Escape From the Heat: Thermal Stratification in a Well-Mixed Estuary and Implications for Fish Species Facing a Changing Climate

Climate change may cause organisms to seek thermal refuge from rising temperatures, either by shifting their ranges or seeking microrefugia within their existing ranges. We evaluate the potential for thermal stratification to provide refuge for two fish species in the San Francisco Estuary (SFE): Chinook Salmon (Oncorhynchus tshawytscha) and Delta Smelt (Hypomesus transpacificus). We compiled water temperature data from multiple monitoring programs to evaluate spatial, daily, hourly, intra-annual, and inter-annual trends in stratification using generalized additive models. We used our data and models to predict the locations and periods of time that the bottom of the water column could function as thermal refuge for salmon and smelt. Periods in which the bottom was cooler than surface primarily occurred during the peak of summer and during the afternoons, with more prominent stratification during warmer years. Although the SFE is often exceedingly warm for fish species and well-mixed overall, we identified potential for thermal refugia in a long and deep terminal channel for Delta Smelt, and in the periods bordering summer for Chinook Salmon. Thermal stratification may increase as the climate warms, and pockets of cooler water at depth, though limited, may become more important for at-risk fishes in the future.

Spatiotemporal variation in Lake Sturgeon movement and habitat selection in Missouri river tributaries : implications for the management and recovery of populations at range margins

Lake Sturgeon were nearly extirpated from Missouri by the 1970s leading the Missouri Department of Conservation (MDC) to list the species as endangered within the state. Recovery efforts commenced with the publication of the first edition of Missouri's Lake Sturgeon Recovery Plan in 1984. Since, growing populations of mature individuals have been documented; however, information gaps regarding habitat selection and movement in the Missouri River Basin portion of its range hinder MDC's efforts to establish a self-sustaining population. In this project we focused on the following research objectives: (1) elucidate the factors that influence movement patterns of Missouri River Lake Sturgeon, (2) investigate survival and dispersal of stocked age-0 Lake Sturgeon from four stocking locations, (3) define seasonal habitat selection in multiple life stages of Lake Sturgeon and illustrate spatial availability of suitable habitats across the study area. I monitored movements 96 subadult and adult Lake Sturgeon over 3.5 years and 187 age-0 juvenile Lake Sturgeon during the fall/winter of two years. Missouri River tributaries were important habitats for adult and subadult Lake Sturgeon throughout the year. Lake Sturgeon use of the Osage River was greater in all months compared to the Gasconade River. Use of the Osage River was highest in the summer and lowest in the winter, and in the Gasconade River it was lowest in the summer and highest in the spring. In each month tributary occupancy was [greater than] 70 [percent]. Spring upstream migrations occurred in each tributary and were correlated with above average discharges and temperatures from 13 to 19 [degrees]C. Fall migrations only occurred in the Osage River but were also correlated with intermediate temperatures and above-average discharges. A few individuals were detected as far upstream as Bagnell Dam at river km (rkm) 129 in the Osage River or rkm 241 in the Gasconade River. In the summer and winter, tributary habitat use for [approximately] 95 [percent] of Lake Sturgeon was restricted to three reaches of deep pool habitat in the Osage and Gasconade rivers which may serve as thermal refugia. Spawning was not documented in either river, although some aggregations of Lake Sturgeon were observed around rocky shoals during the spring in the Osage River from rkm 50 to 80. Upstream migration distance was variable among individuals and years in the Gasconade River precluding the identification of potential spawning sites. Habitat selection was relatively similar in both tributaries and driven by selection for deep habitats [greater than] 7 m in all seasons. Suitability models suggest that preferred summer and winter deep water refugia may be limited to [less than] 5 [percent] of the mapped portions of each tributary but that preferred depth and coarse substrate for reproduction is relatively common at [greater than] 32 [percent] of tributaries. Dispersal directions and distance for age-0 Lake Sturgeon differed among the four stocking sites. Individuals stocked at upstream sites mainly dispersed downstream to overwinter in similar locations as individuals stocked at downstream sites from rkm 10 to 50. Overwinter survival rates were estimated from 40-55 [percent] and were not significantly different among stocking sites. Age-0 Lake Sturgeon selected shallower depths than adults or subadults in both rivers and slightly swifter current velocities in the Gasconade River. Habitat suitability models for the juveniles were able to predict age-0 habitat use and suggest that the greatest availability of nursery habitat occurs in the lower 11 km of the Osage River or in the reach from rkm 9 to 18 in the Gasconade River. These results can be used to inform management decisions designed to meet restoration objectives for Lake Sturgeon in Missouri and across its southern range margins such as: Managing hydrology of regulated rivers, protecting fish when they are vulnerable to harvest or injury in seasonal refugia, promoting resilience under shifting thermal and hydrologic regimes due to climate change, revealing sampling locations for population monitoring or spawning validation, allocating future stocking to suitable habitats, and directing habitat restoration and protection efforts.

