PROJECT 4: HATCHERY TROUT EVALUATIONS

Idaho Department of Fish and Game (IDFG) hatcheries are integral to managing coldwater sportfishing opportunities in Idaho. Current hatchery production capacity and funding are not increasing, while demand for hatchery catchable-sized trout (i.e., ~250 mm in length) remains steady or is increasing. A comprehensive evaluation of hatchery catchable trout exploitation rates (i.e., return-to-creel) and total catch rates (i.e., harvested fish plus released fish) in Idaho’s predominant put-and-take fisheries has been lacking. This project is intended to (1) evaluate exploitation and catch rates of the most-stocked water bodies statewide, and (2) conduct research experiments focusing on hatchery rearing techniques to increase return-tocreel of catchable Rainbow Trout Oncorhynchus mykiss. In 2011 and 2012, IDFG released nearly 68,000 non-reward tagged hatchery Rainbow Trout across more than 60 water bodies, including lakes, reservoirs, community ponds, and rivers. A portion of these releases were intended to evaluate return-to-creel rates of fish reared at high (0.3 lbs/ft/inch), medium (0.23 lbs/ft/inch), and low (0.15 lbs/ft/inch) raceway densities. The statewide average total length (± 95% confidence intervals) of tagged catchable Rainbow Trout released in 2012 was 254 ± 0.3 mm. Average harvest and total catch for catchable Rainbow Trout across all evaluated waters was 20.0% (± 3.1%) and 25.9% (± 4.0%) respectively, for all tags released in 2012 and reported within 365 days of release. On average, harvest and total catch for 15 community ponds was 41.9% (± 6.9%) and 53.6% (± 8.7%), respectively. Mean total catch of Rainbow Trout remained significantly different across in-hatchery rearing densities for both the second year post-release of 2011 tags and for the first year post-release of 2012 tags. The low density treatment had the highest total catch on average, and was significantly different from mediumand high-density groups. However, the higher return-to-creel of fish raised at low densities is not sufficient to offset the reduced number of fish raised and stocked. Additionally, we evaluated returns-to-creel based on strain, release season, length, and length-rank at release. Clearsprings diploid catchables had higher total catch rates than Hayspur diploids, while triploid groups from Troutlodge and Hayspur were not significantly different. For all water body types combined, summer and fall release groups had the highest catch rates, followed by spring. Return-to-creel increased with increasing fish length, and from 200 mm to 305 mm there was roughly a 5% increase in catch rates for each 25 mm increase in length at stocking. Individual fish’s lengthrank within a release group showed a mixed correlation to catch from 2011 to 2012. Future work should further evaluate length and length-rank in relation to catch rates, and explore rearing options that result in maximizing the size-at-release vs. rearing cost relationship as well as reduced size variation.