Number Of Smolts Research Articles

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.

Can the creation of new freshwater habitat demographically offset losses of Pacific salmon from chronic anthropogenic mortality?

Over 1 billion USD are devoted annually to rehabilitating freshwater habitats to improve survival for the recovery of endangered salmon populations. Mitigation often requires the creation of new habitat (e.g. habitat offsetting) to compensate population losses from human activities, however offsetting schemes are rarely evaluated. Anadromous Pacific salmon are ecologically, culturally, and economically important in the US and Canada, and face numerous threats from degradation of freshwater habitats. Here we used a matrix population model of coho salmon (Oncorhynchus kisutch) to determine the amount of habitat offsetting needed to compensate mortality (2-20% per year) caused by a range of development activities. We simulated chronic mortality to three different life stages (egg, parr, smolt/adult), individually and simultaneously, to mimic impacts from development, and evaluated if the number of smolts produced from constructed side-channels demographically offset losses. We show that under ideal conditions, the typical size of a constructed side-channel in the Pacific Northwest (PNW) (3405 m2) is sufficient to compensate for only relatively low levels of chronic mortality to either the parr or smolt/adult stages (2-7% per year), but populations do not recover if mortality is >10% per year. When we assumed lower productivity (e.g.; 25th percentile), we found that constructed channels would need to be 2.5-4.5 fold larger as compared to the typical size built in the PNW, respectively, to maintain population sizes. Moreover, when we imposed mortality to parr and smolt/adult stages simultaneously, we found that constructed side-channels would need to be between 1.8- and 2.3- fold larger that if the extra chronic mortality was imposed to one life stage only. We conclude that habitat offsetting has the potential to mitigate chronic mortality to early life stages, but that realistic assumptions about productivity of constructed side-channels and cumulative effects of anthropogenic disturbances on multiple life stages need to be considered.

Smolt Transportation Influences Straying of Wild and Hatchery Snake River Steelhead into the John Day River

AbstractBarge transportation of steelhead Oncorhynchus mykiss smolts through the Snake and Columbia rivers can increase the probability of straying by returning adults at the individual scale. However, the effect of barge transportation on a major population group has not been evaluated. We estimated the proportion of hatchery‐origin steelhead spawners present in the John Day River from 2004 to 2018 via observation of adults on spawning ground surveys and in traps. Despite no hatchery releases within the John Day River, up to 42% of the observed spawners in nature were of hatchery origin. The proportion of hatchery‐origin spawners in the John Day River was best explained by the number of smolts that were barge transported from the Snake River 2–3 years prior to spawning and natural‐origin spawner abundance. At an individual scale, barged hatchery steelhead from the Snake River, when compared with hatchery and wild steelhead that were allowed to migrate in‐river as smolts, were 73 times more likely to be detected in the John Day River during their adult return migration. Barged hatchery and barged wild steelhead exhibited no difference in the probability of adult detection in the John Day River. Our study demonstrates how a salmonid recovery effort in one portion of the Columbia River basin had an unintended and potentially adverse impact on non‐target populations. To minimize stray spawners (both hatchery and wild) in the John Day River, transportation of Snake River steelhead smolts should replicate spawning years 2013–2018 (i.e., the 2010–2016 smolt cohorts), with transportation not beginning until May 1 and with the total number of transported hatchery smolts maintained at a level no greater than 2.3 million annually.

Marine mortality in the river? Atlantic salmon smolts under high predation pressure in the last kilometres of a river monitored for stock assessment

AbstractThe River Bush (Northern Ireland) is an index river for the estimation of Atlantic salmon, Salmo salar L., stock size, population dynamics and marine survival rates. Marine survival estimates are based on the number of smolts counted at a trap 3.5 km upstream of the river outlet. The survival from release to coastal inshore waters for acoustic‐tagged smolts released at the Bushmills trap varied between 32% and 68%, with both year and brightness during river exit playing a significant role in explaining the variations in survival. This constitutes an important survival bottleneck. Contrary to true marine mortality, this significant loss of smolts in the river and nearshore environments could be reduced by focused management actions. More studies on other rivers, where smolts are enumerated above the head of tide, could further partition smolt and post‐smolt mortality, help differentiate true marine survival and help understand fluctuations in adult returns.

