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

Abstract

I monitored the population structure, abundance and growth, as well as the reproductive and production characteristics of Thysanoessa spinifera in Barkley Sound from March 1991 through March 1997. The years 1992 and 1993 were anomalously warm, 1992 due to an El Nino Southern Oscillation (ENSO). Summer upwelling was particularly strong in 1992 and downwelling in the winter of 1994-95. Mean annual larval (<10 mm) abundance was 2.4, 0.6, 0.4, 6.3 and 1.9 times greater in 1992 through 1996 respectively than in 1991. Adult abundance declined steadily since 1991 and in 1996 was about 0.14 of the pre-ENSO levels. Growth conditions, as indicated by the condition factor, varied seasonally but not interannually. There were no apparent interannual differences in length-at-maturity, sex ratio and spawning season length. I found no apparent relationship between parental and larval abundances. Indications are that the number of spawnings declined as of 1993. Mean annual larval biomass fluctuated. Biomass ranged between 0.19 (1994) and 6.9 (1996) times the 1991 value. Total production (growth + moult) was between 0.24 (1994) and 4.9 (1996) the 1991 value. Adult biomass and productivity fell continuously after 1991 and, in 1996, were about 0.13 of the pre-warm ocean years. Population P:B ratios fluctuated between 14.4 and 44.7, with the highest ratio in 1996.