Anadromous Life History Research Articles

Impact of freshwater rearing history on Atlantic salmon gill response to viral stimulation post seawater transfer

Land-based recirculating aquaculture systems (RAS) have risen in prevalence in recent years for Atlantic salmon production, enabling intensive production which allows increased growth and environmental control, but also having the potential for reducing water use and eutrophication. The Atlantic salmon has an anadromous life history with juvenile stages in freshwater (FW) and on-growing in seawater (SW), enabled by a transformational process known as smoltification. The timing of smoltification and transfer of smolts from FW to SW is critical under commercial production with high mortalities during this period. The impact of FW rearing system on immune function following seawater transfer (SWT) is not well understood. In this study parr were raised in either RAS or a traditional open-LOCH system until smolting and then transferred to a common marine environment. Two-weeks post-SWT fish were immune stimulated with a viral mimic (poly I:C) for 24 h to assess the ability to mount an antiviral immune response, assessed by whole transcriptome analysis of gill tissue, an important immune organ in fish. We show that unstimulated smolts reared in the LOCH had higher immune gene expression than those reared in RAS as determined by functional analysis. However, following stimulation, smolts reared in the RAS mounted a greater magnitude of response with a suite of immune genes displaying higher fold induction of transcription compared to LOCH reared smolts. We suggest RAS smolts have a lower steady state immune-associated transcriptome likely due to an unvarying environment, in terms of environmental factors and lack of exposure to pathogens, which shows a compensatory mechanism following stimulation allowing immune ‘catch-up’ with those reared in the LOCH. Alternatively, the RAS fish are experiencing an excessive response to the immune stimulation.

Ancestry and genetic structure of resident and anadromous rainbow trout (Oncorhynchus mykiss) in Argentina.

Since the first introduction from North America more than a century ago, rainbow trout (Oncorhynchus mykiss) have rapidly established self-sustaining populations in major river basins of Patagonia. Many generations later, only the freshwater resident life history is expressed in the Chubut and Negro rivers of northern Argentinian Patagonia, whereas both the resident and anadromous life histories are found in the Santa Cruz River of southern Argentina. Despite previous studies that have tried to identify the sources of these introduced populations, uncertainty still exists. Here we combined data from many single-nucleotide polymorphisms and microsatellite loci in O. mykiss populations from Argentina and North America to evaluate putative source populations, gene flow between Argentinian river basins, and genetic diversity differences between Argentinian and North American populations. We found that populations from northern and southern Patagonia are highly differentiated and have limited gene flow between them. Phylogeographic analysis also confirmed that they have separate origins, with the northern populations most closely related to the domesticated rainbow trout strains that are raised worldwide and the Santa Cruz River populations most closely related to North American populations from California and Oregon that have an anadromous component. In addition, fish with different life histories in the Santa Cruz River were found to constitute a single interbreeding population. No evidence was found of reduced genetic variation in introduced rainbow trout, suggesting multiple contributing sources. In spite of these advances in understanding, significant questions remain regarding the origins and evolution of the introduced O. mykiss in Patagonia.

The Lifetime Migratory History of Anadromous Brook Trout (Salvelinus fontinalis): Insights and Risks from Pesticide-Induced Fish Kills

Brook trout populations in Prince Edward Island, Canada, have experienced over 50 pesticide-related fish kills since the 1960s. Life history evaluation of large sea-run brook trout recovered following two fish kill events was compared with a reference river using strontium:calcium otolith microchemistry. This study examined the dual hypotheses that anadromous brook trout are more likely to arise from sea-run mothers, and that freshwater entry timing makes them vulnerable to pesticide-induced fish kills. A total 89% of the fish exhibited an anadromous life history, and 77% of these were offspring of anadromous mothers, suggesting that anadromy is dominant in progeny of sea-run mothers. This study adds to our understanding of the maternal inheritance of anadromy in sea-run brook trout populations. Additionally, freshwater entry precedes the majority of fish kill events, illustrating that the overlap between migration and pesticide runoff contributes to the cumulative population risks to sea-run brook trout.

