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Location map of the Scott Creek watershed and a detailed view of the lagoon study area (Santa Cruz County, California, USA)
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We investigated how extreme drought conditions influenced the abundance, growth, movement, and seawater readiness of juvenile coho salmon (Oncorhynchus kisutch) and steelhead trout (Oncorhynchus mykiss) in a small central California coastal lagoon. In 2015, the seasonal sandbar at the mouth of Scott Creek formed over 2 months earlier than average,...
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... Drought can cause lower growth, higher mortality, and a host of other issues for juveniles during the summer (e.g., Harvey et al. 2006;Grantham et al. 2012). Low spring and winter streamflows caused by droughts can hinder smolts from entering the ocean (Osterback et al. 2018), and can similarly hinder adults on their in-bound spawning migration (Keefer et al. 2018). Steelhead are also one life-history form of a highly diverse species that include sympatric anadromous and resident life history types, a range of ages, and varying degrees of iteroparity that can all vary by location and year (Busby et al. 1996). ...
... Smolt migrate from streams to the ocean from approximately March to May. Low streamflows during that time can lead to sandbar formation or shallow conditions at the stream mouths, which can restrict the outmigration window and cause high mortality from predation or lagoon entrapment (Osterback et al. 2018). Low spring streamflows can also indirectly influence nearshore mortality by limiting growth prior to ocean entry, as they do in California Chinook Salmon (Munsch et al. 2019). ...
... One possible factor is that spring is a critical growth period for steelhead in this region. Juvenile steelhead on the central CA coast have the highest growth rates during winter and spring , and because larger smolts have higher survival Osterback et al. 2018), the spring growth surge may have significant impacts on overall smolt survival and population productivity. The second possible factor is that low spring flows limit the ocean-migration window. ...
Weather extremes, such as drought, are predicted to be a strong determinant of species persistence under climate change. Yet predictions often fail to consider that variation in streamflow responses, variation in population dynamics, or adaptations to drought could buffer species against extremes. In this study, we examined the responses of eight California (USA) steelhead populations to a severe drought from 2012 to 2016. We observed that streamflows were highly synchronous across the region in all seasons and did not appear to buffer drought impacts. Population dynamics were variable across the region and did appear to buffer the region from drought impacts. Some populations had very low productivity for 4 years associated with the drought, while others had slightly below-average productivity for only 2 years. Population synchrony was associated with spring-smolt flow, temperature, and drought over time, but was not associated with winter-spawner or summer-juvenile drought, suggesting populations may be adapted to drought conditions. Our results highlight how regional buffering and adaptation can be important mechanisms against climate extremes both now and into the future.
... The seasonal lagoon provides feeding habitat and refuge from predators for the oversummer rearing of salmonids (Hayes et al., 2008). When water quality degrades in the lagoon, coho and steelhead have been reported to retreat back upstream to the lower reaches of Scott Creek (Osterback et al., 2018). ...
... The National Oceanic and Atmospheric Administration (NOAA) and University of California Santa Cruz operate a salmonid life cycle monitoring station (Adams et al., 2011) in lower mainstem Scott Creek (~1.5 km upstream from the Pacific Ocean), which includes a weir trap to intercept and count upstream migrating adults and a smolt trap (100 m upstream from the weir) to count downstream-migrating juveniles. A small conservation hatchery located ~3 km upstream from the weir on one of the Scott Creek tributaries ( Figure 1) (Osterback et al., 2018), in terms of both abundance and biomass. ...
... The independent variables (i.e., environmental and hatchery parameters) included were daily mean water temperature, log10-transformed stream discharge, photoperiod, whether the sample was collected on a wet day, whether the creek mouth was open or closed, whether hatchery-origin smolts were released in the previous three days (for the O. kisutch models only), and whether the November 21, 2019, hatchery-origin parr release had occurred (for the O. kisutch models only). All variables had VIFs less than 5. (Osterback et al., 2018;Quinn, 2018), We observed significant associations between eDNA and a number of environmental factors. Status of the creek mouth and photoperiod are two of these; both vary at low frequencies and likely represent the seasonality observed in the eDNA data. ...
