Research Document - 2011/132
Synthesis of biological and harvest information used to assess populations of northern form Dolly Varden (Salvelinus malma malma) in Canada. Part I: Rat River
By M.-J. Roux, K. Howland, C.P. Gallagher, and R.F. Tallman
Abstract
Biological information of anadromous northern form Dolly Varden (Salvelinus malma malma) from the Rat River, Northwest Territories, collected primarily between 1989 and 2008 along with harvest statistics are synthesized in order to assess the population and examine for trends over time. Data include length, weight, age, growth, sex, maturity, mortality, and estimated and reported subsistence harvest. Biological data were collected from a harvest monitoring program (1989, 1995-2008) where designated local subsistence harvesters (termed ‘monitors’) dead-sampled from their own or other harvester’s catches during the return migration of Dolly Varden from the Beaufort Sea in August and September. Catch and effort data from harvest monitor’s nets, and total harvest numbers from all fishing activity in the monitor’s vicinity were also collected. Additionally, biological data were collected from live-sampling of Dolly Varden at the spawning/overwintering area of the Rat River during tagging (1995-2007) and recapturing (2007) of fish for mark-recapture studies to estimate population abundance.
The population size was estimated to have been between 10,140 and 11,190 based on periodic mark-recapture studies between 1989 and 1997, with a notable decrease in abundance observed in 2004 when the estimated size was approximately 2,911 Dolly Varden. By 2007 abundance had increased to between 8,488 and 14,886 fish (based on the results from two recapture methods for the same year). Changes in annual catch-per-unit-effort data from harvest monitor’s nets were consistent with increases and declines in population abundance.
Males and females examined from the total sample (all maturity stages) captured in 102 mm mesh gill nets decreased 67 and 53 mm in mean length, respectively, during the decline in population abundance. Dolly Varden rebounded to pre-decline lengths following the subsequent increase in population size. Female current-year spawners captured in 102 mm mesh increased in mean length by 77 mm between 2001 and 2004, while both males and female spawners captured in the seine net on the spawning grounds also increased in mean length. Length frequency distributions demonstrate that the lowest mean and modal sizes were observed in the subsistence harvest when the population abundances was at its lowest in 2004, while length distributions from Dolly Varden sampled at the spawning grounds in that same year were among the highest observed. There is some indication from seining in 2007 that a strong year class, based on the relatively high abundance of juveniles, was the main contributor to increased population size. Age data (only available from the harvest monitoring program) indicate that younger male and female Dolly Varden from all maturity stages were harvested in 102 mm and 114 mm mesh gillnets at Rat River fishing locations following the significant decline in the abundance in 2004. There was no evidence of differences in growth among years based on the analyses of von Bertalanffy curves, however growth information from recaptured tagged fish suggest otherwise. Age-at-first-maturity of Dolly Varden captured in the subsistence fishery between 1989 and 2008 ranged between 3 and 5 years for both males and females. The abundance of spawners, both male and female, in the Rat River has decreased over the years. The data suggest that although the abundance of males relative to females has remained consistent over the years, males have been spawning less frequently than females, opting to alternate years when spawning occurs. Annual mortality rates of Dolly Varden from the Rat River between 1989 and 2008 averaged 38% and were not significantly correlated with time. The available information suggests resilience in the population, however, the capacity of the population to adapt to additional stresses (e.g., impacts of climate change on habitat conditions) which may threaten the long-term viability of the population is unknown.
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