Juvenile Coho Salmon Research Articles

Colonization of a temperate river by mobile fish following habitat reconnection

AbstractMobile species are particularly affected by artificial barriers requiring large investments to restore connectivity. However, few large‐scale, long‐term studies have investigated the ecological outcomes of restoring connectivity for these species. Our study, spanning 15–20 years, quantified response trajectories, which represent temporal trends following disturbance, of three native salmonids colonizing 20 km of protected habitat following restoration of fish passage at Landsburg Dam, Cedar River, WA, in 2003. Built in 1901, the dam blocked the upriver movement of native anadromous coho and Chinook salmon and nonanadromous mountain whitefish for 102 years. Restoration effectiveness was also assessed by comparing temporal trends in freshwater productivity of juvenile coho and Chinook salmon in the Cedar River after restoration to a nearby undammed subbasin. We also compared summer densities of juvenile coho and Chinook salmon, and mountain whitefish above the dam measured a decade after restoration to undammed reference systems. Anadromous salmon and nonanadromous mountain whitefish populations increased linearly or nonlinearly following restoration. The positive, asymptotic response represented by adult Chinook salmon counts indicates a slowing in population recovery rate, plateauing a decade after restoration. In contrast, annual abundance of adult coho salmon increased at a constant rate, indicating additional capacity 15 years post‐restoration. Salmonid compositional diversity, driven largely by juvenile coho salmon, also increased nonlinearly, plateauing in a decade. We observed substantial spatial variation in the temporal response, as juvenile coho salmon and mountain whitefish population expansion slowed linearly with upstream distance from the restoration site. There was evidence that some of the annual variation in salmonid biomass in summer was a result of discharge variability in winter and spring, with biomass declining as flow variability increased. Species reintroduction and establishment had no discernible effect on stream‐rearing salmonids living above the dam before restoration, while increasing coho freshwater productivity at the subbasin scale. Results from our study showed recolonization by three mobile species, each with a unique life history, takes at least a decade or more and was dependent on species and life stage, size of the spawning population, distance from restoration site, and annual variability in streamflow.

Differential heat shock protein responses in two species of Pacific salmon and their utility in identifying heat stress

Abstract Rapid and accelerating warming of salmon habitat has the potential to lower productivity of Pacific salmon (Oncorhynchus species) populations. Heat stress biomarkers can indicate where warming is most likely affecting fish populations; however, we often lack clear classifications that separate individuals with and without heat stress needed to make these tools operational. We conducted a heat exposure experiment with trials lasting 12 or 36 h using juvenile Chinook salmon (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch) to validate heat stress biomarkers in white muscle. Following habituation to 13°C, individuals were exposed to water temperatures that increased to 15°C, 17°C, 19°C, 21°C or 23°C. Heat shock protein 70 abundance (HSP70 measured by ELISA) and transcription of 13 genes (mRNA measured by qPCR) including three heat shock protein genes (hsp70, hsp90, hsp27) were measured. A distinct heat stress response was apparent by 21°C in juvenile Chinook salmon and 23°C in juvenile coho salmon using HSP70. A threshold for heat stress classification in Chinook salmon of > 2 ng HSP70 mg.1 total protein identified heat stress in 100% of 21 and 23°C treated individuals compared to 4% in cooler treatments. For coho salmon, > 3 ng HSP70 mg.1 total protein identified heat stress in 100% of 23°C treated individuals compared to 4% in cooler treatments. Transcription from a panel of genes separated individuals between cooler and stressful temperature experiences (≥21°C for Chinook salmon and ≥23°C for coho salmon) with ~ 85% correct classification. Our findings indicate that juvenile Chinook salmon were more temperature-sensitive than juvenile coho salmon and support the use of a HSP70 threshold sampled from muscle for assessing heat stress in individual wild Pacific salmon with an option for non-lethal biopsies for spawning adults.

