This report, about "Dissolved Oxygen Requirements of the Eggs of Bull Trout, Salvelinus confluentus. and Mountain Whitefish, Prosopium williamsoni. has been submitted as a partial fulfillment of contractual obligations to the Northern River Basins Study. The main objective of the study was to identify the effects upon survival, embryonic development, hatching success, and quality of larvae of bull trout and mountain whitefish eggs incubated at various levels of hypoxia at low water temperature.
In early October 1993, fertilized bull trout eggs were collected from the Hill Creek Hatchery in British Columbia and whitefish eggs were collected from the upper reaches of the Athabasca River near the confluence of the Snaring River. The eggs were transported to the Freshwater Institute, in Winnipeg, Manitoba, and incubated at nominal dissolved concentrations of 3, 5, 7, 9, and 13.5 mg/1 at 2°C. These oxygen concentrations are equivalent to oxygen saturations of 22.3, 37.2, 52.0, 66.9, and 100 percent of air saturation for that locality. The period of incubation under hypoxic conditions extended from November 27, 1993 to May 5, 1994. Each dissolved oxygen treatment was replicated thrice for whitefish and, because of insufficient numbers of eggs, once for bull trout. The oxygen treatments contained 28 incubators: replicate one consisted of 14 bull trout incubators (15 eggs/incubator) and 14 whitefish incubators (50 eggs/incubator); replicates two and three consisted of 26 whitefish incubators (50 to 60 eggs/incubator) and 1 or 2 bull trout incubators (10 to 14 eggs/incubator). Dissolved oxygen and water temperature were monitored daily and egg mortality and hatching were recorded at least three times weekly. At several intervals during embryonic development the oxygen consumption, residual oxygen levels, egg and alevin weights and lengths were recorded from the bull trout. In addition to these records, cardiac rates and measurements of egg and yolk size, and of eye diameter and interorbital distance were obtained for whitefish. Following hatch the thermal tolerance of bull trout and feeding success and thermal tolerance of whitefish from each dissolved oxygen treatment were examined.
Exposure to reduced oxygen did not result in any increase in egg mortality of either species. Survival rates were high in all treatments and mortality during the period of hypoxia (Nov. 27, 1993 to May 5, 1994) was less than 10% in hypoxic and normoxic treatment groups of both species. Exposure to hypoxia reduced oxygen consumption by the embryos of both species with the degree of reduction being directly related to the severity of the hypoxia. Residual oxygen levels (the amount of oxygen remaining when the embryo died from hypoxia) were very low and ranged from 0.2 to 0.7 mg/1 (1.5 to 5.5% of saturation) in bull trout eggs and alevins and from 0.4 to 1.0 mg/1 (2.8 to 7.5% of saturation) in whitefish. Cardiac rate was reduced by 8 to 22% in whitefish embryos exposed to dissolved oxygen of 3 mg/1 but was unaffected at higher oxygen levels. Mortality during hatching was insignificant in both species with hatching success generally exceeding 95 % except for one replicate at 3 mg/1 where 86% hatched. The timing of hatch in bull trout eggs was not strongly influenced by hypoxia but the level of development as illustrated by the amount of yolk remaining and smaller body size in the hatched alevins was reduced in embryos incubated at < 7 mg/1 of dissolved oxygen. In whitefish, however, hatching was delayed in embryos incubated at reduced oxygen levels such that the time from fertilization to 50% hatch was 151, 174, 185, 204, and 221 days in eggs from the 13.5, 9, 7, 5, and 3 mg/1 treatments, respectively. Whitefish larvae from all treatments were of similar size.