The influence of precocious sexual maturation on survival to adulthood of river stocked Baltic salmon, Salmo salar, smolts

During three consequtive years, 1975–1977, Individually tagged Baltic salmon Salmo salar smolts of sexually immature male and female fish (n = 35027, mean size: 15.2 cm) and precocious males (n = 6518, mean size: 14.2 cm) were released into Umeälven (Ume river), northern Sweden. Rate of survival (% captured adults) based on 3714 recoveries was significantly higher (p < 0.01) for smolts from immature fish (10.2%) than those from smolts of early maturing males, i.e. precocious males (2.2%). corresponding to an average yield of 474 and 85 kg per KHX) smolts released, respectively. Gain in survival was on average 2.5% and 1.4% per cm increase in smolt size for immature smolts and smolts from precocious males, respectively. The poor survival among smolts of precocious males is suggested to he related to an interaction between sexual maturation and smolting linked to incompletely resorbed gonads leading to a non migratory behaviour. These non migratory males are then suggested to suffer heavily by predation in the river.
The two smolt categories had a similar growth pattern in sea. Smolts from precocious males did not mature early in sea indicating no relation to grisling, i.e. sexually maturing fish returning after first winter in sea. Adult weight of fish returning the fourth summer after release was related to smolt size (P < 0.05). Our Response Surface Model (RSA) predicted that large smolts (19.0 cm) had a higher specific growth rate over their life-span compared to small smolts (<15.0 cm), 0.86% d⁻¹ and 0.46% d⁻¹, respectively. Large smolts (19.0 cm) attained a size of 3.0 kg during their second winter in sea about six months earlier than small smolts (13.0 cm). The paper discusses alternative release strategies that can be employed if the ultimate goal of salmon stocking is maximizing yield.     

Colonization by Pinus sylvestris of a former middle-geolittoral habitat on the Umeälven river in northern Sweden, following river regulation for hydro-electric power

The colonization by Scots pine of a former middle-geolittoral habitat on the Umeälven river, northern Sweden, laid almost completely dry in 1958, was compared with that of a similar habitat, subject to natural changes in water-level, on the Vindelälven river. The geographical position, altitude, exposition, slope and type of vegetation were similar at both sites. The percentage cover of field and bottom layers of the middle geolittoral are considered to have decreased after the fall in water-level of the Umeälven river. The pines were dated by making annual ring counts. During the period 1962–67 a significant increase in pine colonization took place at the Umeälven site above the new, regulated, high-water level. The tree densities (pines m^-2) in 1978 on middle geolittoral sites of the Umeälven and Vindelälven rivers were 4.9 and 1.4, respectively.     

Seasonal changes in androgen levels in stream- and hatchery-reared Atlantic salmon parr and their relationship to smolting

In stream-reared Atlantic salmon Salmo salar , plasma androgens were significantly greater in mature male parr than immature males and females in October, but had declined by January and did not differ significantly from immature fish throughout the spring. Immature fish in March were significantly larger and had greater gill Na⁺, K⁺-ATPase activity than their previously mature counterparts. Bimodal growth distribution was seen in hatchery-reared Atlantic salmon and a proportion of the male fish in the lower mode matured. Plasma testosterone (T) and 11-ketotestosterone (11-KT) were significantly elevated from September to December in mature male (1+ year) parr. In January, plasma androgens had declined in mature males and did not differ significantly from immature fish. By May all the hatchery fish were large enough to smolt and a proportion of the previously mature males had increased gill Na⁺, K⁺-ATPase activity. Therefore elevated androgens in the previous autumn do not prevent smolting. Parr with higher plasma T and 11-KT in April and May, that are presumably beginning to mature, had lower gill Na⁺, K⁺-ATPase activity, indicating that future maturation and associated increases in androgens may inhibit smolting.     

