U.S. Dept Commerce/NOAA/NMFS/NWFSC/Publications

NOAA Tech Memo NMFS NWFSC-30:
Genetic Effects of Straying of Non-Native Hatchery Fish into Natural Populations


HOMING, STRAYING, AND COLONIZATION

Thomas Quinn

School of Fisheries
University of Washington
Box 357980
Seattle, WA 98195, USA

Introduction

The intent of this contribution is to define what is meant by homing and straying, and to describe patterns of homing and straying in salmon populations. I will explore what practices or other factors in hatcheries might encourage straying, and then outline the consequences of straying. By way of historical perspective, it was accepted by about the 1870s that most salmon homed back to their natal streams and rivers to spawn, although some biologists remained unconvinced until the 1930s. A report for the U.S. Commission of Fish and Fisheries stated that ". . . it is an established fact that adult [salmon] will always return to the place where they first made acquaintance with the water, passing directly by the mouths of streams or tributaries better adapted to their purpose, to gain their original home" (U.S. Commission of Fish and Fisheries 1874, p. lxxxii). Biologists also recognized that salmon could swim several hundreds of miles up a river to their natal areas. A later report to the Commission pointed out that a stream near Elko, Nevada "is one of the many that form the headwaters of the Columbia River, and to this point, eighteen hundred miles from its mouth, the salt-water salmon come in myriads to spawn . . ." (U.S. Commission of Fish and Fisheries 1876, p. xxviii-xxix). Milner noted "[t]he generally accepted fact in the habits of anadromous fishes that they are disposed to return to almost the exact locality where they passed their embryonic and earlier stages of growth . . . . Observations of the shad brought to the large markets shows considerable difference in the physiognomy and general contour of those from different rivers. The suggestion is natural that they are distinct and separate colonies of the same species, and thus slight characteristics are perpetuated because they breed in-and-in and do not mix with those of other rivers" (Milner 1876, p. 323).

By the 1930s, a number of biologists were aware of life-history differences among salmon populations (Moulton 1939; e.g., Clemens et al. 1939). In fact, this realization gave the first indication, before tagging studies, that salmon homed to particular localities. Salmon from various populations showed differences in morphology, body size, egg size, fin ray counts, oil content, and so on. In the 1950s to 1970s, knowledge of homing by Pacific salmon was greatly enhanced by the work of Arthur Hasler and his students (reviewed in Hasler and Scholz 1983). Their studies formed the basis for much of what we know about salmon homing. However, straying was not investigated as a behavior pattern in its own right because most salmon homed, and the focus of the research was on the sensory mechanisms of homing. Consequently, the ecological and evolutionary importance of straying from one population to another has received comparatively little attention until recently. Today, we know that there is extensive variation among populations in many traits and that this variation often has clear adaptive value. Such local adaptations have presumably evolved because homing leads to reduced levels of gene flow between habitats, and because there is genetic control of the traits that adapt the salmon for those habitats. It has been hypothesized (Quinn 1984) that adaptations evolve most rapidly in stable habitats and that homing is likely to be positively associated with the intricacy of adaptations for freshwater habitat, and with variation in age at return. Homing and straying have adaptive value for individuals; the relative advantages may depend on environmental conditions, other life-history traits, and perhaps the relative frequencies of homing/straying (Quinn 1984, Kaitala 1990).

Homing and Straying: Definitions and Qualifications

Just what is meant by homing and straying? For a wild fish, home is the natal stream where it incubated, hatched, and emerged. Home is thus, essentially, the redd. However, when humans study salmon homing, the definition of home is influenced by how and where juvenile fish are collected and marked, and how they are recaptured as adults. These factors also influence the perception of how accurately salmon home. One might think of salmon homing through a hierarchy of spatial scales, including first a river basin, then a major tributary, a stream, and a particular point in the stream. Homing will necessarily be more accurate when measured at broader spatial scales. At the final level, the interaction between homing and spawning site selection (Blair and Quinn 1991, Hendry et al. 1995) or mate choice determines the final destination of the fish. The definitions of straying or homing, therefore, depend on the spatial scale of interest. Most research has not been sufficiently explicit in considering the spatial definition of home, and the transition between homing and spawning site selection.