Supportive breeders of Atlantic salmon Salmo salar have reduced fitness in nature

AbstractWild Atlantic salmon (Salmo salar) are in decline over the entire distribution area, and populations are enhanced by supportive breeding. Parental fish are sampled in rivers, crossed and the offspring reared in hatcheries until smolting when released, ready for ocean migration. Naturally reproducing salmon entering the River Imsa from the ocean were monitored from 1976 through 2013, and since 1982, supportive breeders spawning in the river were also monitored. The supportive breeders were first‐generation salmon, reared for 1—2 years in a hatchery before being released at the mouth of the river so they could migrate to sea (i.e., sea‐ranching). Wild and sea‐ranched salmon live in the ocean for 1—2 years before they return to the river for spawning. In years when only wild adults were present, mean number of smolts produced per 100 m2 river area and female breeder was 0.47. When there were 5% wild females, the mean production was only 0.088 (19%). The gradual decrease in offspring production with decreasing proportion of wild females (r2 = .41) indicates that the reduced offspring production was caused by inferior spawning behavior of hatchery females or reduced survival of their offspring in nature. Previous experimental evidence suggests that wild males fertilize the eggs of wild but also hatchery females. It is discussed how epigenetic effects caused by hatchery environments influence the developing juveniles, lead to phenotypic changes that may reduce their fitness in nature even after free ranging for a year or more in the ocean before they return and spawn.

Assessment of Early Migration Dynamics of River‐Specific Hatchery Atlantic Salmon Smolts

AbstractMany Atlantic Salmon Salmo salar populations within the southern extent of the species’ range are at critically low abundances, while others have been extirpated. The focus of many ongoing recovery efforts is on maximizing the number of smolts that reach the ocean, where marine survival is low, primarily through hatchery supplementation and improvement of in‐river hydropower system management. However, estuaries also are known to be sites of high mortality, although in many cases the correlates of this mortality are poorly characterized. We acoustically tagged hatchery smolts (n = 666) during 2001–2005 to evaluate migration performance through freshwater, estuarine, and bay reaches of the small but tidally dynamic Dennys River, Maine, USA, to investigate potential drivers contributing to low returns within the system. Migration behaviors (e.g., migration timing and tidal use) were typical for Atlantic Salmon smolts, but reversals during migration were observed upon entry into the estuary environment more frequently in this system than in regional rivers. We used Cormack–Jolly–Seber mark–recapture models to estimate apparent survival within the study environment during this period. We found two distinct periods of survival, with much lower survival in 2003–2005 than in 2001 and 2002 (more than a 40% decrease). Among the variables considered, temperature and migratory behavior had the largest effects on survival. We found that survival increased by about 28% across the range of temperatures observed during these years; additionally, survival increased by about 15% for fish that reversed direction during migration compared to fish that did not. These results indicate that extreme environmental changes during the transition through this coastal system constitute a significant obstacle to Atlantic Salmon restoration stocking efforts.

Trophic Feasibility of Reintroducing Anadromous Salmonids in Three Reservoirs on the North Fork Lewis River, Washington: Prey Supply and Consumption Demand of Resident Fishes

AbstractThe reintroduction of anadromous salmonids in reservoirs is being proposed with increasing frequency, requiring baseline studies to evaluate feasibility and estimate the capacity of reservoir food webs to support reintroduced populations. Using three reservoirs on the north fork Lewis River as a case study, we demonstrate a method to determine juvenile salmonid smolt rearing capacities for lakes and reservoirs. To determine if the Lewis River reservoirs can support reintroduced populations of juvenile stream‐type Chinook Salmon Oncorhynchus tshawytscha, we evaluated the monthly production of daphnia Daphnia spp. (the primary zooplankton consumed by resident salmonids in the system) and used bioenergetics to model the consumption demand of resident fishes in each reservoir. To estimate the surplus of Daphnia prey available for reintroduced salmonids, we assumed a maximum sustainable exploitation rate and accounted for the consumption demand of resident fishes. The number of smolts that could have been supported was estimated by dividing any surplus Daphnia production by the simulated consumption demand of an individual Chinook Salmon fry rearing in the reservoir to successful smolt size. In all three reservoirs, densities of Daphnia were highest in the epilimnion, but warm epilimnetic temperatures and the vertical distribution of planktivores suggested that access to abundant epilimnetic prey was limited. By comparing accessible prey supply and demand on a monthly basis, we were able to identify potential prey supply bottlenecks that could limit smolt production and growth. These results demonstrate that a bioenergetics approach can be a valuable method of examining constraints on lake and reservoir rearing capacity, such as thermal structure and temporal food supply. This method enables numerical estimation of rearing capacity, which is a useful metric for managers evaluating the feasibility of reintroducing Pacific salmon Oncorhynchus spp. in lentic systems.Received April 24, 2016; accepted July 28, 2016 Published online October 11, 2016

