Smolt Stage Research Articles

Differentiation in juvenile growth and bimodality patterns between northern and southern populations of Atlantic salmon (Salmo salar L.)

Juvenile Atlantic salmon, Salmo salar, from two contrasting populations that had been reared under identical conditions differed in freshwater growth rates and the development of bimodality in length–frequency distributions. Segregation by size started at least a month earlier in the northern (River Shin, northern Scotland) than in the southern population (River Narcea, northern Spain). Northern fish initially grew faster and entered the upper modal group at a larger size (about 100 mm) than did southern fish (about 90 mm). However, the percentage of fish in the upper modal group was greater for the southern population and they grew fastest over winter and during the spring leading up to smolting, and were larger at the smolt stage. By late winter, the individual growth rates of upper modal fish were inversely correlated with their body length in December. These results suggest the existence of genetic differences between populations in the expression of growth bimodality in juvenile Atlantic salmon. This may indicate that size and growth rate thresholds determining the developmental pathway associated with age at smolt metamorphosis may vary between populations as a function of both smolt size and expected growth opportunity during winter and spring.

Discrimination of Norwegian farmed, ranched and wild‐origin Atlantic salmon, Salmo salar L., by image processing

Abstract A method of distinguishing between farmed, ranched and wild‐origin Atlantic salmon, Salmo salar L., using scale morphology is proposed. Circuli spacing and scale texture data, as expressed as a Fourier transform of transmission luminescent patterns. were extracted by image processing. Spacing patterns and texture features were most distinct for wild salmon compared with the other two groups. Three‐group quadratic discriminant function models were developed using different combinations of data types. The most efficient model to separate the three groups had a classification efficiency of 74%. When models were simplified to two groups, farmed and wild, efficiency increased to 90%, thus reflecting the feature overlap between farmed and ranched groups. The method may be a useful tool for more objective and efficient classification of wild versus husbandry‐origin salmon. However, it should be stressed that farmed salmon that escape at the smolt stage are still problematic.

Technology for cultivation of Atlantic salmon, Salmo salar L.: problems and possibilities

. The concept of farming Atlantic salmon, Salmo salar L., beyond smolt stage in sea water started in Norway and Scotland in 1967–1968, using natural enclosures or sea water ponds. Cage culture has since 1969 developed to be the dominating method. Atlantic salmon is cultivated in more than nine countries with a production of 235400t in 1990. Incubation of eggs, hatching, first feeding and smolt production have to a large degree been standardized. Various types of incubator systems and holding units are briefly presented and discussed. There has recently been a trend to move ongrowing cage farms from inshore and sheltered waters to more open and offshore locations. The various types of cages and supplemental equipment for the two options are compared and evaluated. The offshore trend is a more expensive and difficult technology than existing inshore farming methods. Sea enclosures and net enclosures are less attractive. There is new interest in pump ashore systems. Large land-based farms have been built in six countries with varying success. The amounts of water required are enormous and constructions costs are high, but running costs are comparable to cage culture. Great improvements have taken place for both smolt farms and pump ashore farms in treatment of water, use of heat exchangers, filtration of water, disinfection, aeration, oxygenation, photoperiod and light control, automatic feeding, internal transport, handling of fish and surveillance and control systems for fish and water. The problems and prospects of farming are discussed in relation to new and old technology.

The status of wild and hatchery propagated Swedish salmon stocks after 40 years of hatchery releases in the Baltic rivers