For transplanted fish, the ancestral locality or the hatchery where they are reared and the locality where they were released could both be considered homes. While there is some tendency to return to the ancestral area (McIsaac and Quinn 1988, Pascual and Quinn 1994), salmon generally return to the site where they were released (Ricker 1972). For salmon released from a hatchery, the incubation, rearing, and release sites may be the same; in this case, home is the hatchery. When planted from the hatchery to a river, salmon tend to return to the point of release (e.g., Donaldson and Allen 1958, reviewed in Quinn 1993). Fish released in the lower portion of a river tend to be caught only in the lower portion of that river, and fish released in the middle or upper portion of a river tend to be caught in all parts of the river downstream from the release site (steelhead trout (Oncorhynchus mykiss): Wagner 1969, Cramer 1981, Slaney et al. 1993; Atlantic salmon (Salmo salar): Hvidsen et al. 1994, Potter and Russell 1994).

The other side of the homing "coin" is straying. Adult salmon move into non-natal streams for a variety of reasons. We know from radio-tracking data that some fish do not home directly to their natal streams, although these streams may be their final destination (e.g., Berman and Quinn 1991). Upriver migration is characterized by a certain amount of exploratory movement into non-natal streams. If a fish makes an exploratory run up a stream, is caught in a hatchery weir, and is spawned in the hatchery, this constitutes straying from a functional point of view. The fish's genes are incorporated into the hatchery gene pool regardless of whether the fish would have left the hatchery had it been allowed to do so. Consequently, it may be difficult to accurately estimate straying frequencies using data from hatcheries. However, it is clear that some salmon spawn in rivers other than their own and so stray in the truest sense (Quinn et al. 1991).

Estimates of Straying

While many studies have provided data on the proportion of salmon that stray, almost all of these studies have been on single species, and little information exists on comparative straying rates among species. In one of the few such studies, Shapovalov and Taft (1954) reported higher levels of straying by coho salmon (O. kisutsch) than by steelhead into two small creeks in California. It has been speculated that pink salmon (O. gorbuscha) stray more than other species, but hard evidence is lacking. High levels of intraspecific variability may mask interspecific differences. The available information for coho and chinook salmon (O. tshawytscha), for which we have the most data, indicates large amounts of homing variability among populations, even within a small geographical area.

Another problem with the literature on homing is that wild salmon are tagged less frequently than hatchery-produced fish, and when wild salmon are tagged the data are seldom analyzed to produce estimates of straying. Consequently, little is known about straying in wild salmon populations, and most estimates of straying come from hatcheries. Hatchery-produced salmon may not stray with the same frequency as wild salmon, but so few studies have been conducted on hatchery and wild fish in the same areas that we cannot be certain (see below). Many experiments designed to estimate straying are also poorly controlled or are not replicated. In many studies, measuring variability in homing was incidental to other goals, so the data are often confounded with factors besides straying. Most studies also fail to account for straying in and out of a population; in many cases, only the dispersal of strays from the marking site is documented.

As a rough estimate, 90% +/- 10% of salmon home, and this does not include the "pathological" levels of straying that were shown earlier in the workshop for some Snake River hatcheries (Crateau, this volume) and have been documented for some hatcheries on the lower Columbia River (e.g., Grays River chinook salmon: Pascual et al. 1995). However, the overall estimate of 80-100% homing is based largely on data from hatcheries.

Straying in hatchery vs. wild populations

It is difficult to determine from the data at hand whether straying differs between hatchery and wild populations, because studies of hatchery populations greatly outnumber studies of wild populations. Comparisons between wild and hatchery-produced Pacific salmon were conducted by McIsaac (1990) and Labelle (1992). McIsaac (1990) studied fall-run chinook salmon in the Lewis River and found that wild-caught juveniles homed at a higher rate than members of the population that had been incubated and reared in the hatchery. Moreover, short-term rearing of wild fish in a hatchery increased their rate of straying, relative to wild fish not held in the hatchery. On the other hand, Labelle's (1992) study of coho salmon on the east coast of Vancouver Island did not find a significant difference in straying rates between hatchery-produced and wild fish. Studies of Atlantic salmon also did not find differences between the straying rates of hatchery and wild fish (Jonsson et al. 1991, Potter and Russell 1994).

