No abstract

Within their native range in western North America, resident redband trout Oncorhynchus mykiss gairdneri occupy stream habitat from high mountains to low desert. To better understand the temperature tolerance, growth, and stress physiology of native redband trout populations and compare the resilience and responses to reciprocal environments of stocks adapted to desert or montane conditions, we conducted controlled laboratory trials. We tested groups of age‐0 progeny from naturally reproducing desert and montane fish stocks in temperature cycles that simulated the summer conditions typical in regional desert and montane stream habitats. The diel cycles ranged from 8°C to 16°C for montane treatments and from 18°C to 26°C for desert treatments, and our tests were repeated over 2 years. We evaluated survival, growth, feed efficiency, plasma cortisol, heat shock protein levels, and body proximate composition in samples of fish collected during and at the completion of the trials. All of the stocks tested had high survival under all conditions, regardless of their geographic origin. We found no differences consistently attributable to desert or montane origin. Growth rates and protein and lipid efficiencies varied among stocks, between temperature treatments, and between replicate years. We found that the expression of heat shock protein 70 (hsp70) was consistently higher in all stocks maintained at desert temperatures regardless of source, but the absolute quantity of proteins measured varied among populations. We conducted an additional short‐term trial to evaluate the responses of different stocks to upper lethal temperature cycles that approached a daily maximum of 30°C. Although desert‐ and montane‐adapted populations of redband trout were equally dynamic and adaptive in desert or montane diel temperature cycles, we conclude that the desert stocks will be more at risk from increasing temperatures and reduced stream flows in the summer months as climate changes.

Evaluating rearing techniques that maximize angler returns to creel of hatchery trout is an essential tool in shaping hatchery management practices and maximizing the public use of hatchery products. The goal of this study was to determine the effects of raceway rearing density on return to creel of catchable‐sized (mean TL ≈ 252 mm) hatchery Rainbow Trout Oncorhynchus mykiss. In 2011 and 2012, Idaho Department of Fish and Game reared catchable‐sized trout targeting three maximum density indices (0.15, 0.25, and 0.30 lb·ft−3·in−1) at three different state fish hatcheries. Each hatchery stocked fish into the same 11 lakes and reservoirs to evaluate return‐to‐creel rates by rearing density. Although there was a trend of lower angler catch for fish reared at a higher raceway density, the relationship was not statistically significant. Instead, angler catch was significantly influenced by fish size at stocking and the surface area of the water being stocked, whereby larger fish and smaller waters had higher return‐to‐creel rates. At the densities tested in this study, we concluded that fish size at stocking is more important than rearing density in determining return‐to‐creel rates for hatchery catchable Rainbow Trout. Rearing trout at a lower density reduces total hatchery production while not providing a sufficient increase in returns to creel to offset the decreased production.

Catchable-sized hatchery trout (hereafter, catchables) have become a staple component of many fisheries management programs throughout North America. Due to their size, catchables create immediate fisheries once they are stocked, and fisheries managers have gradually shifted towards stocking fewer, larger trout. However, the cost of growing larger fish may reduce the efficiencies of catchable stocking programs overall. We grew catchable-sized Rainbow Trout Oncorhynchus mykiss to two target average sizes (254 and 305 mm total length) at a production scale, while tracking feed expenditures to examine the costs and benefits associated with increased size-at-stocking. Although larger catchables cost 31% more in feed expenditures than those reared to a smaller average size, catch (by anglers) of larger fish increased by 100% relative to smaller fish. Consequently, if target stocking size was changed from 254 to 305 mm and feed costs were held constant by reducing the total number of fish stocked, anglers would benefit by catching larger and more fish, despite the reduction in number of fish stocked. In lentic systems, larger catchables were reported by anglers more quickly than smaller fish, so managers must consider interactions between stocking size and residence time for lentic systems supported by catchables. In lotic systems, overall catch by anglers was much lower than catch at lentic waterbodies, and all catchables were either reported by anglers quickly or failed to be reported at all regardless of size-at-stocking. Producing larger catchables for hatchery-supported fisheries serves to benefit angling and would likely increase angler satisfaction while improving efficiencies associated with hatchery catchable stocking programs.