Higher Early Marine Mortality of Steelhead Associated with Releases of Hatchery Coho Salmon but Not Chinook Salmon

AbstractUnderstanding the drivers of mortality during critical life history periods is an important part of increasing our capacity to rebuild depressed salmonid populations. For threatened steelhead Oncorhynchus mykiss in Puget Sound, Washington, early marine predation has been implicated as a key source of mortality. Yet, the agents that mediate predation pressure are poorly understood. In this study, we characterize abundances of juvenile Coho Salmon O. kisutch and Chinook Salmon O. tshawytscha in Puget Sound and relate these abundance patterns to weekly steelhead survival to better understand whether pulses of hatchery‐released salmonids mediate steelhead survival. We found that weekly abundances of hatchery Coho Salmon and Chinook Salmon smolts vary by several orders of magnitude across weeks, indicating that large resource pulses are available to salmonid predators. We further found that weekly steelhead survival was significantly negatively related to abundances of hatchery‐released Coho Salmon but not Chinook Salmon, which had considerably smaller body sizes than both Coho Salmon and steelhead smolts. Together, our results suggest that releases of Coho Salmon into Puget Sound mediate mortality of steelhead smolts, possibly via increased predation pressure by shared predators.

Modelling the reestablishment of coho salmon (Oncorhynchus kisutch) in Klamath River tributaries after dam removal

AbstractThe planned removal of four dams on the Klamath River (anticipated 2024) will be the largest river restoration effort ever undertaken on the planet. Dam removal will restore access to >50 km of the Klamath River mainstem for coho salmon, but mainstem habitat may not be suitable for rearing juvenile coho salmon. Instead, small tributaries may provide most rearing habitat for reestablishing coho salmon. We used four approaches to evaluate six Klamath River tributaries above existing dams to assess their potential to support juvenile coho salmon: (1) We measured summer temperature regimes and evaluated thermal suitability. (2) We applied an Intrinsic Potential (IP) model to evaluate large‐scale geomorphological constraints on coho salmon habitat. (3) We used the Habitat Limiting Factors Model (HLFM) to estimate rearing capacity for juveniles given current habitat conditions. (4) We developed an occupancy model using data from reference tributaries to predict coho salmon rearing distribution. All six streams had summer temperatures cooler than the mainstem Klamath River. However, five of the streams have barriers that will restrict coho salmon to within 5 km of the confluence with the Klamath River and two were disconnected mid‐summer. Despite these constraints, the tributaries will likely produce coho salmon. Most streams had high IP in their lower reaches, the HLFM model estimated a total capacity of 105,000 juvenile coho salmon, and the occupancy model predicted juvenile coho salmon will rear throughout the accessible reaches. Protection and habitat enhancement for these tributaries will be important for coho salmon reestablishment post‐dam removal.

Coho salmon <i>Oncorhynchus kisutch</i> (Walbaum) on the continental coast of the Okhotsk Sea

Coho salmon is a common object of ichthyofauna in the water bodies of the continental coast of the Okhotsk Sea, with the main stocks concentrated in the central part of the region. The species exhibits mostly the anadromous life strategy in the ontogenesis, though the resident groups are known in three lakes located in the basins of Inya, Ul’beya and Okhota Rivers. Temporal structure of anadromous coho salmon is demonstrated for the first time within the region: the stock is presented by two forms, as early and late ones. Spawning runs of these forms could be indicated by changes of qualitative parameters of fish. Coho salmon of the early and late forms reproduce on spawning grounds with different hydrological and thermal regimes. Juvenile coho salmon spend from 1 to 3 years in the fresh water before the seaward migration. Duration of marine period of their life is usually 1 year, but rarely they return for spawning either in the year of seaward migration or after two winters in the sea. Variability of abundance and qualitative parameters of coho salmon is conditioned mainly by environments of reproduction and feeding, with some impact of epigenetic difference in size and age between the life strategies in ontogenesis.

