Seawater Acclimation and Parr-Smolt Transformation of Juvenile Atlantic Salmon, Salmo solar L.

Transcription

1 Environment Canada Environnement Canada Fisheries Service des ;Aches. and-marini Service ef des sciences de la mer Canada. Fisheries and Marine Service. Resource Development Branch. TECHNICAL REPORT SERIES 1DFO Llb MPO 1111 Seawater Acclimation and Parr-Smolt Transformation of Juvenile Atlantic Salmon, Salmo solar L. by G. J. Farmer, D. Ashfield, and J. A. Ritte Technical Report Series No. MAR/ T-77-3 Freshwater and Anadromous Division Resource Branch Maritimes Region

2 SEAWATER ACCLIMATION AND PARR-SMOLT TRANSFORMATION OF JUVENILE ATLANTIC SALMON, SALMO SALAR L. G.J. FARMER, D. ASHFIELD AND J.A. RITTER MAX, 1977 TECHNICAL REPORT SERIES NO. MAR/T-77-3 FRESHWATER AND ANADROMOUS DIVISION RESOURCE BRANCH FISHERIES AND MARINE SERVICE DEPARTMENT OF FISHERIES AND THE ENVIRONMENT HALIFAX, NOVA SCOTIA

3 iii CONTENTS ABSTRACT RESUME INTRODUCTION 1 METHODS 1 RESULTS 4 DISCUSSION 11 ADDENDUM 15 APPENDIX 17 ACKNOWLEDGEMENTS 19 REFERENCES 21

4 V ABSTRACT Changes in the osmotic concentration of serum, urine and intestinal fluid of one- and two-year-old Atlantic salmon smolts and pre-smolts were examined during exposure to salinity increasing from 0.1 to 31 parts per thousand. Both age-groups acclimated equally well to sea water, the marine osmoregulatory capabilities being developed prior to parr-smolt transformation. Euryhalinity Was attained during the parr stage when fish had reached a fork length of cm. The timing of the parr-smolt transformation was synchronous for both one- and two-year-old juvenile salmon as demonstrated by changes in lipid and moisture content and coefficient of condition (K). Coefficient of condition decreased from February until early June, and lipid from March until July, minimum lipid levels being attained 40 days later than minimum K values. No seasonal changes in protein or ash content were demonstrable for either salmon ageclass. Changes in K values best reflected the migratory disposition of hatchery salmon, migratory activity being most pronounced as K approached a minimum during May and decreasing during early June as K increased. Hatchery release dates are discussed in terms of changing lipid content, coefficient of condition and migratory activity. RESUME On a etudie les changements survenus dans la concentration osmotique de serum, d'urine et de liquide intestinal chez des tacons et pre-tacons de saumon Atlantique ages d'un et deux ans et experimentalement exposes a des salinites variant entre 0.1 et 31 parties par mille. Les deux groupes d'age se sont egalement acclimates a l'eau de mer, les capacites osmoregulatoires marines ayant ete acquises avant la transformation parrtacon. L'euryhalinite fut atteinte au cours du stade parr lorsque les poissons mesuraient de 12 a 13 cm de longueur a la fourche. La transformation parr-tacon s'est produite simultanement chez les deux groupes d'age, comme l'ont demontre les changements survenus dans la teneur en lipides et en humidite et dans le coefficient de condition (K). Le coefficient de condition a baisse de fevrier jusqu'au debut juin et la teneur en lipides, de mars jusqu'en juillet. Les plus bas niveaux de lipides furent atteints 40 jours plus tard que les valeurs minimales du K. Nul changement saisonnier a pu etre decele dans la teneur en proteines ou en cendres pour l'un ou l'autre des groupes d'age de saumon. Les changements dans les valeurs du K ont, le mieux, fait ressortir les tendances migratoires des saumons eleves en pisciculture. L'activite migratoire s'accrat en mai lorsque la valeur du K avait presqu'atteint un minimum et decrat au debut juin alors qu'augmentait la valeur du K. Les dates de relachement pour tacons de pisciculture en fonction des changements dans la teneur en lipides, le coefficient de condition et l'activite migratoire sont discutees.