The Baltic stocks of Atlantic salmon have been subjected to human exploitation for many centuries. During the last century, however, these salmon populations have been subjected to especially dramatic changes in spawning stream access, fishing patterns, and fishing pressure. At the end of the nineteenth century, salmon spawned in 60–70 rivers in the Baltic proper. From 1940 onwards many of the large salmon rivers were dammed, most of them below the lowest spawning rapids for salmon. Methods for rearing salmon up to the smolt stage were developed, and the first smolt releases started around 1950. The number of smolts released in Swedish rivers gradually increased to about 2–2.5 million by the middle of the 1980s. During the last 10 years other countries have started salmon release programs, resulting in an annual release of about 3 million smolts. Smolt releases have been successful, showing high survival rates of stocked fish. Today, only about 20 suitable rivers, most of them in Sweden, are available for natural spawning runs. The runs are very weak.Before the Second World War the Baltic salmon fishery mainly consisted of coastal and river catches of spawning migrators ascending the rivers. However, since the end of the war an offshore drift gillnet fishery has been developed, and nowadays makes up the major part of the total catch. As a consequence, the spawning runs of wild salmon have decreased dramatically. One hundred years ago the natural production of salmon from the Baltic salmon rivers was around 7–8 million smolts annually. In the early 1970s, the wild smolt run to the Baltic included about 2 million fish. At present, only about 0.5 million wild smolts are recruited to the whole Baltic proper, which is only about 20% of the smolt production capacity still remaining in the rivers.The offshore fishery on mixed populations in the central areas of the Baltic proper leads to an extremely high fishing pressure on the Baltic salmon stocks. Wild stocks are unable to cope with the present exploitation rate. As a result, the number of wild spawners in natural streams has gradually been diminishing. In the absence of a compensatory program, a reduction in stock-size of the magnitude shown by wild salmon in the Baltic would decrease catch-per-unit-effort to such a degree that there would be no economic incentive for a commercial salmon fishery. At present, however, about 90% of the salmon smolts leaving Baltic rivers are of hatchery origin. Owing to their extremely high egg-to-smolt survival rates the reared stocks can withstand this high exploitation. Thus the success of hatchery enhancement programs in combination with a lack of effective regulations, allowing a high-catch-per unit effort in the offshore fishery, is largely responsible for the sad fate of remaining wild salmon stocks in the Baltic.Any biologically sound management program for the Baltic salmon would have to include regulations prohibiting or severely restricting the offshore fishery on mixed stocks. Salmon stocks should be exploited in relation to the capacity of individual populations, by a more terminal fishery at the coast or in rivers. However, an established offshore fishery in the main Baltic, including (at least) seven nations, has so far made any kind of regulation difficult.

Effect of habitat types on survival, spatial distribution and production of an allopatric cohort of Atlantic salmon, Salmo salar L., under conditions of low competition

An allopatric cohort of Atlantic salmon, Salmo salar L., introduced to a small previously fishless stream was studied from parr to the smolt stages. In May 3900 0+ parr (mean total length 30mm) were planted at three different densities in habitats with slow, intermediate and fast water velocities. During the first year, high mortality occurred during the first 7 weeks after planting in May, and in September–October. Survival from May 1985 to April 1986, before the smolt emigration, was24.8%. The smolt yield 1 year after planting was 15.5%. It is suggested that the high survival was caused by low competition. Most of the redistribution of the fish took place during the first months. Type of planting habitat affected the timing of redistribution. The parr left slow‐flowing, deep habitats with fine substrate soon after planting, while redistribution was slowest in the fastest flowing habitats with coarse substrate. The observed avoidance of slow, deep habitat types in the absence of interspecific competition, suggests that this may be a fixed behavioural response, and not due to competition. Long movements, up to 800 m, were recorded only within the first 7 weeks after planting. The effect of planting densities on population density was most pronounced immediately after planting in the fast and also intermediate habitats. Planting density effects declined and were not detectable after 1 year. The effect of habitat type on fish numbers and biomass was pronounced irrespective of planting densities. Growth was fastest in the intermediate habitat, and at the lower planting densities. Production was 7.2 g m−2 the first summer‐autumn. Due to smolt emigration, few fish remained in the stream the second summer‐autumn, and the production was 1.0 g m−2.

