A REVIEW OF BULL TROUT (SALVELINUS CONFLUENTUS) LIFE-HISTORY AND HABITAT USE IN RELATION TO COMPENSATION AND IMPROVEMENT OPPORTUNITIES

McPhail, J. D. , and J. S. Baxter. 1996. A review of bull trout (Salvelinus confluentus) life-history and habitat use in relation to compensation and improvement opportunities. Fisheries Management Report No. 104, 35 p. The bull trout (Salvelinus confluentus) is a char endemic to western North America. It has had a confused taxonomic history, and its specific distinction from the Dolly Varden (Salvelinus malma) is still in doubt. In the areas where the two nominal species overlap there is evidence of hybridization and even introgression. This taxonomic problem has management implications, and will only be solved by studies in the areas of geographic overlap. The bull trout spawns in the fall (September to October) in flowing water. The threshold spawning temperature is around 9°C. Courtship and spawning behaviour are similar to other char. The female chooses the deposition site and digs the redd. The degree of sexual dimorphism varies among populations, but in most populations males develop bright spawning colours and a kype, while females are less colorful. The eggs are about 5-6 mm in diameter and optimal incubation temperature ranges from 2 to 4°C. In the wild, fry emerge approximately 220 days after egg deposition. Newly emerged fry are secretive and hide in the gravel along stream edges, and in side channels. Juveniles are found mainly in pools, but also in riffles and runs. They maintain focal sites near the bottom and are strongly associated with instream cover, especially overhead cover. Juveniles feed primarily on aquatic insects taken from the bottom or from drift. As they grow, their diet shifts to fish, and most adults (except for stream residents) are piscivores. Like many char, the bull trout occurs as a number of life-history forms. The stream-resident form lives out its life in small headwater streams. It is often dwarfed and reaches sexual maturity at a small size, and sometimes at an early age. The fluvial form lives as an adult in large rivers but spawns in small tributary streams. It often attains a large size, reaches sexual maturity at about five, and undergoes long migrations between mainstem rivers and small tributary spawning streams. The lacustrine-adfluvial form has a similar life-history. It spawns in tributary streams but lives as an adult in lakes. It grows to a large size, usually reaches sexual maturity in about its fifth year, and often makes long migrations between lakes and spawning streams. A fourth possible life-history type is anadromy. The evidence for the existence of anadromous bull trout is still slim, but in the Puget Sound-Strait of Georgia region they probably occur. There has been no formal attempt to document the life-history of anadromous bull trout. The evidence for homing in migratory bull trout is equivocal, and some populations probably home with great fidelity while others show a high rate of straying. The genetic structuring of bull trout populations suggest that many populations have been through genetic "bottlenecks". Genetic variability within populations is low, but genetic differences among populations often are marked. This, along with striking inter-population differences in nuptial colouration and sexual dimorphism, suggest the existence of distinct stocks. Adult and juvenile densities are often low, and the species is sensitive to environmental degradation and over-fishing. Human activities that create migration barriers, increase siltation, and increase variation in natural temperature and flow regimes within streams, are particularly harmful. Bull trout do not do well in competition with introduced salmonids, and there is evidence that introduced lake trout have replaced bull trout in a number of lakes. Bull trout are declining in numbers throughout their range, especially at the southern edges of their distribution where a number of populations have become extinct. Unfortunately, opportunities for enhancement are limited, and the major hope for restoring bull trout numbers lies with regulation (e.g., closures, gear restrictions and harvest limits) and public education.