Cold Tolerance and Fatty Acid Composition of Striped Bass, White Bass, and Their Hybrids

—Cold tolerance of striped bass Morone saxatilis, white bass M. chrysops, palmetto bass (female striped bass 3 male white bass), and sunshine bass (female white bass 3 male striped bass) were compared under controlled laboratory conditions. Two groups of each taxon were acclimated at 208C in a recirculating-water system housed in an environmental chamber and were fed either a natural or prepared diet for 84 d. The fatty acid composition of the natural diet was 13% more unsaturated than that of the prepared diet. Fish fed the natural diet subsequently had unsaturated : saturated fatty acid ratios 10–25% higher than fish fed the prepared diet. After being subjected to identical simulated cold fronts (108C drop in surface water temperature, as if the fish were confined in cages or pens), all groups of fish fed the prepared diet suffered high mortality (50–90%) whereas there was zero mortality among the groups receiving the natural diet. White bass and sunshine bass fed the prepared diet had higher survival rates (50% and 40%, respectively) compared with their striped bass and palmetto bass counterparts (10% and 20%, respectively). The lower incipient lethal temperature was higher for fish fed the prepared diet (5.9, 4.8, 2.5, and 1.98C for striped bass, palmetto bass, sunshine bass and white bass, respectively) than for those fed the natural diet (near 0.08C, but 1.88C for sunshine bass). Both studies reflect a maternal affect on cold tolerance, with white bass being most tolerant. We demonstrated that diet-induced muscle fatty acid composition directly affects cold tolerance of striped bass, white bass, and their hybrids. Striped bass Morone saxatilis have been cultured and propagated for more than 100 years in the United States. Subsequent research revealed that hybrids of the striped bass and white bass Morone chrysops were better suited for culture (Bishop 1968; Logan 1968; Williams 1971, 1976; Ware 1975; Bonn et al. 1976; Kerby and Joseph 1979), yet retained the appealing taste of the striped bass (Lareau 1987). Production of hybrid striped bass is currently one of the fastest growing segments of commercial aquaculture in the United States. There have been relatively few studies of the culture of striped bass or hybrid striped bass in cages. Harrell et al. (1988) obtained nearly 100% survival of striped bass overwintered in cages in freshwater ponds on the Eastern Shore of Maryland. However, striped bass reared in cages in a brackish-water environment in New York had unacceptably high mortality rates that were attributed to low water temperatures during the fall and winter (Valenti et al. 1976). Recently, Hogans (1994) demonstrated that striped bass could be successfully overwintered in cages in marine water. Wil* Corresponding author: [email protected] 1 Present address: Department of Wildlife and Fisheries, Mississippi State University, Mississippi State, Mississippi 39762–9690, USA. Received December 10, 1998; accepted April 13, 1999 liams et al. (1981) and Woods et al. (1983) demonstrated that hybrid striped bass could be successfully grown in cages in estuarine waters in South Carolina and North Carolina, respectively. Culture of striped bass or hybrid striped bass in the Midwest has not been as extensively established as in the coastal regions of the United States. When raising hybrid striped bass in cages, several producers have reported sudden losses of hybrids when the water temperature rapidly decreased by several degrees in a relatively short period of time (Valenti 1989; A. M. Kelly and C. C. Kohler, personal observation). The rapid onset of cold temperatures has been reported as the cause of death in several species of fish (Verril 1901; Storey 1937; Galloway 1941; Gunther 1941; Ash et al. 1974; Coutant 1977; Mitchell 1990). It is believed that the lipid composition in the fish muscle plays a vital role in the ability of fish to adapt from one temperature to another (Hazel 1984; Greene and Selivonchick 1987; Henderson and Tocher 1987). Phospholipids are the class of lipids in which the most obvious changes occur. As environmental temperatures decrease, the invariable response is an increase in fatty acid unsaturation (Johnston and Roots 1964; Caldwell and Vernberg 1970; Hazel 1979; Cossins and Prosser 1982). Conversely, as ambient temperatures increase, phospholipid saturation must also increase to avoid excess fluidity. 279 COLD TOLERANCE AND FATTY ACIDS The dynamics of lipid composition of cells occurs in order to maintain a constant fluid matrix for enzymes associated with membranes (Greene and Selivonchick 1990). Different species of fish differ in their patterns of fat deposition and mobilization, which in turn affects the temperature range in which the species can grow and survive. For example, the Nile tilapia Oreochromis niloticus does not store excess lipids in the musculature but rather relies on visceral deposits that it is incapable of mobilizing at low temperatures, which results in high mortalities between 88C and 6.58C (Satoh et al. 1984). Viola et al. (1988) demonstrated that the common carp Cyprinus carpio, which is capable of mobilizing lipids from muscular and visceral deposits, is able to survive to 4.58C under the same conditions. Diets influence the fatty acid composition in several species of fish (Henderson and Tocher 1987; Lovell 1989; Seo et al. 1994), and the ability of a fish to alter its lipid composition when placed in colder water is one factor that determines survival. For example, summer harvest syndrome is an anomaly seen in goldfish Carassius auratus when they are harvested in the summer and placed in tanks containing water that is colder than the pond water (Mitchell 1990). The death of these fish is thought to be a result of the fat that the goldfish consume or produce (Mitchell 1990). Goldfish with high concentrations of saturated body fat are less tolerant of temperature change than fish with high concentrations of unsaturated body fat. Similarly, rainbow trout Oncorhynhcus mykiss that have been fed diets high in saturated fats stiffen and die when placed in cold water (Mitchell 1990). In these fish, the fat apparently hardens in the colder water, causing the fat-impregnated muscles to stiffen and the fish to become exhausted and lose movement. Although it has been hypothesized that temperature is closely linked to membrane composition, relatively few studies have been conducted to determine if a correlation exists between lipid composition and cold tolerance. This study was designed to determine the effect of a sudden temperature change (a simulated cold front) on striped bass, white bass, and their hybrids fed either a natural or prepared diet, as well as to determine their lower incipient lethal temperature. The association of fatty acid composition and unsaturated: saturated fatty acid ratios in these fish were examined with respect to their tolerance to cold.