Winter growth and survival of juvenile Atlantic salmon (Salmo salar) in experimental raceways

We used experimental raceways to determine overwinter mortality of wild-reared immature and mature post-young-of-the-year Atlantic salmon (Salmo salar). Secondarily, we investigated the effects of differing treatments (velocity and shelter) on winter growth and survival. Overall survival from November to April was 94%, and survival of immature (98%) and mature (90%) parr, although statistically different, was very similar. Immature parr grew more in length than mature parr, and both immature and mature parr in higher velocity (12 cm·s–1) raceways grew more than those in lower velocity (0.6 cm·s–1) raceways. Stomach contents were twofold greater in parr occupying higher velocity raceways than those in lower velocity raceways. Caloric content of immature and mature parr did not differ in any of five monthly samples. Lowest caloric content occurred in early February and increased between February and March when water temperatures were well below those considered optimal for growth. Although ice cover was present, isolating parr from conditions that occur in natural settings may have helped parr achieve nearly 2.5 times greater survival than parr in the wild. Further, whereas previous studies showed parr select habitats to minimize energetic loss, our results show a distinct advantage for parr to expend energy to feed during winter.

Energy allocation among developmental stages, age groups, and types of Atlantic salmon (Salmo salar) spawners

Relationships between energy density (kJ·100 g–1) and body length (mm), which varied between life-history stages of Atlantic salmon (Salmo salar), were described by power functions. The variation was chiefly caused by differences in lipid concentrations. Energy densities of maturing and rematuring parr were higher than those of similarly aged smolts. Monthly specific growth rate was higher in maturing parr than in salmon at sea. Mature parr males allocated relatively more energy to gonadal development and less to soma development than anadromous males. Spawning expenditure increased from 34 to 53% for 55- to 72-cm-long females. In males, similar losses were from 47 to 49% of their total energy content at river entry. More energy was used in reproduction in a long river than in a short one. Male allocation to gonads relative to soma was highest in the shorter river. Energy allocation pattern varied among developmental stages. Very energy-rich male parr rematured in subsequent years without going to sea; less energy-rich ones smolted and migrated to sea.

Differential energy allocation of alternative male tactics in masu salmon (Oncorhynchus masou)

To compare energy allocation to gonad, relative to soma, between alternative male tactics (mature parr and migratory males) in masu salmon (Oncorhynchus masou), we quantified energy contents in somatic and testicular tissues for the two tactics from two lake-run populations (Shikaribetsu Lake and Shumarinai Reservoir) in Hokkaido, northern Japan. Mass-specific testicular energy (kJ·g–1) was lower in migratory males than in mature parr in the Shikaribetsu population, but not in the Shumarinai population. The consistent pattern was observed in somatic energy, suggesting that mass-specific energy reflects population environment rather than inherent allocation pattern. Despite this confounding effect of specific energy, total testicular energy (kJ), relative to total somatic energy, was higher in mature parr in both populations. We suggest that strong sperm competition involved with sneak spawning is responsible for an increased testicular investment in mature parr.

Cross-tributary analysis of parr to smolt recruitment of Atlantic salmon (Salmo salar)

We used estimates of Atlantic salmon (Salmo salar) parr and smolt density, estimated in three tributaries of the West River, Vermont, U.S.A., to determine (i) if smolt recruitment is density dependent or independent of parr density, (ii) if the proportion of parr migrating as smolts and cohort survival differ among tributaries, and (iii) the effect of parr maturity on smolt production and recruitment variability. We found that parr to smolt recruitment was best described with a linear function providing no evidence for density dependence in the recruitment dynamics of parr and smolts at the tributary scale. The proportion of age-1 parr recruiting to age-2 smolts did not systematically differ among tributaries or years (overall mean ± 95% CL: 18 ± 11%, range = 9-37%), and mean age-1 to age-2 survival ranged less than twofold among tributaries (27-46%) and was independent of cohort density. Survival of age-1 mature (39%) and immature (33%) parr was similar, but probability of smolting for mature parr (0.21) was threefold less than for immature parr (0.76). Quantifying smolt recruitment pathways involving parr maturation helped elucidate the population-level effect of parr maturation on smolt production and recruitment variability.

