Production of Triploid Lake Trout by Means of Pressure Treatment JOSEPH R. KOZFKAY* Idaho Department of Fish and Game, Resident Fisheries Research Section, Nampa, Idaho 83686, USA ERIC J. WAGNER Utah Division of Wildlife Resources, Fisheries Experimental Station, Logan, Utah 84321, USA DWIGHT APLANALP Idaho Department of Fish and Game, Grace Fish Hatchery, Grace, Idaho 83241, USA Abstract.—To induce triploidy in lake trout Salvelinus namaycush, we treated a production-sized group at Story State Fish Hatchery, Wyoming, with 65,500 kPa at 300 degree-minutes (8 C·min) for 5 min and investigated four different pressure treatments (62,053 kPa at 3008 C·min for 5 min; 65,500 kPa at 200, 300, or 4008 C·min for 5 min) at Saratoga National Fish Hatchery, Wyoming. For the production group, eggs hatched and survived at a higher rate when held in chilled water (10.68 C) than ambient water (11.58 C). For the experimental groups, mean survival of pressuretreated groups to the eyed egg, hatching, and feeding fry stages was at least 15% lower than that of the control group. However, due to high variability, there was no statistical difference in survival among the experimental groups. All fry (n 5 178) sampled from the four experimental treatments and the single production group were triploid. Introductions of lake trout Salvelinus namaycush throughout the western United States have led to reduction or elimination of native or recreationally important species (Ruzycki et al. 2003). However, introduced lake trout also support high-yield or trophy fisheries (Martinez and Wiltzius 1995), making them popular with recreational anglers. To maintain harvest rates, fisheries may be supplemented with hatchery-produced fish. Due to their long life span (Donald and Alger 1986) and highly piscivorous feeding behavior (Eby et al. 1995), introduced lake trout populations may be difficult to manage (Johnson and Martinez 2000). The use of sterile lake trout in hatchery-supported fisheries may allow better control of predator population densities and predator–prey ratios, thereby improving management of this species. Techniques for inducing triploidy (i.e., functional sterility) in salmonids have been widely studied, especially in Oncorhynchus spp. and Salmo spp. (Ihssen et al. 1990; Benfey 1999). Much of this research was undertaken to improve the performance of salmonids for commercial aqua- * Corresponding author: jkozfkay@idfg.state.id.us Received November 5, 2004; accepted November 18, 2004 Published online April 29, 2005 culture. Triploidy also provides advantages for recreational fisheries management in that normal gonadal maturation is disrupted, and therefore triploid salmonids are unable to reproduce (Thorgaard 1983). This reduces the potential for hybridization between wild and stocked fish and allows hatchery-supported populations to be more easily maintained at desired densities. Few studies have been conducted on techniques to induce triploidy in char Salvelinus spp., and we are aware of no studies conducted to induce triploidy in lake trout. For brook trout Salvelinus fontinalis, pressure treatments of 62,053 kPa were most successful (100% triploidy) when applied at 200 degree-minutes after fertilization (8 C·min) for 5 min (Benfey et al. 1997). For Arctic charSalvelinus alpinus, 65,500-kPa pressure treatments at 2258 C·min and 3008 C·min yielded 100% triploidy with ‘‘excellent’’ survival (Keefe and Benfey 1995). Gillet et al. (2001) produced 100% triploidy in Arctic char by means of a 65,500-kPa treatment at 3208 C·min for 5 min, and survival was approximately 90% that of controls. Additionally, thermal treatments have been used to induce triploidy in brook trout (Dube et al. 1991; Galbreath and Samples 2000); however, during two of our pilot studies, survival of thermally treated lake trout eggs94 KOZFKAY ET AL. was less than 10%. Using these published pressure treatments as a starting point, we sought to develop a triploidy induction technique that (1) provided high induction rates in lake trout and (2) provided adequate survival to the eyed egg, hatching, and feeding fry stages relative to that of untreated eggs

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