The timing, sex and age composition of the wild and reared Atlantic salmon ascending the Simojoki River, northern Finland

In the Simojoki River in the northern Gulf of Bothnia, reared salmon stocked as smolts produced considerable numbers of ascending one‐sea‐winter (1 SW) males, whereas the proportion of male 1 SW salmon was low among spawning migrants of wild or reared parr origin. The sex ratio among ascending wild fish and reared salmon stocked as parr was similar, with females predominating, while reared salmon stocked as smolts were mainly males. The multi‐sea‐winter (MSW) salmon entered the river annually within a fairly short time period from the beginning of the migration season, independent of their sex or origin. 1 SW males migrated into the river significantly later in the season than MSW males. The results indicate that the delayed opening of the fishing season in the Gulf of Bothnia is effective in reducing the harvest of MSW salmon at sea. However, as the timing of the ascent may vary by several weeks from year to year, the effect of this regulation bound to certain calendar days may also vary considerably from year to year.     

Effects of life history variation on size and growth in stream-dwelling Atlantic salmon

A large size variation amongst life histories for stream-dwelling Atlantic salmon Salmo salar was found and the relative effect of life histories on size varied over time. As early as December (age 0+ years), fish that later smolted at age 2+ years were significantly larger than fish that did not smolt at age 2+ years. In contrast, there were no mass differences at age 0+ years between fish that would mature or not at age 1+ years (October). The mass differences between smolts and non-smolts persisted until smolting, and differences between mature and immature fish first appeared in May (age 1+ years). Following September (age 1+ years), there was also a significant interaction between smolting and maturity. Previously mature and immature age 2+ year smolts were not significantly different in size, but immature age 2+ year non-smolts were much lighter than mature age 2+ year non-smolts. Based on mass differences, the apparent 'decision' to smolt occurred c . 5 months before (winter, age 0+ years) the decision to mature (late spring, age 1+ years). In addition to strong seasonal growth variation, sizes of freshwater Atlantic salmon were largely structured by the complex interaction between smolt-age and maturity.     

Individual growth and smoltification of juvenile masu salmon, Oncorhynchus masou Brevoort, under rearing conditions

In a reared population of individually marked juvenile masu salmon, individual growth was monitored from the first autumn in 1983 to the following spring. The potential smolts were not significantly greater in mean fork length and body weight than the potential parr in late August of the first year, but they then grew faster until March of the second year. As a result, the potential smolts formed the upper mode of the bimodal length distribution after February. Especially in autumn (October and November) the specific growth rates of potential smolts were significantly greater than those of parr, and the bimodality in growth rate distribution was more distinct for males than females. These suggest that there are two groups having different growth rates in autumn of the first year and that sufficient growth in this period may play an important role in smoltification in the following spring.     

Effects of sexual maturation, castration, and androgen implants on growth in one- and two-year-old parr in a Baltic Atlantic salmon (Salmo salar L.) stock

The effects of maturation, castration, and androgen implants on growth in 1- and 2-year-old Baltic salmon ( Salmo salar L.) were studied in a stock originating from the Umeälven (Ume river) in northern Sweden. 1 + male parr that later matured were larger than non-maturing parr in the spring. Later in the summer, during the period of gonadal growth, the growth rate of maturing parr was depressed compared to that of non-maturing males. Implantation of capsules with 11-ketoandrostenedione (OA) or testosterone (T) in sexually immature 1-year-old fish, accelerated the specific growth rate in spring and early summer, particularly considering growth in weight, resulting in an increase in the condition factor. In contrast, the growth rate was depressed from late June onwards, the period when the gonads normally develop in 1 + parr. T, but not OA, also promoted sexual maturation. Castration or OA treatment did not affect total summer growth in 2-year-old salmon. This study suggests a causal relationship between androgens and the growth pattern observed in maturing 1 + male parr.     