Strontium isotopes reveal diverse life history variations, migration patterns, and habitat use for Broad Whitefish (Coregonus nasus) in Arctic, Alaska.

Conservation of Arctic fish species is challenging partly due to our limited ability to track fish through time and space, which constrains our understanding of life history diversity and lifelong habitat use. Broad Whitefish (Coregonus nasus) is an important subsistence species for Alaska’s Arctic Indigenous communities, yet little is known about life history diversity, migration patterns, and freshwater habitat use. Using laser ablation Sr isotope otolith microchemistry, we analyzed Colville River Broad Whitefish 87Sr/86Sr chronologies (n = 61) to reconstruct movements and habitat use across the lives of individual fish. We found evidence of at least six life history types, including three anadromous types, one semi-anadromous type, and two nonanadromous types. Anadromous life history types comprised a large proportion of individuals sampled (collectively, 59%) and most of these (59%) migrated to sea between ages 0–2 and spent varying durations at sea. The semi-anadromous life history type comprised 28% of samples and entered marine habitat as larvae. Nonanadromous life history types comprised the remainder (collectively, 13%). Otolith 87Sr/86Sr data from juvenile and adult freshwater stages suggest that habitat use changed in association with age, seasons, and life history strategies. This information on Broad Whitefish life histories and habitat use across time and space will help managers and conservation planners better understand the risks of anthropogenic impacts and help conserve this vital subsistence resource.

Diversity of life history traits, growth, and lipid storage in migratory variants of steelhead and rainbow trout (Oncorhynchus mykiss) in Kamchatka, Russia

Partial migration in salmonids provides access to freshwater and marine feeding environments varying in productivity. To test the hypothesis that partial migration drives differences in growth and energy storage with differential consequences for females than males, we assigned Oncorhynchus mykiss sampled on the western Kamchatka Peninsula, Russia, to one of five life history contingents based on scale patterns. Stable isotopes, obtained from a subset of fish, indicated different foraging by fish showing oceanic and coastal anadromous life histories. In younger fish, body size was largest in anadromous fish feeding on the high seas, intermediate for those feeding in coastal environments, and smallest for nonanadromous fish feeding in rivers. Older fish converged on large (oceanic) and small (coastal and freshwater) body sizes. Somatic lipid content, also collected for a subset of fish, averaged 2.5% across all life histories. Percent lipid was highest in females of ocean and males of coastal and freshwater life histories. Taken together, migratory patterns predictably influenced growth but may limit compensatory lipid storage in females, especially for the rare life histories.

Counting the Parts to Understand the Whole: Rethinking Monitoring of Steelhead in California’s Central Valley

Steelhead (Oncorhynchus mykiss expressing an anadromous life history) in the Sacramento and San Joaquin rivers and their tributaries in California’s Central Valley (CCV) belong to a Distinct Population Segment (DPS) that is listed as threatened under the US Endangered Species Act. Although contemporary management and recovery plans include numerous planned and ongoing efforts seeking to aid in DPS recovery—such as gravel augmentation, manipulation of spring flows, and restoration of rearing and spawning habitat—a paucity of data precludes the possibility of evaluating the effect of these actions on populations of Steelhead in CCV streams. Knowledge gaps relating to historic and current abundance, population-specific ratios of resident and anadromous life-history expression, and the influence of hatchery-reared fish remain largely unaddressed. This is partly a result of aspects of Steelhead biology that make them difficult to monitor, including the multitude of factors that contribute to the expression of anadromy, polymorphic populations, and migration periods that coincide with challenging field conditions. However, these gaps in understanding are also partly the result of an institutional focus on Chinook Salmon (Oncorhynchus tshawytscha) and a pervasive notion that actions benefiting Chinook populations will also benefit Steelhead populations. To evaluate these gaps and to suggest approaches for assessing DPS recovery actions, we review available data and existing monitoring efforts, and consider the actions necessary to inform the development of targeted O. mykiss monitoring programs. Current management and recovery goals focus on abundance estimates of Steelhead only, yet current monitoring is insufficient for reliable estimates. We argue that a reallocation of monitoring resources to better understand the interaction between resident O. mykiss and Steelhead would provide better data to estimate the vital rates needed to evaluate the effects of recovery actions.

