Factors Affecting the Homing of Fall Chinook Salmon from

Columbia River Hatcheries

Pascual, Miguel, A., Thomas Quinn and Howard Fuss. 1995. Factors affecting the homing of fall chinook form Columbia River hatcheries. Trans. Am. Fish. Soc. 124: 308-320

Abstract.—Straying results in an exchange of individuals between wild and hatchery salmon populations and is important because it could affect their genetic differences. Understanding what factors affect straying could help in the development of procedures for controlling the influence of specific stocks on nearby populations. We explore the effects of release date and the transportation of juvenile chinook salmon Oncorhynchus tshawytscha on straying by adults from two Columbia River hatcheries. Early and late releases from Washougal Hatchery produced much higher straying levels than intermediate release dates, and this pattern was consistent among brood years. Fish reared at Grays River Hatchery and released from Washougal Hatchery strayed significantly

more than local (i.e., Washougal) fish of the same brood year and release date. Our

results and those from previous studies indicate that the sequence of imprinting events is characterized by the combination of the time at which the fish experience given locations and the physiological state of the fish at that time. Changes in release date and transportation disrupt the sequence and timing of these events, with varying effects on homing. Hatchery rearing and release techniques can sometimes be modified to control straying and minimize the impact of hatchery fish on other hatchery and wild stocks.

Quotes from text: The distinctive migration and homing behavior of salmon gives rise to populations or stocks (Rich 1939; Ricker 1972) that are distinguished by a suite of local phenotypic adaptations (Taylor 1991) and genetic variation at selectively neutral loci (e.g., Utter et al. 1989). The alternative of homing is straying; some fish do not return to their natal streams but spawn elsewhere. Under natural conditions straying is an integral part of salmonid behavior and is important for natural populations because

it leads to the colonization of new habitat (Milner and Bailey 1989), avoidance of adverse

local conditions (Leider 1989), and increases in genetic heterogeneity (Utter 1991).

Olfactory imprinting by juveniles is the prerequisite for salmon homing and appears to occur at the time of smolt transformation, although salmon also remember odors experienced before that stage (Quinn 1993). Displacement of juveniles affects

homing; releases progressively farther away from the rearing site tend to produce more strays (Solazzi et al. 1991; Hansen et al. 1993). Harden Jones (1968) proposed that maturing salmon reverse the sequence of their outward migration as juveniles; disruption in this sequence can result in straying. According to this hypothesis, outplanted salmon

return first to the odors of their release site, and if the odors of the rearing site are then detected, they continue their migration to the rearing site (Quinn et al. 1989).

Homing is affected by odors learned during juvenile stages, but there is also a genetic component. Salmon reared and released at locations different from those of their ancestors sometimes show weaker homing responses than locally adapted fish (Bams 1976) and may tend to return to their ancestral place of origin, even if they never experienced its waters (Mclsaac and Quinn 1988). This behavior may be based on innate preferences for particular physical characteristics of the environment, such as temperature, flow, or substrate composition (Quinn 1993). In addition, there is evidence that, in a given population, older fish are more likely to stray than younger ones (Quinn and Fresh 1984; Quinn et al. 1991; Unwin and Quinn 1993).

Olfactory imprinting seems to occur at specific times in the fish's development, hence the time of release has the potential to affect straying. Atlantic salmon Salmo salar released in winter strayed more than those released at other times of the year (Hansen and Jonsson 1991), and introduced chi-nook salmon Oncorhynchus tshawytscha in New Zealand showed low levels of straying when released in the summer but strayed more commonly when released during fall and winter (Unwin and Quinn 1993).

Straying can result in an exchange of individuals between wild and hatchery stocks, which is a concern for conservation and management because it can reduce the genetic differences among populations that are essential for their fitness (Hindar et al. 1991). Whether or not strays affect local gene pools depends on the degree of assortative mating and the survival differential between populations (Quinn 1993). There are indications that transplanted stocks are less productive than local ones (Ricker 1972; Reisenbichler 1988) and that salmonids of hatchery origin are less productive during natural spawning than wild fish (Chilcote et al. 1986; Leider et al. 1990).

Wild and hatchery fish also display different reproductive behavior (Jonsson et al. 1991; Fleming and Gross 1992). The degree of introgression of hatchery and wild stocks, although a matter of concern because of its potential effects on wild stocks, remains largely unknown. Because of the possible effects of hatchery fish on wild populations and the conspicuous decline of many salmon stocks in the U.S. Pacific Northwest over the last 50 years (Nehlsen et al. 1991), the role of hatcheries in resource management is evolving. The original view that hatcheries could make up for al fish production lost because of fishing and habitat alteration is giving way to one in which hatcheries are integrated with natural production (Lichatowich and Mclntyre 1987).

The questions are how to produce hatchery fish with rates of smolt-to-adult survival in the natural environment similar to those of wild fish and how to minimize potentially negative interactions of hatchery with wild fish. Straying is a necessary condition for introgression of hatchery and wild stocks, and its study can lead to the establishment of some principles for salmon management. It is relevant, therefore, to determine whether hatchery rearing and release practices affect straying. For example, efforts to increase survival by manipulating release date (e.g., Bilton et al. 1982) or by moving fish from one hatchery to another (Peck 1993) may affect straying. In this paper we explored the effects of release date and transportation of fish between hatcheries on straying by analyzing the recoveries of experimental groups of chinook salmon released from two Columbia River hatcheries. Although these two facilities may not be representative of salmon hatcheries in general, controlled and replicated rearing and release experiments conducted there allowed us to test the following specific hypotheses: (1) straying can be minimized by selection of appropriate release dates, (2) fish transported from one hatchery to another are more likely to return to the release site than to the rearing site or to show a higher overall level of straying than fish released at the rearing site, and (3) straying increases with age at maturity. Because of the complex interactions of release date, brood year, and release location with age at return and homing, we analyzed the data using generalized linear models (GLM) rather than conventional pairwise comparisons.