Juvenile salmon habitat use drives variation in growth and highlights vulnerability to river fragmentation

AbstractWidespread stream network fragmentation from dams and culverts has altered habitat connectivity in river ecosystems and presents an acute threat to migratory fish. To support watershed management for an iconic migratory fish group, we assessed juvenile salmon growth outcomes across habitat use strategies and characterized how these life histories may be impacted by stream connectivity loss. Juvenile coho salmon (Oncorhynchus kisutch) in the Big Lake drainage, Alaska, USA, were individually tracked over 2012–2013 and categorized into habitat use behaviors, with fish either remaining in streams throughout freshwater residency or migrating seasonally to overwinter in lake habitats. Size, growth rate, and body condition of smolts (n = 1113) were compared across habitat use strategies. Juvenile coho salmon that moved seasonally to lake overwintering habitats, the most frequently observed strategy, grew faster and were significantly larger as smolts compared to their counterparts who remained in streams exclusively (spring Age 1 fish: 18% larger by weight, 9% faster growth rate; spring Age 2+ fish: 26% heavier, 11% faster growth). Environmental data from a subset of overwinter lakes indicate that greater foraging opportunity and lower energy costs may be implicated in growth advantages conferred by lentic overwintering strategies. Habitat use strategies requiring seasonal migrations, however, increased exposure to stream connectivity loss, and fish blocked from accessing a potential overwinter headwater lake by a culvert and dam had lowest body condition among study groups. Stream network fragmentation restricts access to preferred overwinter habitats, and our findings suggest this may constrain freshwater rearing strategies associated with strong juvenile coho salmon growth. As size at smolt has been implicated as a driver of salmon survival through ocean residency, reduced freshwater habitat connectivity during juvenile stages may have deleterious impacts on later marine life stages. Consequently, conservation of stream connectivity across lentic and lotic habitats represents an important watershed management priority for juvenile salmon.

Riverscape View of Fish Assemblages, Habitat, and Stream Temperatures during Summer Low Flows in the Chehalis River, Washington

The Chehalis River is a coastal river in southwestern Washington State where decision makers are considering options to reduce flood damage and improve freshwater habitats. This study focused on the mainstem river above and below the location of a potential dam, one of the flood-reduction options being considered. Our objectives were to describe spatial patterns of fish species, physical habitat, and stream temperature, and identify associations between fish species and the physical environment at two spatial scales. Data were collected in spatially continuous reaches following a “riverscape” approach, and patterns in the data were described using three different ordination techniques. Most of the variation in fish species composition occurred at the sub-basin scale (entire study area), with additional but minor patterns at the reach (1-km) scale. At the sub-basin scale, fish species composition was organized in an upstream (salmonid) to downstream (cyprinid) replacement pattern and was best explained by the combination of river location, habitat, and temperature. At the reach scale, physical habitat and stream temperature differentiated juvenile trout versus coho salmon in the salmonid-dominated upstream extent of our study area, and dace versus shiner in the cyprinid-dominated downstream extent. We conclude that salmon and steelhead are particularly vulnerable to habitat loss upstream of the proposed dam because they disproportionately use this area as summer rearing habitat. Protection and restoration of headwater areas, such as our study area in the Chehalis River, will be critical to support salmonid populations into the future.

Erratum for the Report "A ubiquitous tire rubber-derived chemical induces acute mortality in coho salmon," by Z. Tian, H. Zhao, K T. Peter, M. Gonzalez, J. Wetzel, C. Wu, X. Hu, J. Prat, E. Mudrock, R. Hettinger, A. E. Cortina, R. G. Biswas, F. V. C. Kock, R. Soong, A. Jenne, B. Du, F. Hou, H. He, R. Lundeen, A.

After publication of the Report “A ubiquitous tire rubber–derived chemical induces acute mortality in coho salmon,” which revealed 6PPD-quinone to be the primary causal toxicant toward coho salmon, a commercial standard of this molecule became available, and the authors developed and published an isotopic analytical method for more accurate quantification of extracts, environmental samples, and fish exposures (1). The authors found a ~15-fold increase in peak areas using the commercial standard, indicating that the previous standards overestimated both the reported median lethal concentration (LC50) and the environmental concentrations of 6PPD-quinone in the study by a factor of 8.3. Using new exposures with the commercial standard and the isotopic method for quantification, LC50 values to juvenile coho salmon were subsequently revised to a lower value of 95 ng/L. Although the absolute concentrations in Figs. 3 and 4 of the Report shift lower when using the updated calibration, the relative relationship between environmental concentrations and LC50 presented in Fig. 4A are not changed, and the conclusions and implications of the paper are otherwise not affected.