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., down a release ladder. 2. Effect of fish developmental strategy on speed and pattern of movement

The behaviour of different types of Atlantic salmon smolt migrating down a release ladder were compared. Immature S2 smolts were faster through a release ladder, and their journey times less variable than S1 and mature male S2 fish. Journey time variability was less among stripped than among unstripped mature male smolts. The mean residence time of passive objects (oranges) in each pool of the ladder was correlated with pool surface area, whilst that of fish was correlated with pool volume. The general pattern of movement of smolts through the ladder was consistent with the hypothesis of non-directed passive displacement of fish near the water surface. In fact, fish spent most of their time nearer the bottom of the pools with the result that their migration speed was slower than that of passive objects. Pools towards the top of the ladder had the highest volume and surface area and therefore offered the greatest hindrance to passive displacement. Immature S2 smolts may have been more active in seeking a passage through this barrier. In 1986 and 1987 no significant change in ladder travel time or fish condition factor was detected over the period May to July. In 1986 fish condition factors and variance in ladder travel time increased significantly in August. Immature S2 smolt migration may have been less nocturnal. Mature male smolts were affected more by high daytime water temperatures than immature S2 and S1 smolts.     

Changes in growth hormone and prolactin messenger ribonucleic acid levels during seawater adaptation of amago salmon (Oncorhynchus rhodurus).

To examine the changes in secretion of growth hormone (GH) and prolactin (PRL) with reference to their osmoregulatory roles, changes in pituitary mRNA levels and plasma concentrations of these hormones were examined during seawater adaptation in silvery juveniles (smolts) and precociously mature males (dark parr) of amago salmon (Oncorhynchus rhodurus). Transfer to seawater increased plasma sodium levels in both smolts and dark parr. Smolts adjusted their plasma sodium to the level associated with seawater-adaptation (165 mEq/liter) within 3 days, whereas no adjustment was seen in dark parr; the latter failed to survive in seawater for more than 3 days. In smolts, plasma GH levels increased significantly 1 day after transfer, whereas there was no significant change in dark parr. An increase in GH mRNA levels was observed in smolts in association with increased plasma GH, whereas there was no change in dark parr. In contrast, a reduction in plasma PRL levels was consistently observed in both smolts and dark parr after transfer to seawater. However, there was no significant change in PRL mRNA levels in either smolts or dark parr. These results suggest that both gene expression and release of GH are activated by seawater transfer only in smolts with adequate seawater adaptability, whereas PRL gene expression is decreased after seawater transfer regardless of seawater adaptability.     

Compensatory sea growth of male Atlantic salmon, Salmo salar L., which previously mature as parr

The proportion of mature male parr in 11 families of Atlantic salmon, Sulrno sulur, reared under similar conditions in fresh water varied from 0–43%. The mature males were smaller than their siblings in December as 1 + and in late March. After individual tagging and transfer to a sea cage in early April. the previously mature males grew faster than previously immature salmon during the next 6 months. This compensatory growth resulted in almost equal size between the two groups. The results are discussed in relation to the different life strategies of salmon.     

Seasonal pattern in age, sex and body condition of Barn Owls Tyto alba killed on motorways

The Barn Owl Tyto alba was the most common owl killed on motorways in northeastern France. The possible causes of this mortality and the age, sex and body condition of the road-killed birds in 1991–1994 have been investigated. The number of birds killed on roads was highest in the period from early autumn to late winter, i.e. during the non-breeding period, and showed a pattern similar to that of the temporal difference between sunset, which varies with day length, and peak of traffic, the occurrence of which is constant throughout the year. An autumnal mortality peak, concomitant with the post-fledging dispersal, was mainly of immature birds, especially females. A second mortality peak in late winter was composed mainly of mature birds, with an equal proportion of males and females. From autumn to winter, there was no significant change in body mass in the different age and sex categories of birds killed on roads, except for mature males which had a significantly lower body mass in winter. From early autumn to late winter, the mean body mass of immature owls killed on motorways did not differ significantly from that of captive immatures fed ad libitum. This suggests that the immature birds were in good body condition. In contrast, the body mass of road-killed mature females was significantly lower than that of captive mature females over the same time periods. In mature males in late winter, a drop in body mass in both road-killed and captive birds suggests an endogenous seasonal phenomenon. Except for mature females, Barn Owls killed on roads in 1991–1994 were in good body condition. This does not support the idea that only birds in poor body condition were killed. We conclude that the mortality of Barn Owls on motorways in autumn and winter was probably related to the concomitance between the peak of traffic and the onset of hunting activity and the large number and dispersal of immature individuals during the same period.     

