Back-calculation of growth trajectories from otolith microstructure is a valuable tool for understanding mechanisms underlying variability in growth among fish populations. We analyzed fish ...length/otolith radius relationships for Snake River spring/summer Chinook and Snake River fall Chinook salmon, listed as separate “Evolutionarily Significant Units” (ESUs) under the US Endangered Species Act, to determine whether these ESUs shared relationships. In addition, we analyzed otoliths from seven separate populations within the Snake River spring/summer Chinook ESU to assess the variability in relationships among populations, which are much more closely related than ESUs. We also examined several potential functional forms for the equations. We found that the separate ESUs had significantly different fish length/otolith radius relationships, but that variability in otolith growth rate could not explain the difference. Relationships among populations within the spring/summer Chinook ESU did not vary nearly as much as those between ESUs. The quadratic model and the power model fit the data equally well, and constraining these models to pass through a biological intercept (estimated fish length and otolith radius at hatching) resulted in only a slight decrease in model fit. To test the ability of the models to back-calculate fish lengths, we predicted the length at tagging for 17 PIT-tagged fall Chinook that were measured at release and at recapture. The back-calculation demonstrated little bias (<1 mm FL, on average) and relatively small standard deviation (∼3.5 mm) for the best model. When we repeated the back-calculation with data from both ESUs combined, bias increased substantially (to 15 mm FL), demonstrating the importance of determining the proper taxonomic level at which to combine data within a species.
Ecological theory traditionally describes predator–prey interactions in terms of a law of mass action in which the prey mortality rate depends on the density of predators and prey. This simplifying ...assumption makes population-based models more tractable but ignores potentially important behaviors that characterize predator–prey dynamics. Here, we expand traditional predator–prey models by incorporating directed and random movements of both predators and prey. The model is based on theory originally developed to predict collision rates of molecules. The temporal and spatial dimensions of predators–prey encounters are determined by defining movement rules and the predator's field of vision. These biologically meaningful parameters can accommodate a broad range of behaviors within an analytically tractable framework suitable for population-based models. We apply the model to prey (juvenile salmon) migrating through a field of predators (piscivores) and find that traditional predator–prey models were not adequate to describe observations. Model parameters estimated from the survival of juvenile chinook salmon migrating through the Snake River in the northwestern United States are similar to estimates derived from independent approaches and data. For this system, we conclude that survival depends more on travel distance than travel time or migration velocity.
Climate strongly influences the population dynamics of many species, but intrinsic and extrinsic factors such as density‐dependence and anthropogenic impacts can confound the effects of climate. ...Further, the temporal scale of climate response is determined by the unique characteristics of a species’ life history, and determining the most appropriate climate indicator at the proper scale is a challenge faced by population ecologists. We focused on how climate influences juvenile survival of bocaccio (Sebastes paucispinis), a threatened Pacific rockfish, because its abundance has declined >90% in the last 25 years, ostensibly as the result of overfishing. Bocaccio recruitment is episodic, with strong recruitment events apparently related to climate conditions. We developed a sequence of models that related log of juvenile survival to the predictor variables population density and climate, as measured by the Northern Oscillation Index. A model that contained only population density as a predictor variable explained only 1.4% of the variance, while a model that included only climate indices explained 52%. Including density additively with climate did not improve model fit. However, a model that included an interaction between density and climate explained more than 68% of the variance. In addition, models that represented climate as monthly indices fit the juvenile survival data much better than those that averaged climate over 2‐ or 3‐month periods. Our results suggest that climate affects bocaccio recruitment as a series of pulses corresponding to particular life‐history events, with population density mediating the magnitude of the climate effect during the settlement stage.
Although evolutionary change within most species is thought to occur slowly, recent studies have identified cases where evolutionary change has apparently occurred over a few generations. ...Anthropogenically altered environments appear particularly open to rapid evolutionary change over comparatively short time scales. Here, we consider a Pacific salmon population that may have experienced life‐history evolution, in response to habitat alteration, within a few generations. Historically, juvenile fall Chinook salmon (Oncorhynchus tshawytscha) from the Snake River migrated as subyearlings to the ocean. With changed riverine conditions that resulted from hydropower dam construction, some juveniles now migrate as yearlings, but more interestingly, the yearling migration tactic has made a large contribution to adult returns over the last decade. Optimal life‐history models suggest that yearling juvenile migrants currently have a higher fitness than subyearling migrants. Although phenotypic plasticity likely accounts for some of the change in migration tactics, we suggest that evolution also plays a significant role. Evolutionary change prompted by anthropogenic alterations to the environment has general implications for the recovery of endangered species. The case study we present herein illustrates the importance of integrating evolutionary considerations into conservation planning for species at risk.