6PPD-Quinone: Revised Toxicity Assessment and Quantification with a Commercial Standard

Stormwater exposure can cause acute mortality of coho salmon (Oncorhynchus kisutch), and 6PPD-quinone (6PPD-Q) was identified as the primary causal toxicant. Commercial standards of 6PPD-Q recently became available; their analysis highlighted a systematic high bias in prior reporting concerning 6PPD-Q. A 6PPD-Q commercial standard was used to re-confirm toxicity estimates in juvenile coho salmon and develop a liquid chromatography-tandem mass spectrometry analytical method for quantification. Peak area responses of the commercial standard were ∼15 times higher than those of in-house standards, and the updated LC50 value (95 ng/L) was ∼8.3-fold lower than that previously reported. These data support prior relative comparisons of the occurrence and toxicity while confirming the substantial lethality of 6PPD-Q. While environmental concentrations are expected to be lower, 6PPD-Q also was more toxic than previously calculated and should be categorized as a “very highly toxic” pollutant for aquatic organisms. Isotope dilution-tandem mass spectrometry methods enabled accurate quantification (limits of quantification of <10 ng/L) within environmental samples.

Declines of Juvenile Coastal Cutthroat Trout and Coho Salmon over Fifteen Years in a Salmon-Bearing Stream in the Salish Sea

Pacific salmon are ecologically, economically, and culturally important species indigenous to British Columbia, Canada. Unfortunately, some populations of Pacific salmon have been declining due to climate change, habitat loss, overfishing, and anthropogenic development. As such, considerable resources have been invested to study, restore, protect, and monitor Pacific salmon and their habitat. Since 2004, Lyall Creek, a salmon-bearing stream in the Salish Sea in British Columbia, Canada, has been monitored. Using 2-pass electrofishing, we investigated changes in observed densities over 15 years (2004–2019) for juvenile coho salmon (Oncorhynchus kisutch) and juvenile coastal cutthroat trout (Oncorhynchus clarkii clarkii) that inhabit Lyall Creek. A statistically insignificant but general decreasing trend was observed for juvenile coho salmon densities over 15 years in Lyall Creek. Juvenile coastal cutthroat trout densities in Lyall Creek exhibited a statistically significant decline. More research is required to understand what anthropogenic and environmental factors are negatively influencing Pacific salmon densities in Lyall Creek. Specifically, water availability may be an important stressor affecting these salmon populations.

Wild juvenile salmonid abundance in Wisconsin tributaries indicates limited contributions to Lake Michigan fisheries

Natural reproduction of salmonids occurs in many Lake Michigan tributaries, yet little is known about abundance and the potential contribution of wild fish hatching in Wisconsin tributaries. The objectives of our study were to determine if: 1) abundance of wild juvenile salmonids (primarily adfluvial rainbow trout, Oncorhynchus mykiss, referred to as steelhead) varied among selected Wisconsin streams based on available spawning and age-0 habitat; 2) stream temperature regimes could limit survival of juvenile salmonids, and 3) wild juvenile salmonids outmigrate from Wisconsin tributaries into Lake Michigan or larger tributaries. In 2016 and 2017, juvenile salmonid abundance was estimated in six Wisconsin tributaries to Lake Michigan by multiple-pass depletion sampling using backpack electrofishing. Habitat assessments included steelhead redd surveys, age-0 habitat surveys, and stream temperatures were monitored using in-stream loggers. Passive integrated transponder (PIT) tagging and PIT antennas were used to detect outmigration from three streams (Willow, Stony and Hibbard creeks). Population estimates for individual streams ranged from 75 to 2276 for juvenile steelhead and from 0 to 243 for juvenile coho salmon, Oncorhynchus kisutch. No correlation was detected between juvenile steelhead abundance and quality age-0 habitat. Stream temperatures rarely exceeded the thermal limit for steelhead (27 °C). Outmigration rates for three streams ranged from 0.6% to 3.1%, but these estimates were considered minimum values. Low abundance of wild juvenile steelhead and coho salmon alone suggest that the contributions of these tributaries to Lake Michigan fisheries are likely small. Furthermore, relying on returns of wild steelhead produced in these streams is probably insufficient to maintain stream fisheries.

