Life history theory provides a framework to understand environmental change based on species traits that capitalize on stable, predictable, or stochastic environmental conditions. We evaluated life history strategies and temporal trends in abundance from 1975 to 2017 for 28 fish species within the Potomac River of eastern North America. Multivariate analysis identified life history strategies defined by small‐bodied species with extended spawning seasons and early maturation (opportunistic strategists), large‐bodied species with delayed maturation and long lifespans (periodic strategists), and intermediate‐sized species exhibiting parental care (equilibrium strategists). Linear mixed models detected temporal trends in abundance for 13 species (46%), of which nine increased and four decreased over time. Increasing species were characterized by opportunistic life history strategies (e.g., banded killifish, Fundulus diaphanous; mosquito fish, Gambusia holbrooki), whereas decreasing species were characterized by periodic or equilibrium strategies (e.g., smallmouth bass, Micropterus dolomieu; river chub, Nocomis micropogon). Recent introductions can account for temporal increases in some cases, but most increasing species are native to the study area. Observed increases in opportunistic native species and decreases in periodic and equilibrium species indicate that environmental conditions have become less stable and less predictable over time, consistent with observed increases in spring peak flows in the study area. Our study indicates the importance of environmental stochasticity for fish community responses to land use and climate change and demonstrates the utility of life history theory in this regard.
Advances in video technology enable new strategies for stream fish research. We compared juvenile (age‐0) and adult (age‐1 and older) Brook Trout Salvelinus fontinalis abundance estimates from underwater video with those from backpack electrofishing and dive count methods across a series of stream pools in Shenandoah National Park, Virginia (n = 41). Video methods estimated greater mean abundance of adult trout than did one‐pass electrofishing, but video estimates of adult abundance were not different than estimates from three‐pass electrofishing or dive count methods. In contrast, videos underestimated the abundance of juvenile trout; we suggest that this is because predator avoidance behaviors by juvenile trout limit their use of microhabitat locations visible to cameras. Integrated abundance estimates from two cameras increased correspondence to comparison methods relative to estimates from single cameras, demonstrating the importance of an expanded field of view for video sampling in streams. Geomorphic features helped to explain methodwise differences: more adult Brook Trout were estimated with video than with three‐pass electrofishing as riffle crest depth and boulder composition increased, indicating habitat associations with trout escapement from electrofishing. Our results demonstrated that video techniques can provide a robust alternative or supplement to traditional methods for estimating adult trout abundance in stream pools.
Abstract. In mountain headwater streams, the quality and resilience of summer cold-water habitat is generally regulated by stream discharge, longitudinal stream channel connectivity and groundwater exchange. These critical hydrologic processes are thought to be influenced by the stream corridor bedrock contact depth (sediment thickness), a parameter often inferred from sparse hillslope borehole information, piezometer refusal and remotely sensed data. To investigate how local bedrock depth might control summer stream temperature and channel disconnection (dewatering) patterns, we measured stream corridor bedrock depth by collecting and interpreting 191 passive seismic datasets along eight headwater streams in Shenandoah National Park (Virginia, USA). In addition, we used multi-year stream temperature and streamflow records to calculate several baseflow-related metrics along and among the study streams. Finally, comprehensive visual surveys of stream channel dewatering were conducted in 2016, 2019 and 2021 during summer low flow conditions (124 total km of stream length). We found that measured bedrock depths along the study streams were not well-characterized by soils maps or an existing global-scale geologic dataset where the latter overpredicted measured depths by 12.2 m (mean) or approximately four times the average bedrock depth of 2.9 m. Half of the eight study stream corridors had an average bedrock depth of less than 2 m. Of the eight study streams, Staunton River had the deepest average bedrock depth (3.4 m), the coldest summer temperature profiles and substantially higher summer baseflow indices compared to the other study steams. Staunton River also exhibited paired air and water annual temperature signals suggesting deeper groundwater influence, and the stream channel did not dewater in lower sections during any baseflow survey. In contrast, Paine Run and Piney River did show pronounced, patchy channel dewatering, with Paine Run having dozens of discrete dry channel sections ranging from 1 to greater than 300 m in length. Stream dewatering patterns were apparently influenced by a combination of discrete deep bedrock (20+ m) features and more subtle sediment thickness variation (1–4 m) depending on local stream valley hydrogeology. In combination, these unique datasets show the first large-scale empirical support for existing conceptual models of headwater stream disconnection based on spatially variable underflow capacity and shallow groundwater supply.
Throughout their range, Brook Trout (Salvelinus fontinalis) occupy thousands of disjunct drainages with varying levels of disturbance, which presents substantial challenges for conservation. Within the southern Appalachian Mountains, fragmentation and genetic drift have been identified as key threats to the genetic diversity of the Brook Trout populations. In addition, extensive historic stocking of domestic lineages of Brook Trout to augment fisheries may have eroded endemic diversity and impacted locally adapted populations. We used 12 microsatellite loci to describe patterns of genetic diversity within 108 populations of wild Brook Trout from Tennessee and used linear models to explore the impacts of land use, drainage area, and hatchery stockings on metrics of genetic diversity, effective population size, and hatchery introgression. We found levels of within-population diversity varied widely, although many populations showed very limited diversity. The extent of hatchery introgression also varied across the landscape, with some populations showing high affinity to hatchery lineages and others appearing to retain their endemic character. However, we found relatively weak relationships between genetic metrics and landscape characteristics, suggesting that contemporary landscape variables are not strongly related to observed patterns of genetic diversity. We consider this result to reflect both the complex history of these populations and the challenges associated with accurately defining drainages for each population. Our study highlights the importance of genetic data to guide management decisions, as complex processes interact to shape the genetic structure of populations and make it difficult to infer the status of unsampled populations.
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