Fecundity and water flow influence the dynamics of Atlantic salmon

AbstractPopulations are retained at reduced levels by resource competition and environmental stochasticity. In the Norwegian River Imsa, the relationship between fecundity of Atlantic salmon (Salmo salar) spawners and number of smolts per unit river area was investigated for cohorts spawned from 1976 to 2011. Annual number of smolts produced per unit area was best described by a multiplicative model and increased with the fecundity of the females as proxy for number of eggs deposited and the minimum water flow in August towards the end of the first growth season. Mean monthly water temperature, or water flow in any other month during the first year, had no significant effect on number of smolts produced. At sea, there was an almost linear relationship between number of emigrating smolts and returning adults, possibly because population abundance of Atlantic salmon is low relative to the carrying capacity in the ocean. Thus, both number of eggs spawned and minimum water flow in late summer influenced population abundance in the present river.

Changes in smolt traits of Atlantic salmon (Salmo salar Linnaeus, 1758) and linkages to parr density and water temperature

Smolt traits (length, age) and timing of smolt migration of wild Atlantic salmon, Salmo salar L., were investigated in the Simojoki River, northern Baltic Sea. The aim was to determine whether they responded to changes in parr length, parr density and temperature from 2000 to 2014. Annual electrofishing surveys and smolt numbers determined parr densities by springtime trapping in the river mouth. During the smolt trapping period captured parr and smolts were aged from scales. Water temperature was measured daily. Mean length decreased from 137 mm (TL) to 129 mm among 2-year-old smolts, and from 150 mm to 139 mm among 3-year-olds. Median date of the smolt migration was 10 days earlier, from early June to late May during the study period, linked to the rise in air temperature in May at the nearby Kemi-Tornio airport. However, the median day temperature and the mean daily water temperatures during the second (Q2) and third (Q3) migration quartiles did not change. This implied that migration began when a suitable water temperature was reached, independent of the date.

Growth and Smoltification of Three Norwegian Strains of Atlantic Salmon Salmo salar Reared under Different Thermal Regimes

AbstractA comparative study was performed with juvenile Atlantic salmon Salmo salar from three stocks in Western Norway that differ in their natural conditions. One is from warm, lowland river conditions (Årdal), one is from cold glacial river conditions (Stryn) and the last one (Suldalslågen) is from a hydropower‐regulated river. The salmon parr were tagged and reared at 4, 5.5 and 7 °C and simulated natural water temperature (SNT) for river Suldalslågen. Size distribution was unimodal at 4 °C, with a change to a bimodal distribution, representing potential 1+ and 2+ smolts, at the other temperature regimes. The relative biomass of 1+ smolts varied between the stocks as Stryn (cold glacial river) stock had the highest number of smolts at 7 °C and the Suldalsågen stock (hydropower‐regulated river) displaying the highest number at the SNT regime. Overall, the Stryn stock, originating from cold river conditions, seemed to be well adapted to growth and smoltification at cold temperatures, whereas salmon parr from river Suldalslågen seem to be better adapted to the natural temperature regime (SNT) of this river than the other two stocks. This was reflected in the gill Na+,K+‐ATPase as the Suldalslågen stock showed increasing activity from 16 April (4.2 µmol ADP mg protein−1 h−1) to 10 May (9.2 µmol ADP mg protein−1 h−1), and at the end of the experiment, enzyme activity in Suldalslågen stock was significantly higher than both Stryn (5.7) and Årdal (5.9 µmol ADP mg protein−1 h−1) stock. In contrast, the warm lowland stock, Årdal, fish had low Na+,K+‐ATPase activity with no distinct peak at any of the sampled dates from March through May. Copyright © 2016 John Wiley & Sons, Ltd.