Counting the Parts to Understand the Whole: Rethinking Monitoring of Steelhead in California’s Central Valley

Steelhead (Oncorhynchus mykiss expressing an anadromous life history) in the Sacramento and San Joaquin rivers and their tributaries in California’s Central Valley (CCV) belong to a Distinct Population Segment (DPS) that is listed as threatened under the US Endangered Species Act. Although contemporary management and recovery plans include numerous planned and ongoing efforts seeking to aid in DPS recovery—such as gravel augmentation, manipulation of spring flows, and restoration of rearing and spawning habitat—a paucity of data precludes the possibility of evaluating the effect of these actions on populations of Steelhead in CCV streams. Knowledge gaps relating to historic and current abundance, population-specific ratios of resident and anadromous life-history expression, and the influence of hatchery-reared fish remain largely unaddressed. This is partly a result of aspects of Steelhead biology that make them difficult to monitor, including the multitude of factors that contribute to the expression of anadromy, polymorphic populations, and migration periods that coincide with challenging field conditions. However, these gaps in understanding are also partly the result of an institutional focus on Chinook Salmon (Oncorhynchus tshawytscha) and a pervasive notion that actions benefiting Chinook populations will also benefit Steelhead populations. To evaluate these gaps and to suggest approaches for assessing DPS recovery actions, we review available data and existing monitoring efforts, and consider the actions necessary to inform the development of targeted O. mykiss monitoring programs. Current management and recovery goals focus on abundance estimates of Steelhead only, yet current monitoring is insufficient for reliable estimates. We argue that a reallocation of monitoring resources to better understand the interaction between resident O. mykiss and Steelhead would provide better data to estimate the vital rates needed to evaluate the effects of recovery actions.

Anadromy efficacy of native kokanee in Alturas Lake, Idaho

Eaton KR, Tardy KA. 2022. Anadromy efficacy of native kokanee in Alturas Lake, Idaho. Lake Reserv Manage. XX:XX–XX. Partial migration is a complex process that can have profound impacts on ecosystems and populations. Oncorhynchus nerka ecotypes in the Sawtooth Valley include migratory and resident populations, with each exhibiting partial migration. Kokanee can exhibit anadromous behavior and therefore are valuable for O. nerka recovery. In the Sawtooth Valley, Alturas Lake supports a native kokanee population for which the ability to produce outmigrants could influence conservation decisions. In this study, we used contemporary genetic tools to identify the life history origin of Alturas Lake O. nerka outmigrants. Additionally we use population and survival estimates to determine the extent and success of any present kokanee migration. We found that in Alturas Lake, all juvenile outmigrants are progeny of native kokanee. We also found that increased population abundance is related to a higher number of outmigrants. Furthermore, juvenile survival estimates of Alturas kokanee outmigrants were similar to juvenile Redfish Lake sockeye salmon survival estimates. These similar survival estimates and return of anadromous kokanee adults throughout program history indicate that the Alturas Lake kokanee population is successful in exhibiting anadromous behavior. While kokanee are not a focus for O. nerka recovery, this study shows the importance of partial migration in a native kokanee population in preserving the anadromous life history strategy. Our results highlight that in locations with no sockeye salmon population, like Alturas Lake, native kokanee populations are vital to O. nerka recovery and should be monitored and protected accordingly. Supplemental data for this article is available online at https://doi.org/10.1080/10402381.2022.2026541.