The effects of social status on life-history variation in juvenile salmon

Under good growing conditions, juvenile Atlantic salmon metamorphose into the migratory smolt stage at 1+ or 2+ years of age. The life-history decision on whether or not to migrate at 1+ years is made in July–August of the previous year. After this time, populations develop a bimodal size distribution, the larger fish (upper modal group) being the 1+ smolts and the lower modal group being fish that will smolt at 2+. Fish of high social status are more likely to become 1+ smolts. We examined the causal nature of this relationship by manipulating status within a laboratory population of sibling fish. The absolute status of individual fish was estimated within 2 weeks of first feeding. Relative status was then manipulated by dividing the population into two, half containing the fish with the highest absolute status (high ranking) and the remaining half of fish of lowest absolute status (low ranking). The status of individually marked fish was then determined within each of the two groups. Individual growth rates were monitored until smolting strategies were apparent. There was a complete overlap in the sizes of subsequent upper and lower modal group parr in early June, but from late June onwards fish in the upper modal group grew faster. The high- and low-ranking groups did not differ either in mean growth rates or in the proportions of fish adopting the alternative smolting strategies. However, they differed in the factors that influenced an individual's developmental strategy: within the high-ranking group, relative social status in June was a significant predictor of whether a fish would smolt aged 1+, whereas length at that time was not. In contrast, no relationship between status and smolting strategy was found in the low-ranking group, where differences in status were less clear-cut and had less influence on growth. Instead, age of smolting could be predicted from early growth rate. These results demonstrate that the influence of status on smolting depends on the extent to which fish of high status suppress the growth of those lower in the hierarchy.

Seasonal changes in thyroid hormones and osmoregulation in mature male parr and 1- and 2-year-old immature parr of Baltic salmon (Salmo salar L.)

One-year-old Atlantic salmon improved their seawater adaptability in late spring, at a time when 2-year-old fish smoltify, though not to the same degree as the latter. One-year-old males that matured precociously were less able to adapt to seawater than immature fish. Both 1- and 2-year-old immature salmon increased their plasma levels of thyroxine when entering the smolt stage; however, both the increase and the levels were much lower in the 1-year-old fish. The seasonal patterns of plasma triiodothyronine and thyroxine were studied in immature parr and precocious males held in fresh and brackish water. The most consistent difference was that the levels of triiodothyronine and thyroxine were higher in mature males than in immature fish during winter.

Choice of feeding station in Atlantic salmon, Salmo salar, parr: effects of predation risk, season and life history strategy

The current speed at which underyearling salmon parr held feeding stations was examined from late summer to early winter in laboratory flume tanks that offered a choice between (a) areas with high water flow, high food availability but high predation risk and (b) areas with low flow, little food but shelter from predators. In August, those fish that would become smolts aged 1 + (and which by late winter formed the upper modal group, UMG, of the bimodal size distribution) adopted positions in faster currents than did the fish which would take a further year to reach the smolt stage (the lower modal group, LMG). However, the chosen current speed of UMG fish decreased through the period of study, so that by December all fish were found in areas of low flow, and hence little food. Both date and water temperature had independent effects on the chosen current speed of UMG fish.The effect of predation risk was investigated using a model trout. A brief sight of this predator caused 47% of fish that had been in the main, exposed currents to move to slacker, sheltered areas; they took 1 h, on average, to return to their previous position. The fish that remained in position upon seeing the predator reduced their rate of tail beating, presumably increasing crypticity. Eventual UMG parr were less likely than were LMG fish to move away upon seeing a predator. The fish moved to faster currents than normal 2–3 h after seeing the predator, possibly compensating for the earlier reduction in feeding rate.

Production of Single-Domain Magnetite Throughout Life by Sockeye Salmon, Oncorhynchus Nerka

