Environmental Biology of Fishes 70: 145–154, 2004. © 2004 Kluwer Academic Publishers. Printed in the Netherlands. Effects of temperature on the early development, growth, and survival of shortnose sturgeon, Acipenser brevirostrum, and Atlantic sturgeon, Acipenser oxyrhynchus, yolk-sac larvae Ryan S. Hardy a & Matthew K. Litvak b,∗ a Idaho Department of Fish and Game, 2750 Kathleen Avenue, Coeurd’Alene, ID 83815, U.S.A. (e-mail: rhardy@idfg.state.id.us) b Department of Biology and Center for Coastal Studies and Aquaculture, University of New Brunswick, P.O. Box 5050, Saint John, N.B., Canada, E2L 4L5 ∗ Corresponding author (Tel.: 506-648-5565; Fax: 506-648-5811; e-mail: litvak@unbsj.ca) Received 11 February 2002 Accepted 2 September 2003 Key words: escape response, early life history, yolk utilization efficiency, yolk utilization rate Synopsis We reared shortnose and Atlantic sturgeons at different temperatures after hatch and measured yolk utilization rate and efficiency (YUE), maximum standard length, survival and development of escape response. Newly hatched Atlantic sturgeon, were smaller in size, more efficient at utilizing yolk (incorporating yolk to body tissue) and reached developmental stages sooner than shortnose sturgeon reared at the same temperatures (13–15 ◦ C). Within each species, decreasing temperature delayed yolk absorption, escape initiation, time to reach maximum size, and time to 100% mortality. However, YUEs and the size of the larvae at these ‘stages’ were independent of rearing temperature for both species. These results suggest that even as temperature drives metabolic processes to speed up development, these two species are still extremely efficient at transferring yolk energy to body tissues. The lower efficiencies experienced by larval shortnose may reflect difference in yolk quality between the two species and/or the Atlantic sturgeon’s higher conversion efficiency. The ability of these two sturgeon species to develop successfully and efficiently under a wide range in temperatures may provide a competitive advantage over more stenothermic species and explain their persistence through evolutionary time. Introduction Mortality in many fish species is very high during early development due to their small size, limited swimming ability, and sensitivity to variations in their surround- ing environment (Rice et al. 1987, Miller et al. 1988). During these vulnerable stages, fishes may be adversely affected by environmental factors that influence sur- vival and subsequent year-class strength (Houde 1994, Claramunt & Wahl 2000). Size of a fish at the time of the switch from endogenous to exogenous feeding, referred to as the ‘critical period’ in development, may be a very impor- tant factor affecting year-class strength of a species (Hjort 1914, Cushing 1972). Therefore, examination of environmental factors that affect the rate of yolk utilization, efficiency to which yolk energy is trans- ferred to body tissues, and body size at each stage of development is important to our understanding of recruitment. Water temperature is considered to be the most important environmental factor influencing fish devel- opment (Chambers & Leggett 1987). Limited informa- tion exists on how water temperature affects the early development of sturgeon. The few studies performed on sturgeon (Wang et al. 1985, 1987, Gershanovich & Taufik 1992) included larvae of different sizes at the start of the experiment. Since eggs of many sturgeon species are spawned within a narrow range of tem- peratures and have a relatively short incubation period