Protecting a displaced species in an altered river: a case study of the endangered Sacramento River winter-run Chinook Salmon

Endangered Sacramento River winter-run Chinook Salmon (Oncorhynchus tshawytscha) exist as a single population that spawns in the Sacramento River downstream of Shasta Dam near Redding, CA. Displaced from their historical habitat after dam construction circa 1940, their survival depends on cold water released from Shasta Reservoir. Managing and recovering the species is further complicated by their anadromous life history, habitat loss and degradation, largescale water supply management, and climate change. The California Department of Fish and Wildlife and other resource agencies coordinate closely to protect the species from extinction, confronting challenges with collaborative restoration and science-driven management. As climate change brings more frequent droughts, warmer weather, and increased variability in precipitation, Sacramento River winter-run Chinook Salmon recovery will require greater collaboration and a shift to more holistic restoration actions that promote and maintain the diversity and resilience of the species and its habitats.

Quantifying lost and inaccessible habitat for Pacific salmon in Canada’s Lower Fraser River

AbstractLoss of connectivity caused by anthropogenic barriers is a key threat for migratory freshwater species. The anadromous life history of salmonids means that barriers on streams can decrease the amount of habitat available for spawning and rearing. To set appropriate targets for restoration, it is important to know how different populations have been impacted in terms of the location and extent of historically available habitat that has been lost or has become inaccessible. Using mapped and predicted barriers to fish passage in streams and diking infrastructure, the amount of both floodplain and linear stream habitat that remains accessible today was estimated for 14 populations of salmon in the Lower Fraser River, British Columbia, Canada’s most productive salmon river. To place these estimates within a historical context, the floodplain area was estimated using vegetation records from the 1850s, and lost streams were estimated using a digital elevation model‐derived stream network. To bolster areas where little mapping has been done, current barrier data were used to predict locations likely to have barriers. Accessibility to floodplain was poor across the entire region with only 15% of the historical floodplain remaining accessible. Linear stream habitat ranged in accessibility from 28% to 99% across populations based on mapped barriers. Inclusion of predicted barriers revealed an additional 33 km of potentially inaccessible stream habitat and the modeled stream network located approximately 1700 km of stream length that has been completely lost. Comparing habitat accessibility and barrier density against the assessed status of populations revealed insights useful for understanding the impact of barriers on spawning and rearing and guiding the allocation of restoration effort. Applying methods for addressing missing data, such as lost streams and unmapped barriers, was essential for estimating the accessibility of habitat within a historical context. While much emphasis has been placed on the role of marine conditions in wild Pacific salmon recovery, the magnitude of habitat loss in the Fraser cannot be ignored and suggests it is a major driver of observed salmon declines.

Freshwater early life growth influences partial migration in populations of Dolly Varden (Salvelinus malma malma)

Populations of northern Dolly Varden (Salvelinus malma malma) exhibit partial seaward migration, yet little is known about this phenomenon in Dolly Varden populations. Our study analyzed data from three different Dolly Varden populations in the western Canadian Arctic in order to determine if: (1) differences in size-at-first seaward migration exist between fish that migrate at early and late ages among populations inhabiting different river systems, and (2) annual growth influences anadromous or resident life history choice. Otolith strontium analysis and back-calculation were used to determine age- and size-at-first seaward migration, respectively. Differences in age- and size-at-first seaward migration were determined across river system and migration age. Back-calculated fish lengths were compared using a mixed effect model to determine how early growth influences migratory tactics (early or late aged smolt, or resident). Our results indicate that fish exhibiting faster early growth migrated in earlier years and at smaller sizes than slower growing fish, however size- and age-at first seaward migration varied by river system. Faster growing Dolly Varden tended to become either residents or early smolts, while slower growth was associated with smolting later in life. This is contrary to life history theory where the fastest growing fish in a population should mature as a resident. Our results indicate factors other than growth may be influencing life history ‘decisions’ in Dolly Varden. Future work on growth efficiencies and metabolic rates is needed to assess how they affect migratory behaviours.

