Growth and physiological parameters of whiting (Sillaginodes punctata) in relation to salinity By C. A. Meakin and J. G. Qin School of Biological Sciences, Flinders University, Adelaide, SA, Australia Summary This study assessed the impact of salinity on whiting (Sillag- inodes punctata) in an attempt to understand the mechanisms by which salinity could potentially influence habitat selection and growth of King George whiting in southern Australia. The experiment included whiting of two age classes, young of the year (YOY) and 2 + year, at three salinities (30, 40, 50 ppt). YOY whiting showed no significant difference in length or weight gain, specific growth rate, feed intake, food conversion ratio or condition factor when exposed to the three salinities for 72 day. Plasma osmolality of YOY whiting was not significantly different at any salinity, although it was signifi- cantly lower than that of 2 + year whiting. The 2 + year whiting showed significantly higher plasma osmolality than the YOY. Blood plasma potassium and chloride levels of 2 + year fish at 50 ppt were significantly higher than those at 30 ppt and 40 ppt. Blood sodium levels at 50 ppt were significantly higher than at 30 ppt but the sodium level at 40 ppt was not different from 30 ppt or 50 ppt. Haematocrit of 2 + whiting was significantly higher at 30 than at 50 ppt while haematocrit at 40 ppt was not different from 30 or 50 ppt. The 2 + year-old whiting had a more pronounced increase in plasma osmolality and plasma ions at high salinities, indicating poorer osmoreg- ulatory capacity in older fish. This study provides physiolog- ical evidence to partially explain habitat occupancy and growth in relation to salinity of different age groups of whiting in southern Australia. Introduction King George whiting (Sillaginodes punctata) inhabit Austra- lian waters from southern New South Wales to southern Western Australia. Most research on whiting has focused on migratory and recruitment patterns (Hamer and Jenkins, 1997; Hyndes et al., 1998; Fowler et al., 2000; Jenkins, 2005) habitat use (Connolly, 1994; Jenkins and Wheatley, 1998), swimming ability (Jenkins and Welsford, 2002) and the development of the digestive system (Chen et al., 2003). Studies have shown that the growth of this species is slow, taking 3–4 years to reach maturity at a size of 30–37 cm (Hyndes et al., 1998; Fowler et al., 2002). Settlement of post-larvae occurs in the shallow waters of the Gulf St Vincent and Spencer Gulf, South Australia, and as the fish grow, they gradually move out into the deeper waters of the gulfs and migrate south towards their spawning grounds (Fowler et al., 2002). Besides ecological and behavioural studies, evidence of environmental factors influencing the physiology of whiting is rare. Fish are often subjected to environmental stresses from changes in temperature, salinity and oxygen concentrations. Prolonged exposure to environmental stressors leads to a tertiary stress response including slow growth, immune mal- function and low reproductive output (Barton, 2002). The Port River and Barker inlet estuary system north of Adelaide is one of the main nursery areas for whiting (Jones, 1984). This estuary system receives very little freshwater and is subjected to large seasonal fluctuations in temperature and salinity. Salinity levels in both Gulf St Vincent, and the Barker inlet in summer have exceeded 40 ppt (Fielder et al., 2007). Jones et al. (1996) reported salinity and temperature fluctua- tions in the Barker inlet ranging from 36 ppt and 15°C in July, to 41 ppt and 26°C in January, while on rare occasions salinity can fall to as low as 30 ppt (Jackson and Jones, 1999). Juvenile whiting found in the upper Spencer Gulf have been reported in water up to 48 ppt (Ham and Hutchinson, 2002). Boeuf and Payan (2001) commented that it is unlikely for most marine fish to grow at optimal rates in seawater that exceeds 35 ppt, and Imsland et al. (2008) observed that even stenohaline species can benefit from being reared in lower concentrations of saltwater. Salinity is one of the key environmental factors regulating the physiological responses of numerous fish species and the salinity tolerance of fish depends on the environment and developmental stage (Boeuf and Payan, 2001). For instance, California halibut (Paralichthys californicus) exhibit age-dependent tolerance to salinity and inhabit different environments as they grow (Madon, 2002). However, little is known on the salinity tolerance of King George whiting, despite its substantial contribution to fisheries in South Australia. Apart from human influence such as fishing pressure, the impact of changing environments on the whiting fishery has not been paid much attention. In the last decade, South Australia has experienced record extended heat waves, with daytime air temperatures in excess of 45°C and minimum overnight temperatures exceeding 30°C along with long-term meteorological drought (National-Climate-Centre, 2008, 2009). Drought-induced salinity changes have been shown to affect estuary catch rates (Gillson et al., 2009) and the recent focus on climate change has highlighted issues regarding climate induced changes in fish abundance and distribution (Hughes, 2000; Lotze et al., 2006). King George whiting are found in locations with a wide range of salinities and tend to occupy different habitats at different ages. As the movement of whiting larvae is primarily determined by ocean currents, the ambient salinity at settlement locations should have potential influence on the growth rates and recruitment success of this species. Therefore, the aim of this study was to investigate the impact of salinity on the growth response of juvenile whiting, and to assess the osmoregulatory abilities of young of the year J. Appl. Ichthyol. 27 (2011), 1316–1321 Ó 2011 Blackwell Verlag, Berlin ISSN 0175–8659 Received: August 24, 2010 Accepted: January 6, 2011 doi: 10.1111/j.1439-0426.2011.01781.x U.S. Copyright Clearance Centre Code Statement: 0175–8659/2011/2706–1316$15.00/0 Applied Ichthyology Journal of