Kokanee Stock Status and Contribution of Cabinet Gorge Hatchery, Lake Pend Oreille, Idaho, 1991 Annual Progress Report.

Abstract

Initially, rehabilitation of kokanee Oncorhynchus nerka met with apparent success reaching a peak abundance of 10.2 million fishTl988. However, a decline of 47% followed from 1988 through 1991 to 5.4 million fish. The decreased population was attributed to poor recruitment of wild fish, poor egg take, thus, low stocking of hatchery fry (7.3 million in 1990 and 5.0 million in 1991 compared to about 13.0 million in 1981), and poor survival of fish ages 3 and 4 (average survival of the older fish was only 35% in 1990 compared to 72% in prior years but it was 68% in 1991). In addition, standing stocks of kokanee have remained relatively stable (x = 8.6 kg/hectare) since 1986 despite the dramatic changes in density. Prior to this study (1985) standing stocks were substantially higher (x = 13.6 kg/hectare). The kokanee population is probably operating below carrying capacity. Hatchery fry comprised 59% of the total kokanee recruitment in 1991 (93% of fry biomass). This contribution of 1.06 million fry ranked fifth behind 1988 (3.74 million), 1989 (2.25 million), 1982 (1.89 million), and 1990 (1.56 million) since hatchery supplementation began in the 1970s. Survival of hatchery fry was 21% in 1991, the second highestmore » since this investigation began. Two release strategies were tested in 1991 of which the best survival was recorded for the Sullivan Springs release at 23% while the early Clark Fork River release continued to have lowest survival at 18%. Survival of hatchery reared kokanee fry is still below the goal of 30% and it appears that this goal may not be attainable most years. Statistical analysis between number of days from fry release to recapture day and fry survival did not reveal a relationship (P = 0.43). Survival of fry from late releases is higher (P = 0.05) than early releases but no difference (P L 0.71) was detected between stocking locations. Good survival of fry from the Sullivan Springs releases was attributed to large size of kokanee fry (55 mm), warm water temperatures of July, and higher cladoceran densities compared to June. River release Lower survival of the early Clark Fork is attributed to the exceptionally high river flows (1,984 cm /s or 70,000 ft /s) and low density of zooplankton. Age of adult kokanee sampled at Sullivan Springs was 1% age 2, 46% age 3, and 53% age 4. The high proportion of age 3 kokanee resulted in lower average fecundity. Recovery of about 160 fin clipped kokanee at Sullivan Springs provided evidence of imprinting. About 0.4% of the marked kokanee released in 1988 returned to spawn at age 3. This failure of a spawning run to Clark Fork River and low ( 610 mm) while an additional 14,800 rainbow trout Oncorhynchus mykiss were released. The harvest of kokanee is at 33% of the management goals, but the harvest of large Gerrard rainbow trout was the best in 15 years.« less