Species-specific vulnerability to angling and its size-selectivity in sympatric stream salmonids

In mixed fisheries where multiple species are caught, to manage resources sustainably, knowledge about the species-specific vulnerability to fishing is equally or even more important than knowledge of size selectivity of the gear. We compared the vulnerability to bait recreational angling in four salmonid species in natural streams in Japan. The ranking of species-specific angling vulnerability was as follows (from highest to lowest): rainbow trout (Oncorhynchus mykiss), masu salmon (Oncorhynchus masou), white-spotted char (Salvelinus leucomaenis), and southern Asian Dolly Varden (Salvelinus curilus). In all species, larger individuals were more vulnerable to angling, but there were differences in the size dependence between species. In rainbow trout and Dolly Varden (which have a nonanadromous life history in the study area), the probability of being caught monotonically increased with body size, while the vulnerability to angling in masu salmon and white-spotted char (which have an anadromous life history in the study area) showed a domed-shaped pattern. We found that across the species the catch per unit effort showed a hyperstable relationship with population density. Therefore, diminishing local populations are prone to collapse, and this collapse would be hard to foresee based on catch rate data alone.

Is Marine Survival for Puget Sound’s Wild Steelhead Really That Bad? A Nisqually River Case Study Evaluating Estimates of Productivity and Survival of Oncorhynchus mykiss

AbstractWe highlight the uncertainty that exists around estimating productivity and survival for one population of threatened steelhead Oncorhynchus mykiss (anadromous Rainbow Trout) with the goal of strengthening our understanding of the well‐documented poor marine survival of Puget Sound steelhead. We evaluated how sensitive estimates of productivity and survival were to the uncertainty associated with smolt trap enumerations, redd survey methodology, estimates of fish per redd, and estimates of smolt production by resident Rainbow Trout in order to clarify causes of poor steelhead survival in this area. We show that from 2004 to 2014, estimates of both freshwater productivity and marine survival were highly sensitive to estimates of fish per redd used to expand redd counts, as well as error around smolt abundance estimates. Regardless, uncertainty associated with these inputs did not explain the low survival and high productivity observed for Nisqually River steelhead relative to other populations. In addition, we identified progeny from anadromous mothers upstream of what was previously considered a barrier, and we also documented that a proportion of steelhead smolts (N = 4/43) originated from resident mothers (Rainbow Trout). While these results indicated an underestimation in the total number of steelhead redds counted each year and an overestimation of steelhead progeny enumerated at smolt traps, they had little impact on estimates of survival and productivity. Results from the current study support previous work reporting poor marine survival for populations of Puget Sound steelhead and highlight the sympatric relationship between resident and anadromous life histories of O. mykiss. Overall, our study supports a management strategy that protects both the anadromous and fluvial forms of O. mykiss, prioritizes habitat improvements that promote freshwater productivity, and increases research focused on identifying causes of poor marine survival.

The various ways that anadromous salmonids use lake habitats to complete their life history

Despite the preponderance of exorheic lakes in rivers home to anadromous salmonids, little research has focused on how salmon, trout, and char use lakes as part of their anadromous life histories. The literature on this subject has so far revealed that some parr move into lakes to feed and grow before smoltification but that smolts moving through lakes tend to have high mortality in disproportion to what is observed in other habitats they migrate in or through. Adults have been observed using lakes for behavioural thermoregulation prior to spawning, and kelts of iteroparous species often exploit lakes to overwinter before returning to sea to recondition. We summarized knowledge on lakes as salmonid habitat and identified knowledge gaps about the use of lakes by anadromous salmonids related to whether lakes are barriers that structure genetics of populations, whether mortality in lakes is compensatory or additive, and whether systems with lakes have higher rates of repeat spawning among iteroparous salmonids. Human activities that alter lakes require further study to understand how changes in temperature, oxygen, ice, or circulation affect navigation and fate.

The ontogeny of Na+ balance during rapid smoltification in pink salmon (Oncorhynchus gorbuscha).