Regional and temporal patterns of straying

Coded wire tagging has provided a large database which can be used for homing studies (van der Haegen and Doty 1995). These data show that spatial patterns of straying vary from one river to another. The proportion of salmon that stray is not the same in all hatcheries in a region such as the lower Columbia River. In addition, the proportion of the total number of straying salmon entering a given river is not simply explained by its distance from the hatchery of origin. For example, Cowlitz River spring-run chinook salmon strayed more often to the Lewis River than to the Kalama River, even though the Kalama River is closer to the Cowlitz River than is the Lewis River (Quinn and Fresh 1984).

It appears that salmon do not stray merely because they are fatigued and cannot reach their natal spawning areas. In many cases, they stray to localities above their river of origin. The proportions of salmon straying into and out of a hatchery can vary considerably. Quinn et al. (1991) found variation from 9.9-27.5% in the proportions of fall-run chinook salmon straying from five lower Columbia River hatcheries. More dramatic, however, was the variation in attractiveness of rivers to strays. Virtually no salmon strayed into the Washougal and Abernathy Hatcheries, but about 30% of the marked salmon entering the Kalama and Lewis Rivers were strays (Quinn et al. 1991). Expanded examination by Pascual and Quinn (1994) confirmed these patterns of variation in straying and found that fish seemed more likely to enter rivers or hatcheries similar to their home than to less similar sites. For example, salmon produced in tributaries of the Columbia River seemed to stray into other tributaries rather than to hatcheries along the mainstream of the river.

In addition to differences in straying among rivers, straying can also differ from year to year. Interannual variability may be associated with catastrophic events such as the eruption of Mount St. Helens (Leider 1989). Less dramatic environmental changes such as variation in flow and temperature may also contribute to temporal variability in straying, but definitive studies do not seem to have been conducted on these subjects. There is some evidence that temporal variation in straying is associated with population size (Quinn and Fresh 1984). In years when many fish returned to the Cowlitz River hatchery, homing was better than in years when fewer fish returned. This suggests that the dynamics of small populations may be different from those of larger populations. This is an important issue and it needs to be evaluated with other data sets. There is also interannual variation in straying from a site, perhaps related to water quality, rearing conditions, or the number of returning salmon. The tendency of hatchery-produced salmon to enter their hatchery, as opposed to spawning in the river, can also vary greatly from year to year (Nicholas and Downey 1983).

Age at return also contributes to variability in straying. Older chinook salmon tend to stray more than younger fish (Quinn and Fresh 1984, Quinn et al. 1991, Unwin and Quinn 1993, Pascual et al. 1995). The difference in the rate of straying by chinook jacks and by 4- or 5-year-old fish may be an order of magnitude (Quinn and Fresh 1984). Age-specific straying rates have also been observed for coho salmon (Labelle 1992), but not for Atlantic salmon (Potter and Russell 1994). Perhaps, the longer a fish is out to sea, the more it forgets the olfactory cues it needs to return to its natal locality. The turnover of sensory epithelial cells associated with odor recognition (Nevitt et al. 1994), changes in the odors of river water, or some unknown evolutionary mechanism may be responsible for this age effect. Hatchery practices can also influence the age structure of the spawning population, which may in turn influence straying.

Straying and colonizing new areas

Little is known about the relationship between straying and the colonizing of unoccupied areas. Although most translocations of salmon have been notoriously unsuccessful, some have succeeded. For example, the inadvertent translocation of pink salmon into the Great Lakes resulted in a rapid colonization of Lake Superior and other Great Lakes (Kwain 1987). The translocation of chinook salmon to one river in New Zealand quickly led to unaided colonization of several other rivers within 15 years, but the present level of straying among rivers is not high enough to account for the wide-spread colonization that apparently took place after the initial introduction (Unwin and Quinn 1993, Quinn and Unwin 1993).