5 INTRODUCTION Parr-smolt transformation of juvenile anadromous salmonids has been associated with the development of marine osmotic and ionic regulatory mechanisms. However, some studies have demonstrated that these mechanisms develop prior to transformation coincident with the attainment of a specific size (Conte and Wagner 1965; Conte et al.1966; Wagner 1974). Wagner (1974) suggests that parr-smolt transformation and the development of the marine hypoosmoregulatory mechanisms of steelhead trout (Salm gairdneri) may be two distinct and unrelated physiological processes. In this context, transformation has been associated by Conte (1969) with a type of biological clock informing smolting species of the precise time to seek the marine environment. At the Mactaquac Hatchery, New Brunswick, 10%-20% of the juvenile salmon attain a fork length of 14.5 cm at one year of age, the minimum size considered suitable for release. The remainder are held for an additional year before liberation as larger two-year-old juveniles. Adult returns from the release of yearling salmon have been considerably lower than for those released at two years of age initiating the present study to determine the timing of parr-smolt transformation of both ageclasses and the time or size at which their marine osmoregulatory mechanisms develop. Groups of one- and two-year-old juvenile Atlantic salmon (>14.5 cm) were exposed to increasing salinity (up to 31 40) during February-March as pre-smolts and again during May as smolts to determine their marine osmoregulatory capabilities. Changes in the osmotic concentration of serum, urine and intestinal fluid of euryhaline teleosts during seawater exposure have been documented (Parry 1966; Shehadeh and Gordon 1969; Conte 1969; Hickman and Trump 1969) and were used as criteria for determining the success of seawater acclimation. Changes in coefficient of condition, migratory activity and proximate composition of both one- and two-year-old juveniles were also followed during the February-September period to determine the timing of transformation and the attainment of the post-smolt condition (Wagner 1968; Fessler and Wagner 1969; Foda 1974). Data were used to predict release dates for both juvenile age-classes with respect to the development of their marine hypoosmoregulatory mechanisms and the occurrence of parr-smolt transformation. METHODS The two groups of juvenile salmon used during the study were each comprised of yearling and two-year-old fish from the Mactaquac Hatchery and were progeny of spring-run parents captured in the Saint John River, New Brunswick. Each of the groups arrived in the laboratory three weeks prior to the commencement of the seawater acclimation studies which began during the fourth week of February and again during the first week of May. The transportation, holding and sampling procedures

6 2 for both the February and May groups were identical. Upon arrival in the laboratory, 100 salmon of each age-class were added to each of four 3,000-liter circular tanks, the two agegroups within a tank being separated by a partition constructed of small-mesh seine material. The freshwater within the tanks was maintained at 4.0±0.5 C and the fish were subjected to a natural photoperiod. All groups were fed daily to satiation with finely chopped Atlantic cod (Gadus morhua) having a proximate composition of 17.7± protein, 2.0±0.1% lipid and 80.1±2.6% moisture. In all tanks, fish had resumed feeding within one week of arrival from Mactaquac. Initial weights and fork lengths for both groups are presented in Table 1. The oxygen concentration of fresh water in the experimental tanks ranged from 7.8 to 10.6 ppm, total hardness from 12.1 to 16.6 ppm, ph from 6.1 to 6.6 and turbidity from 0.3 to 1.6 jtu. Residual chlorine was non-detectable. TABLE 1. Weight and fork length of juvenile salmon used in the March and May seawater acclimation studies. February May Age Fork length Weight Fork length Weight (years) (cm) (g) (cm) (g) ± ± ± ± ± ± ± ±18.9 During February and again during May, four yearling and four two-year-old salmon, deprived of food for 72 hr, were randomly selected from each of the three freshwater experimental tanks; and samples of their blood, urine and intestinal fluid were obtained prior to sacrificing them for proximate analysis. The salinity within each of the tanks was then increased 2 40/ day and 12 salmon of both age-classes were sampled at salinities of and 31 /m. A final sampling date occurred after exposure to 31 /00 for one week. After the last sampling date, salmon in the experimental tanks were held at 31 /00 for one month and the weights and fork lengths of any mortalities recorded. Salmon in the fourth tank acted as controls and were held in fresh, water throughout the study. Upon removal from tanks for sampling, salmon were positioned ventral side up on a laboratory counter and the area of the cloaca was rinsed with deionized water and blotted dry. Urine was then collected by placing the finely tapered tip of a 1 ±1 SD used here and throughout text.