Pink salmon hatch in fresh water, but their highly anadromous life history requires them to migrate into the ocean immediately after gravel-emergence, at a very small size. During their down-river migration these larvae undergo rapid smoltification that completely remodels their osmoregulatory physiology. At this time, the larvae reportedly have high whole-body Na+ contents and we hypothesised that the active accumulation of internal Na+ occurs in preparation for ocean entry. Using a comparative approach, the present study characterised the ontogeny of Na+ regulation in larvae of the anadromous pink salmon and the fresh-water rainbow trout. Our results indicate that larvae from both species actively accumulated Na+; however, whole-body Na+ content was higher in rainbow trout larvae compared to pink salmon. The time-course of this response was similar in the two species, with highest Na+-uptake rates ([Formula: see text]) shortly after yolk sac absorption, but the mechanism of Na+ accumulation differed between the species. Rainbow trout larvae greatly increased [Formula: see text] to overcompensate for a large simultaneous increase in Na+-efflux rate ([Formula: see text]), whereas pink salmon mounted a smaller increase in [Formula: see text] while maintaining tight control over [Formula: see text], which is supported by a significantly lower paracellular permeability. Our results indicate that the transient accumulation of internal Na+ is not a unique feature of the highly anadromous life history in pink salmon and may be a common ontogenetic pattern during larval development in salmonids; and perhaps it is associated with the development of the cardiovascular system during the larvae's transition to a more active lifestyle.

Loss of Coilia nasus habitats in Chinese freshwater lakes: An otolith microchemistry assessment

Loss of valued diadromous fishes and their habitats is one of the most critical problems in aquatic habitat connection and resource management worldwide. In China, the Poyang, Dongting, Gaobao, Gucheng, Dongping, and Taihu lakes were known to be historical migratory spawning sites of the anadromous estuarine tapertail anchovy Coilia nasus. However, except for Poyang Lake, it is believed that these lakes are no longer used by anadromous fish owing to overfishing, water pollution, and loss of connectivity. To confirm this assumption, we used an electron probe microanalyzer to analyze elemental strontium (Sr) and calcium (Ca) microchemical patterns in the otoliths of C. nasus individuals sampled from these lakes, in accordance with our previous analysis of the otolithic patterns of the same species sampled from habitat areas characterized by different salinity gradients. The results of line transect analysis of Sr/Ca ratios and Sr X-ray intensity maps of the otoliths indicated that all individuals from Dongting, Gaobao, Gucheng, Taihu, and Dongping lakes were characterized by a freshwater-resident life history. In contrast, individuals from Poyang Lake exhibited both freshwater-resident and anadromous life histories. The findings of this pilot study suggest that anadromous C. nasus can be found in Poyang Lake but are unlikely to be found in Dongting, Gaobao, Gucheng, Dongping, or Taihu lakes, despite these lakes being historical distribution areas or even spawning sites. This anchovy can possibly be used as a good model species for understanding the aforementioned global problem. Given that C. nasus is a commercially important species, restoration of its natural habitats and maintenance of their connections are recommended for its management and conservation.

Evaluating Artificial Propagation Release Strategies for Recovering Endangered Snake River Sockeye Salmon

AbstractAs a growing number of aquatic organisms become imperiled due to anthropogenic and environmental threats, there is an increasing need to implement captive propagation programs to conserve some species. These captive propagation programs can aid in the recovery of species, such as for Sockeye Salmon Oncorhynchus nerka, by providing individuals that can be used to supplement wild populations. The Snake River Sockeye Salmon Captive Broodstock Program implemented a comprehensive monitoring and evaluation plan to evaluate adult recruitment using three juvenile release strategies from excess eggs of captive females. Hatchery‐produced eyed eggs, presmolts, and smolts were released into the upper Salmon River basin to increase the abundance of individuals expressing an anadromous life history as a way to support population augmentation. A mixed‐effects logistic regression model used to evaluate egg‐to‐adult returns between release strategies indicated that release strategies differed from one another. Full‐term smolt production was observed to produce the highest recruitment among the release strategies and has become the focus of recovery efforts involving juvenile releases. The estimated odds of a fish (egg) returning as an adult were 9.7 times greater for smolts reared at the Oxbow Fish Hatchery and 3.6 times greater for smolts reared at the Sawtooth Fish Hatchery relative to eggs that were hatchery reared and released as presmolts or placed into in‐lake incubator boxes. The variation observed in productivity and in expression of life history traits among release strategies indicates that juvenile releases are not one size fits all, and we recommend undertaking a similar assessment to match observed outcomes with desired objectives when initiating supplementation programs.

