Juvenile Coho Research Articles

Migration in drought: Receding streams contract the seaward migration window of endangered salmon

AbstractProlonged migration windows buffer migratory animal populations against uncertainty in resource availability. Understanding how intensifying droughts from climate change influence the migration window is critical for biodiversity conservation in a warming world. We explored how drought affects the seaward migration of endangered coho salmon (Oncorhynchus kisutch) near the southern extent of their range in California, USA. We tracked stream departures of juvenile coho, measuring streamflow and temperature in seven streams over 13 years, spanning a historic drought with extreme dry and warm conditions. Linear mixed effects models indicate that, over the range of observations, a decrease in seasonal streamflow (from 4.5 to 0.5 mm/day seasonal runoff) contracted the migration window by 31% (from 11 to 7 weeks). An increase from 10.2 to 12.8°C in mean seasonal water temperature hastened the migration window by three weeks. Pacific salmon have evolved to synchronize ocean arrival with productive ocean upwelling. However, earlier and shorter migration windows during drought could lead to mismatches, decreasing fitness and population stability. Our study demonstrates that drought‐induced low flows and warming threaten coho salmon in California and suggests that environmental flow protections will be needed to support the seaward migration of Pacific salmon in a changing climate.

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.

Urban Roadway Runoff Is Lethal to Juvenile Coho, Steelhead, and Chinook Salmonids, But Not Congeneric Sockeye.

We compared the sensitivity of closely related Pacific salmon and steelhead (Oncorhynchus spp.) to untreated urban stormwater runoff across three storm events. Juvenile coho, sockeye, steelhead, and Chinook were exposed for 24 h to untreated urban runoff and then transferred to clean water for 48 h. As anticipated from previous studies, coho were highly susceptible to runoff toxicity, with cumulative mortality rates ranging from 92%–100% across the three storms. By contrast, juvenile sockeye were unaffected (100% survival), and cumulative mortality rates were intermediate for steelhead (4%–42%) and Chinook (0%–13%). Furthermore, coho died rapidly following the onset of stormwater exposure (generally <4 h), whereas mortality in Chinook and steelhead was delayed by 1–2 days. Similar to previous findings for coho, steelhead and Chinook did not recover when transferred to clean water. Lastly, significant mortality occurred in coho even when roadway runoff was diluted by 95% in clean water. Our findings extend the urban runoff mortality syndrome in salmonids and point to a near-term need for sublethal studies in steelhead and Chinook to more precisely understand stormwater risks to threatened species recovery efforts in the western United States.

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.

Field experiments to assess passage of juvenile salmonids across beaver dams during low flow conditions in a tributary to the Klamath River, California, USA.

Across Eurasia and North America, beaver (Castor spp), their dams and their human-built analogues are becoming increasingly common restoration tools to facilitate recovery of streams and wetlands, providing a natural and cost-effective means of restoring dynamic fluvial ecosystems. Although the use of beaver ponds by numerous fish and wildlife species is well documented, debate continues as to the benefits of beaver dams, primarily because dams are perceived as barriers to fish movement, particularly migratory species such as salmonids. In this study, through a series of field experiments, we tested the ability of juvenile salmonids to cross constructed beaver dams (aka beaver dam analogues). Two species, coho salmon (Oncorhynchus kisutch) and steelhead trout (O. mykiss), were tracked using passive integrated transponder tags (PIT tags) as they crossed constructed beaver dam analogues. We found that when we tagged and moved these fishes from immediately upstream of the dams to immediately downstream of them, most were detected upstream within 36 hours of displacement. By the end of a 21-day field experiment, 91% of the displaced juvenile coho and 54% of the juvenile steelhead trout were detected on antennas upstream of the dams. In contrast, during the final week of the 21-day experiment, just 1 of 158 coho salmon and 6 of 40 (15%) of the steelhead trout were still detected on antennas in the release pool below the dams. A similar but shorter 4-day pilot experiment with only steelhead trout produced similar results. In contrast, in a non-displacement experiment, juveniles of both species that were captured, tagged and released in a pool 50 m below the dams showed little inclination to move upstream. Further, by measuring hydraulic conditions at the major flowpaths over and around the dams, we provide insight into low-flow conditions under which juvenile salmonids are able to cross these constructed beaver dams, and that multiple types of flowpaths may be beneficial towards assisting fish movement past instream restoration structures. Finally, we compared estimates of the number of juvenile salmonids using the pond habitat upstream of the dam relative to the number that the dam may have prevented from moving upstream. Upstream of the dams we found an abundance of juvenile salmonids and a several orders of magnitude difference in favor of the number of juveniles using the pond habitat upstream of the dam. In sum, our study suggests beaver dams, BDAs, and other channel spanning habitat features should be preserved and restored rather than removed as perceived obstructions to fish passage.