7 microcapillary tube against the urino-genital papilla and applying gentle pressure on the bladder (Shehadeh and Gordon 1969). The cloaca was again rinsed with deionized water, blotted dry, and a microcapillary tube inserted into the terminal area of the intestine. Intestinal fluid approximating in composition that normally excreted by fish (Shehadeh and Gordon 1969) usually filled the capillary without application of pressure on the abdomen. Blood samples, taken from the caudal artery by syringe, were introduced into microcapillary tubes which were sealed at one end and then left for a half hour at room temperature to allow clot formation. The capillary tubes were then centrifuged for five minutes, and the portion containing the serum was separated and sealed at both ends. Osmolarity of urine and intestinal fluid was determined immediately after collection and that of serum within three hours using a Clifton nanoliter osmometer standardized daily with deionized water and sodium chloride solutions of known osmolarity. None of the salmon were anaesthetized prior to sampling. After completion of the May acclimation study, salmon which had been maintained in freshwater were held until September at temperatures similar to those at Mactaquac, exposed to a natural photoperiod and fed to satiation once daily with chopped cod. Twelve salmon of each of the two ageclasses were sacrificed once monthly from June to September to follow changes in their proximate composition and coefficient of condition. Proximate composition of salmon was determined after freeze-drying whole fish to constant weight and grinding them through a 20-gauge screen in a laboratory mill. Lipid content was estimated by extraction from 1-g subsamples for four hours with chloroform-methanol (2:1) (Folch et al. 1957), and ash by heating 1-g samples at 550 C for 24 hours (A.O.A.C. 1965). The ammonia released from samples after Kjeldahl digestion was measured with an ammonia-sensing gas electrode (Orion #95-10) (Ray, unpublished data)1 to provide a measure of nitrogen content. Protein content was calculated by multiplying the nitrogen estimates by Although the use of this method resulted in the inclusion of non-protein nitrogen in the estimates, the magnitude of error does not significantly influence the validity of the results without correction (Brett et al. 1969; Love 1970; Beamish 1972). Carbohydrate content was not determined because it does not amount to more than 0.3% wet weight (Dean and Goodnight 1964). The osmoregulatory ability of salmon parr (2-37 g) was assessed using 200 underyearling fish received from Mactaquac during June. The parr were held in a 3,000-litre freshwater 3 1Ray, S. Dept of Fisheries and the Environment, Fisheries and Marine Service, Biological Station, St. Andrews, New Brunswick.