Effect of stripping on the length, condition and gonadal state of mature male Atlantic salmon, Salmo salar L., parr during autumn and winter

Hatchery-reared mature male Atlantic salmon parr were shorter than immature parr of the same age through the winter period and this difference increased prior to smolting in spring. Stripping had no effect on growth. In autumn mature male parr had a higher condition factor (CF) than immature parr, but from early January no difference was observed between unstripped mature males and immature parr. Stripping reduced the CF of male parr but the difference had disappeared by the time of smolting when the CFs of both mature groups were lower than that of immature fish. The quantity of expressible milt in stripped males decreased from c . 3% body weight in November and December to <0.05% by 2 March, after which no further milt expression was recorded. Milt expression in previously unstripped males was about 1% of body weight on 2 March, similar to that of stripped males 1 month earlier.     

Spawning seasonality and body sizes at sexual maturity in the bluespine unicornfish,Naso unicornis (Acanthuridae)

We herein evaluate several reproductive metrics of Hawaiian Archipelagic populations of the bluespine unicornfish (Naso unicornis), an economically and ecologically important, broadly distributed tropical Pacific reef fish, based on multi-year, fishery-dependent and fishery-independent collections. Sex-specific spawning seasonality was characterized for fish collected mostly from Oahu (Main Hawaiian Islands, MHI) using a gonadosomatic index. Histological slides preparations were used to score gonad developmental phase and to classify individuals of either sex as immature or mature. Sex-specific median body lengths at maturity (L₅₀) were estimated by logistic fits of proportion mature versus length class. Spawning was highly seasonal in Hawaii, with a single brief (May–June) peak spawning period. Proportionate gonad-to-body weight values were relatively low, averaging only about 0.1 % and 0.6 % across all months of year and 0.16 % and 1.03 % during May–June for males and females, respectively. Median lengths at sexual maturity differed between the sexes. L₅₀ values for fish collected throughout all months of year were 30.1 ± 0.5 (standard error) cm Fork Length (FL) for males and 35.5 ± 0.7 cm FL for females. Spawning seasonality and L₅₀ estimates for bluespine unicornfish in Hawaii suggest that the species spawns several months earlier in the calendar year and matures at larger body lengths in Hawaii versus Guam, the Northern Mariana Islands, and Pohnpei in the Federated States of Micronesia. Estimated lengths at sexual maturity are compared to the minimum length (14 inches or 35.6 cm FL) mandated for this species in Hawaii: median size at maturity occurs at a length appreciably less than (males) or approximately equal to (females) minimum legal size. A likely disproportionately large contribution of old females to population replenishment is discussed relative to the minimum size limit.     

Carlin tag recoveries as an indicator of predation on salmon smolts by goosanders and red-breasted mergansers

Between 1984 and 1990 a total 221 Carlin tags used to mark salmon Salmo salar smolts in the River North Esk, NE Scotland, were recovered from the stomachs of goosanders Mergus merganser and red-breasted mergansers M. serrator . Both Carlin-tagging and adipose-clipping affected the predation of salmon smolts by sawbill ducks. The mean (± S.D.) sizes of tagged smolts taken by both species were similar (117 ± 3 mm) and significantly smaller than the mean sizes of smolts in the river, possibly due to a reduction in the swimming performance of small smolts bearing tags. Large adipose-clipped smolts (±mean smolt size) were predated significantly more than unclipped smolts, but no such difference was observed for small smolts (相似文献   

