Osmoregulation in the brook trout, Salvelinus fontinalis—II. Effects of size, age and photoperiod on seawater survival and ionic regulation

Abstract

1. 1. Brook trout ( Salvelinus fontinalis) of a single genetic stock, and hatched at the same time, were raised under two photoperiod and two feeding regimes to obtain fish of the same age but with different sizes and photoperiod experiences. In 11 experiments over 1.5 firs, fish were gradually exposed to 32 ppt seawater for 20 days to investigate the ontogeny of salinity tolerance. 2. 2. Daily changes in plasma osmolarity, [Na +], [Cl −], [K +], [Mg 2+], thyroxine, hematocrit and gill Na +,K +-ATPase during adaptation to 10, 20 and 32 ppt were examined in one experiment. 3. 3. Size was the primary determinant of seawater survival ( r 2 = 0.77) the effect of size on seawater survival slowed after fish reached a fork length of 14 cm. The effect of age on seawater survival ( r 2 = 0.65) was through its covariance with size. 4. 4. Photoperiod affected seawater survival only through its influence on the timing of male maturation, which decreased salinity tolerance. 5. 5. Regulation of plasma osmolarity, [Na +], [Cl −], [K 2+], [Mg 2+] and hematocrit in sea water increased linearly with size over the entire range of sizes (6–32 em). 6. 6. Gill Na +,K +-ATPase activity after 20 days in seawater decreased with increasing size of brook trout, possibly reflecting decreased demand for active ion transport in larger fish. 7. 7. Plasma thyroxine concentrations declined in seawater, but no definitive role of this hormone in seawater adaptation was found. 8. 8. Size dependent survival and osmoregulatory ability of brook trout is compared to other salmonids and a conceptual model is developed.