8 4 tank at 15±0.5 C and fed finely chopped cod once daily. Measurements of body moisture, and osmolarity of serum and intestinal fluid were then made for 15 parr randomly selected from the tank. Salinity was increased 2 /00/day, and 15 parr were sampled at 15.5 /0 and again after five days exposure to 310/00. The remaining fish were then held for two months at and a record was kept of the mortality occurring within the tank. This procedure was repeated during November with a different group of underyearling parr. Twelve yearling and 12 two-year-old juveniles were sampled at each salinity during the February and May seawater acclimation studies. It was calculated that true differences of 15 mosm/1 for serum, 25 mosm/1 for intestinal fluid, and 1.5% wet weight or less for each of protein, lipid, ash and moisture could be detected at the 95% level with 90% assurance using a sample size of that magnitude (Steel and Torrie 1960). Each sample of 12 salmon was composed of three groups of four fish, each of the groups being taken from different experimental tanks receiving the same treatment (salinity). The data were subjected to stepwise multiple regression analysis to determine the relationship between the dependent and independent variables, the acceptance of a given independent variable being judged by its contribution to the minimization of the residual mean square. Where little or no linear trend was apparent, treatment means were compared using Scheffe's test after analysis of variance (Steel and Torrie 1960). RESULTS The influence of ambient salinity, juvenile age (1 or 2 yr) and month of saltwater exposure on the osmolarity of the three body fluids was determined by multiple regression techniques. The serum osmolarity of both yearling and two-year-old juvenile salmon increased linearly with ambient salinity, values in never exceeding those in fresh water by more than,9%. For example, serum osmolarity of the yearlings during February- March increased from 329±9 mosm/1 in fresh water to 359±18 mosm/1 in sea water; the corresponding values for the two-year-old salmon were 333±11 and 351±10 mosm/l. The only significant variable describing changes in serum osmolarity was ambient salinity; neither month of exposure nor age were significant (P>0.05). The relationship between serum osmolarity and ambient salinity can be described by the regression: (1) O s = 0.769S The coefficient of multiple determination (R2) is and the multiple correlation coefficient is (1,191 d.f.), where Os is serum osmolarity (mosm/1) and S is ambient salinity ( /m). Osmolarity of intestinal fluid also increased linearly with increasing salinity, values at never exceeding those

9 recorded at by more than 15%. Values for both ageclasses of salmon at the different salinities were similar to the osmolarity of serum. For example, osmolarity of intestinal fluid for yearlings during February-March increased from 316±17 in fresh water to 340±26 mosm/1 in sea water; the corresponding values for two-year-old fish were 317±33 and 334±15 mosm/1. Similar values for both age-classes were observed during the May experiment. The relationship between osmolarity of intestinal fluid, ambient salinity and salmon age can be described by the regression: (2) OI = S A The coefficient of multiple determination (R2) is and the multiple correlation coefficient is (2,143 d.f.), where OI is osmolarity of intestinal fluid (mosm/1), S is salinity ( 40) and A is salmon age (years). Month of seawater exposure was not a significant variable, while salmon age explained only 1.6% of the variance in OI. 5 Variable Partial F R2 change S A Significant P<0.05. Osmolarity of urine from salmon of both age-classes showed an exponential increase with increasing ambient salinity. The values recorded for fish in fresh water demonstrated that urine was dilute, whereas values in sea water were markedly higher and similar to the osmolarity of intestinal fluid. For yearlings during February-March, urine osmolarity increased from 32±9 in fresh water to 133±9 in half sea water and to 331±36 mosm/1 in sea water. Corresponding values for the two-year-old fish were 25±9, 104±41 and 327±20 mosm/1. Urine osmolarity for both age-classes was slightly greater at and 15.5 Xm salinity during May than observed during February-March, while values at 31 4, were identical. Values for the yearlings during May at 0.1 4, and salinity were 41±6, 159±45 and 329±31 mosm/1, respectively; and for the two-year-old fish, values were 44±8, 171±43 and 323±36 mosm/1. The relationship between urine osmolarity, ambient salinity and month of the year can be described by the regression: 1 _2 (3) logic Ou = x 10 x S M x 10 (SxM) The coefficient of multiple determination (R2) is and the multiple correlation coefficient is (3,165 d.f.), where

