Factors influencing the relative fitness of hatchery and wild spring Chinook salmon (Oncorhynchus tshawytscha) in the Wenatchee River, Washington, USA.(Report)

Introduction

Artificial propagation is a commonly used tool to conserve a wide variety of threatened species (Mallinson 1995). Hatchery propagation, in which fish are bred and reared for part of their lives in captivity before being released into the wild, is widely used to supplement wild salmon populations (Naish et al. 2007). For example, over 4 billion anadromous juvenile salmon are released annually into the North Pacific Ocean from hatcheries in North America and Asia (Beamish et al. 1997). Similar hatchery programs and large-scale closed-pen fish farming operations exist for Atlantic salmon (Salmo salar) in the North Atlantic Ocean and Baltic Sea. Hatcheries are increasingly intended to contribute to conserving natural salmonid populations, as well as to produce fish to mitigate for lost harvest opportunities (National Research Council 1996). In particular, supplementation projects, in which natural spawning by hatchery fish is intended to augment a natural population's abundance, have become common throughout the Pacific Northwest (Naish et al. 2007) and Europe (Fleming et al. 2000).

A key biological uncertainty about the effects of hatchery production on natural populations is the degree to which hatchery-produced fish can reproduce in the natural environment (Reisenbichler and McIntyre 1977; Ford 2002; Araki et al. 2008). Evaluating relative reproductive success is therefore critical for determining if the considerable investment society has made in hatchery supplementation is actually contributing to the recovery of salmon populations (Mobrand et al. 2005). Accurately measuring the biological causes of variance in reproductive success is important not only for determining the benefits of conservation hatcheries, but also for evaluating the risks from fish that stray from "production" type hatcheries. The presence of large numbers of hatchery fish on spawning grounds can obscure the status of natural populations because their reproductive success is unknown (McClure et al. 2003) and may lead to reduced short-and long-term natural productivity because of genetic deterioration of the natural population as a result of interbreeding between naturally produced fish and hatchery fish (Lynch and O'Hely 2001; Ford 2002). By quantifying the reproductive success of hatchery fish relative to that of fish from the natural population, the viability of natural populations receiving hatchery fish can be more accurately evaluated.

Even when the relative reproductive success of hatchery-produced fish is quantified, the causes of fitness differences between hatchery and wild fish often remain unknown. Conceptually, there could be a wide variety of reasons why hatchery fish might have lower fitness than wild fish spawning in the same stream (Araki et al. 2008). Even in cases where hatchery fish have low fitness, the conservation implications are likely to vary depending on why the hatchery fish are less fit. For example, if reduced fitness is largely due to environmental effects such as release location, this would probably lead to fewer conservation concerns than if fitness reductions were due to genetic differences in behavior or physiology.

In a recent review, Araki et al. (2008) concluded that hatchery-produced steelhead (i.e., sea-run rainbow trout, Oncorhynchus mykiss) generally have lower reproductive success in the natural environment than wild steelhead. In contrast, they noted that few data are available on the relative reproductive success of hatchery and wild Chinook salmon (Oncorhynchus tshawytscha) despite the extensive use of hatchery supplementation for this species (Independent Scientific Advisory Board 2003, 2005; Araki et al. 2008). Recently, several papers have reported on studies of hatchery Chinook salmon breeding success in laboratory or seminatural environments (Fritts et al. 2007; Pearsons et al. 2007; Schroder et al. 2008), but there are no published studies of the relative fitness of this species in the wild.

In this study, we assess the relative reproductive success of naturally spawning hatchery-and natural-origin spring run Chinook salmon in the Wenatchee River, Washington, USA, by employing a genetic pedigree analysis and determine the degree to which differences in reproductive success between hatchery and natural Chinook salmon can be explained by biological characteristics such as run timing, morphology, and spawning location.

