Research Document - 2010/115

Synthesis of biological and harvest information used to assess populations of northern form Dolly Varden (Salvelinus malma malma) in Canada. Part II: Big Fish River

By C.P. Gallagher, M-J. Roux, K.L. Howland, and R.F. Tallman

Abstract

Biological information of anadromous northern form Dolly Varden (Salvelinus malma malma) collected from the Big Fish River, Northwest Territories, between 1972 and 2000 along with harvest statistics are synthesized in order to assess the population and examine for any trends over time. Data include abundance, length, age, growth, sex, maturity, mortality, and estimated and reported subsistence harvest. Inconsistent sampling methods and locations, and the many instances of low sample sizes of males and females, particularly for ageing structures, among study years preclude robust assessment of trends of biological parameters. Population abundance estimates suggest a decline in population size from the 1970s to the mid-1980s, which remained low throughout the 1990s. Male Dolly Varden captured by seining in the 1990s were often smaller than females. Reliable estimates of female age-at-maturity varied between 4 and 6 years, while male age-at-maturity was 6 years. Female spawners outnumbered males while the proportion of spawners among sampling years did not appear to change. Sex ratios were variable, however females generally outnumbered males 2:1. Trends in annual mortality were limited due to the paucity of ageing data in males; female mortality ranged between 0.42 and 0.37 over time. While Dolly Varden from the Big Fish River once yielded harvests of approximately ≥8,000 kg the current harvest may now be <100 fish. Although there was limited data suitable for assessment purposes, the results among years were standardized for capture method and location and any biases that could influence interpretations of the assessment were identified. The inconsistent approaches used in the past underscore the need for a consistent approach and methodology for future assessment and monitoring of Dolly Varden from the Big Fish River.

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