A new low-cost approach to 3-D water temperature monitoring

<p>Water temperature is one of the key factors controlling aquatic ecosystems and influencing physical, chemical and biological processes. Detailed observations of spatial and temporal patterns in water temperature are important for assessing e.g. variations in thermal refugia, impacts of climate change and for developing appropriate management strategies. Freshwater  temperatures are still mostly analysed based on single point measurements, but these do not reflect the spatial thermal variability within waterbodies (i.e. stream and lake) and therefore could lack information on thermal refugia. 2-D images of freshwater temperature in varying spatial resolution are increasingly obtained by space- and airborne methods such as UAV (unmanned aircraft vehicles). While these UAV methods offer the necessary spatial resolution at the surface, they require in situ measurements to obtain absolute temperature values and don’t provide information on vertical thermal variability. Approaches that bridge this gap do exist (e.g. fibreoptic cables), but high demand on resources and high costs limit widespread use.</p><p>The aim of this work was to develop a low-cost, custom-build, fully flexible 3-D temperature sensor system that can be used for calibration and validation of thermal UAV observations, but also adds information on water temperature with depth. The design of our floating sensor system (with a maximum of 72 sensors) offers high flexibility in horizontal/vertical spacing and logging time intervals (ms to h). Here we present the first results of our prototype, which was calibrated using Solinst Leveloggers (accuracy ± 0.05ºC) and tested under various ambient conditions, both in the laboratory and in a lab-in-field experiment in a relatively shallow lake (maximum measurement depth of 1.50 m) in NE Scotland. We also evaluated the use of this system with UAV imagery at the lake.</p><p>The results show a quick response of the individual sensors to temperature changes and indicate suitability of the system for validating and calibrating thermal UAV images. For a set-up with 12 vertical arrays (6 sensors at different depths for each array) and arranged as a grid, preliminary data indicated the value for a 3-D approach as not all thermal patterns at depth were captured by surface measurements. Next, the transferability of the sensor system to a stream will be tested and applied to a stream water management case. Together with UAV thermal imagery, the new sensor system could have the potential for a wide range of research and management applications (e.g. thermal habitats, groundwater upwelling, infiltration of cooling water).</p>

Field-based oxygen isotope fractionation for the conservation of imperilled fishes: an application with the threatened silver shiner Notropis photogenis

Identifying the realized thermal habitat of animals is important for understanding life history and population processes, yet methods to estimate realized thermal use are lacking for many small-bodied organisms, including imperilled fishes. Analysis of oxygen isotopes provides one solution, but requires the development of species-specific fractionation equations. To date, such equations have generally been limited to commercial or game fish species. Here, we developed a field-based fractionation equation for the threatened silver shiner Notropis photogenis to better understand the thermal ecology of the species in an urban watershed. Archived otoliths were analyzed for oxygen isotope values (δ18O). There was a significant linear relationship between otolith isotope fractionation and water temperature, described by δ18Ootolith(VPBD) - δ18Owater(VPBD) = 32.03 - 0.21(°C). Results indicate that otolith isotope techniques can be used to identify the average relative temperature occupied by silver shiner, representing the first investigation of oxygen isotopes to understand thermal occupancy of the species. This field-based equation provides an opportunity to understand how silver shiner may respond to alterations in stream temperatures resulting from urbanization and climate effects and may be useful in identifying thermal refugia for the species. Field-based, species-specific fractionation equations can provide insights into the thermal ecology of many small-bodied fishes, which are increasingly imperilled due to thermal stressors.