Materials and methods

Study population

Our study population consists of the spring run Chinook salmon that spawn in the Wenatchee River, Washington (Fig. 1). The population is listed as "endangered" under the Endangered Species Act (Federal Register 70:37160). Starting in 1989, hatchery supplementation has been used in an attempt to increase population abundance, and in recent years the hatchery program has produced >50% of the individuals in the population that spawn naturally. The hatchery program's focus is primarily on the Chiwawa River, a major tributary of the Wenatchee River. Wild and hatchery-origin broodstock for the supplementation program are collected at a weir in the Chiwawa River, and the offspring of those fish are released back into the Chiwawa River as yearlings. Although juvenile fish are released only in the Chiwawa River, as adults they return to spawn in all of the major spawning areas throughout the watershed (Murdoch et al. 2008).

The population exhibits a "stream-type" life-history pattern (Healey 1991) in which adults return to fresh water in the spring several months prior to spawning, and juveniles migrate to the ocean during the spring 1 year following their emergence from the gravel (Healey 1983). In wild populations of Chinook salmon in the Columbia River, including the Wenatchee River, most anadromous males become sexually mature between ages three to five, and most females become mature at ages four or five (Myers et al. 1998). Another characteristic of stream-type Chinook salmon is that some male fish mature at 1 or 2 years of age without migrating to the sea (Rich 1920; Burck 1967; Mullan et al. 1992), but little is known about the reproductive success of these early maturing males.

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Adult and juvenile trapping and sampling

In 2004 and 2005, beginning in early April and ending in early August, essentially all migrating spring Chinook salmon were trapped and sampled (scales, caudal fin clip) at Tumwater Dam, located at river kilometre (rkm) 43.7 on the Wenatchee River (Fig. 1, Table 1). Tumwater Dam is located below all of the major spring Chinook salmon spawning areas in the watershed, so we obtained samples from essentially all of the potential breeders, with the possible exception of those mature male parr that never migrated below Tumwater Dam and a very small number of adults that migrated after the trapping period ended. Biological data were collected from all adult salmon sampled (Table 1). Each fish was identified to gender, scanned for passive integrated transponder (PIT) tags (fish without a PIT tag had one inserted during sampling) and coded wire tags and the presence or absence of the adipose fin (indicating hatchery or wild origin; all hatchery fish released in the Wenatchee watershed have a clipped adipose fin). Fork and post orbital to hypural plate length were measured to the nearest centimetre and weight to the nearest 0.01 kg. Subsequent identification of PIT-tagged carcasses recovered on the spawning grounds permitted the comparison of carcass recovery distributions of individual hatchery-and naturally produced fish. Spawning location was based upon location of carcass recovery and linked back to fish identity at Tumwater Dam by PIT tag information. Spawning ground surveys of all potential spawning habitat were conducted twice a week throughout the entire spawning season (Murdoch et al. 2008). Carcass recovery location of each PIT-tagged fish was recorded using handheld GPS devices and converted to river kilometre using ArcView 9.2 (ESRI, Redlands, California).

Table 1. Summary of 2004 and 2005 adult and juvenile Chinook salmonsamples, stratified by year, origin, and life stage.     Adults                     Yearling       Subyearling                                juveniles      juvenilesYear  Origin    Males  Females   C    N    LW   C    N2004  Hatchery   1502     270     Wild        437     376    744  203  647  576  447     Unknown      18      132005  Hatchery   1573    1724     Wild        235     238              889     Unknown      15      17Note: Trap locations are as follows: C, Chiwawa River; N, Nason Creek;LW, Lower Wenatchee River.

Juvenile Chinook salmon samples (caudal fin clip) were taken from fish collected in rotary screw traps located on the lower Wenatchee River (rkm 9.6), Chiwawa River (rkm 1.0), and Nason Creek (rkm 0.8) (Fig. 1, Table 1). All rotary traps were located downstream from the majority of spawning habitat in each stream. The primary collection location was the lower Wenatchee River, which is below all spawning areas and operated from early February through August, although most yearling smolts were captured prior to 1 July. Depending on river discharge levels, one or two screw traps (1.5 m diameter) were operated, and trap efficiency ranged between 1% and 3%. Because of spring runoff, traps operated during 83% and 92% of the trapping period in 2006 and 2007, respectively. At tributary trap locations, yearling (age 1) juveniles were sampled approximately daily from early March to late June. In addition, separate …