A greater understanding of eDNA behavior in the environment is needed before it can be employed for ecosystem monitoring applications. The objectives of this study were to use autonomous sampling to conduct long‐term, high‐frequency monitoring of the eDNA of native salmonid species in a Californian coastal stream, describe temporal variation of eDNA on multiple scales and identify environmental factors that drive this variation, and evaluate the ability of the eDNA datasets to detect rare species and represent organismal abundance. Using high‐throughput autonomous environmental sample processors (ESPs) and qPCR, we enumerated eDNA concentrations from 674 water samples collected at subdaily intervals over 360 days at a single site. We detected eDNA from two imperiled salmonids (coho salmon Oncorhynchus kisutch and steelhead/rainbow trout O. mykiss) in most samples; O. kisutch eDNA was generally in lower concentration and more variable than O. mykiss eDNA. High‐frequency (i.e., subdaily and daily) variability in salmonid eDNA concentrations showed occasional patchiness (i.e., large differences between consecutive samples), while seasonal differences were observed consistent with the ecology of the species at this site. Salmonid eDNA concentrations were significantly associated with creek discharge, photoperiod, and whether the creek mouth was open or closed by a seasonal sandbar. The release of hatchery‐origin O. kisutch parr into the stream was associated with a significant increase in eDNA concentration for the remainder of the study. We compared eDNA signals with fish abundance data collected from traps located at the site. Fish were detected more often by eDNA than from trapping. Significant positive associations between fish abundance and eDNA concentrations were observed for O. mykiss; however, no such associations were observed for O. kisutch. This study adds to our knowledge on the occurrence and behavior of fish eDNA in lotic systems and informs future biomonitoring efforts using automated sampling technology. The objective of our study was to conduct high‐frequency autonomous sampling of salmonid eDNA in a coastal stream, elucidate the temporal patterns and environmental correlates of the eDNA signal, and compare the signal with a measure of abundance obtained from a fish trap collocated at the study site. We sampled one to three times per day for approximately one year, collecting nearly 700 samples. The eDNA signals showed high variability on short time scales and were aligned with expected salmonid seasonal trends in this ecosystem. Salmonid eDNA was significantly associated with creek discharge, photoperiod, and whether the creek mouth was open or closed by a seasonal sandbar. Comparing eDNA with fish counts from traps, we found that eDNA detected fish more often than traps but that eDNA concentrations were weakly or not at all correlated with fish abundance.
... This demonstrates that warm habitats can be valuable for cold-water fish even if they lack fine-scale temperature variation and are distant from cooler habitats. In addition, riverscape ecology theory predicts that fish should move multiple times per year to link foraging, spawning, and refuge habitat (Fausch et al., 2002), yet formally described life histories typically include just 1 migration to each habitat type annually Dodson et al., 2013;Thurow, 2016) (but see Hayes et al., 2011;Osterback et al., 2018). Within stratified lakes, there is some evidence that fish seeking summer refuge in the cool, deep middle of the lake migrate in spring and fall to seasonally warm foraging habitats near shore (although the fall migration may be influenced more by spawning than foraging [Guzzo et al., 2017]). ...
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.
... kisutch) and steelhead trout (O. mykiss) in Scott Creek, California also demonstrate upstream movements (Osterback et al., 2018). Atlantic salmon (Salmo salar) juveniles may also exhibit this upstream migration pattern (Hutchings, 1986). ...
Background: Anadromous salmonids present a marvellous opportunity to study animal movement, with some juveniles in the Yukon and Amur rivers travelling more than 2000 km from their natal areas to the ocean. During their freshwater residence, juvenile salmonids, regardless of river of origin or migration distance, balance the pressures of feeding, predator avoidance, and migration to survive. Questions: What are the choices of current and swimming velocities that stream-dwelling juvenile salmonids use to optimize lifetime reproductive success? How are these influenced by maximum current velocity in the stream or river that they inhabit? Mathematical methods: I developed a dynamic optimality model that treats current and swimming velocities as decision variables. The state variables are downstream river location and fish size. I solve the optimality model using optimal control theory and apply it to juvenile ocean-type Chinook salmon in the Hanford Reach, Columbia River, Washington. Results: Five fundamental behaviours or movement phases result from the optimality model: rapid upstream migration, appetitive ('foraging') upstream movement, station holding, appetitive downstream movement, and rapid downstream migration. These fundamental behaviours were not specified a priori, but emerge when optimizing lifetime reproductive success over the full range of possible behaviours. The appetitive and station holding behaviours are broadly characterized as foraging/ predator avoidance. Rapid migration is favoured over foraging/predator avoidance whenever the magnitude of the marginal value of displacement exceeds the marginal predation risk of displacement. If, during foraging/predator avoidance, the maximum current velocity rises above the swimming speed that maximizes growth, station holding is optimal; otherwise, appetitive movement, which carries greater predation risk, might be optimal. The two types of downstream movement predicted by the optimality model (appetitive movement and rapid downstream migration) describe the movements of the 'ocean-type' and 'stream-type' races of Chinook salmon populations of the Columbia River. In the Hanford Reach application, optimal movements begin with station holding, then switch to downstream appetitive movement or rapid downstream migration, depending on the maximum current velocity. Juveniles accelerate as they migrate downstream. I describe an experiment to test the influence of current velocity on foraging behaviour and a field study to characterize juvenile upstream migrations.