Although single-domain particles of biogenic magnetite have been found in different species of pelagic fishes, nothing is known about when it is synthesized, or about whether the time during life when it is produced is correlated with the development of responses to magnetic field stimuli. We have investigated production of biogenic magnetite suitable for use in magnetoreception in different life stages of the sockeye salmon, Oncorhynchus nerka (Walbaum). Sockeye salmon were chosen because responses in orientation arenas to magnetic field stimuli have been demonstrated in both fry and smolt stages of this species. We found significant quantities of single-domain magnetite in connective tissue from the ethmoid region of the skull of adult (4-year-old) sockeye salmon. The ontogenetic study revealed an orderly increase in the amount of magnetic material in the same region of the skull but not in other tissues of sockeye salmon fry, yearlings and smolts. The physical properties of this material closely matched those of magnetite particles extracted from the ethmoid tissue of the adult fish. We suggest that single-domain magnetite particles suitable for use in magnetoreception are produced throughout life in the ethmoid region of the skull in sockeye salmon. Based on theoretical calculations, we conclude that there are enough particles present in the skulls of the fry to mediate their responses to magnetic field direction. By the smolt stage, the amount of magnetite present in the front of the skull is sufficient to provide the fish with a magnetoreceptor capable of detecting small changes in the intensity of the geomagnetic field. Other tissues of the salmon, such as the eye and skin, often contained ferromagnetic material, although the magnetizations of these tissues were usually more variable than in the ethmoid tissue. These deposits of unidentified magnetic material, some of which may be magnetite, appear almost exclusively in adults and so would not be useful in magnetoreception by young fish. We suggest that tissue from within the ethmoid region of the skull in pelagic fishes is the only site yet identified where magnetite suitable for use in magnetoreception is concentrated.

Artificial reconditioning, spawning and survival of Atlantic salmon, Salmo salar L., kelts in salt water and survival of their Fl progeny

. Atlantic salmon kelts were successfully reconditioned in salinities of 16% and 28%o using a moist pellet diet. Two groups of reconditioned kelts were re-acclimated to fresh water in the autumn tu prevent super chitling in salt water during the winter. The feeding response and growth of salmon kelis. both years, was highest when salinities were maintained at 28% during the winter and 16%during the summer. However, other salinities also provided acceptable growth rates. Two-sea-winter salmon kelts reconditioned at a slower rate in all groups than 1-sea-winter salmon kelts. Kelt survival was 93.1% the first year and 82.7% the second year. Gonnd maturation occurred in 17.9% of the female 1-sea-winter kelts and 40.6% of the male 1-sea-winter kelts after the first year. At the end of the second year. 83.3% of the 1-sea-winter female kelts and 96.3% of the l-sea-winter male kelts matured and spawned. Reconditioned salmon kelts with a mean fork length of 68.4 cm produced 7261 eggs per fish with a mean egg diameter of 5.70 mm and a fecundity of 2262 eggs.kg−1, Survival (Fl) from the green egg stage to the alevin and smolt stages was 73.5% and 38.8% respectively. The smolt to adult return rule based on unadjusted tug recaptures was 2.20%.

Movement and Survival of Presmolt Steelhead in a Tributary and the Main Stem of a Washington River

Abstract Presmolt steelhead (Salmo gairdneri) were marked in Gobar Creek from 1981 to 1984 and movements between this creek and the main stem of the Kalama River in Washington were examined. Some of these migratory parr travelled a considerable distance down the river, whereas others remained in the tributary. We found that migrant parr were capable of surviving to the smolt stage. Most migrant parr remained in downstream areas prior to smolting, with few returning upstream and emigrating as smolts from Gobar Creek. Of all juvenile steelhead from Gobar Creek that survived to become smolts, most emigrated from the creek to downstream areas as parr and subsequently outmigrated from these downstream areas the following year. The freshwater survival of a majority of the steelhead spawned in tributaries may depend largely on the availability of suitable rearing environments in downriver areas.

Lakeward Migrations by Juvenile Atlantic Salmon, Salmo salar

Atlantic salmon parr, Salmo salar, undertook lakeward migrations from outlet streams in the Wings Brook system, eastern Newfoundland. Lakeward movement occurred from May through November, intensifying over a 6- to 8-wk period during spring. Parr remained in the lakes where they either matured (usually males) or smoltified (predominantly females) before returning to the streams. Mature parr emigrated from the lakes during autumn whereas smolts entered the streams during spring. Lakes provided conditions for increased parr growth and survival to the smolt stage relative to the riverine environment, and appeared to represent important overwintering habitat. Smolt production occurred almost entirely within the lakes. Parr movement into lakes could not be explained by physical environmental factors. Lakeward migrations appear to have an adaptive basis and can be explained as an evolutionarily stable reproductive tactic.

