Variability in Burbot Cohort Abundance at Juvenile and Adult Stages in Columbia Lake, British Columbia

Abstract

This study examined changes in the abundance of juvenile (age-0 and age-1) and adult Burbot Lota lota in Columbia Lake, British Columbia, for the 1991–1999 cohorts. The objectives were to quantify the degree of variation in cohort abundance at different life stages and investigate the timing of recruitment limitation. Adult spawner abundance and age composition were monitored at a tributary spawning site from 1996 to 2001. Juvenile cohort abundance was estimated from 1997 to 1999, providing age-0 abundance indices for the 1997–1999 cohorts and age-1 indices for the 1996–1998 cohorts. The number of tributary spawners declined from about 1,500 in 1996 and 1997 to 86 in 1999 and then rebounded to 995 by 2001. Adult length frequency and age composition showed that this fluctuation reflected periodic influxes of strong cohorts that dominated the spawning population. The strongest new cohort (1999) observed at the tributary in 2001 came from the smallest number of tributary spawners. Substantial differences in the juvenile abundance of cohorts spawned from similar numbers of adults (1996, 1997) suggested a density-independent effect at the egg or larval stage. The 1999 cohort, however, was more abundant than several other cohorts at the adult stage but not at age 0, suggesting a second limiting period after the first growing season. Possible explanations for the exceptional survival of the 1999 cohort include a temporary expansion of interstitial habitat caused by unusually high water levels and low abundance of older Burbot. As large fluctuations in cohort abundance appear to be characteristic of some Burbot populations, inclusion of age composition is recommended for population assessments.

Received September 4, 2012; accepted January 30, 2013

ACKNOWLEDGMENTS

This study was funded by the Fish and Wildlife Compensation Program–Columbia Basin, a joint initiative of BC Hydro, the Province of British Columbia, and Fisheries and Oceans Canada that was created to help sustain and enhance fish and wildlife populations in the Canadian portion of the Columbia River basin. Provincial employees Bill Westover and Jay Hammond initiated the study. Don McPhail of the University of British Columbia provided knowledge and guidance throughout. Martin Stapanian, Steve Martell, Vaughn Paragamian, and two other reviewers provided constructive comments on earlier drafts. Field assistance was provided by Joyce Hutchinson, Peter Mylechreest, Harald Manson, Tara Fleming, Mark Phillpotts, Larry Taylor, Derek Whyte, Mark Stevenson, Brian Reesor, Ed Russel, Kevin Heidt, and Colin Spence.

Notes

The term “Kootenai” represents the U.S. spelling, the term “Kootenay” the Canadian spelling. In this article the former is used when it refers to locations or entities in the United States, the latter when it refers to locations in Canada.