| Summary: | During 1960-61 the abundance and distributions of four species of euphausiids (Euphausia pacifica Hansen, Thysanoessa spinifera Holmes, Thysanoessa longipes Brandt and Thysanoessa raschii M. Sars) and the developmental stages of one species, E. pacifica were studied each month in relation to temperature and salinity in Indian Arm, British Columbia, using the method of T-S-P diagrams. Euphausia pacifica probably is a resident species and as such was the most tolerant towards environmental conditions and their fluctuations in Indian Arm, followed in order of decreasing tolerance by the expatriate species T. spinifera, T. longipes and T. raschii. All species, (whether resident or expatriates), were useful biological indicators of oceanographic changes in Indian Arm, particularly with reference to the detection of outside waters entering the inlet. Field data indicated that temperature and salinity may have been contributory "regulatory factors" with regard to the vertical distribution of euphausiids in Indian Arm, particularly in the region of maximum temperature and salinity change in the thermocline and halocline, between about 10 m and the surface. In contrast, the occurrences and distribution of euphausiids in intermediate-depth and deeper waters and the general absence of adult specimens of E. pacifica, T. spinifera and T. longipes from the deep waters, below 120 m, and of T. raschii from waters below 60 m have suggested that regulatory factors other than temperature and salinity were also operative. Nauplii and metanauplii of E. pacifica were markedly restricted in their distribution to deeper water when compared with the broad vertical distribution of eggs, later developmental stages (calyptopii, furcilia) and the adults of this species, a feature which may be similarly caused. In the laboratory, experiments were conducted in attempts to determine if the variations in euphausiid distributions found in the field resulted from different reactions of species to temperature, salinity and/or combinations of temperature-salinity, or to some other property or properties acting within particular temperature-salinity ranges. In the laboratory specimens were induced to migrate vertically in temperature which increased and salinity which decreased towards the surface. The numbers migrating decreased progressively with increase in the temperature and with decrease in the salinity. The strongest effects occurred when the rate of change of either property was the maximum obtainable and when temperature and salinity gradients were combined. These experiments indicated, also, that specimens would migrate into higher temperatures and lower salinities than usually obtained in the field (and this, despite the much steeper gradients employed in the laboratory). In a second series of experiments temperature and salinity conditions were kept constant, or nearly so, and specimens (of E. pacifica) were induced to migrate from water originating in one area ("home') into water from another area ("foreign"). In general, specimens showed a preference towards the properties of "home" water and would "avoid" the "foreign" waters. In survival experiments specimens survived in larger numbers and for longer periods in "home" waters than in "foreign" waters or in mixtures of the two. There were indications of a seasonal fluctuation in survival of specimens. On the basis of the findings from the field and laboratory investigations, it is postulated that properties unique to different waters, and the reaction of euphausiids towards these "unique properties", were important in the occurrences and distribution of euphausiids in Indian Arm and their migration and survival in the laboratory. Science, Faculty of Zoology, Department of Graduate
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