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.

Treading Water: Tire Wear Particle Leachate Recreates an Urban Runoff Mortality Syndrome in Coho but Not Chum Salmon.

Tire tread wear particles (TWP) are increasingly recognized as a global pollutant of surface waters, but their impact on biota in receiving waters is rarely addressed. In the developed U.S. Pacific Northwest, acute mortality of adult coho salmon (Oncorhynchus kisutch) follows rain events and is correlated with roadway density. Roadway runoff experimentally triggers behavioral symptoms and associated changes in blood indicative of cardiorespiratory distress prior to death. Closely related chum salmon (O. keta) lack an equivalent response. Acute mortality of juvenile coho was recently experimentally linked to a transformation product of a tire-derived chemical. We evaluated whether TWP leachate is sufficient to trigger the acute mortality syndrome in adult coho salmon. We characterized the acute response of adult coho and chum salmon to TWP leachate (survival, behavior, blood physiology) and compared it with that caused by roadway runoff. TWP leachate was acutely lethal to coho at concentrations similar to roadway runoff, with the same behaviors and blood parameters impacted. As with runoff, chum salmon appeared insensitive to TWP leachate at concentrations lethal to coho. Our results confirm that environmentally relevant TWP exposures cause acute mortalities of a keystone aquatic 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.

Characterizing juvenile salmon predation risk during early marine residence.

Predation mortality can influence the distribution and abundance of fish populations. While predation is often assessed using direct observations of prey consumption, potential predation can be predicted from co-occurring predator and prey densities under varying environmental conditions. Juvenile Pacific salmon Oncorhynchus spp. (i.e., smolts) from the Columbia River Basin experience elevated mortality during the transition from estuarine to ocean habitat, but a thorough understanding of the role of predation remains incomplete. We used a Holling type II functional response to estimate smolt predation risk based on observations of piscivorous seabirds (sooty shearwater [Ardenna griseus] and common murre [Uria aalge]) and local densities of alternative prey fish including northern anchovy (Engraulis mordax) in Oregon and Washington coastal waters during May and June 2010–2012. We evaluated predation risk relative to the availability of alternative prey and physical factors including turbidity and Columbia River plume area, and compared risk to returns of adult salmon. Seabirds and smolts consistently co-occurred at sampling stations throughout most of the study area (mean = 0.79 ± 0.41, SD), indicating that juvenile salmon are regularly exposed to avian predators during early marine residence. Predation risk for juvenile coho (Oncorhynchus kisutch), yearling Chinook salmon (O. tshawytscha), and subyearling Chinook salmon was on average 70% lower when alternative prey were present. Predation risk was greater in turbid waters, and decreased as water clarity increased. Juvenile coho and yearling Chinook salmon predation risk was lower when river plume surface areas were greater than 15,000 km2, while the opposite was estimated for subyearling Chinook salmon. These results suggest that plume area, turbidity, and forage fish abundance near the mouth of the Columbia River, all of which are influenced by river discharge, are useful indicators of potential juvenile salmon mortality that could inform salmonid management.

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.

Prey tell: what quillback rockfish early life history traits reveal about their survival in encounters with juvenile coho salmon

Predation is a major source of mortality in the early life stages of fishes and a driving force in shaping fish populations. Theoretical, modeling, and laboratory studies have generated hypotheses that larval fish size, age, growth rate, and development rate affect their susceptibility to predation. Empirical data on predator selection in the wild are challenging to obtain, and most selective mortality studies must repeatedly sample populations of survivors to indirectly examine survivorship. While valuable on a population scale, these approaches can obscure selection by particular predators. In May 2018, along the coast of Washington, USA, we simultaneously collected juvenile quillback rockfish Sebastes maliger from both the environment and the stomachs of juvenile coho salmon Oncorhynchus kisutch. We used otolith microstructure analysis to examine whether juvenile coho salmon were age-, size-, and/or growth-selective predators of juvenile quillback rockfish. Our results indicate that juvenile rockfish consumed by salmon were significantly smaller, slower growing at capture, and younger than surviving (unconsumed) juvenile rockfish, providing direct evidence that juvenile coho salmon are selective predators on juvenile quillback rockfish. These differences in early life history traits between consumed and surviving rockfish are related to timing of parturition and the environmental conditions larval rockfish experienced, suggesting that maternal effects may substantially influence survival at this stage. Our results demonstrate that variability in timing of parturition and sea surface temperature leads to tradeoffs in early life history traits between growth in the larval stage and survival when encountering predators in the pelagic juvenile stage.