The Natural Regulation and Relevance of Wild and Hatchery Coho Salmon Production in the Strait of Georgia

The commercial and recreational fishery for Coho Salmon Oncorhynchus kisutch in the Strait of Georgia in the 1970s and 1980s that annually averaged 733,800 individuals declined in the 1990s and was closed in 1998. We observed that above a threshold, ocean carrying capacity is regulating production and not the number of juvenile Coho Salmon entering the Strait of Georgia. We show that a 52% reduction in releases of Coho Salmon from Canadian hatcheries from 2010–2018 did not change the average abundance of hatchery fish surviving through to September of their first ocean year. Our explanation that above a threshold number of smolts, ocean carrying capacity is regulating production rather than escapements is relevant to rebuilding efforts and the use of hatcheries. The results add to the accumulating evidence that production of Pacific salmon Oncorhynchus spp. is closely related to the capacity for rapid growth in the first months in coastal ecosystems.

Variation in predator diet and prey size affects perceived impacts to salmon species of high conservation concern

Management of protected species is difficult when objectives include the recovery of both predator and its prey. Ideally, identifying trade-offs between competing objectives involves evaluating management alternatives with a quantitative model that integrates information on both species, but data are often limited. We used new predator diet data and simulation modeling to update our understanding of seal predation on juvenile Chinook (Oncorhynchus tshawytscha) and coho salmon (Oncorhynchus kisutch) in the Puget Sound. Under prey size assumptions used by previous studies, estimates suggest predation could be a significant source of mortality for Chinook salmon (mean: 37% of ocean age-0 juveniles), but varied considerably among years (range of median estimates: 22%–49%). However, when we estimated prey size from otoliths recovered from seal scats, the impact (numbers of fish consumed) decreased by 71%. Predation on coho salmon was estimated to be relatively low under both scenarios (6%–8% of ocean age-0 juveniles) with a 21% decrease in consumption using otolith-derived prey size. Our analysis highlights the importance of updating model inputs and re-evaluating assumptions of multi-species models used for ecosystem-based fisheries management.

Juvenile Coho and Chinook Salmon Growth, Size, and Condition Linked To Watershed‐Scale Salmon Spawner Abundance

AbstractAnadromous Pacific salmon Oncorhynchus spp. are semelparous, and resource subsidies from spawning adult salmon (marine‐derived nutrients [MDN]) benefit juvenile salmonids while they rear in freshwater. However, it is unclear if juvenile salmon populations respond predictably to the abundance of spawning salmon at the watershed scale. To address whether hypothesized benefits to rearing juveniles scale up to population and watershed scales, we examined juvenile Coho Salmon Oncorhynchus kisutch and Chinook Salmon O. tshawytscha growth, fork length, condition, and abundance as a function of MDN assimilation throughout the Unalakleet and North rivers in western Alaska. Additionally, a mark–recapture experiment provided abundance estimates of Coho Salmon smolts emigrating from these two rivers. Prior to spawning, residual MDN from past years offered little advantage to juvenile salmon. However, after the arrival of spawning adults, juveniles demonstrated a positive relationship between MDN and fish size, growth, and condition in fall and winter. Out‐migrating smolts also benefitted from MDN resources via increased size and growth rates. Coho Salmon smolt abundance was unrelated to total spawner biomass, but a positive relationship between MDN assimilation and smolt abundance suggested a possible effect on overwinter survival. Furthermore, similar trends in spawner biomass and the abundance of age‐1 smolts suggested that age at smolting was influenced by MDN. These relationships support the hypothesis that salmon spawner abundance during Coho and Chinook Salmon rearing is an important factor in the juvenile productivity of these species.