Early male maturity in two stocks of chinook salmon ( Oncorhynchus tshawytscha) transplanted to an experimental hatchery in southeastern Alaska

Progeny of 1976-brood chinook salmon ( Oncorhynchus tshawytscha) from the Chickamin and Unuk rivers in southeastern Alaska were reared to the smolt stage in a hatchery 250 km northwest of their natal streams. As fingerlings, the fish were separated into eight groups to test the effect of stock, time of release, smolt size at release, and culture salinity on marine survival and rates of early male maturity (mature males returning earlier than the youngest mature females; i.e., ≤4 years in age). A total of 13 397 Chickamin River and 22 508 Unuk River smolts were tagged and released from the hatchery in 1978. From 1978 to 1982, 1373 fish from the Chickamin River and 1634 fish from the Unuk River stock were recovered in fisheries or as maturing fish returning to the release site. Age-5 chinook salmon of both sexes from the Chickamin River were larger than those from the Unuk River stock ( P < 0.05), but age-6 fish of both stocks were similar in size, regardless of sex. Rates of early male maturity were 5.4–16.7% in the Chickamin River stock and 70.6–96.0% for the Unuk River stock, and stock was significant ( P < 0.030) in determining early-male maturity. Smolt size at release, culture salinity, and time of release did not affect maturation age of males in either stock.

Some physiologic distinctions between freshwater and diadromous forms of the Atlantic salmon, Salmo salar L.

Parr of the diadromous and freshwater forms of the Atlantic salmon, Salmo salar L., were cultured through the smolt stage in photoperiod and temperature cycles that were delayed 3 months relative to those in nature. Such environmental manipulations caused a delay of approximately 3 months in smoltification of diadromous parr, as measured by changes in water and lipid content and salinity tolerance. These changes did not occur in parr of freshwater salmon exposed to the same modified photoperiod and temperature cycle. The implications of this are discussed.

Precocious Male Atlantic Salmon Parr (Salmo salar) in Insular Newfoundland

The incidence of sexual precocity in male Atlantic salmon parr (Salmo salar) was examined in selected rivers in insular Newfoundland from 1974 to 1977. The incidence was variable but generally high, particularly in eastern rivers. Examination of age-specific lengths revealed that the faster growing males became precocious but percent precocity for these varied among river systems. Precocity did not influence growth of parr in comparison to immatures up to the smolt stage. No difference was found in the condition (using gutted weights) of mature and immature parr. However, condition deteriorated in precocious fish compared to immature fish with smoltification. Parr sex ratios frequently differed significantly from 1:1. Possible reasons are discussed in terms of sex related distribution patterns and mortality of precocious parr. From smolt runs examined, it appears that too few precocious parr migrate as smolt to contribute to the grilse population. As a result of the high mortality, rivers with a high percentage of sexually precocious males tend to have a corresponding high percentage of adult females.

Effects of Eubothrium salvelini (Cestoda: Pseudophyllidea) on the growth and vitality of sockeye salmon, Oncorhynchus nerka

Sockeye salmon (Oncorhynchus nerka) fry infected experimentally with the cestode Eubothrium salvelini, as well as noninfected controls, were studied in the laboratory for approximately 1 year through their development to the smolt stage. Fish were weighed and measured at 10-day intervals and tested for stamina in an active capacity tunnel. Parasitism had a deleterious effect on the growth, survival, and swimming performance of the fish.