10 6 Ou is urine osmolarity (mosm/l), S is salinity (V o) and M is month of the year. The most significant variable determining Ou was S, the M and SxM variables accounting for only 3% of the variance in Ou. Salmon age was not a significant variable (P>0.05). Variable Partial F R2 change S 1, M SxM 'Significant P<0.05. During the FebruarymalMay acclimation experiments, the twoyear-old juveniles exhibited 100% survival in each of the four tanks, while mortality occurred among a small percentage of the yearlings (0%-8%). The mortality among the yearlings was attributable to aggressive behaviour which resulted in scale loss and wounding. Serum and intestinal fluid were obtained from a number of salmon parr inadvertently included with the experimental fish. Both body fluids were elevated, the value of 495±41 (N=22) for serum and of 471±49 (N=17) for intestinal fluid representing a 50% increase from values recorded in fresh water. Attempts to obtain samples of urine were unsuccessful. Changes in the osmolarity of serum, urine and intestinal fluid of both ageclasses with increasing salinity are shown in Fig. 1. The ability of salmon parr ranging from 2 to 37 g (6-15 cm fork length) to survive in salinities of 31 Xm during two months exposure was related to their size, all mortality being restricted to fish of 19 g or less (412 cm fork length). Within this weight range, mortality was greatest among the smallest individuals. Parr, unable to acclimate to 31 40, showed elevated osmolarity of serum and intestinal fluid coupled with decreased body moisture. Values for serum and intestinal fluid were 447±29 and 422±27 mosm/1 respectively, considerably greater than the values of 351±14 and 336±14 mosm/1 recorded for these fluids taken from acclimated parr. Total body moisture for acclimated fish was 73.7±0.7% wet weight and for those unable to acclimate to 31 40, 69.8±1.5%. The relationship between serum osmolarity, parr weight and ambient salinity can be described by the regression equation: (4) Os = S x10-1 (S2xW) x 10 3 (s2 xte)

11 7 The coefficient of multiple determination (R2) is and the multiple correlation coefficient is (3,92 d.f.), where Os is serum osmolarity (mosm/1), S is salinity ( /,m ) and W is parr weight (g). Variable Partial F R2 change S S2xW S2 xw 'Significant P< Parr <12cm Serum Intestinal fluid Ii 1;400 = Serum C)300 E Intestinal fluid Urine, May 0 Urine, March] O 16 Salinity ( FIG. 1. Changes in the osmotic concentration of serum, urine and intestinal fluid of juvenile Atlantic salmon (>14.5 cm) exposed to increasing salinity, and mean values ±1 SD for parr <12 cm. (Values were predicted from Equations 1, 2 and 3.) 32 The relationships among serum osmolarity, parr weight and ambient salinity were demonstrated using Equation 4 (Fig. 2). Serum osmolarity of parr in fresh water remained constant throughout the weight range (2-37 g), increasing little with salinity to 15 /.. At 31 40, elevation of osmolarity above freshwater values was most marked among the smallest individuals (52% increase), whereas values for the heaviest parr showed little increase (6%).

12 8 500 g 460 E 8 E 420 g 380 O a) (i)?9 0. FIG. 2. The influence of wet weight and ambient salinity on the serum osmolarity of underyearling Atlantic salmon parr. (Values were predicted from Equation 4). The relationship: (5) LF = W (r=0.98), where LF is fork length (cm) and W is weight (g), was derived from measurements of experimental fish, allowing interpretation of Fig. 2 in terms of fork length rather than weight. For parr greater than 19 g (12 cm fork length), there were no apparent signs of osmotic stress such as extreme elevation of osmolarity of serum and intestinal fluid or decreased body moisture. Changes in coefficient of condition (K) and proximate composition of yearling and two-year-old salmon during the period days from January 1 are summarized (Tables 2 and 3). The relation between the cumulative number of days from January 1, salmon age and changes in K, moisture and lipid

13 TABLE 2. Changes in coefficient of condition (K) and proximate composition (% wet weight) of yearling Atlantic salmon during the period days from January 1. (Values are means of 12 fish with one SD in parentheses.) Parameters Days from January K (0.049) (0.060) (0.038) (0.035) (0.045) (0.030) (0.029) (0.068) Moisture (0.626) (1.201) (0.544) (0.655) (0.770) (0.619) (0.843) (1.240) Protein (0.757) (0.673) (0.499) (0.629) (0.674) (0.734) (0.273) (0.867) Lipid (0.667) (0.933) (0.524) (0.556) (0.526) (0.599) (0.591) (1.114) Ash (0.075) (0.153) (0.068) (0.072) (0.065) (0.056) (0.071) (0.067)