... Following mouth closure, estuaries often transition through multiple ecological states due to episodic changes in key environmental parameters such as lagoon volume, water temperature, dissolved oxygen (DO) concentration, and salinity (Shapovalov and Taft 1954;Boughton et al. 2017). While freshwater predominates, conditions can become brackish when occasional large waves overtop the beach crest and deliver seawater into the lagoon (Smith 1990;Nylen 2015;Osterback et al. 2018). Episodic overtopping events can substantially alter lagoon water chemistry and result in stratified conditions (both vertical and longitudinal) which can last days to months at a time depending on lagoon bathymetry and the volume of freshwater inflow delivered from the watershed (Behrens et al. 2013(Behrens et al. , 2016Nylen 2015). ...
... Additionally, empirical studies have demonstrated that lagoonrearing juvenile steelhead often attain critical size for ocean entry (fork lengths > 150 mm) at a relatively early age, experience enhanced marine survival, and disproportionately recruit to the adult breeding population Hayes et al. 2011). However, lagoons may expose juvenile steelhead to elevated predation risk (Hayes et al. 2011;Frechette et al. 2013;Osterback et al. 2013) and periods of physiological stress due to degraded water quality (Smith 1990;Boughton et al. 2017;Osterback et al. 2018). ...
... Recently, Osterback et al. (2018) provided evidence that juvenile salmonids (both steelhead and the more thermally sensitive coho salmon, Oncorhynchus kisutch) were able to survive and add substantial body mass in the Scott Creek (Santa Cruz County, CA, USA) lagoon during an extended (> 7 months) drought-related closure event, despite periods of high water temperature (> 20 °C) and hypoxia (DO < 5 mg L −1 ). It remains unclear, however, how juvenile salmonids cope with such extremes when they occur. ...
In California (USA), seasonal lagoons provide important oversummer rearing habitat for juvenile steelhead trout (anadromous Oncorhynchus mykiss ). However, key water quality parameters such as temperature and dissolved oxygen concentration can periodically approach or exceed the physiological tolerances of steelhead during the protracted dry season. A field study employing distributed temperature sensing technology, water quality monitoring, habitat mapping, and mark-recapture sampling was conducted to examine how shifting environmental conditions affected the performance and behavior of juvenile steelhead in the Scott Creek estuary/lagoon (Santa Cruz County). Abiotic conditions were driven by episodic inputs of seawater to the typically freshwater lagoon. During midsummer, the water column was vertically stratified which reduced suitable lagoon rearing habitat by approximately 40%. Nevertheless, steelhead abundance, growth, and condition factor were high during the summer and decreased in autumn following lagoon destratification and cooling. Unlike previous work, this study identified limited emigration from the lagoon to riverine habitat during the summer. Instead, juvenile steelhead exhibited crepuscular movement patterns within the lagoon, with peaks in upstream (to upper lagoon habitat) and downstream (to lower lagoon habitat) movement occurring at dawn and dusk, respectively. This study underscores that habitat complexity and connectivity are critical for juvenile steelhead production and persistence and provides insight into steelhead habitat use and behavior in seasonal lagoons.
... Growth opportunities in lagoons can be very high during summer when the rest of the watershed has limited growth opportunities . Small spring emigrants have been observed moving into lagoons during spring, doubling in size during the summer, and then either smolting or returning to the stream Osterback et al. 2018). Even though the Scott Creek lagoon represents only 5% of the watershed area, it plays an outsized role in the population carrying capacity and overall dynamics. ...