Migration of hatchery reared juvenile Atlantic salmon, Salmo salar L., smolts down a release ladder. 3. Reactions on exiting the ladder

Twenty-eight hatchery-reared Atlantic salmon ( Salmo salar L.) smolts were tracked as they left a release ladder and moved through the estuary of the Lussa River and out to sea. Smolt passage through the estuary was fastest at high water and slowest at low water; the reverse of the situation observed for passive objects floating in the surface water layer. At low tide the water channel was effectively a freshwater stream and the smolts behaved in it accordingly, attempting to maintain station. At high water smolts moved downstream with the current close to the water surface. The degree of variation in individual speed through the estuary was the main difference observed between immature S2, stripped mature male S2, unstripped mature male S2 and S1 smolts. Smolts moved directly out of the estuary with no apparent check in their progress. Sea contact times in the vicinity of the river mouth, for both passive objects and smolts, were longest around high water and shortest around low tide. In the sea no differences were observed in the behaviour of the four different smolt types. While passive objects showed no tendency to move off in a particular direction, smolts showed a significant inclination to head off on a southerly bearing.     

Life histories of Atlantic salmon altered by winter temperature and summer rearing in fresh- or sea-water

Increased growth during winter increased the incidence of age 1+ Salmo salar smolts in spring. High condition factor in spring and good growth in summer was associated with a high incidence of sexually mature males in autumn. In two experiments, groups (n=160–237 per group) of individually identified parr, either ungraded (lower and upper modal groups: LMG, UMG) or size-graded (LMG only), were reared at either 10, 6 or 3 °C overwinter (Nov to May). At 10 °C, up to 51% of parr originally in the LMG became smolts in spring at age 1+. In contrast, at either 6 or 3 °C (control), < 6% of LMG parr became smolts. The probability of being recruited into the UMG overwinter was positively related to initial body size, and was increased by size-grading. Smolt recruitment was two-fold higher among females compared to males; a proportion of males by age 0+ had already opted to mature at age 1+ rather than smolt at age 1+. In contrast, smolting at age 1+ was not inhibited in males previously mature at age 0+. During summer (May to Nov), all experimental groups were reared at ambient temperature, each subdivided between fresh water (max 21 °C) or seawater (max 15 °C). Good growth in seawater of winter recruits into the UMG confirmed they had completed smolting. Mortality in seawater among parr was 41–83%, and among smolts was 10–22%. Specific growth rate during summer was inversely related to winter rearing temperature. The incidence of sexual maturity in autumn at age 1+ among male parr was positively related to winter rearing temperature, fork length and condition factor in May, but there was large variation among individuals with respect to body size and maturity. Summer rearing in seawater reduced growth and the incidence of maturation. Parr and post-smolt maturity was 84–99% and 100% in fresh water respectively, 21–58% and 0% in seawater.     

Sexually mature and immature yearling male European ground squirrels: a comparison of behavioral and physiological parameters

Early puberty may increase lifetime reproductive success, but may also entail diverse costs. In male European ground squirrels (Spermophilus citellus), age at sexual maturity varies among individuals. We compared sexually active and inactive yearling males under standardized conditions in outdoor enclosures. Non-reproductive yearling males hibernated significantly longer than mature males and emerged when the mating period had already started. Testosterone concentrations peaked in the precocious males during premating and mating, and were depressed in the non-reproductive individuals throughout the study period. Emergence body mass was similar in both groups, although sexually immature males were significantly heavier at the end of the active season. Additionally, non-reproductive yearlings grew faster than precocious individuals. Male–male aggression peaked during mating and the following 4 weeks. Intense aggressive interactions like chases and fights were mostly initiated by reproductive males and were directed towards mature competitors. Aggressive behavior in immature males was mainly expressed as spatial displacements. Non-agonistic interactions with females were only observed in sexually active males. Concentrations of fecal cortisol metabolites did not differ between the two male groups and were elevated in the second half of the active season. Leucocyte numbers were reduced during mating in mature males, whereas in immature animals, cell numbers did not change significantly. Our results indicate that early puberty in European ground squirrels involves lower growth rates, high intrasexual aggression, lower prehibernatory body mass, and probably worse immunological condition during mating.     

