tropical and southern hemisphere organisms highlights the urgent need for more studies on the rapid evolutionary effects of dams.

The construction of dams on large rivers has negative impacts on native species. Environmental flows have been proposed as a tool to mitigate these impacts, but in order for these strategies to be effective they must account for disparate temperature and flow needs of different species. We applied a multi‐objective approach to identify trade‐offs in dam release discharge and temperature for imperiled fishes with contrasting habitat requirements, while simultaneously meeting the needs of human water users. Using the Sacramento River (California, USA) as a case study, our model suggests that current management aimed at providing high discharge for downstream water users and cold water for endangered winter‐run Chinook salmon (Oncorhynchus tshawytscha) has detrimental impacts on threatened green sturgeon (Acipenser medirostris), which require warm water for juvenile growth. We developed an optimal dam release scenario that can be used to meet the needs of salmon, sturgeon and human water users. Our results show that dam releases can be managed to successfully achieve these multiple objectives in all but the most severe drought years. Synthesis and applications. This study shows that managing dam releases to meet the needs of a single species can have detrimental effects on other native species with different flow and temperature requirements. We applied a multi‐objective approach to balance environmental requirements of multiple species with the needs of human water users. Our findings can be used to guide management of Shasta Dam and our approach can be applied to achieve multi‐object management goals in other impounded rivers.

We reared white sturgeon Acipenser transmontanus under laboratory conditions and found that a random‐forest model containing scute counts and total length predicted age significantly better than total length alone. Scute counts are rapid, inexpensive and non‐lethal meristics to gather in the field. This technique could improve age estimates of imperilled sturgeon populations.

Feeding at early fish life stages is a key determinant of survival to recruitment. To understand the environmental and developmental determinants of early life stage feeding in ESA‐threatened green sturgeon (Acipenser medirostris), we performed a diet study in a highly managed section of California's Sacramento River, where temperature and discharge are controlled by dam releases. Utilising field collections from 2012 to 2016, we assessed the impacts of temperature, discharge and morphological development on the composition and number of prey items in larval green sturgeon diets. Results show that there are more empty stomachs at colder temperatures. Higher discharge conditions decreased prey taxon richness and counts, especially the abundance of cyclopoid copepods in diets. Fish smaller than 30 mm had teeth on the oral jaws and showed a strong reliance on zooplankton prey. The developmental loss of teeth in fish greater than 30 mm was associated with decreased zooplankton consumption and increased richness of benthic macroinvertebrates in diets. Our results show that river management through dam releases has the potential to impact the earliest life stage of green sturgeon by reducing the prevalence of favoured zooplankton prey in diets.

The Southern Distinct Population Segment of Green Sturgeon Acipenser medirostris spawns in the Sacramento River, California, and is listed as a threatened species under the U.S. Endangered Species Act. We estimated the spawning run size and population size in 2010–2015 by using dual‐frequency identification sonar (DIDSON) sampling, underwater video camera species identification, and acoustic tag detections. Spawning run size varied from 336 to 1,236 individuals. We estimated the total population size to be 17,548 individuals (95% confidence interval [CI] = 12,614–22,482). The estimated number of adults was 2,106 (95% CI = 1,246–2,966), the estimated number of juveniles was 4,387 (95% CI = 2,595–6,179), and the estimated number of subadults was 11,055 (95% CI = 6,540–15,571). This study provides the first estimate of Sacramento River Green Sturgeon run size and initiates a time series of abundance that can inform Endangered Species Act recovery processes. Furthermore, these absolute abundance estimates provide a context for evaluating the significance of impacts, such as bycatch in coastal fisheries or entrainment in water diversions, where the number of impacted individuals is known.