In addition to translocations, some natural colonization by salmon also occurs. For example, in Glacier Bay, Alaska, new habitat appears as the glacier recedes, and new habitat is colonized as it becomes suitable for spawning (Milner 1987, Milner and Bailey 1989). Straying and the ability to colonize new areas over evolutionary time is important, but little research has been done on this topic. It appears that soon after colonization, and coincident with small population sizes, straying rates may be high; however, after populations become established, only modest rates of straying occur.

Hatchery Practices and Straying

Some hatchery practices might promote straying, the most obvious being the long-standing practice of transporting individuals from one locality to another. Salmon are commonly displaced from hatcheries to "seed" nearby habitat. Most fish reared at one facility through their juvenile stages, but released at another site, return to the site of release and not to the rearing facility (e.g., Donaldson and Allen 1958, Quinn et al. 1989, reviewed by Quinn 1993). Several researchers have studied the details of the timing of imprinting and have found that fish can be imprinted not only at the smolt stage, but also to a lesser extent at earlier stages (Dittman et al. 1994, 1996). Therefore, if a rearing hatchery is in one watershed and the release site is in another watershed, fish tend to return to the release site. As the distance between the rearing facility and the release site gets closer, larger numbers of fish return to the rearing facility, especially if the facility and release site are in the same watershed (Quinn 1993). However, the amount of "straying" from the release site is only roughly correlated with geographical distance. The release site's position within the watershed also affects homing. Johnson et al. (1990) reported that "almost all returning [coho salmon] released as yearlings at a site 23 km upstream from the rearing hatchery returned to the rearing site, whereas only 7-26% of adults originally released in a tributary 11 km downstream from the rearing hatchery returned to the rearing site" (p. 427).

In the Columbia River system, smolts are also displaced to improve their post-release survival. They may be taken from their hatchery ("point of origin" transportation) or captured during their downstream migration, trucked or barged around dams, and then released downriver. Point of origin transportation is usually accomplished by taking the fish by truck, or by truck and then by barge. Coho salmon trucked from the Little White Salmon Hatchery to Youngs Bay returned to Youngs Bay, not to the hatchery (Vreeland et al. 1975). Coho trucked 9 km downstream from Willard Hatchery and then barged to a release point below Bonneville Dam showed improved survival but impaired homing (McCabe et al. 1983). Releases in salt water also tend to increase straying. Solazzi et al. (1991) trucked coho salmon (reared at least in part at Big Creek Hatchery) to release sites below Bonneville Dam (river km 234), and Tongue Point (rkm 29). In addition, smolts were taken by boat in tanks receiving ambient water to the bar of the river (rkm 2), 19 km offshore in the river's plume, 19 km offshore outside the river's plume, and 38 km offshore in non-plume water. These six locations, progressively farther from the rearing site, produced the following proportions of salmon that returned to rivers outside the Columbia River system: <0.1%, 3.4%, 4.1%, 6.1%, 21.0%, and 37.5%. However, salmon captured as migrants and trucked long distances (e.g., from Ice Harbor Dam to Bonneville Dam) may return to the rearing site (Ebel et al. 1973, Slatick et al. 1975). Overall, the displacement studies indicate that maturing salmon tend to reverse the sequence of their outward migration as juveniles. This will lead them to the river or hatchery where they began life. Displaced salmon return first to the odors of their release site and will continue to the rearing site if its odors can be detected. If not, they seem to seek the nearest river or hatchery.

The date of release also influences homing. Fish released too early might be expected to stray more because they have not had time to imprint, or because their endocrine physiology is not synchronized with migration. Studies of Atlantic salmon (Hansen and Jonsson 1991) and of chinook salmon in the lower Columbia River (Pascual et al. 1995) and in New Zealand (Unwin and Quinn 1993) show that fish released after the smolt stage may also stray more frequently than earlier releases. It appears that exposure to site-specific water without migration is not sufficient for imprinting and will not lead to accurate homing, hence salmon held too long stray even though they were given a full opportunity to imprint (Dittman et al. 1996).