14 TABLE 3. Changes in coefficient of condition (K) and proximate composition (% wet weight) of two-year-old Atlantic salmon during the period days from January 1. (Values are means of 12 fish with one SD in parentheses.) Days from January 1 Parameters K (0.044) (0.060) (0.050) (0.041) (0.048) (0.017) (0.039) (0.174) Moisture (0.688) (0.649) (1.108) (1.057) (1.043) (0.866) (1.481) (0.653) Protein (0.830) (0.582) (0.494) (0.344) (0.743) (0.570) (0.417) (0.894) Lipid (0.798) (0.792) (0.969) (1.052) (0.893) (0.678) (1.147) (0.629) Ash (0.095) (0.084) (0.095) (0.119) (0.135) (0.085) (0.050) (0.032)

15 11 content were described using multiple regression techniques (Appendix). K values for salmon of both year-classes decreased from Day 56, the first sampling day, to reach a minimum near Day 160 (early June). Thereafter, K increased until the end of the experiment on Day 259. Changes in K were best described by the day and day2 variables, whereas salmon age was non-significant indicating the synchronous nature of this trend for the two age-classes. Lipid content for salmon of both age-classes remained constant between Days 56 and 84 and then decreased to a minimum near Day 200 (July); lipid levels at that time were half those recorded on Day 56. Thereafter, lipid increased until the last sampling date in September. Moisture content of both age-classes was inversely related to lipid, values increasing from Day 56 to Day 200 when they began to decrease. Throughout the experimental period, the sum of moisture and lipid content was about 80% for both the yearling and the twoyear-old juveniles. Changes in lipid and moisture were best described by the day and day2 variables. Age, while significant, explained a relatively small proportion of the variance associated with lipid and moisture content. In contrast, protein content of both age-classes remained constant between Days 56 and 259. Within both groups, the majority of means for various sampling dates were grouped within a homogeneous subset using Scheffe's test (P<0.05) after analysis of variance. Menton protein content for the two-year-old salmon (17.9%) was slightly greater than that for the yearlings (16.7%). Similarly, ash content remained constant throughout the experimental period for salmon of both age-classes. No differences were demonstrable for the two-year-old salmon, while the majority of the means for the yearlings were grouped within a homogeneous subset using Scheffe's test (P<0.05). DISCUSSION Most species of marine and anadromous teleosts hypoosmoregulate in sea water to maintain the salt concentration of their extracellular fluid below that of the environment. Blood concentration of migratory teleosts moving from fresh water to sea water has been observed to increase by only 5%-10% (Parry 1966). Water loss affected by the external salt gradient is replenished by ingestion of sea water at the rate of 0.3%-1.5% body weight/hr (Conte 1969). Sea water in the gut is then converted from a solution hyperosmotic to plasma containing primarily NaC1 to one nearly isosmotic to plasma containing mostly MgSO4. For rainbow trout held in sea water, 91% and 99% of the sodium and chloride, respectively, and 80% of the water are absorbed from the gut after ingestion (Shehadeh and Gordon 1969). Most divalent ions remain in the gut and are excreted via the mucous tubes, feces and kidney, while the gill epithelium is the principal site for the elimination of excess monovalent ions (Conte 1969).. In the marine environment, decreases in urine formation and hence water loss are a result of a reduction