Effects of experimental flow manipulations on water quality, hypoxia, and growth of Threatened Salish sucker (Catostomus sp. cf. catostomus) and juvenile coho salmon (Oncorhynchus kisutch)1

The Salish sucker (Catostomus sp. cf. catostomus) is a federally Threatened species in Canada, inhabiting small lowland streams along with juvenile coho salmon (Oncorhynchus kisutch) in southwestern British Columbia and adjacent Washington State. Experiments were conducted in two ponds to determine the effects of reduced flow on dissolved oxygen, growth of stocked Salish sucker and juvenile coho salmon (Oncorhynchus kisutch), and fish movement using passive integrated transponder (PIT) tags. Sequential flow reduction in the first experiment resulted in a significant decrease in dissolved oxygen (9.1 to 3.1 mg·L−1) and Salish sucker growth (23% decline), although lower dissolved oxygen and growth were confounded with decreased temperature. Flow reduction in a second split-pond experiment including coho salmon resulted in significantly lower dissolved oxygen (6.9 to 2.9 mg·L−1), a 51% reduction in growth of coho salmon, but no detectable change in Salish sucker growth. PIT tag detections demonstrated increased use by both species of a well-oxygenated refuge in the flow reduction treatment, indicating sublethal effects of hypoxia on fish behaviour and habitat use. These experiments demonstrate the induction of severe hypoxia by reduced flows under eutrophy, with the potential for negative impacts on Salish sucker and juvenile coho salmon growth and habitat use.

Evaluating Coexistence of Fish Species with Coastal Cutthroat Trout in Low Order Streams of Western Oregon and Washington, USA

When multiple species of fish coexist there are a host of potential ways through which they may interact, yet there is often a strong focus on studies of single species without considering these interactions. For example, many studies of forestry–stream interactions in the Pacific Northwest have focused solely on the most prevalent species: Coastal cutthroat trout. To examine the potential for interactions of other fishes with coastal cutthroat trout, we conducted an analysis of 281 sites in low order streams located on Washington’s Olympic Peninsula and along the central Oregon coast. Coastal cutthroat trout and juvenile coho salmon were the most commonly found salmonid species within these streams and exhibited positive associations with each other for both presence and density. Steelhead were negatively associated with the presence of coastal cutthroat trout as well as with coho salmon and sculpins (Cottidae). Coastal cutthroat trout most frequently shared streams with juvenile coho salmon. For densities of these co-occurring species, associations between these two species were relatively weak compared to the strong influences of physical stream conditions (size and gradient), suggesting that physical conditions may have more of an influence on density than species interactions. Collectively, our analysis, along with a review of findings from prior field and laboratory studies, suggests that the net effect of interactions between coastal cutthroat trout and coho salmon do not appear to inhibit their presence or densities in small streams along the Pacific Northwest.

Connectivity between lentic and lotic freshwater habitats identified as a conservation priority for coho salmon

Abstract Juvenile Pacific salmon exhibit diverse habitat use and migration strategies to navigate high environmental variability and predation risk during freshwater residency. Increasingly, urbanization and climate‐driven hydrological alterations are affecting the availability and quality of aquatic habitats in salmon catchments. Thus, conservation of freshwater habitat integrity has emerged as an important challenge in supporting salmon life‐history diversity as a buffer against continuing ecosystem changes. To inform catchment management for salmon, information on the distribution and movement dynamics of juvenile fish throughout the annual seasonal cycle is needed. A number of studies have assessed the ecology of juvenile coho salmon (Oncorhynchus kisutch) during summer and autumn seasons; catchment use by this species throughout the annual cycle is less well characterized, particularly in high‐latitude systems. Here, n = 3,792 tagged juvenile coho salmon were tracked throughout two complete annual cycles to assess basin‐wide distribution and movement behaviour of this species in a subarctic, ice‐bearing catchment. Juvenile coho salmon in the Big Lake basin, Alaska, exhibited multiple habitat use and movement strategies across seasons; however, summer rearing in lotic mainstem environments followed by migration to lentic overwinter habitats was identified as a prominent behaviour, with two‐thirds of tracked fish migrating en masse to concentrate in a small subset of upper catchment lakes for the winter. In contrast, the most significant tributary overwintering site (8% of tracked fish) occurred below a culvert and dam, blocking juvenile fish passage to a headwater lake, indicating that these fish may have been restricted from reaching preferred lentic overwinter habitats. These findings emphasize the importance of maintaining aquatic connectivity to lentic habitats as a conservation priority for coho salmon during freshwater residency.