Although imprinting is a large component of homing, homing is not entirely a learned behavior. Local populations may home better than transplanted ones (pink: Bams 1976; chinook: McIsaac and Quinn 1988). Salmon may home better to their natal site than to a new site (chinook: McIsaac and Quinn 1988, Pascual and Quinn 1994; coho: Labelle 1992), and transplanted populations may show some tendency to return to their ancestral location (chinook: McIsaac and Quinn 1988, Pascual and Quinn 1994).

Interactions Between Hatchery Strays and Wild Salmon

If a hatchery produces a large number of salmon, straying by even a small percentage of them has the potential to disrupt the genetic composition of nearby wild populations. For example, the proportion of strays from an ocean-ranching facility (Oregon Aqua-Foods) was low, about 6%, but these strays accounted for about 74% of the fish in nearby Yaquina Bay tributaries (Nicholas and Van Dyke 1982). In this case, not only might there be genetic interactions, but simple stock assessment is compromised. A census of natural spawning areas would overestimate the size of wild populations, because the absolute number of strays--a small percentage of the larger hatchery population--was large relative to the local population.

While there is concern that strays from hatcheries will influence wild gene pools, wild salmon may also stray into a hatchery. Nicholas and Van Dyke (1982) estimated that 2,022 (64.7%) of the 3,124 wild coho salmon returning to the Yaquina River watershed in 1981 entered the Oregon Aqua-Foods hatchery. Such decoying of wild salmon into hatcheries both reduces the number of wild fish in the stream and contributes to genetic mixing.

Gene flow from hatchery strays may dilute beneficial genes in populations of locally adapted wild fish, or disrupt adaptive gene complexes. However, salmon mating is non-random. Factors contributing to differential reproductive success include intrasexual competition, some degree of mate choice, differences in aggressiveness between wild and hatchery fish, size effects, different return times, and so on. Differences between homing salmon and strays in distribution within a river system (e.g., Atlantic salmon: Jonsson et al. 1990) might also tend to reduce genetic interactions. Finally, since salmon can discriminate siblings from non-relatives (coho: Quinn and Busack 1985), and can distinguish fish in their own population from those of other populations (sockeye: Groot et al. 1986; coho: Quinn and Tolson 1986), the magnitude of interbreeding may not be equivalent to the proportions of wild and hatchery-produced fish. Wild fish may actively reject siblings and non-native hatchery fish as mates on natural spawning grounds.

Tallman and Healey (1994) studied small chum salmon (Oncorhynchus keta) populations on Vancouver Island and indicated that the level of genetic exchange between strays was lower than that inferred by the presence of strays in spawning areas. Simply counting stray hatchery fish on spawning grounds may not provide a reliable estimate of the genetic interaction between hatchery-produced and wild populations. However, genetic consequences may occur if hatchery strays spawn with locally adapted wild fish (Taylor 1991, this volume) because domestication selection and non-native stock in the hatchery might reduce the fitness of wild fish. If hatchery fish have experienced domestication selection or are a non-native stock, then they may reduce the fitness of wild fish with whom they mate (Reisenbichler and McIntyre 1977, Reisenbichler 1988, Leider et al. 1990, Hindar et al. 1991, Johnsson and Abrahams 1991).



Conclusions

Salmon as a group generally home to natal sites to spawn. Homing occurs in diverse groups of salmonids with life-history patterns differing in duration of freshwater residence, anadromy, iteroparity or semelparity, and spawning habitat. Straying between natural populations appears to be an integral part of the evolutionary biology of salmonids and may be important for colonizing new habitats or for avoiding unfavorable habitats. However, intra-specific variation ("local adaptation") presumably results from the scarcity of strays or their high mortality rate, or both, relative to locally adapted salmon. This is consistent with the generally poor survival of transplanted salmon, relative to native populations or to the population in its home environment.

It is unclear whether some species of salmon stray more than other species, but the amount of straying within a species varies considerably among populations, and older salmon tend to stray more than younger fish. It is also not clear whether hatchery-reared salmon generally stray more than wild salmon. The degree of homing in outplanted salmon often differs from that in locally-reared and released salmon, and appears to be determined by complex interactions between rearing location, release site, date, endocrine events, and migration itself. Straying fish tend to enter nearby rivers, although there are many exceptions. Homing, and therefore straying, may be influenced by such factors as water temperature, flow, presence of other salmon, habitat quality, and so on. It is not clear, however, whether fish that stray actively identify their natal breeding grounds, then migrate elsewhere, or whether strays are unable to find their natal site. The propensity to stray itself may be a genetically controlled trait, in addition to genetically based metabolic and physiological traits that make homing possible.