The diet of red-breasted mergansers (Mergus serrator) during the smolt run in N.E. Scotland: the importance of salmon (Salmo salar) smolts and parr

Salmon ( Salmo salar ) formed the largest proportion by mass offish found in the stomachs of red-breasted mergansers ( Mergus serrator ) shot during the smolt run in north-east Scotland. Salmon parr represented approximately two-thirds by mass of juvenile salmon eaten, whilst smolts were present in a smaller proportion than previously predicted. The median lengths of parr and smolts eaten were 70 and 115 mm, respectively; the latter being significantly smaller than the smolt population sampled during annual production estimates on the river.     

Compensatory releases reduce genetic differentiation among Atlantic salmon populations in the Baltic Sea: evidence from the River Ume‐Vindelälven

About 90% of salmon smolts in the Baltic Sea derive from hatchery enhancement programmes designed to compensate for loss of catches and genetic resources due to the damming of salmon rivers. The potential threat of genetic homogenization from extensive hatchery releases, however, has not been thoroughly investigated. We provide evidence that straying from deliberate releases poses a threat to indigenous populations by identifying the origin of 127 fin‐clipped (hatchery) salmon caught in the River Ume‐Vindelälven during 1997 to 2000, using mtDNA and six microsatellite loci. The analysis of eight potential donor stocks revealed that compensatory releases from the R. Ångerman and R. Luleälven hatcheries have resulted in a significant amount of straying to the river Ume‐Vindelälven (at least 10 and 12 migrants per year). As predicted to due to increased migration, the analysis of temporal samples from the wild population of R. Vindelälven showed a decreasing trend in genetic differentiation estimates (measured as Fst) relative to hatchery the hatchery strains of R. Ångermanälven and R. Luleälven. Our results suggest that gene flow from compensatory releases poses a serious threat to the genetic makeup of the existing wild populations in the Baltic.     

Body composition and energy allocation in life-history stages of brown trout

Energy contents of immature parr and smolts, and mature resident and anadromous brown trout Salmo trutta sampled from a small stream in southern Norway were estimated from lipid, protein and carbohydrate concentrations. In immatures the lipid concentrations were highest in parr in the autumn. Mean lipid concentrations increased significantly with age in parr sampled in autumn (1·3% in age 0+ to 3·4% in age 3+), whereas they did not in smolts. The lipid concentrations of parr in spring were not significantly different from those of similarly aged smolts. By contrast, the relative water content (%) decreased with age in parr in the autumn and increased with age in smolts, mean values being slightly higher in smolts (78%) than in parr (77%). Protein and carbohydrate concentrations did not vary with age in the immature fish, mean protein concentrations being 18·0, 17·5 and 16·8% in parr in the autumn and spring, and in smolts, respectively. In residents, the concentrations of lipids were lower and of water higher, in age group 1 than in older fish, whereas there was no significant variation with age amongst anadromous trout. The energy concentration of 2+ smolts (349 kJ 100 g^-1) was similar to that of 0+ parr in the autumn. Mean somatic energy density in autumn was 1·1 times higher in freshwater residents than in parr at age 1+ (407 and 387 kJ 100 g^-1) and marginally different at age 2+ (462 and 426 kJ 100 g^-1, respectively). The energy contents per unit mass of residents were 1·3–1·6 times that of similar aged smolts. Mean somatic energy density of anadromous trout (504 kJ 100 g^-1) was higher than that of residents (455 kJ 100 g^-1). Somatic energy, lipid and protein concentrations were correlated highly with water contents of all life stages, age and sex groups.