Catchment-wide survival of wild- and hatchery-reared Atlantic salmon smolts in a changing system

We developed a hierarchical multistate model to estimate survival of Atlantic salmon (Salmo salar) smolts in the Penobscot River, USA, over a decade during which two mainstem dams were removed from the catchment. We investigated effects of (i) environmental factors, (ii) rearing history, and (iii) management actions, including dam removal, turbine shutdown, and installation of new powerhouses. Mean ± SD smolt survival per kilometre was higher through free-flowing reaches of the catchment (0.995 ± 0.004·km−1) than through reaches containing dams that remain in the system (0.970 ± 0.019·km−1). We observed maximum survival between 12 and 17 °C and at intermediate discharges (1200 m3·s−1). Smolt survival increased concurrent with dam removal and decreased following increases in hydropower generation. The greatest increase in smolt survival followed seasonal turbine shutdowns at a dam located on the largest tributary to the Penobscot River, while other shutdowns had little influence. Our model provides a useful tool for assessing changes to survival of migratory species and will be useful for informing stocking plans to maximize numbers of smolts leaving coastal systems.

Growth and survival of pacific coho salmon smolts exposed as juveniles to pesticides within urban streams in western Washington, USA

Pesticides are frequently detected in urban streams, with concentrations often exceeding those reported in surface waters within agricultural areas. The authors studied growth, survival, and return rates of coho salmon (Oncorhynchus kisutch) smolts exposed to a pesticide mixture ("cocktail") representative of the pesticides most frequently reported within urban streams in western Washington State, USA, in fall through early spring. Exposure concentrations were selected to represent a reasonable worst-case scenario based on field monitoring data. Smolts were continuously exposed to pulses of the cocktail either from fertilization through swim-up (2007-2008) or from fertilization through smoltification (2007-2008 and 2008-2009), coded wire tagged, and released in 2008 and 2009. Pre-release endpoints (growth, survival, sex ratio, brain acetylcholinesterase activity, and gonado- and hepatosomatic indices) were not affected. However, the number of returning adults exposed to the cocktail to swim-up (0.90%, n = 42) was more than double that of unexposed controls (0.38%, n = 26) in 2008, whereas in 2009, fish exposed through smoltification returned in lower numbers (0.15%, n = 18) than controls (0.37%, n = 30). Variability in return rates among treatments between years was comparable to that observed in previous whole life cycle studies with Pacific salmon and other contaminants. Results suggest that exposure to pesticides in urban streams does not directly impair early life stages of coho salmon, and that additional studies incorporating releases of larger numbers of smolts across several years are necessary to adequately quantify effects on return rates.

Wild chinook salmon survive better than hatchery salmon in a period of poor production

The population dynamics of chinook salmon (Oncorhynchus tshawytscha) from the Cowichan River on Vancouver Island, British Columbia, Canada are used by the Pacific Salmon Commission as an index of the general state of chinook salmon coast wide. In recent years the production declined to very low levels despite the use of a hatchery that was intended to increase production by improving the number of smolts entering the ocean. In 2008, we carried out an extensive study of the early marine survival of the hatchery and wild juvenile chinook salmon. We found that both rearing types mostly remained within the Gulf Islands study area during the period when most of the marine mortality occurred for the hatchery fish. By mid September, approximately 1.3% of all hatchery fish survived, compared to 7.8%–31.5% for wild fish. This six to 24 times difference in survival could negate an estimated increased egg-to-smolt survival of about 13% that is theorized to result through the use of a hatchery. Estimates of the early marine survival are approximate, but sufficient to show a dramatic difference in the response of the two rearing types to the marine nursery area. If the declining trend in production continues for both rearing types, modifications to the hatchery program are needed to improve survival or an emphasis on improving the abundances of wild stocks is necessary, or both. The discovery that the juvenile Cowichan River chinook salmon remain within a relatively confined area of the Gulf Islands within the Strait of Georgia offers an excellent opportunity to research the mechanisms that cause the early marine mortalities and hopefully contribute to a management that improves the production.