Growth Limitation of Marine Fish by Low Iron Availability in the Open Ocean

It is well-established that phytoplankton growth can be limited by the vanishingly low concentrations of dissolved iron found in large areas of the open ocean. However, the availability of iron is not typically considered an important factor in the ecology of marine animals, including fish. Here we compile observations to show that the iron contents of lower trophic level organisms in iron-limited regions can be an order of magnitude less than the iron contents of most fish. Although this shortfall could theoretically be overcome if iron assimilation rates were very high in fish, observations suggest this is not the case, consistent with the high recommended iron contents for mariculture feed. In addition, we highlight two occurrences among fish living in iron-poor regions that would conceivably be beneficial given iron scarcity: the absence of hemoglobin in Antarctic icefish, and the anadromous life history of salmon. Based on these multiple lines of evidence, we suggest that the iron content of lower trophic level organisms can be insufficient to support many fish species throughout their life cycles in iron-poor oceanic regions. We then use a global satellite-based estimate of fishing effort to show that relatively little fishing activity occurs in High Nitrate Low Chlorophyll (HNLC) regions, the most readily-identified iron-poor domains of the ocean, particularly when compared to satellite-based estimates of primary production and the observed mesozooplankton biomass in those waters. The low fishing effort is consistent with a low abundance of epipelagic fish in iron-limited regions, though other factors are likely to contribute as well. Our results imply that the importance of iron nutrition extends well beyond plankton and plays a role in the ecology of large marine animals.

Environmental DNA for the enumeration and management of Pacific salmon.

Pacific salmon are a keystone resource in Alaska, generating annual revenues of well over ~US$500million/year. Due to their anadromous life history, adult spawners distribute amongst thousands of streams, posing a huge management challenge. Currently, spawners are enumerated at just a few streams because of reliance on human counters and, rarely, sonar. The ability to detect organisms by shed tissue (environmental DNA, eDNA) promises a more efficient counting method. However, although eDNA correlates generally with local fish abundances, we do not know if eDNA can accurately enumerate salmon. Here we show that daily, and near-daily, flow-corrected eDNA rate closely tracks daily numbers of returning sockeye and coho spawners and outmigrating sockeye smolts. eDNA thus promises accurate and efficient enumeration, but to deliver the most robust numbers will need higher-resolution stream-flow data, at-least-daily sampling, and a focus on species with simple life histories, since shedding rate varies amongst jacks, juveniles, and adults.

Magnetic map in nonanadromous Atlantic salmon

Long-distance migrants, including Pacific salmon (Oncorhynchus spp), can use geomagnetic information to navigate. We tested the hypothesis that a "magnetic map" (i.e., an ability to extract positional information from Earth's magnetic field) also exists in a population of salmon that do not undertake oceanic migrations. This study examined juvenile Atlantic salmon (Salmo salar) originally from a nonanadromous population in Maine transferred ∼60 years ago to a lake in central Oregon. We exposed juveniles to magnetic displacements representative of locations at the latitudinal boundaries of the Pacific salmon oceanic range in the North Pacific and at the periphery of their ancestral oceanic range in the North Atlantic. Orientation differed among the magnetic treatments, indicating that Atlantic salmon detect map information from the geomagnetic field. Despite no recent history of ocean migration, these fish displayed adaptive orientation responses similar to those observed in native Pacific salmonids. These findings indicate that use of map information from the geomagnetic field is a shared ancestral character in the family Salmonidae and is not restricted to populations with anadromous life histories. Lastly, given that Atlantic salmon are transported throughout the world for capture fisheries and aquaculture, such a robust navigational system is of some concern. Escaped individuals may have greater potential to successfully navigate, and thus invade, introduced habitats than previously suspected.