World Aquacluture Magazine - September 2020

56 SEP TEMBER 2020 • WORLD AQUACULTURE • WWW.WA S.ORG Hybridization Experiments At the coldwater hatchery at SUNYCobleskill, NewYork, brook trout females were spawned with Arctic char males to create an F 1 hybrid Arctic char. Subsequently, only fish that resembled Arctic char, those without dorsal and caudal vermiculations (Scott and Crossman 1973), were bred to form the next generation. Arctic char hybrids were reared under ambient temperature and photoperiod using the same incubators, tanks and conditions described above for pure-strain Arctic char. The study reported here is based on 12 years of data and includes 354 spawnings and 1,409,276 hybrid eggs. Since the beginning of this study, ten generations of hybrid Arctic char have been reared. Under ambient conditions at SUNYCobleskill, hybrid Arctic char spawned at 8-10 C and brook trout at 10-12 C. Spawning occurred primarily in October for brook trout and November for Arctic char hybrids. Reproductive Success Throughout the ten generations of this study, reproductive success of hybrid Arctic char was greaterthan pure-strain Arctic char at New York photoperiods and groundwater temperatures (Fig. 7). Average fertility was 83 percent, eye-up rate was 45 percent and hatching rate was 35 percent. The fertilization, eye-up and hatching rates of hybrid Arctic char were significantly better than pure strain Arctic char at NewYork temperatures and latitude. Previous studies of Arctic char × brook trout hybrids reported decreased fertility and sterility (Phillips and Ráb 2001, Ocalewicz et al . 2014). This may be due to differences in chromosome number causing problems with their pairing during meiosis (Tave 1993). It was hypothesized that only fish that produced viable gametes would leave progeny, thus, in subsequent generations, fertilization and hatching success would be improved. The percent eye-up and hatching rate for hybrid Arctic char eggs was more than double that of pure-strain Arctic char (Fig. 7). There was also a significant trend for improving fertilization rates with each gen- eration of hybrids (Fig. 8, Pearson Product Moment Correlation P<0.05). Appearance The appearance of Arctic char and their hybrids is quite variable. The primary visual characteristic separating Arctic char from brook trout is the presence of dorsal vermiculations on the brook trout (Scott and Crossman 1973). Therefore, the focus of broodstock selection was to only spawn hybrids without dorsal vermiculations (Fig. 9). After nine generations, 89 percent of hybrids had the appearance of an Arctic char, 53 percent had no dorsal markings, while 36 percent had dorsal spots. Further, 49 percent of the F 9 hybrids had at least one red spot with a blue halo, considered to be a brook trout trait (Scott and Crossman 1973). Furthermore, hybrid Arctic char had significantly greater girth than either brook trout or pure strain Arctic char. The coefficient of condition indicated that there was more variance in 200 hybrid Arctic FIGURE 7. Egg incubation descriptive statistics for pure Arctic char and ten generations of hybrids. 0 20 40 60 80 100 Fertilization Eye Up Hatch Percent Pure Arctic Char F1-F10 Hybrids FIGURE 8. Improvement in fertility rate of hybrid Arctic char between generations. y = 66.341x 0.1113 R² = 0.4141 0 10 20 30 40 50 60 70 80 90 100 0 1 2 4 5 6 7 8 9 10 % Fertilization Hybrid Generation FIGURE 10. Comparison of condition factor (length-weight relationship) among Arctic char, brook trout and hybrids. 0 5 10 15 20 25 30 35 40 45 1.4-1.5 1.3-1.4 1.2-1.3 1.1-1.2 1.0-1.1 .9-1 .8-.9 .7-.8 Percent Condition Hybrids Arctic Char Brook Trout FIGURE 9. Variable appearance of ninth generation Arctic char female (top) and male (bottom).