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Marine Ecology Progress Series

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MEPS 173:163-180 (1998)  -  doi:10.3354/meps173163

Interannual variations in the population biology and productivity of Euphausia pacifica in Barkley Sound, Canada, with special reference to the 1992 and 1993 warm ocean years

R. W. Tanasichuk*

Department of Fisheries and Oceans, Pacific Biological Station, Nanaimo, British Columbia V9R 5K6, Canada

ABSTRACT: I monitored the size composition, abundance, growth, reproductive biomass and productivity characteristics of Euphausia pacifica in Barkley Sound, Canada, from March 1991 through March 1997. Sea temperatures were anomalously warm in 1992 (El Niño Southern Oscillation, ENSO) and in 1993; the strongest upwelling over the study period occurred in 1992. Mean annual E. pacifica larval abundance was 6 times greater in 1992 than in 1991, the pre-ENSO year. Adult abundance in 1992 and 1993 was twice that for 1991 due to the strong recruitment moving through the population. Larval abundance fluctuated substantially since the warming while adult abundance returned to pre-ENSO levels in 1994. Growth conditions, as indicated by variations in the condition factor, did not vary significantly over the 6 yr study period. Length-at-maturity increased over the spawning season but interannual variations were too small to influence adult abundance. The sex ratio fluctuated randomly around a grand mean of 0.56 males within and between years. I present a procedure for defining spawning events as a function of larval development time and its variability. I found a number of discrete spawnings annually and suggest that there has been no change in spawning frequency or duration over the study period. I found no significant change in the size of any larval stage over the study period. The relationship between parental and larval abundance follows a Ricker stock-recruit relationship; larval abundance decreased at high parental abundances. Mean annual larval biomass and productivity (growth + moult) ranged from being the same as to 9 times greater than 1991 levels. Mean adult biomass was 1.2 and 2 times greater in 1992 and 1993 respectively than in 1991; productivity was 1.8 and 2.3 times higher in 1992 and 1993 respectively and has returned to pre-ENSO levels. P:B ratios for total production ranged from 55 to 89 and from 8.6 to 12.4 for larvae and adults respectively. Population P:B ratios fluctuated between 10.7 and 23.3.

KEY WORDS: Euphausia pacifica · Euphausiid · Population biology · Productivity · Ocean variability

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