Efficacy of environmental DNA sampling to detect the occurrence of endangered coho salmon (Oncorhynchus kisutch) in Mediterranean‐climate streams of California's central coast

AbstractAnalysis of environmental DNA (eDNA) has emerged as an important tool for investigating the occurrence and distribution of fish and other species in aquatic environments. However, in lotic systems, uncertainty remains about how environmental factors influence the downstream transport, degradation, and dilution of eDNA, and hence how detection probability varies with distance from the eDNA source. We conducted cage experiments and paired eDNA and snorkel surveys to evaluate the potential for eDNA methods to detect endangered coho salmon in Mediterranean‐climate streams of the Santa Cruz Mountains, California. Juvenile coho salmon at two densities were placed in cages, and water samples were collected and analyzed (qPCR) at seven locations from 10 to 1,000 m downstream. Although we detected coho salmon eDNA up to 1,000 m downstream of the cage, the vast majority of detections were within 200 m, and detections at greater distances occurred only at the higher fish density. In field collections from 21 stream reaches across the Santa Cruz Mountains, eDNA methods and snorkeling produced identical outcomes in terms of reach‐level detection of coho salmon. Probability of occurrence in a water sample and detection in a qPCR replicate both increased with observed fish density; however, even at the lowest densities, a protocol of three water samples and two qPCR replicates resulted in a &gt;90% cumulative probability of detection. Overall, our studies suggest that detection probability decreased rapidly with distance in Santa Cruz Mountain streams, likely because low stream discharges, slow transport velocities, and strong interaction between the water column and stream substrate result in rapid degradation or settling of eDNA. Our findings thus support the use of eDNA methods for detecting rare species, but also indicate that local conditions strongly influence transport dynamics and hence need to be considered when developing survey designs.

Acute cerebrovascular effects in juvenile coho salmon exposed to roadway runoff

Urban stormwater is responsible for recurrent mortality events of coho salmon (Oncorhynchus kisutch) documented in the US Pacific Northwest. Currently, the toxic mode of action is unknown; however, mortality is often associated with large increases in hematocrit (Hct). We found that hemoconcentration and red cell swelling may offer a partial explanation for increased Hct, based on increased total hemoglobin and reduced mean cell hemoglobin concentration of the blood in runoff-exposed juvenile coho salmon, relative to controls. No significant changes were seen in plasma total thiols or antioxidant power, discounting osmoregulatory dysfunction and plasma dehydration as a driver of hemoconcentration. To test for plasma leakage, we injected Evans blue dye complexed with bovine serum albumin (EBD–BSA) intracardially and circulated for 5 min, followed by a 15 min washout with saline. Coho salmon exposed to roadway runoff showed significant accumulation of EBD–BSA in brain and olfactory rosette regions compared with controls, demonstrating plasma leakage from the cerebrovasculature. Our results suggest blood–brain barrier disruption underlies symptom development in coho salmon exposed to roadway runoff.

Amino acid cues emanating from Pacific salmon eggs and ovarian fluid.

The eggs of salmonid fishes are an important food source for many aquatic predators that detect eggs using olfaction. Moreover, chemicals from eggs and ovarian fluid aid sperm cells in detecting and locating eggs for fertilization, and ovarian fluid is attractive to conspecific males. Thus chemicals from eggs and ovarian fluid may facilitate reproduction but may also attract egg predators. The authors sampled mature females of three Pacific salmon species - Chinook (Oncorhynchus tshawytscha), coho (Oncorhynchus kisutch) and sockeye (Oncorhynchus nerka) - and determined the proportional representation of amino acids, potent fish odorants, from their eggs and ovarian fluid (Chinook and coho salmon only). They then tested juvenile coho salmon, an egg predator, for responses to ovarian fluid and egg odours using the electro-olfactogram (EOG) recording technique. The amino acid compositions of the salmon species were significantly and positively correlated with each other, and the interspecific differences were comparable to those between individuals of the same species. The egg water samples were, on average, dominated by lysine, alanine and glutamine (12.6%, 12.4% and 10.9%, respectively). The ovarian fluid samples were dominated by lysine (20.5%), followed by threonine (9.7%), glycine (9.2%) and arginine (8.8%). EOG recordings demonstrated the ability of juvenile coho salmon to detect the chemical traces of eggs and ovarian fluid. It is concluded that salmon eggs are a potent source of odours for potential predators but likely not highly differentiated among salmon species.