Size dependent early salinity tolerance in two sizes of juvenile white sturgeon, Acipenser transmontanus B. Mojazi Amiri a,b , D.W. Baker b, ⁎, J.D. Morgan c , C.J. Brauner b a Department of Fisheries & Environmental Sciences, Faculty of Natural Resources, University of Tehran, 31585-4314 Karaj, Iran b Department of Zoology, 6270 University Boulevard, University of British Columbia, Vancouver, B.C., Canada V6T 1Z4 c Faculty of Science and Technology, Vancouver Island University, 900 Fifth Street, Nanaimo, B. C., Canada V9R 5S5 abstract article info Article history: Received 20 December 2007 Received in revised form 26 August 2008 Accepted 29 August 2008 Keywords: Ionoregulation Osmoregulation Salinity White sturgeon Seawater transfer The objective of this study was to investigate the influence of size on salinity tolerance in 1 year old juvenile white sturgeon. Two sizes of sturgeon (10 and 30 g) from the same spawning event (thus reducing confounding effects of genetic make-up and size) and reared in the same environment were exposed to a salinity of 0, 8,16, 24, or 32 ppt for up to 120 h. Both 10 and 30 g fish exhibited N 93% mortality within 24 h after transfer to 24 or 32 ppt, regardless of whether they were transferred directly from freshwater (FW) or following a 48 h pre-treatment period at 16 ppt. Direct transfer from FW to 16 ppt was associated with 25 to 30% mortality, indicating that these fish have some ability to tolerate large changes in salinity for up to 5 days at this stage. Following exposure to 8 and 16 ppt, an elevation in plasma osmolarity, [Na + ], and [Cl - ] was observed between 24 and 72 h in both 10 and 30 g sturgeon, but plasma ions and osmolarity in surviving fish at 120 h were not significantly different between groups held at 0, 8, and 16 ppt. Despite being unprepared for either direct or stepwise transfer to salinities of 24 ppt or greater, size confers some ionoregulatory advantage, as mortality occurred more slowly and the degree of ionoregulatory perturbation was less in 30 g than 10 g fish over the course of the exposures. It is not known whether the apparent advantage of size is related to a size-dependent development of ionoregulatory capacity or due to social status which can also influence ionoregulatory capacity, but age and genetic differences did not likely contribute to this size effect. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Categorizing sturgeon species by patterns of anadromy has been based on actual distributions of sturgeon as collected from the wild (e.g., Auer, 1996; Boreman, 1997; Bain, 1997; Krayushkina, 1998). However, the absence of collection of small sturgeon in marine or estuarine environments does not preclude the possibility that individuals are exploiting these resource-rich areas (Bain, 1997). Two problems are associated with attempting to classify endemic populations by pattern of anadromy. The first is that many populations have been prevented from accessing the ocean due to dam construc- tion. This limitation imposes a potadromous life cycle in populations that may historically have exhibited other patterns. The second problem is that smaller juvenile sturgeons are very difficult to catch (Wilson and McKinley, 2004). While this difficulty is obvious from a cursory examination of the distribution literature on many sturgeon species (e.g., Kynard, 1997), the reasons for this are not as clear. However, they may be related to the phototropic response of young sturgeon (Cech and Doroshov, 2004), or the tendency of sturgeon to hide in gravel or under rocks (e.g., Peake, 1999). On the other hand, it is possible that sampling has either not occurred where these animals are, or failed to target sturgeon of these small sizes. White sturgeon (Acipenser transmontanus Richardson, 1836) has been classified as a semi-anadromous species found along the Pacific coast of North America in the watersheds of the Fraser, Columbia, Sacramento, and San Joaquin rivers (Cech and Doroshov, 2004). Despite this classification, some land locked populations exist in freshwater reservoirs and tributaries of the Columbia River (Doroshov et al., 1997). Lower Columbia and Sacramento populations are thought to spend their adult life in the sea (salinity between 22 and 33‰), but migrate to fresh water during spawning periods (Bemis and Kynard, 1997; Krayushkina, 1998; Wilson and McKinley, 2004), in a pattern qualitatively similar to some salmonid species, albeit over a much longer period, as females may not return to spawn until 15–25 years old. Although sturgeons are chondrosteans, they are thought to exhibit most key teleost features of osmoregulation (Potts and Rudy, 1972), with similar osmoregulatory mechanisms (Gershanovich et al., 1991; Cech, 2000; Wright, 2007) for adapting to saline environments. As with many other species of sturgeon, little is known about early life history of this species: for example, it is not clear at what developmental stage white sturgeon enter estuaries. However, it appears that juvenile white sturgeon may remain in fresh water for Aquaculture 286 (2009) 121–126 ⁎ Corresponding author. Tel.: +1 604 822 3378; fax: +1 604 822 2416. E-mail address: baker@zoology.ubc.ca (D.W. Baker). 0044-8486/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.aquaculture.2008.08.037 Contents lists available at ScienceDirect Aquaculture journal homepage: www.elsevier.com/locate/aqua-online