16 12 in the rate of glomerular filtration. Thus, the kidney functions chiefly as an excretory device for magnesium and sulfate ions, while conserving water in addition to monovalent ions and other filtered plasma constituents (Hickman and Trump 1969). Conversely, the kidney of freshwater teleosts acts as a water excretory device, urine being dilute because of the need to conserve filtered ions. During both acclimation experiments, serum osmolarity of one- and two-year-old juveniles in sea water increased by K9% from values recorded in fresh water, and values for intestinal fluid increased by <15%. In contrast, salmon parr, unable to acclimate to sea water, showed a 50% elevation of both these body fluids from freshwater values. Both Gordon (1959) and Parry (1961) report that blood concentrations of euryhaline teleosts acclimated to sea water show an increase of only <10% above those in fresh water. Data from the present study suggest that both age-classes acclimate equally well to sea water and that completion of parr-smolt transformation is not requisite for successful seawater acclimation. Salinity was found to be the only significant variable describing changes in serum osmolarity, while salmon age and month of seawater exposure were non-significant. Similarly, changes in osmolarity of intestinal fluid during seawater exposurewere best described by ambient salinity, age explaining only a small percentage of the variance in osmolarity and month of exposure being non-significant. Although month of seawater exposure did not significantly influence acclimation, salmon used in the two experiments were markedly different. Those employed during February-March had high lipid and K values characteristic of the pre-smolt condition, while salmon used during May had low lipid and K values distinctive of smolts. Changes in urine osmolarity of both ageclasses during seawater exposure also followed a pattern typical of a euryhaline teleost possessing the capacity for marine hypoosmoregulation. Values for yearlings during May increased from 41±6 mosm/1 in fresh water to 329±31 mosm/1 in sea water, and for the two-year-old fish, from 44±8 to 323±36 mosm/l, similar to the increase of 57 to 304 mosm/1 observed for the euryhaline southern flounder (Paralichthys lethostigma) acclimated from fresh water to sea water (Hickman 1968). The most significant variable describing changes in urine osmolarity in the present study was ambient salinity; the month variable accounted for only 3% of the variance, while juvenile age was non-significant. Euryhalinity was achieved during the parr stage when salmon had exceeded 12 cm fork length. This agrees with Wagner (1974), who found that seawater acclimation of juvenile rainbow trout was independent of the onset of parrsmolt transformation and occurred when trout had attained a size of cm. Thus, for Atlantic salmon and rainbow trout, parr-smolt transformation appears to be associated more with a type of biological clock informing smolting species of the time to seek the marine environment, rather than representing the initial development of the marine hypoosmoregulatory mechanisms.

17 During May-June when parr-smolt transformation was well advanced, urine osmolarity of salmon in fresh water was greater than observed during the February-March period. Values for one- and two-year-old juveniles during February were 32±9 and 25±9, respectively, increasing to 41±6 and 44±8 during May and 52±8 and 48±9 mosm/1 during June. A slight decrease to 41±5 and 37±6 mosm/1 for the one- and two-year-old salmon, respectively, was recorded during July. Holmes and Stainer (1966) observed that during parr-smolt transformation of rainbow trout, rate of urine flow and ionic excretion declined to half the pre-smolt values, the decrease being attributable to a reduction in the glomerular filtration rate. The decline in renal excretory rate was accompanied by an increase in urine osmolarity, values of 43±7 for pre-smolts increasing to 56±7 during transformation and then declining to 35±3 mosm/1 when the ouvert signs of transformation were lost. At that time, the glomerular filtration rate and renal excretory pattern had returned to that found for pre-smolting fish. The changes occurring in the renal excretory pattern of trout held in fresh water during transformation tended towards the pattern known to exist for seawater acclimated fish, and was interpreted by Holmes and Stainer (1966) as a pre-adaptation to the marine environment. In this context, the increase in urine osmolarity observed for juvenile salmon during May and June was coincident with the peak of migratory activity at Mactaquac. Coefficient of condition (K) of one- and two-year-old salmon decreased from February until the second week of June before starting to increase. For salmon of both year-classes, there was an inverse relation between lipid and moisture, lipid reaching a minimum during July and moisture a maximum at that time. The minimum lipid levels recorded during July were half those observed during February, and were attained 40 days after minimum K values were reached. Changes in K, lipid and moisture content with time were synchronous for both salmon year-classes, as demonstrated by the low significance of the age variable in the regression analysis. Love (1970) has found that decreases in lipid content of fish are accompanied by increases in water content so that their sum is approximately constant, a phenomenon observed for salmon in the present study. During depletion, lipid reserves of fish must reach critically low levels before proteins are utilized (Love 1970). Despite a 50% lipid loss during transformation, protein content of salmon in the present study remained constant, indicating that low levels of lipid resulting in utilization of this component had not been attained. The 1975 release of salmon at Mactaquac was made during the first half of May when coefficient of condition approached a minimum, but when lipid had decreased by only 17% from the levels recorded during February. It is estimated that by mid-june at least 40% of the lipid reserves would have been depleted had the fish been retained until that time. Changes in coefficient of condition with time reflect the migratory disposition of salmon at Mactaquac, the peak of migratory behaviour in the holding ponds occurring during May, particularly the last half of May, and being complete by early June 13