Possibilities of using the length differentiation of hatchery sea trout, Salmo trutta m. trutta L., parr to predict numbers of one-year smolts

Growth and smoltification were followed during the first year of life in 14 full sibling families of sea trout, Salmo trutta m. trutta L., originating from the Vistula stock. Until January, the families were reared separately, and then together after they had been PIT tagged. The degree of smoltification was evaluated in the second spring. Smolts were noted in each family. The percentage of these, which ranged from 14 to 66%, depended on mean fish length in October (r 2 = 0.338) and January (r 2 = 0.346). In either October or January the fish that smolted next spring were, on average, longer than those that did not, but this difference was not manifested by the bimodality of length distribution, and it did not allow predicting the number of smolts based on division of distributions of fish length in autumn or winter into Gaussian components.

Night stocking facilitates nocturnal migration of hatchery‐reared Atlantic salmon, Salmo salar, smolts

Abstract Hatchery‐reared salmon, Salmo salar L., smolts are generally stocked during daylight hours, but the natural migration of smolts tends to occur at night. Recapture rates and timing of migration were compared between Atlantic salmon smolts stocked during the day and during the evening. Timing of release had no significant effect on the number of smolts recaptured, but had a strong effect on nocturnal behaviour. When stocked in the evening (but not during the day) hatchery‐reared smolts moved almost exclusively during the night. This study suggests that timing the release to coincide with the natural time of smolt migration may provide valuable acclimatisation and facilitate nocturnal smolt passage.

Influence of shelters and food resources on smoltification of juveniles of the Atlantic salmon Salmo salar

Influence of feeding conditions and of the presence of a various number of shelters in basins on development of differentiation of juveniles of Salmo salar at the age 1+ and formation of smolts were studied under experimental conditions. The experiments had been carried out on groups of ten fish. The influence of investigated factors turned out to be significant. Under normal feeding four times a day, the quantity of formed smolts depends on the number of shelters: the more shelters that are available, the fewer smolts appear in groups. The presence of one shelter is a strong stimulant enhancing aggressive activity towards conspecific individuals, resulting in destruction of some fish in the group. Under feeding once a day, i.e., under food shortage, more fish perished, and the number of smolts decreased as the fish did not accumulate a sufficient energy potential for the morphophysiological transformation. One of the potential mechanisms of development of differentiation of fish juveniles into parrs and future smolts is discussed, based on domination of some fish possessing the individual territory and exclusion of subdominant specimens into open water as a result of social interaction.

Pike predation on hatchery‐reared Atlantic salmon smolts in a northern Baltic river

Abstract – The effect of pike Esox lucius predation on the mortality of newly stocked Atlantic salmon Salmo salar smolts was investigated in the Pyhäjoki River, Finland. The number of smolts eaten by pike was assessed by estimating the size of the pike population (mark–recapture experiment) and studying the stomach contents of pike. Before recapturing the pike, approximately 39,700 smolts were stocked upstream of the 2.5‐km‐long (89‐ha) research area. The estimated size of the >40‐cm pike population was 1507 (95% CL 1012–4731) individuals (17 pike and 29.8 kg·ha−1). Pike were estimated to eat 29% of the released smolts during 1 week. The diet of the pike in the research area consisted almost entirely of smolts, whereas in the reference area with no stocked smolts, the meal sizes were significantly smaller and the importance of smolts as prey was substantially lower. Pike <40 cm had not eaten any smolts, probably indicating a size refuge for the smolts, or alternatively fear of intraspecific interactions or cannibalism of pike.

