During some years Neocalanus plumchrus overwinters in Saanich Inlet while in other years they are absent from the fjord (Harrison et al., 1983). The cause of this variation is not known but the annual development of anoxic conditions followed by an intrusion of dense, oxygenated water, was suspected to influence their distribution and abundance. Vertical and horizontal haul samples collected from Saanich Inlet indicated the overwintering population in 1985 (September 1985 to March 1986) was sparse; in September 1986 the population of N. plumchrus was comparably low. In the spring and summer, N. plumchrus is introduced from the Strait of Georgia and Juan de Fuca Strait, and in some years may arise from reproduction within the inlet. So few adults were collected at the three stations in Saanich Inlet in January-March 1986 (<0.20 m⁻³) that their potential contribution to the spring population was considered negligible. Decline in the overwintering population in September 1985 and 1986 appears to be correlated with the occurrence of an extensive deep water renewal. The distribution of N. plumchrus during early stages of their deep water migration (June to August), is influenced by the low oxygen concentrations in the bottom of the inlet (i.e. 0.10-0.30 mL L⁻¹). During the renewal, copepods occurred above the oxygen minimum (75 m) possibly due to their displacement or their avoidance of the low oxygen zone. Subsequently, they were exposed to tidal transport out of the inlet and perhaps to increased predation. On two occasions in which N. plumchrus was present during the winter in Saanich Inlet (1969 & 1974), a high volume of dense water intruded, disrupting the copepod layer during the mixing of the two water masses. Even though a greater volume of water left the inlet some of the copepods could have remained in the water, below sill depth. Neocalanus plumchrus successfully overwinters and reproduces in Sechelt Inlet which is well-oxygenated but has a very shallow sill (15 m) that 'locks' the copepods into the inlet. The fifth copepodite stages also occupy deeper depths in Sechelt Inlet than in Saanich Inlet, even though the bottom depths are comparable. In low oxygen tolerance experiments many factors such as the period of captivity, and the region of origin (i.e. Saanich Inlet versus the Strait of Georgia) caused variable results. Although the minimum oxygen level tolerable during their migration could not be determined, N. plumchrus tolerated levels as low as 0.56 mL L⁻¹ (12% mortality). Sediment trap samples indicate that a massive die off could not account for the loss of N. plumchrus from Saanich Inlet. / Science, Faculty of / Zoology, Department of / Graduate