18 14 (McAskill, personal communication)1. Coincidentally, condition approaches a minimum during May and then increases during early June, indicative of the attainment of the post-smolt condition or non-migratory stage. Judging from changes in migratory behaviour, K values and lipid content, the first half of May would be appropriate for release of salmon from.mactaquac. Additionally, the marine hypoosmoregulatory mechanisms for fish of >12-13 cm are fully developed at that time. In the past, two-year-old juveniles have generally been released during the last half of May and yearlings during June, assuming transformation of the younger fish occurred at a later date. Since changes in migratory activity, K values and lipid content with time are synchronous for one- and two-year-old salmon, release dates for both groups should be similar. The pattern for the peak of migratory activity to precede the time of greatest lipid depletion associated with transformation ensures the availability of reserve energy for downstream migration, seawater acclimation and the period of adaptation to new food sources. Since the greater part of natural mortality of salmon smolts occurs during downstream migration and the first few months of marine life (Carlin 1968; Ricker 1976), it is presumed that additional stored energy during this period of acclimation to a new environment is of importance in decreasing mortality. In this context, Peterson (1973) reports that salmon smolts fed a diet containing 16% lipid for one month prior to release rather than the usual diet containing 6% lipid provided the second highest return ever recorded at the Bergeforesen Hatchery, Sweden. 1McAskill, J. Dept of Fisheries and the Environment, Fisheries and Marine Service, Mactaquac Hatchery, Fredericton, New Brunswick.

19 15 ADDENDUM During a subsequent study (1976), K values of smolts at Mactaquac decreased from February until May before increasing, the attainment of minimum K values prior to a subsequent increase occurring about 20 days earlier than in the present study. Differences in the timing of changes in K values between the two studies may be attributable to differences between laboratory and hatchery holding conditions. Smolts sampled during the 1976 study were held at Mactaquac throughout the experimental period (Days ) while those in the present study (1975) were moved from Mactaquac to the laboratory on Day 120 to follow the changes associated with parrsmolt transformation.

20 17 APPENDIX RESULTS OF MULTIPLE REGRESSION ANALYSIS (Moisture and lipid are expressed as % wet fish weight, day as the cumulative number of days from January 1 and age as years.) Dependent variable - Coefficient of condition (K): Independent variable B Partial F R 2 change _2 Day x * Day x * Constant *Significant P<0.05. Multiple correlation coefficient = (2,257 d.f.). Dependent variable - Moisture: Independent variable B Partial F R 2 change 1 Day 0.891x10_3 53.5* Day x * Agee * Constant *Significant P<0.05. Multiple correlation coefficient = (3,256 d.f.). Dependent variable - Lipid: Independent variable B Partial F R2 change 1 Day x10_ * Day x * Age * Constant *Significant P<0.05. Multiple correlation coefficient = (3,187 d.f.).

21 19 ACKNOWLEDGEMENTS We are grateful to the staff of the Mactaquac Hatchery for their cooperation during the study.

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