The Effect of an Introduced Summer Steelhead Hatchery Stock on the Productivity of a Wild Winter Steelhead Population

AbstractWe investigated the effect of a hatchery program for summer steelhead Oncorhynchus mykiss on the productivity of a wild winter steelhead population in the Clackamas River, Oregon. We used a suite of Ricker and Beverton–Holt stock–recruitment models that incorporated species interaction variables to demonstrate that when high numbers of hatchery summer steelhead adults were present the production of wild winter steelhead smolts and adults was significantly decreased. We found that large releases of hatchery smolts also contributed to the decrease in wild adult productivity. Averaged over the results of our models, a 50% decline in the productivity parameter (the number of recruits per spawner at low densities) and a 22% decline in the maximum number of recruits produced in the basin were observed when high numbers of hatchery fish were present. We concluded that over the duration of the hatchery program, the number of hatchery steelhead in the upper Clackamas River basin regularly caused the total number of steelhead to exceed carrying capacity, triggering density‐dependent mechanisms that impacted the wild population. The number of smolts and adults in the wild winter steelhead population declined until critically low levels were reached in the 1990s. Hatchery fish were removed from the system in 2000, and early results indicate that the declining trends have reversed.

Evaluation of Hatchery versus Wild Sockeye Salmon Fry Growth and Survival in Two British Columbia Lakes

AbstractWe evaluated the relative success of sockeye salmon hatchery fry stocking in two British Columbia–Alaska transboundary lakes (1,622‐ha Tatsamenie Lake and 492‐ha Tahltan Lake). Fry stocking began in the late 1980s and is still under way. During the study period, survival patterns in the two lakes were different. At Tatsamenie Lake, wild egg‐to‐fry survival was higher than hatchery egg‐to‐fry survival (11.3% versus 4.3%) and wild egg‐to‐smolt survival was higher than hatchery egg‐to‐smolt survival (5.8% versus 2.5%). We found no relationship between fry survival and stocking date, spawner abundance, or food availability in Tatsamenie Lake, but we did find a significant positive relationship between early‐spring fry length and egg‐to‐fry survival. Also, we found that in net‐pen experiments, larger fry survived better. From this, we concluded that hatchery fry stocked into Tatsamenie Lake may suffer from size‐mediated mortality and that net‐pen supplemental feeding could be used to overcome this problem. However, because eggs used for hatchery production come from the lake, fry stocking can only be justified when hatchery fry survival exceeds wild fry survival. At Tatsamenie Lake, this goal has not yet been attained. At Tahltan Lake, wild egg‐to‐fry survival was lower than hatchery egg‐to‐fry survival (1.5% versus 6.3%) and wild egg‐to‐smolt survival was lower than hatchery egg‐to‐smolt survival (3.6% versus 12.8%). Hatchery fry release date and food availability both failed to explain these differences in fry and smolt survival at Tahltan Lake. However, there was a strong negative relationship between total female escapement and wild egg‐to‐smolt survival. This supported the results of an earlier investigation, which suggested that Tahltan Lake was spawning‐site limited. We concluded that at Tahltan Lake, hatchery fry stocking increased smolt numbers, is biologically justified, and should be continued.

Genetic architecture of growth and early life‐history transitions in anadromous and derived freshwater populations of steelhead

Heritabilities of growth, precocious maturation and smolting were measured in 75 families of juvenile steelhead or rainbow troutOncorhynchus mykiss, progeny of within and between line matings (crosses) of wild, anadromous steelhead and wild, resident (lake) rainbow trout originally derived from the same anadromous stock 70 years earlier. The tagged yearling progeny were combined by line in common freshwater rearing containers and graded into three categories: mature, smolt or rearing (undifferentiated) at age 2 years. Heritabilities of precocious male maturity, smolting and growth were moderate to high, and the genetic correlation between growth and smolting was low. Smolting and precocious male maturity were highly variable among families within lines and significantly different between lines. Each of the four lines produced significant numbers of smolts at age two. Smolting and maturation were negatively genetically correlated, which may explain the persistence of smolting in the lake population despite strong selection against lake smolts; balancing selection on male maturation age may help to maintain variation for smolting. The high heritability of smolting, coupled with the inability of smolts that leave the lake to return to it indicates that the genetic potential for smolting can lie dormant or be maintained through a dynamic interaction between smolting and early maturation for decades despite complete selection against the phenotype. The results have significant implications for the preservation of threatened anadromous stocks in fresh water and the inclusion of resident fish of formerly anadromous populations, currently trapped behind long‐standing barriers to migration, as one component of the same population.