Life History Diversity in Klamath River Steelhead

Oncorhynchusmykiss exhibits a vast array of life histories, which increases its likelihood of persistence by spreading risk of extirpation among different pathways. TheKlamathRiver basin (California–Oregon) provides a particularly interesting backdrop for the study of life history diversity inO. mykiss, in part because the river is slated for a historic and potentially influential dam removal and habitat recolonization project. We used scale and otolith strontium isotope (Sr/Sr) analyses to characterize life history diversity in wild O. mykiss from the lower Klamath River basin. We also determined maternal origin (anadromous or nonanadromous) and migratory history (anadromous or nonanadromous) of O. mykiss and compared length and fecundity at age between anadromous (steelhead) and nonanadromous (Rainbow Trout) phenotypes of O. mykiss. We identified a total of 38 life history categories at maturity, which differed in duration of freshwater and ocean rearing, age at maturation, and incidence of repeat spawning. Approximately 10% of adult fish sampled were nonanadromous. Rainbow Trout generally grew faster in freshwater than juvenile steelhead; however, ocean growth afforded adult steelhead greater length and fecundity than adult Rainbow Trout. Although 75% of individuals followed the migratory path of their mother, steelhead produced nonanadromous progeny and Rainbow Trout produced anadromous progeny. Overall, we observed a highly diverse array of life histories among Klamath River O. mykiss. While this diversity should increase population resilience, recent declines in the abundance of Klamath River steelhead suggest that life history diversity alone is not sufficient to stabilize a population. Our finding that steelhead and Rainbow Trout give rise to progeny of the alternate form (1) suggests that dam removal might lead to a facultatively anadromous O. mykiss population in the upper basin and (2) raises the question of whether both forms of O. mykiss in the KlamathRiver should bemanaged under the same strategy. Oncorhynchus mykiss displays a vast array of life histories. The species exhibits anadromous (steelhead) and nonanadromous (Rainbow Trout) forms, both of which are capable of spawning repeatedly in a lifetime (Shapovalov and Taft 1954; Behnke 1992; Busby et al. 1996; Willson 1997). Steelhead and Rainbow Trout can occur in sympatry, with (Seamons et al. 2004; Kuzishchin *Corresponding author: [email protected] Present address: Trout Unlimited, Post Office Box 771233, Steamboat Springs, Colorado 80477, USA. Received June 20, 2015; accepted October 16, 2015 227 Transactions of the American Fisheries Society 145:227–238, 2016 American Fisheries Society 2016 ISSN: 0002-8487 print / 1548-8659 online DOI: 10.1080/00028487.2015.1111257 D ow nl oa de d by [ U ni ve rs ity o f C al if or ni a D av is ] at 1 1: 50 0 9 M ar ch 2 01 6 et al. 2007; McPhee et al. 2007; Pearsons et al. 2007) or without (Zimmerman and Reeves 2000; Narum et al. 2004) interbreeding, and can give rise to progeny of the alternate form (Viola and Schuck 1995; Riva-Rossi et al. 2007; Courter et al. 2013). Rainbow Trout reside in freshwater for the duration of their lives, display varying degrees of movement (e.g., residency, potadromy), and reach sexual maturity at an age of 1–10 (typically 2–3) years (Willson 1997; Moyle 2002). Steelhead spend 1–7 (typically 2–3) years in freshwater, from several months to 6 (typically 2) years at sea, and then return to freshwater to spawn (Busby et al. 1996; Willson 1997; Savvaitova et al. 2000; Hodge et al. 2014). In total, O. mykiss displays at least 36 different life histories (Moore et al. 2014). Life history diversity increases the likelihood of population persistence by spreading the risk of extirpation among life histories (Stearns 1992; Watters et al. 2003; Fox 2005). Research suggests, for example, that the stability of Pacific salmon Oncorhynchus spp. and steelhead abundance is positively related to life history diversity (e.g., Greene et al. 2010; Shindler et al. 2010; Moore et al. 2014). Accordingly, the conservation and enhancement of diversity are important components of Pacific salmonid recovery (McElhany et al. 2000; Ruckelshaus et al. 2003; Beechie et al. 2006). Facilitating the recolonization of historical habitat above dams may be a particularly effective strategy for increasing life history diversity and thus population resilience (Beechie et al. 2006). The Klamath River basin (California–Oregon) provides an interesting backdrop for the study of life history diversity in O. mykiss for several reasons. First, a dam removal and habitat recolonization project of unprecedented scale is slated to occur on the main stem. The Klamath River has been divided by one or more impassable dams since 1918, and today four dams near the California–Oregon border block anadromous fishes from accessing more than 650 km of historical habitat in the upper basin (Hamilton et al. 2005; Hamilton et al. 2011). Under the proposed action, all four dams would be removed (USDOI and CDFG 2012). Second, the basin below the dams (hereafter, the lower Klamath River basin) supports California’s most productive steelhead fishery (Hopelain 1998). Last, little is known about the current life history diversity in the lower basin. The last basin-specific and comprehensive evaluation of O. mykiss life histories (Hopelain 1998), which was conducted more than 30 years ago, suggested that steelhead in the lower Klamath River basin exhibit substantial life history diversity. However, since that study was conducted, wild Klamath River stocks have declined (NRC 2008; Qui~nones et al. 2014) and at least one significant change in life history has occurred (incidence of the half-pounder life history declined from 80% to 11% in the Trinity River subbasin; Hodge et al. 2014). Dam removal is predicted to favor increased life history diversity in Klamath River O. mykiss (USDOI and CDFG 2012). Otolith strontium isotope (Sr/Sr) analysis provides a valuable tool for the retrospective determination of life history in fishes. In particular, otolith Sr/Sr ratios can be used to determine both maternal origin (anadromous or nonanadromous; Courter et al. 2013) and migratory history (Kennedy et al. 2002) in polymorphic species. The utility of such analyses is based on several assumptions: (1) that the Sr/Sr ratio of surface water reflects the underlying geology of the watershed and remains constant through time (Kennedy et al. 2000; Bacon et al. 2004), (2) that Sr/Sr ratios in otoliths record the chronological environmental history of individual fish (Kennedy et al. 2000; Kennedy et al. 2002; Bacon et al. 2004), and (3) that the Sr/Sr ratio of ocean water is globally uniform (D 0.70918; Hodell et al. 1991; Kennedy et al. 2002; Miller and Kent 2009) and differs significantly from that of freshwater. The analysis of maternal origin is founded on the additional assumptions that otolith core chemistry reflects the environment in which yolk precursors develop and that yolk formation is mostly complete when anadromous females are still at sea (Kalish 1990; Volk et al. 2000; Zimmerman and Reeves 2002). Assuming these conditions are met, otolith strontium isotope analysis should be a viable technique for determining both maternal origin (anadromous or nonanadromous) and migratory history (anadromous or nonanadromous) of Klamath River O. mykiss. In this study we used scale analysis and otolith strontium isotope analysis to quantify life history diversity in wild O. mykiss from the lower Klamath River basin (California). Specifically, we examined variability in the duration of freshwater and ocean rearing, age at maturation, and incidence of repeat spawning. We also examined the relationship between maternal origin and migratory history of O. mykiss and compared length and fecundity at age between steelhead and Rainbow Trout.