... For subyearling Chinook salmon, mouth closure can delay ocean entry and directly impact early ocean survival (Weitkamp et al. 2015). For salmonids that overwinter in fresh water and may return upstream, such as steelhead trout (Oncorhynchus mykiss) and coho salmon (Oncorhynchus kisutch), mouth closure may not impact ocean entry timing (Shapovalov and Taft 1954;Osterback et al. 2018). Construction of the levees in the Redwood Creek estuary has contributed to the mouth closing earlier than it did historically due to excessive accumulation of sediments and reduction of the tidal prism (Cannata et al. 2006). ...
... Because of this emigration upstream, we expect the estimate of survival to be biased low. To our knowledge, this is the first example of Chinook smolts moving back upstream, similar to results observed for coho salmon and steelhead in other estuaries in California (Koski 2009;Hayes et al. 2011;Osterback et al. 2018). ...
Estuaries are commonly touted as nurseries for salmonids, providing numerous advantages for smolts prior to ocean entry. In bar-built estuaries, sandbars form at the mouth of rivers during periods of low stream flow, closing access to the ocean and preventing outmigration. We evaluated how summer residency in a leveed bar-built estuary affects the growth, survival, and recruitment of a Chinook salmon (Oncorhynchus tshawytscha) population. We performed a mark–recapture study on outmigrants to determine juvenile estuary abundance, growth, and survival. We used returning adult scales and otoliths to determine the relative proportion of summer estuary residents in spawning adults. Juveniles in the estuary grew less after mouth closure, and ultimately summer estuary residents had lower smolt-to-adult survival and contributed disproportionately less to the spawning population than juveniles that reared in the ocean their first summer. Mouth closure may lower food availability and deteriorate estuary conditions by reducing marine prey influx and estuary circulation. This research demonstrates the complexity of estuary dynamics and function as salmonid nurseries, particularly when considering the extensive modification of estuaries.
... A combination of diminished growth rewards and heightened predation risks likely account for reduced upper estuary utilization by juvenile Pescadero steelhead during warm and deoxygenated lagoonal conditions. Using stationary PIT tag antenna, Osterback et al. (2018) demonstrates that juvenile CCC steelhead in the nearby Scott Creek IE (Santa Cruz County, CA) repeatedly moved between warmer lagoon and cooler lower mainstem creek waters during an extended droughtinduced sandbar closed phase in 2015, suggesting individuals actively thermoregulated by moving between adjacent habitat types. Pedersen (1987) reports that juvenile O. mykiss require DO concentrations of approximately 7.0 mg•L −1 for proper growth and food conversion efficiency and food consumption declines below 6.0 mg•L −1 . ...
... Our study adds to a growing body of research about the consequences of extended drought for CCC steelhead (e.g., Smith 1990; Jankovitz 2015; Osterback et al. 2018). The historic California drought persisted until water year 2016 and Jankovitz (2015) reports that poor water quality during summer 2015 "terminated all O. mykiss production and probably survival throughout the [Pescadero] lagoon". ...
... Initially a shallow brackish water epilimnion formed but was lost by late summer when saline conditions, hot water temperatures, and very low DO concentrations prevailed throughout the water column (Jankovitz 2015). Dewatered channels upstream of the Pescadero IE during summer 2015 likely impeded upstream movement and behavioral thermoregulation (Osterback et al. 2018). ...
We used acoustic telemetry and environmental monitoring to elucidate preferred microhabitats of juvenile steelhead trout (Oncorhynchus mykiss) in a Central California intermittent estuary (IE) during historic drought. We collected over half a million fish locations in the Pescadero IE (San Mateo County, CA) across 15 weeks during an extended sandbar-closed period which permitted quantification of fine scale habitat use and movement patterns. Tagged juvenile steelhead expressed strong site fidelity, especially at night when core habitat area - defined as the 50% probability of being present in an area - contracted by over one order of magnitude. The rate of movement was slow overall (~0.4 to 0.6 lengths·s−1) and remained at baseline levels at night (~40 mm∙s−1). The daytime rate of movement generally tracked solar radiation levels and appeared to be moderated by water temperatures. Spikes in the rate of movement occurred during crepuscular periods and the maximum hourly rate of movement (138 mm∙s−1) was observed during the early study period from 10:00 to 11:00 when water temperatures were physiologically optimal (17–18 °C). Water quality worsened upstream when water temperatures exceeded 18 °C and dissolved oxygen concentrations declined below 7.0 mg·L−1. Fish tag detections at stationary receivers in the upper estuary declined linearly with deteriorating water quality conditions. Qualitative analysis of juvenile steelhead habitat utilization indicated a strong preference for two microhabitat features in the estuary during the study; both were shallow (~1.5 m), wind-protected, and possessed cover and sandy substrates that occurred within the fresh or near fresh epilimnion where lagoon water quality was best and benthic prey was likely most abundant. Upstream movement occurred in late fall for over half of the tagged cohort, which likely enhances population resiliency by allowing these fish to escape lethal water quality conditions coincident with the transition from closed to open estuary in late fall. Climate projections for California’s Central Coast predict an increase in extreme dry events and the information presented here can help natural resource managers prepare for the future, such as the critical need to promote development of a sufficiently oxygenated epilimnion during extended sandbar-closed ecosystem states.