Effects of Continuous Zinc Exposure on Sockeye Salmon during Adult-to-Smolt Freshwater Residency

Abstract This chronic zinc toxicity test with sockeye salmon (Oncorhynchus nerka) consisted of a 3-mo adult exposure followed by an 18-mo exposure of embryonic through smolt stages. The zinc concentrations utilized were 30 to 112 μg/liter during the adult-to-smolt exposure period, and an additional 242 μg/liter concentration was used for the embryo-to-smolt exposure period. These zinc concentrations produced no adverse effects on survival, fertility, fecundity, growth, or on the subsequent survival of smolts transferred to seawater. Exposure to 242 μg/liter produced sufficient acclimation to markedly decrease acute mortality at zinc levels lethal to unacclimated sockeye salmon juveniles. The 112- and 242-μg/liter concentrations were 0.15 and 0.32, respectively, of the 749-μg/liter, 96-h LC50 for the 8-mo-old sockeye salmon. An application factor relating 96-h LC50 and “safe” concentrations of zinc to anadromous sockeye salmon in soft water appears to be >0.15 and at least 15 times larger than the often re...

Homing of Rainbow Trout Transplanted in Lake Michigan: A Comparison of Three Procedures Used for Imprinting and Stocking

We compared the homing ability of three groups of rainbow trout (Salmo gairdneri) stocked in Lake Michigan by different procedures. One group of juvenile rainbow trout was imprinted to a synthetic chemical, morpholine, during the presmolt and smolt stages, and a second group was not imprinted. Both groups were stocked directly into Lake Michigan, 1 km north of the Little Manitowoc River. A third group of trout was retained in a pond on the Little Manitowoc River during the presmolt and smolt stages and then released into Lake Michigan at the same location as the other two groups. During the adult spawning migration, morpholine was metered into the Little Manitowoc River. This river and 16 other locations were monitored for returning fish. The morpholine-imprinted fish returned to the Little Manitowoc River in greater numbers and strayed less than did fish from the other two treatment groups. This result is a consequence of exposure to a unique odor cue at the critical period for imprinting.

Review of the Rate of Growth and Mortality of Pacific Salmon in Salt Water, and Noncatch Mortality Caused by Fishing

Mortality (other than landed catch) caused by pelagic gillnetting is estimated to be equal to the catch, for salmon in their penultimate year of life, and equal to about a quarter of the catch for salmon in their final year of life. Mortality caused by trolling for coho (Oncorhynchus kisutch) and chinook salmon (O. tshawytscha) averages about one fish killed (mostly below legal size) for every two that are boated. The natural mortality rate for sockeye salmon (O. nerka) in their final year of life averages about 0.015 per mo and is somewhat more in earlier years of pelagic life; the greater part of natural mortality after the smolt stage occurs during the downstream migration and early months of "coastal" life. For coho and chinook the best natural mortality estimate for the last year of life is 0.013 per mo, and that for pink (O. gorbuscha) and chum (O. keta) is of the same order. Growth rates during the final growing season vary from 0.26 per mo for pink and coho salmon to 0.06 per mo for chinook in their 5th ocean yr. Gains from ceasing to take immature salmon on the high seas range up to 300% of the catch now being taken in that category, while for fish taken in their final year they range up to about 70%, depending on the time of year at which the fishing is done. Gains from transferring existing pelagic net fisheries to the coastal region would be 76% (North American sockeye) and 86% (Asian sockeye) of the weight of fish now caught pelagically. Gains in total yield of existing salmon fisheries (pelagic and coastal) are estimated as 78% for Asian pink salmon and 72% for Asian sockeye. The increase in weight of the total catch from discontinuing ocean trolling for Columbia River chinook salmon and increasing river fishing correspondingly is estimated tentatively as between 63 and 98%.

Amino acid contents of the brain and the muscle of young salmon ( Salmo salar L.) at parr and smolt stages

1. 1. During the parr-smolt transformation of Salmo salar L. in fresh water, variations in the concentration of some free amino acids in the brain and the muscle are observed. 2. 2. An increase of threonine and glutamine in the brain and a decrease of the total free amino acids—particularly glycine and taurine—in the muscle are especially important. 3. 3. The results are discussed in relation with the migratory behaviour and hydromineral regulation.