The hybrid trout is the star of the show in Henrys Lake. The cross between the Henrys Lake female cutthroat and the male rainbow trout grows faster, to a larger size, lives longer and fights harder than any other fish in the lake. Not uncommon in the world of hybrids, the rainbow/cutthroat cross is often referred to as a cuttbow in many bodies of water. However at Henrys Lake, the common terminology is simply Hybrid.
The original rainbow cutthroat hybrid trout in the lake was the result of a cross between Henrys Lake cutthroat and rainbow trout that escaped from private fisheries operated on the North Shore of Henrys Lake by Joe Sherwood and George Rae in the early 1900s. These hybrids were naturally occurring and not very abundant. They attracted quite a following among anglers however and were popular for the natural characteristics of hybrid trout.
Hybridization is the result of crossing two similar, but different species. In many cases hybrids are sterile as in crosses between the horse and the burro or they can be fertile as in crosses between wolves and domestic dogs. Some popular hybrid crosses in the fish world are: splake, which are brook trout lake trout hybrids and tiger musky which are Northern pike muskellunge hybrids. Both of the hybrids are sterile, however the rainbow cutthroat cross or hybrid is fertile. This is where the problem lies with hybrids that are fertile. If they breed in high enough proportion with either of the pure strain parent species then the purity is lost and it can erode the effective quality of the hybrid. The bigger problem in Henrys Lake is diluting the cutthroat trout gene pool with rainbow trout genetics from hybrid trout that spawn with cutthroat trout.
By artificially sterilizing the hybrids that are planted in Henrys, the problem of breeding with pure strain Yellowstone cutthroat is eliminated. Since 2004, the hybrids that are planted in Henrys have been sterilized using a pressure induced procedure. This procedure has been successful at producing sterile hybrids approaching 100% of the time. Prior to 2004, the Henrys Lake hybrids were sterilized using a heat procedure that was not as successful as the current pressure sterilization technique. Every spring spawning season, approximately 750,000 rainbow cutthroat eggs are treated with this pressure technique to produce about 200,000 sterile hybrid fingerlings that will be planted into Henrys Lake in September.
The current rainbow cutthroat hybrid trout in Henrys Lake is an amazing fish. It can achieve an average length of around 20 inches in the creel in some years, with an upper range of more than 30 inches and a weight of up to 15 pounds. Ten pound fish are not uncommon! Quick growth is driven by hybrid vigor, voracious feeding habits and an abundance of food, allowing it to grow to a large overall size.
Idaho Fish and Game plants around 200,000 fingerling hybrids each year into Henrys Lake. Hybrid trout make up approximately 20% of the fish in the creel, although percentages vary from year to year. The greater proportional representation of hybrid trout in the creel, with over six time as many cutthroat being planted in the lake, is testament to their voracious feeding habits. Hybrid trout stomachs have been examined with over 300 scuds, 200 damsel flies, 100 leeches and 50 large snails in different fish. Stomach analysis also alludes to the concept that these trout develop optimal search patterns for food based on relative abundance of food items and the low diversity but high number of particles ingested. The gist is that sometimes it takes a particular fly to catch fish!