Predicting the vertical distribution of the opossum shrimp, Mysis relicta, in Lake Ontario: a test of laboratory-based light preferences

Light and temperature strongly influence the vertical distribution of the mysid shrimp, Mysis relicta. We monitored the vertical movements and depth selection behavior of mysids exposed to different light intensities and light-temperature gradients in the laboratory and derived a mysid light preference function in units relevant to mysid vision: "mylux''. Mysids preferred light levels between 10(-8) and 10(-7) mylux (similar to 10(-6) to 10(-5) lux) and rarely moved into waters of 10(-3) mylux (similar to 0.1 lux) and greater. A model that assumed equal weight and independence of mysid light and temperature preference functions successfully predicted the proportion of mysids found in two different temperature-light combinations in the laboratory. This model also predicted the depth of maximum mysid density to within 2 m on two spring nights and within 5 m on two summer nights of varying moon phase and thermal conditions in Lake Ontario. This study provides novel insights into how temperature and light interact to influence the vertical distribution of mysids. Our model may be used to predict mysid vertical distribution in any deepwater system inhabited by mysids in which the primary mysid predators are visual feeders.