To the extent that there are genetic differences between hatchery and wild salmonids, straying of hatchery-produced salmon to interbreed with wild fish is cause for concern if they are less fit than wild fish. The most obvious and pressing research need is for information linking the straying of adult salmon and the exchange of genes between populations. Thus, data on the relative reproductive success of homing (locally adapted) salmon and strays, whether of wild or hatchery origin, is essential for wise management of salmon populations.

Acknowledgments

I gratefully acknowledge Ernest Brannon and Kees Groot, who supervised my graduate and post-doctoral work, respectively, and with whom I have collaborated over many years on salmon migration and homing research. I also thank two former graduate students, Miguel Pascual and Andy Dittman, who have also collaborated with me and given me many insights into salmon homing.




Citations

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Discussion

Question: Nils Ryman: Is it possible to select for high rates of straying? Has this been tried?

Answer: Tom Quinn: This has not been tried to my knowledge. However, a study of family-specific differences in several fitness traits such as survival, growth, age composition, fecundity, and so on, was made on Atlantic salmon in Iceland. The researchers did find family-specific differences in homing. These family differences in straying could simply reflect genetic differences among families in memory, sensory ability, swimming performance, and so on, and not differences in the direct genetic control of straying.

Question: Audience: Would it be useful to examine physiological changes in fish to estimate homing ability?

Answer: Tom Quinn: I think the patterns would be complex at best. For example, at the School of Fisheries (University of Washington), coho salmon released as zero age smolts have much lower levels of thyroid hormones than is commonly observed in other hatcheries, yet homing is very good (Dittman et al. 1994). Among adults, salmon populations return at very different states of maturity (e.g., spring- and fall-run chinook, summer- and winter-run steelhead). There seems to be no universal relationship between endocrine changes and homing. This is not to say that there is no relationship, only that it may vary considerably among populations and individuals.

Question: Richard Carmichael: Are you aware of information indicating that rates of straying among natural populations may vary between groups of salmon with different life-history patterns? For example, the Grande Ronde Basin harbors two groups of fish. Fish in one group stay their entire life in the area where they were spawned, and fish in the other group move fairly long distances into main-stem rearing areas in fall, then smolt the following year. Do these different life-history patterns produce different levels of homing?

Answer: Tom Quinn: Yes, there is evidence that stream-type and ocean-type sockeye salmon may stray more, or at least show less genetic differentiation than the conventional lake-type (Wood 1995). In the Cowlitz River Hatchery, spring chinook seemed to home more precisely than fall chinook (Quinn and Fresh 1984, Quinn et al. 1991), but the generality of this pattern among hatcheries and its application to wild populations is unclear.

Question: Mike Lynch: Are you saying that perhaps a straying rate of 2-5% might be fairly normal?

Answer: Tom Quinn: Yes. Straying rates range from almost nothing to a lot, depending on species and region. However, I must emphasize the dearth of information on wild populations.

Question: Mike Lynch: Are these estimates of straying rates compatible with those estimated from molecular data?

Answer: Tom Quinn: We seldom have estimates of straying in wild salmon populations for which we also have genetic data (but see Quinn et al. 1987 for sockeye, and Tallman and Healey 1994 for chum). There is no reason to suspect that straying rates and gene flow must be equivalent because poor survival of the progeny of strays, or non-assortative mating or some other process, may mediate the genetic interactions.

Question: Nils Ryman: Is the straying and the occurrence of jacks related? Is migratory behavior abnormal in both cases?

Answer: Tom Quinn: I am not sure I would be willing to say that jacks display abnormal behavior; they still go to sea, return to fresh water, and spawn. They may have a different marine distribution as a consequence of their younger ages, but they still migrate far enough away so they no longer have contact with their natal rivers. To the extent that there are patterns, jacks seem to stray less often than older salmon.

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