Effects of size, age and photoperiod on hypoosmoregulation in brook trout, Salvelinus fontinalis

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution August 1983

Brook trout (Salvelinus fontinalis) raised from eggs under twophotoperiod and two feeding regimes were tested for physiologicalchanges preparatory for transition from freshwater to seawater. Size,age, growth rate, photoperiod, and diel rhythms were examined forpossible influences on plasma osmolarity, [Na+], [Cl-], [K+],[Mg2+], thyroxine concentration, hematocrit, and gillNa+,K+-ATPase activity of brook trout in freshwater. Significantdiel cycles were found in plasma osmolarity, (Na+] and thyroxineconcentration. Significant size and/or age related changes occurredfor plasma osmolarity, [Na+], [K+] and hematocrit, but could explain little of their total variation (0.02 < r2 <0.18). Asexually dimorphic response to photoperiod was observed in hematocritfor both mature and immature fish, with hematocrit of mature femalesdeclining in autumn and hematocrit of immature males increasing inautumn. Gill Na+,K+-ATPase activity did not respond tophotoperiod or feeding treatment and showed no change with size orage. Plasma thyroxine levels responded to feeding and photoperiodtreatment. There was a significant correlation between the percentmean difference in plasma thyroxine and the mean difference in growthrate between high and low feed fish (r2 = 0.51), suggesting arelationship between thyroxine and growth.In 11 experiments over 1.5 yrs, brook trout were gradually exposedto 32 ppt seawater for 20 d to investigate the ontogeny of salinitytolerance. A single experiment examined daily changes in plasmaosmolarity, [Na+], [Cl-], [K+], [Mg2+], thyroxine, hematocritand gill Na+,K+-ATPase during adaptation to 10 ppt, 20 ppt and 32ppt. Size was the primary determinant of seawater survival (r2 =0.77); the effect of size on seawater survival slowed after fishreached a fork length of 14 cm. The effect of age on seawater.survival (r2 = 0.65) was through its covariance with size.Photoperiod affected seawater survival only through its influence onthe timing of male maturation, which decreased salinity tolerance.Hypoosmoregulation of plasma osmolarity, [Na+], [Cl-], [K+],[Mg2+] and hematocrit increased linearly with size over the entirerange of sizes (6-32 cm). Gill Na+,K+-ATPase activity after 20 din seawater decreased with increasing size of brook trout, possiblyreflecting decreased demand for active ion transport in larger fish.Plasma thyroxine concentrations generally declined in seawater, but nodefinitive role of this hormone in seawater adaptation was found.Size dependent survival and osmoregulatory ability of brook trout iscompared to other salmonids and a conceptual model is developed.Decreased salinity tolerance and hypoosmoregulatory ability wasfound in mature male brook trout and was not found in females orimmature males. Lowered salinity tolerance of adult males becomesacute during autumn photoperiod when normal spawning occurs. Plasma[Cl-], [Mg2+], osmolarity and hematocrit are significantly higherin mature males after transfer to seawater, relative to maturefemales. It is postulated that reduced adult male hypoosmoregulatoryability explains skewed sex ratios in anadromous populations, limitsthe extent of anadromy, and was a significant phase in the evolutionof extended salmonid migration.Anadromous brook trout of Riviere a la Truite, Quebec, wereexamined for physiological changes associated with salmonidsmoltification, and compared to non-anadromous brook trout of theMatamek River. There were no significant differences in plasmathyroxine concentration, gill Na+,K+-ATPase activity, hematocritor osmoregulatory ability of anadromous and non-anadromous brooktrout. Moisture content was significantly different between fish fromthe two river systems, but had the same pattern of declining moisturecontent as summer progressed. Silver coloration of brook trout inRiviere a la Truite was significantly associated with larger fish andhigher gill Na+,K+-ATPase activity, but not with changes in plasmathyroxine, moisture content, hematocrit or condition factor. Silvercoloration was absent in Matamek River brook trout. Brook trout athigh salinity estuarine sites had significantly greater gillNa+,K+-ATPase activity and hypoosmoregulatory ability than brooktrout at low salinity sites. Atlantic salmon (Salmo salar) in highsalinity estuarine sites had significantly higher plasma thyroxine andgill Na+,K+-ATPase activity than brook trout. The resultsindicate that smoltification is relatively undeveloped in brook troutand that estuarine residence is important in salinity adaptation andeventual seaward migration.

I gratefully acknowledge the financial support of the W.H.O.I.education office, the Tai Ping foundation and the U.S. Department ofCommerce Sea Grant Program.