We examined avian predation risk of juvenile steelhead trout (Oncorhynchus mykiss) migrating through the Columbia River Estuary in relation to their osmoregulatory physiology, body length, rearing conditions (hatchery or wild), migration timing, and migration year. From 2003 to 2006, mean gill Na+, K+ ATPase activity of migrating wild steelhead was greater than hatchery steelhead. Hatchery steelhead were always longer than wild steelhead. Wild steelhead never had higher plasma [Na+] or osmolality levels than hatchery fish after seawater challenge trials conducted in 2004, 2005, and 2006. More passive integrated transponder (PIT) tags from hatchery fish (19%; 126 of 678 fish) were detected on East Sand Island among bird nesting colonies than PIT tags of wild fish (14%; 70 of 509 fish), presumably consumed by birds. As gill Na+, K+ ATPase activity and migration date within a year increased, the probability of an individual fish being eaten by an avian predator decreased. Length, rear type, and year were not related to predation risk. These results show that physiology and migration timing of juvenile steelhead play an important role in a migrant’s risk to avian predation within an estuary.

Hatchery programs that are designed to aid recovery of natural populations of anadromous salmonids, including steelhead Oncorhynchus mykiss, require locally derived, natural‐origin broodstock. In such programs, achieving smoltification size thresholds may require extending hatchery rearing beyond age 1. We compared out‐migration survival and travel rates of 142,990 PIT‐tagged steelhead smolts released at age 1 (S1 rearing strategy) or age 2 (S2 rearing strategy) over five release years at Winthrop National Fish Hatchery (WNFH, Okanogan County, Washington). An S2 rearing cycle produced larger smolts with more uniform size distributions, resulting in higher survival during the first portion of their out‐migration than for S1 smolts in three of the five release years. The S2 smolts migrated more rapidly to the ocean than S1 smolts in all years except 2011 and arrived in the Columbia River estuary 5.4 d earlier on average than the S1 smolts. The S1 steelhead that did not leave during the volitional release were subsequently forced from the hatchery to measure their survival. Nonvolitional S1 migrants were smaller and had survival rates that were 2.3–66.3 times lower than those of S1 steelhead that left WNFH on their own. The same was true for S2 steelhead, but the magnitude of the survival difference between volitional migrants and fish forced from the raceways was less variable and ranged from 2.5‐ to 4.6‐fold. We conclude that S2 rearing can be a successful strategy for producing smolts from local natural‐origin broodstock, with out‐migration survival and travel times that are equivalent to or better than those of S1 smolts produced from nonlocal broodstock. Received July 25, 2016; accepted March 31, 2017 Published online May 31, 2017

In streams with sympatric populations of steelhead Oncorhynchus mykiss and coastal cutthroat trout O. clarkii clarkii (hereafter, cutthroat trout), life history descriptions and smolt production estimates may be hampered by misclassification of hybrids as steelhead or cutthroat trout. Additionally, important morphological and physiological differences between hybrid and non‐hybird smolts are often unknown. Therefore, we assessed field classification and created classification models to quantify and reduce misclassification rates among migrating steelhead, cutthroat trout, and hybrid smolts. Field misclassifications of smolts with steelhead or cutthroat trout genotypes were low (1% and 2%, respectively). However, field misclassification of fish with hybrid genotypes was high, with 11% of the hybrids being misclassified as steelhead and 42% of the hybrids being misclassified as cutthroat trout. Hybrid smolts were larger, had lower gill Na+, K+‐ATPase activities, and lower condition factors than steelhead but were similar to cutthroat trout smolts in these same measurements. Additionally, statistical classification analyses using morphological traits including subterminal jaw slash intensity, hyoid teeth presence, maxillary length, breaks of pigment along outer margin of adipose fin, condition factor, and migration date improved classification error rates of hybrids from 53% to 21%. In systems with sympatric populations of steelhead and cutthroat trout, we recommend a thorough evaluation of field‐based identification methods with genetic techniques to assess the effectiveness of field‐based classification in addition to examining important life history differences among steelhead, cutthroat trout, and their hybrids.

In 2004 and 2005, steelhead (anadromous rainbow trout Oncorhynchus mykiss) reared at Winthrop National Fish Hatchery (Winthrop, Washington) were tagged with passive integrated transponders and subjected to a volitional or forced release. Gill Na+, K+‐ATPase (NKA) activity, body size, condition factor (K), travel time, and apparent survival were compared between volitional migrants (VMs), forced‐release migrants (FRMs), and volitional‐release nonmigrants (VNMs). Gill NKA activity and body size were similar between VNM, VM, and FRM groups, but K varied by date and migrant group. However, by the end of the volitional‐release period, all three groups had similar K. The VNMs consistently demonstrated lower apparent survival to McNary Dam on the Columbia River than did the VM and FRM groups, which had three to eight times greater survival than did VNMs. Volitional migrants had higher apparent survival to McNary Dam than did FRMs in 2005; however, apparent survival was similar between the two groups in 2004. Apparent survival between McNary and John Day dams was similar among all three groups in 2004 and 2005. Median travel time to McNary Dam was not consistently different among groups and differed between years. By combining results from VNMs and VMs, we were able to compare apparent survival between volitional‐ and forced‐release strategies. In 2004, the forced‐release group had a significantly higher apparent survival to McNary Dam than did volitionally released fish, whereas there was no difference in apparent survival between the two release strategies in 2005. There was little evidence for a survival‐, size‐, or physiology‐related advantage of volitionally released fish over forced‐release fish. The consistently lower apparent survival of VNMs does support the use of volitional release to screen fish that will fail to migrate downstream due either to low survival or residualization in the natal stream.

Smoltification by juvenile Pacific salmonids has been described as a developmental conflict whereby individuals face several life-history decisions. Smoltification occurs as a result of interactions between organismal condition and environmental cues, although some fish may forgo ocean migration and remain in freshwater streams for some time (residualize). We compared the physiological profiles of steelhead that were actively migrating to the ocean (migratory fish) and those that remained in fresh water (residuals) for at least a period of between 2 wk and 3 mo after release from a hatchery facility. In addition, we investigated the physiological characterization of residuals that further differentiated into precocial freshwater residents or parr that will either precocially mature in fresh water or migrate to the ocean in the future. Residuals had higher condition factors and gonadosomatic index than migratory fish and were characterized as less prepared for saltwater due to low levels of gill Na+,K+-ATPase activity and Na+,K+-ATPase α1b-subunit expression. Residuals tended to be males with the highest condition factors. Sex-specific differences are probably reflective of male fish adopting an alternative life-history strategy foregoing outmigration as a result of condition at the time of release. Collection of residuals throughout the fall suggested that residual hatchery fish further diversify into precocially mature fish that will presumably attempt to spawn without ever migrating to the ocean or into parr that will precocially mature or migrate in a future year.