Relationship between Seawater Adaptability and Body Weight in 0+ Landlocked Rainbow Trout.

Abstract

A seawater challenge test was conducted on 0+ age-group landlocked rainbow trout On corhynchus mykiss to estimate the sufficient size for seawater culture. The seawater adaptability of fish of different body weights (averages respectively 50, 100, 150, and 200g) was assessed accoring to mortality, muscle moisture content, gill Na+-K+ATPase activity, and serum Na con centration, at intervals of 0, 6, 12, 24, 48, and 96h after direct transfer into seawater (31.6%). A high mortality occurred among fish in the 50g weight class. Muscle moisture content in the 200g weight class showed less change than in other classes. The serum Na concentration in the 50 and 100g weight classes was higher than in the 150 and 200g weight classes after seawater transfer. These results indicate that the seawater adaptability of fish is size dependent, and that fish over 150g may acquire complete seawater adaptability. Therefore, fish over 150g are of suf ficient size to develop in seawater culture and these fish can be transferred directly into seawater without any acclimation.

Differential Seawater Adaptability in Three Different Sizes of Under-yearling Steelhead Trout

“…The rainbow trout in a 200 g size class maintained significantly higher levels of muscle moisture content after the transfer to seawater than did the other smaller size classes (Gorie, 1993). Again, in comparison to the previous studies on rainbow trout reported by Leray et al (1981) and Gorie (1993), steelhead trout in the current study seem to be able to maintain muscle moisture better than landlocked rainbow trout of the similar size.…”

“…No mortality was observed in the 100 g weight group, while mortality in both the 50 and 150 g weight groups was 2.6% ( Table 1 ). In a previous study of a landlocked O. mykiss strain, Gorie (1993) reported a 29% mortality in the 50 g size class compared to 3% and 6% mortality in 100 and 150 g size classes, respectively, 96 h after seawater transfer. In comparison with the previous study by Gorie (1993) , under our experimental conditions the current mortality results after seawater transfer indicate that steelhead trout has a higher tolerance to seawater (32 ppt) than landlocked O. mykiss of the same size.…”

“…Bath & Eddy (1979) concluded that increased plasma Na + and Cl − concentrations in the adaptation phase is the result of initial increased drinking and diffusing of salts into the gills. In a previous study, Gorie (1993) showed that landlocked rainbow trout over 150 g size develop high level hypoosmoregulatory capacity, but trout of 50 and 100 g sizes did not develop a high level. Thus, in comparing the steelhead trout with its landlocked counterpart, steelhead trout seems to develop a higher level of hypoosmoregulatory capacity at a smaller size than landlocked rainbow trout in terms of plasma ionic regulations.…”

“…In a previous study of a landlocked O. mykiss strain, Gorie (1993) reported a 29% mortality in the 50 g size class compared to 3% and 6% mortality in 100 and 150 g size classes, respectively, 96 h after seawater transfer. In comparison with the previous study by Gorie (1993), under our experimental conditions the current mortality results after seawater transfer indicate that steelhead trout has a higher tolerance to seawater (32 ppt) than landlocked O. mykiss of the same M Lee, J-W Lee size. This agrees with the 96-h LC50 salinity test results reported by Johnsson & Clarke (1988).…”

“…However, if O. mykiss are transferred from freshwater to seawater before they develop efficient osmoregulatory capacity, the fish cannot increase NKA activity to a sufficient level, which results in prolongable elevation of salts (e.g., plasma Na + and Cl − ) and dehydration (e.g., muscle moisture decrease) internally, which could lead to the interruption of normal biochemical processes and physiological homeostasis ( Bystriansky et al, 2006 ; Kennedy et al, 2007 ; Kültz, 2015 ). In a previous study, Gorie (1993) has reported that 0+ landlocked strain of O. mykiss need to reach a 150 g size to acquire a high level of hypoosmoregulatory capacity in seawater. In the current study, we hypothesized that under-yearling steelhead trout acquire a high level of seawater adaptability in a relatively smaller size compared to landlocked rainbow trout.…”