Intraspecific variability in thermal tolerance: a case study with coastal cutthroat trout.

Fish physiological performance is directly regulated by their thermal environment. Intraspecific comparisons are essential to ascertain the vulnerability of fish populations to climate change and to identify which populations may be more susceptible to extirpation and which may be more resilient to continued warming. In this study, we sought to evaluate how thermal performance varies in coastal cutthroat trout (Oncorhynchus clarki clarki) across four distinct watersheds in OR, USA. Specifically, we measured oxygen consumption rates in trout from the four watersheds with variable hydrologic and thermal regimes, comparing three ecologically relevant temperature treatments (ambient, annual maximum and novel warm). Coastal cutthroat trout displayed considerable intraspecific variability in physiological performance and thermal tolerance across the four watersheds. Thermal tolerance matched the historical experience: the coastal watersheds experiencing warmer ambient temperatures had higher critical thermal tolerance compared with the interior, cooler Willamette watersheds. Physiological performance varied across all four watersheds and there was evidence of a trade-off between high aerobic performance and broad thermal tolerance. Given the evidence of climate regime shifts across the globe, the uncertainty in both the rate and extent of warming and species responses in the near and long term, a more nuanced approach to the management and conservation of native fish species must be considered.

Contribution of warm habitat to cold-water fisheries.

A central tenet of landscape ecology is that mobile species depend on complementary habitats, which are insufficient in isolation, but combine to support animals through the full annual cycle. However, incorporating the dynamic needs of mobile species into conservation strategies remains a challenge, particularly in the context of climate adaptation planning. For cold-water fishes, it is widely assumed that maximum temperatures are limiting and that summer data alone can predict refugia and population persistence. We tested these assumptions in populations of redband rainbow trout (Oncorhynchus mykiss newberrii) in an arid basin, where the dominance of hot, hyperproductive water in summer emulates threats of climate change predicted for cold-water fish in other basins. We used telemetry to reveal seasonal patterns of movement and habitat use. Then, we compared contributions of hot and cool water to growth with empirical indicators of diet and condition (gut contents, weight-length ratios, electric phase angle, and stable isotope signatures) and a bioenergetics model. During summer, trout occurred only in cool tributaries or springs (<20 °C) and avoided Upper Klamath Lake (>25 °C). During spring and fall, ≥65% of trout migrated to the lake (5-50 km) to forage. Spring and fall growth (mean [SD] 0.58% per day [0.80%] and 0.34 per day [0.55%], respectively) compensated for a net loss of energy in cool summer refuges (-0.56% per day [0.55%]). In winter, ≥90% of trout returned to tributaries (25-150 km) to spawn. Thus, although perennially cool tributaries supported thermal refuge and spawning, foraging opportunities in the seasonally hot lake ultimately fueled these behaviors. Current approaches to climate adaptation would prioritize the tributaries for conservation but would devalue critical foraging habitat because the lake is unsuitable and unoccupied during summer. Our results empirically demonstrate that warm water can fuel cold-water fisheries and challenge the common practice of identifying refugia based only on summer conditions.

RESUMEN: Un principio central de la ecología de paisaje es que las especies ambulantes dependen de hábitats complementarios, los cuales son insuficientes en aislamiento, pero al combinarse mantienen a los animales durante el ciclo anual completo. Sin embargo, la incorporación de las necesidades dinámicas de las especies ambulantes dentro de las estrategias de conservación todavía es un reto, particularmente en el contexto de la planeación de la adaptación climática. Para los peces de agua fría, generalmente se asume que las temperaturas máximas son limitantes y que los datos estivales son suficientes para predecir refugios y la persistencia poblacional. Pusimos a prueba estas suposiciones en poblaciones de trucha arcoíris (Oncorhynchus mykiss newberrii) de una cuenca árida, en donde el dominio de aguas cálidas e hiperproductivas durante el verano emula las amenazas del cambio climático pronosticadas para los peces de agua fría en otras cuencas. Usamos telemetría para descubrir los patrones estacionales de movimiento y uso de hábitat. Después, comparamos las contribuciones que tienen las aguas cálidas y frías al crecimiento con indicadores empíricos de dieta y condición (contenidos intestinales, proporciones peso-longitud, ángulo de fase eléctrica y huellas de isotopos estables) y un modelo bioenergético. Durante el verano, las truchas sólo estuvieron presentes en manantiales o afluentes fríos (<20°C) y evitaron el Lago Klamath Superior (>25°C). Durante la primavera y el otoño, ≥65% de las truchas migraron al lago (5-50 km) para procurar alimento. El crecimiento durante la primavera y el otoño (media [SD] 0.58% día-1 [0.80%] y 0.34 día-1 [0.55%], respectivamente) compensaron la pérdida neta de energía en los refugios fríos durante el verano (-0.56% día-1 [0.55%]). En el invierno, ≥90% de las truchas regresaron a los afluentes (25-150 km) para desovar. Entonces, mientras que los afluentes perennemente fríos fomentaron los refugios termales y el desove, fueron las oportunidades de alimentación en el lago cálido estacional las que finalmente alentaron estos comportamientos. Las estrategias actuales de adaptación climática pondrían como prioridad de conservación a los afluentes, pero devaluarían el hábitat crítico de alimentación porque el lago está desocupado y no es apto durante el verano. Nuestros resultados demuestran empíricamente que las aguas cálidas pueden promover las pesquerías de aguas frías y desafiar la práctica común de identificar refugios basándose solamente en las condiciones estivales.

A Large-Scale, Multiagency Approach to Defining a Reference Network for Pacific Northwest Streams.

Aquatic monitoring programs vary widely in objectives and design. However, each program faces the unifying challenge of assessing conditions and quantifying reasonable expectations for measured indicators. A common approach for setting resource expectations is to define reference conditions that represent areas of least human disturbance or most natural state of a resource characterized by the range of natural variability across a region of interest. Identification of reference sites often relies heavily on professional judgment, resulting in varying and unrepeatable methods. Standardized methods for data collection, site characterization, and reference site selection facilitate greater cooperation among assessment programs and development of assessment tools that are readily shareable and comparable. We illustrate an example that can serve the broader global monitoring community on how to create a consistent and transparent reference network for multiple stream resource agencies. We provide a case study that offers a simple example of how reference sites can be used, at the landscape level, to link upslope management practices to a specific in-channel response. We found management practices, particularly areas with high road densities, have more fine sediments than areas with fewer roads. While this example uses data from only one of the partner agencies, if data were collected in a similar manner they can be combined and create a larger, more robust dataset. We hope that this starts a dialog regarding more standardized ways through inter-agency collaborations to evaluate data. Creating more consistency in physical and biological field protocols will increase the ability to share data.