For migratory species, duration of migration, or “travel time,” is often a critical variable in determining the cost of migration. Observed travel times are the result of both environmental factors ...such as air or water currents and the behavior of individuals. In an effort to distinguish among these components, I developed a migration model based on an advection‐diffusion equation that characterizes population movements in terms of two biologically meaningful parameters: migration rate and rate of population spread. I applied the model to travel time data from juvenile chinook salmon (Onchorhynchus tshawytscha), which were tagged during their seaward migration. The tagged fish originated from three separate evolutionarily significant units (ESUs) as classified by the U. S. National Marine Fisheries Service. The model was expanded by allowing migration and diffusion rates to vary with fish length and river flow. Variability in travel times explained by these factors was strikingly similar from year to year within ESUs, and the migratory behavior revealed by the analysis was consistent with the life‐history patterns that distinguish the ESUs. The approach presented here is easily adaptable to a wide range of migratory species and may be particularly useful for predicting how at‐risk populations respond to variable conditions in regulated or otherwise disturbed migration habitats.
The ability to manage anthropogenic actions that affect the dynamics of animal populations requires the identification and understanding of life-stage-specific mortality. This understanding can be ...confounded when the expression of mortality is removed, in time or space, from its cause. For years, researchers studying endangered Snake River spring–summer Chinook salmon Oncorhynchus tshawytscha have debated the magnitude of mortality that is related to—but expressed after—passage through the Snake and Columbia River hydropower system (“latent” mortality). We conducted experiments with Chinook salmon to assess the magnitude of latent mortality from two sources: passage through juvenile bypass structures at dams, and transportation with larger juvenile steelhead O. mykiss present in the barge holds. Nearly 129,000 juvenile Chinook salmon (passive integrated transponder tagged) were exposed to different treatment conditions during downstream migration. Study fish were then held in seawater tanks for up to 223 d, and time to mortality was noted for each individual that died. We analyzed survival patterns by using statistical procedures for time-to-event data (i.e., survival analysis). Differential survival between treatment groups was taken to indicate latent mortality caused by the specific treatment. We used a nonparametric Kaplan–Meier analysis to visualize survival patterns and a parametric logistic regression analysis to model the effects of multiple factors. Chinook salmon that were transported with steelhead had significantly lower survival than those that were transported alone. However, there was little evidence for differential latent mortality between Chinook salmon that were not detected at the bypass systems of five dams along the migration route and those that were detected at two to five bypass systems. Our application of survival analyses to individuals subjected to various treatments and held for extended periods produced an effective combination that can be used to test for latent mortality; these results may serve as an initial assessment for further conservation investigations and as a guide to more-targeted research. Received March 9, 2011; accepted September 8, 2011
Columbia River plume fronts De Robertis, Alex; Morgan, Cheryl A.; Schabetsberger, Robert A. ...
Marine ecology. Progress series (Halstenbek),
09/2005, Letnik:
299
Journal Article
Recenzirano
Odprti dostop
Well-defined fronts develop at the seaward edge of riverine plumes where suspended materials and planktonic organisms are concentrated by convergent water flows. Riverine plume fronts have been ...hypothesized to be favorable fish habitats because they can lead to localized prey aggregations. We examined the spatial distribution of juvenile Pacific salmonidsOncorhynchusspp. in and around plankton-rich frontal regions of the Columbia River plume to test the hypothesis that juvenile salmonids aggregate at riverine plume fronts to feed. Juvenile salmonids tended to be abundant in the frontal and plume regions compared to the more marine shelf waters, but this pattern differed among species and was not consistent across the 2 study years. Stomach fullness tended to be higher in the more marine shelf waters than either the front or plume areas, which does not support the hypothesis that salmonids consistently ingest more prey at frontal regions. Many prey organisms were disproportionately abundant at these fronts, but salmon stomach-content analysis did not reveal higher stomach contents at fronts or identify prey groups indicative of feeding in the frontal areas. Although our results indicate that the Columbia River plume influences the distributions of juvenile salmon, our observations do not support the hypothesis that juvenile salmonids congregate to feed at fronts at the leading edge of the Columbia River plume. The short persistence time of these fronts may prevent juvenile salmon from exploiting these food-rich, but ephemeral, features.
Non-indigenous species may be the most severe environmental threat the world now faces. Fishes, in particular, have been intentionally introduced worldwide and have commonly caused the local ...extinction of native fish. Despite their importance, the impact of introduced fishes on threatened populations of Pacific salmon has never been systemically examined. Here, we take advantage of several unique datasets from the Columbia River Basin to address the impact of non-indigenous brook trout, Salvelinus fontinalis, on threatened spring/summer-run chinook salmon, Oncorhynchus tshawytscha. More than 41 000 juvenile chinook were individually marked, and their survival in streams without brook trout was nearly double the survival in streams with brook trout. Furthermore, when brook trout were absent, habitat quality was positively associated with chinook survival, but when brook trout were present no relationship between chinook survival and habitat quality was evident. The difference in juvenile chinook survival between sites with, and without, brook trout would increase population growth rate (λ) by ca. 2.5%. This increase in λ would be sufficient to reverse the negative population growth observed in many chinook populations. Because many of the populations we investigated occur in wilderness areas, their habitat has been considered pristine; however, our results emphasize that non-indigenous species are present and may have a dramatic impact, even in remote regions that otherwise appear pristine.
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