The Kootenai River white sturgeon Acipenser transmontanus

is a landlocked population restricted to a 270 km stretch of the Kootenai River, which flows from western Montana through northern Idaho and into British Columbia. This population has shown an almost complete lack of natural juvenile recruitment since 1974. The total adult population in 1998 was estimated to be about 650 individuals and this fish is currently classified as endangered by the Idaho Department of Fish and Game (http://refuges.fws.gov/fish/KootenaiRiverSturgeon.html). The basis for its lack of juvenile recruitment and declining population numbers is not clear and there is, therefore, a need to better understand the reproductive biology of these animals. As a species, sturgeon are routinely exposed to hypercapnia in their natural environment (Crocker and Cech, 1998). Environmental hypercapnia is associated with an increase in arterial PCO∑ and a decrease in blood pH in the white sturgeon A. transmontanus (Crocker and Cech, 1998; Crocker et al., 2000). We have previously noted that maintenance of salmonid sperm at low pH or high CO2 prevents the onset of motility, and reduces fertility, when sperm are subsequently diluted with water (Bencic et al., 2000a,b, 2001). Further, the maximal pH sensitivity of this capacity for motility corresponds to the pH range over which the buffering capacity of seminal plasma is particularly low (Ingermann et al., 2002). These findings suggest that the functional properties of salmonid sperm may be especially sensitive to hypercapnia and resulting respiratory acidosis, as well as metabolic acidosis, prior to release into the environment. As sturgeon are subjected to environmental hypercapnia, especially in intensive aquaculture settings (Crocker and Cech, 1996, 1998; Farrell et al., 2001), and because fish are occasionally anesthetized with CO2 (e.g. Fish, 1943; Prince et al., 1995), including during gamete harvesting, our previous findings with salmonids suggest that environmental hypercapnia and respiratory and metabolic acidosis could be associated with reductions in sperm motility and possibly fertility in the sturgeon. In contrast, Gallis et al. (1991) reported that motility of sperm from the Siberian sturgeon Acipenser baeri is relatively insensitive to pH changes near physiological semen pH (approximately 8.1). Williot et al. (2000) also found no correlation between sperm motility and pH in A. baeri. Finally, Gallis et al. (1991) reported that semen from A. baeri shows a strong buffering capacity. As preliminary observations from our laboratory suggested the opposite in the white sturgeon A. transmontanus, this study was designed to determine the pH sensitivity of the capacity for motility, the influence of CO2, and the buffering capacity of semen of the Kootenai River white sturgeon A. transmontanus. 2885 The Journal of Experimental Biology 205, 2885–2890 (2002) Printed in Great Britain © The Company of Biologists Limited 2002 JEB4081