Geographic variations in life-history characteristics of white-spotted charr (Salvelinus leucomaenis)

Breeding-population structure, age and size at maturity, age and size at smolting, and riverine growth rate were investigated for three populations of white-spotted charr (Salvelinus leucomaenis) in the Nairo, Haraki, and Takahara rivers in Japan. The breeding populations in the Nairo and Haraki rivers on Hokkaido Island exhibited size polymorphism, comprising smaller mature parr and larger migrants. On the other hand, the population in the Takahara River on central Honshu Island showed a monomorphic structure, being composed of mature parr only. Maturation at the parr stage was positively associated with body size. The patterns of length-specific maturation ratios in the three rivers were similar, although females matured at a larger size than males. Smolt sizes differed between the polymorphic populations in the Nairo and Haraki rivers, but size relationships between the sexes were similar in the two populations. The incidence of smolts of a given length within populations was much higher in the northern Nairo River population than in the more southerly Haraki River population. These observations indicate that among-population variation in the structure of white-spotted charr populations might be related to differences in the relative proportions of mature parr and smolts in the population. We suggest that the relative difference in size thresholds between smolting and maturation determines the relative incidence of mature parr and smolts, both within and among populations and between the sexes, which in turn regulates the breeding-population structure.

Exceptions to semelparity: postmaturation survival, morphology, and energetics of male chinook salmon (Oncorhynchus tshawytscha)

Between 2.1 and 6.8% of fall-run male chinook salmon (Oncorhynchus tshawytscha) reared in two New Zealand hatcheries matured as yearling parr, of similar size to immature siblings. The incidence of mature parr in 58 half-sib families ranged from 0 to 69% of the available males. Although chinook salmon are normally semelparous, about 80% of mature parr survived to mature again at age 2, and all fish held for another year matured again at age 3. All three ages produced milt that successfully fertilized eggs. Morphological development in mature parr and repeat-maturing males was consistent with that of older, first time maturing males. The gonadosomatic index for mature age-2 males was 11.7, 7.2, and 5.4% for repeat-maturing males, freshwater-reared males, and sea-run males, respectively. Muscle energy density for repeat-maturing males (4.45 kJ/g) was lower than for normal males (5.20-5.45 kJ/g) and negatively correlated with the gonadosomatic index. Although we think it unlikely that repeat maturation occurs regularly in the wild, our results indicate that under favorable conditions, chinook salmon can exhibit some iteroparous traits. We hypothesize an evolutionary continuum between semelparity and iteroparity in salmonids, primarily characterized by modifications in a few key energetic and physiological thresholds.

Effect of maturation on parr growth and smolt recruitment of Atlantic salmon

We determined the effect of maturation on parr growth and smolt recruitment of Atlantic salmon (Salmo salar) stocked in Vermont tributaries of the Connecticut River. Both among and within tributaries, mature parr ranged between 28 and 52% of the age-1 parr collected and up to 67% of the age-2 parr collected. Percent age-1 parr maturing in October-November was positively related to mean length the preceding June. In October-November, immature age-1 parr were greater in mean length than age-1 mature parr. Data from parr individually tagged in June and recaptured in October showed immature parr exhibited twofold greater individual growth than maturing parr. Smolt recruitment was highly dependent upon state of maturity the preceding fall; fewer individuals that matured as parr recruited to smolt compared with parr remaining immature. Our study shows, over a broad spatial scale, that variation in incidence of maturation is largely explained by parr size among tributaries and river reaches and, furthermore, empirically demonstrates a direct negative effect of maturation on parr growth and recruitment to smolt. Thus, parr maturation is an important consideration for the enhancement and (or) restoration of Atlantic salmon populations via stream stocking programs.

Plasma levels of five androgens and 17α-hydroxy-20β-dihydroprogesterone in immature and mature male Baltic salmon (Salmo salar) parr, and the effects of castration and androgen replacement in mature parr

During the spawning period, the plasma levels of the androgens 11-ketotestosterone, testosterone, 11β-hydroxytestosterone, 11β-hydroxyandrostenedione, 11-ketoandrostenedione, and of the progestin 17α-hydroxy-20β-dihydroprogesterone were higher in mature compared with immature male Baltic salmon parr. 11-Ketotestosterone was always found to be the predominant androgen in the plasma of mature male parr and mature anadromous males. Mature male parr and large mature anadromous males had similar androgen profiles during the main spawning period, although 11-ketotestosterone tended to be higher in the mature anadromous males and testosterone was always higher in the mature male parr. At the time of the main spawning period, plasma levels of 17α-hydroxy-20β-dihydroprogesterone rose sharply in mature male parr, these levels being similar to those found in the plasma of large mature anadromous males. All measured androgens were low or nondetectable in castrated parr. Treatment of castrated parr with Silastic capsules containing 11-ketoandrostenedione resulted in high plasma 11-ketoandrostenedione levels and even higher levels of 11-ketotestosterone, indicating an extratesticular site of androgen conversion.