... The authors suggest that the behavioral difference observed was likely due to a more advanced smolt transition in Snake vs. Clearwater fish, as demonstrated by higher levels of gill Na+/K+-ATPase activity. Osterback et al. (2018) examined how drought conditions effected the ecology of juvenile coho and steelhead in the coastal lagoon of Scott Creek, a heavily studied watershed in central California. In the drought year of 2015, a seasonal sandbar at the mouth of the lagoon formed two months earlier than normal, trapping salmon for an additional 7 months. ...
A summary of the literature most relevant to the future impacts of climate change on Pacific Salmon from the year 2018
... There is increasing evidence that juvenile coho salmon can occupy and persist in ostensibly thermally stressful habitats when food resources are abundant. For example, Osterback et al. (2018) reported positive growth by juvenile coho salmon in a central California coastal freshwater lagoon despite mean daily water temperatures > 20°C and attributed these results, in part, to high standing stocks of invertebrate prey. It is also likely that local adaptation and (or) acclimation contribute to the ability of coho salmon populations at the southern end of their North American range (i.e., California) to cope with elevated and highly variable thermal regimes. ...
... However, there is mounting evidence that fish often reside in locations that exceed purported thermal optima when such locations (or nearby habitats) exhibit thermal heterogeneity over time and (or) space (Brewitt et al. 2017). Movements between thermally heterogeneous environments are often cyclical, with salmonids dispersing from physiologically optimal cold-water habitat to warmer water to seek enhanced foraging and growth opportunities (Brewitt et al. 2017;Osterback et al. 2018). Conversely, juvenile salmonids rearing at more northern latitudes have been shown to disperse from cooler to warmer habitats to accelerate metabolic processes and growth (Armstrong et al. 2013). ...
... Our results suggest that enhanced food resources may allow juvenile salmonids to persist in habitats that would be deemed suboptimal based on temperature criteria alone. This may be particularly true in watersheds with intrinsically high rates of secondary production or in highly productive seasonal habitats such as coastal freshwater lagoons (e.g., Bond et al. 2008;Hayes et al. 2008;Osterback et al. 2018). Several recent studies have called for a broader inclusion of prey availability into salmonid habitat selection and suitability models (e.g., Wipfli and Baxter 2010;Weber et al. 2014), and the results presented here strongly support this recommendation. ...
Conservation efforts for Pacific salmon (Oncorhynchus spp.) increasingly prioritize maintenance of cool water temperatures that protect all freshwater life stages. However, development of appropriate temperature standards requires a robust understanding of the interactions among water temperature, ecosystem productivity, and fish performance. We used a series of in situ enclosures to examine how natural spatiotemporal gradients in thermal conditions and prey availability affected the summer growth and survival of age-0+ coho salmon (Oncorhynchus kisutch). Coho salmon absolute growth rates peaked at a mean daily average water temperature (mean T) of 16.6 °C and an associated maximum weekly maximum temperature (MWMT) of 21.1 °C. Juvenile growth under these thermal conditions was sixfold greater than the growth rates observed for conspecifics rearing in the coolest study reach (mean T = 13.0 °C; MWMT = 16.0 °C). Even at the highest rearing temperature (mean T = 18.1 °C; MWMT = 24.0 °C), growth rates remained positive and above the study-wide average, although overall survival was reduced. Among the predictor variables examined, invertebrate prey abundance was the predominant factor influencing age-0+ coho salmon growth. These results suggest that abundant prey resources may mitigate the negative effects of elevated water temperature on fish growth in riverine environments. Given the likelihood of increasing stream temperatures with climate change, productive ecosystems may provide critical refuges for juvenile salmonids.
