Background: Advances in ultrasonic telemetry, including the ability to accurately position a transmitter within an array of hydrophone receivers, have led to increased opportunities to investigate a broad spectrum of ecological questions in aquatic systems. The quality and efficiency of positioning a transmitter relies upon factors controlled by the researcher (for example, geometry of the receiver array) as well as environmental conditions (for example, water quality or environmental noise). While the physics of sound wave propagation are well understood, the high amount of environmental variability in and among aquatic habitats makes it difficult to predict exactly how any given ultrasonic signal will behave. To evaluate variability in system performance across different receiver arrays in diverse environments we present positional records for fixed-location tags recorded with a popular positioning array, the VEMCO Positioning System (VPS). Using these records we evaluate the relationships between system performance, measured as both horizontal positioning error and positioning efficiency, and user-controlled and environmental variables. We used generalized linear mixed models to assess performance at a coastal site, a site in a freshwater tidal estuary, and a riverine site. Results: The positioning errors were similar across sites, with median errors ranging from 1.6 to 3.3 m. In contrast, there was large variation in positioning efficiency across sites, with poor positioning efficiency in the coastal habitat (7%), possibly due to high levels of bioacoustic noise, and moderate efficiency in the river (21%) and estuary habitats (27%). Our statistical models indicate that array geometry was consistently the most important predictor of positioning performance. Environmental noise and water movement also emerged as additional predictors of performance at several sites. Conclusions: The results provide insight into VPS performance capabilities and emphasize the importance of testing array geometries. Additionally, water quality parameters should be monitored and receiver mooring designs should be carefully considered before embarking upon a telemetry study. We hope this work will guide future researchers in creating more effective designs for positioning arrays, and facilitate the collection of high quality information about movement and behavior patterns of aquatic organisms.
Dams and diversions in rivers often alter flow regimes, which in turn can affect stream biota, particularly when the novel conditions are dramatically different from those under which native species evolved. Management of regulated rivers can include development of environmental flows that seek to restore ecological integrity. However, designing a flow regime that adequately supports aquatic biodiversity requires quantifiable metrics of hydrologic variability that directly relate to aquatic species diversity. This study examined both regulated and unregulated montane rivers over 3 years within the marked seasonality of California's Mediterranean climate to determine which quantifiable elements of a flow regime were most correlated with changes in the biologic community, using stream invertebrates during the summer low flow period as an indicator. Boosted regression trees were used to assess relationships between quantifiable metrics of flow regimes and the invertebrate community. Non‐metric multidimensional scaling and a multi‐response permutation procedure were used to analyze general trends in invertebrate assemblage patterns between rivers and years. Results indicated that hydrologic alteration corresponded to differences in invertebrate community composition, and that metrics related to the spring snowmelt recession and variability in summer water temperature had the greatest relative influence on invertebrate community diversity. Rivers exhibiting longer duration spring recessions, lower daily rates of change during the spring recession, and stable water temperatures during summer supported relatively higher invertebrate community diversity. These relationships suggest that quantifiable metrics associated with seasonal flow variability and predictability should be an integral part of environmental flow design and monitoring.
We conclude that Bombus may be a key agent of hybridization and introgression in these sympatric milkweed populations, and hybrids are neither preferred nor selected against by pollinators. Thus, we have identified a potential mechanism for how hybrids act as bridges to gene flow between A. exaltata and A. syriaca. These results provide insights into the breakdown of prezygotic isolating mechanisms.
Populations of Chinook Salmon Oncorhynchus tshawytscha in California are in decline due to the combined effects of habitat degradation, water diversions, and climate change. Reduced life history diversity within these populations inhibits their ability to respond to these stressors. Putah Creek, a small creek in California’s Central Valley that once supported Chinook Salmon, is undergoing restoration to provide spawning habitats for this imperiled species. Beginning in 2014, increasing numbers of Chinook Salmon spawned throughout the creek, and emigrating juveniles were observed in the following months. Here we used otolith annual growth bands and microchemistry to investigate the age structure and natal origins of the adult spawners. Most individuals were 2 or 3 years old, and they originated from at least seven different natal sources, overwhelmingly from Central Valley hatcheries (~88%). These findings highlight that straying fall‐run Central Valley Chinook Salmon can rapidly utilize restored habitats, potentially establishing new populations. However, to facilitate local adaptations, straying rates and gene flow will have to be managed over time. Reconnecting migratory pathways and restoring many small and diverse streams, like Putah Creek, provides an opportunity to increase life history diversity, strengthening the recovery and resilience of Chinook Salmon.
Predation is a common cause of early life stage mortality in fishes, with reduced risk as individuals grow and become too large to be consumed by gape-limited predatory fishes. Large-bodied species, such as sturgeon, may reach this size-refuge within the first year. However, there is limited understanding of what this size threshold is despite the value of this information for conservation management. We conducted laboratory-based predation experiments on juvenile green sturgeon, Acipenser medirostris, to estimate vulnerability to predation during outmigration from their natal reaches in California to the Pacific Ocean. Two highly abundant and non-native predatory fish species (largemouth bass, Micropterus salmoides, and striped bass, Morone saxatilis) were captured in the wild to be tested with developing juvenile green sturgeon from the UC Davis Green Sturgeon Broodstock Program. Experimental tanks, each containing five predators, received thirty prey for 24-hr exposures. Between sturgeon prey trials, predators were exposed to alternative prey species to confirm predators were exhibiting normal feeding behaviors. In addition to green sturgeon mortality data, trials were video recorded and predatory behaviors were quantified.Overall, these predator species displayed much lower rates of predation on juvenile green sturgeon than alternate prey. Predation decreased with green sturgeon size, and predation risk diminished to zero once sturgeon reached a length threshold of roughly 20-22 cm total length, or between 38% and 58% of predator total length. Behavioral analyses showed low motivation to feed on green sturgeon, with both predators attempting predation less frequently as sturgeon grew. Results of this study imply that optimizing growth rates for larval and juvenile sturgeon would shorten the time in which they are vulnerable to predation. Future experiments should assess predation risk of juvenile green sturgeon by additional predator species common to the Sacramento-San Joaquin watershed. K E Y W O R D S green sturgeon, largemouth bass, predation, striped bass 1 | INTRODUC TI ON Sturgeon populations across the globe have been experiencing drastic population declines. Consequently, sturgeon are the most threatened group of animals on the IUCN Red List of Threatened Species, with 63% of the species listed as Critically Endangered and 85% at risk of extinction (IUCN 2019). Sturgeon are large-bodied and long-lived, with unique reproductive strategies such as late maturation and infrequent | 15 BAIRD et Al. S U PP O RTI N G I N FO R M ATI O N Additional supporting information may be found online in the Supporting Information section. How to cite this article: Baird SE, Steel AE, Cocherell DE, Poletto JB, Follenfant R, Fangue NA. Experimental assessment of predation risk for juvenile green sturgeon, Acipenser medirostris, by two predatory fishes. J Appl Ichthyol.
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