Smolt Status Research Articles

Differences in smolt status affect the resistance of Atlantic salmon (Salmo salar L.) against infectious pancreatic necrosis, while vaccine-mediated protection is unaffected.

In today's aquaculture of Atlantic salmon (Salmo salar L.), a majority of viral disease outbreaks occur after seawater transfer. A relevant question is how the parr-smolt transformation influences the efficacy of viral vaccines and the innate resistance against viral diseases. In this study, vaccinated and unvaccinated A. salmon parr were exposed to different photoperiodic regimens (1-, 3- or 6-week continuous light-WCL). Fish groups at different stages in the smoltification process were induced, as demonstrated by differences in morphological and physiological smolt parameters. At the time of seawater transfer, the 6-WCL group had reached a more pronounced stage in the smoltification process than the 1-WCL group. In unvaccinated fish, the subsequent cohabitation challenge with infectious pancreatic necrosis virus (IPNV) gave a significantly higher accumulated mortality in the 6-WCL group (87%) compared to the 1-WCL group (39%). In the vaccinated groups, this effect was not apparent and there were no differences in accumulated mortality between the 1 WCL, 3 WCL and 6-WCL groups. These data suggest that the resistance to IPN in A. salmon was negatively influenced by smoltification, while vaccine-mediated protection to IPN was maintained equally well irrespective of smolt status.

Salmonid gene expression biomarkers indicative of physiological responses to changes in salinity and temperature, but not dissolved oxygen.

ABSTRACTAn organism's ability to respond effectively to environmental change is critical to its survival. Yet, life stage and overall condition can dictate tolerance thresholds to heightened environmental stressors, such that stress may not be equally felt across individuals and at all times. Also, the transcriptional responses induced by environmental changes can reflect both generalized responses as well as others that are highly specific to the type of change being experienced. Thus, if transcriptional biomarkers specific to a stressor, even under multi-stressor conditions, can be identified, the biomarkers could then be applied in natural environments to determine when and where an individual experiences such a stressor. Here, we experimentally challenged juvenile Chinook salmon (Oncorhynchus tshawytscha) to validate candidate gill gene expression biomarkers. A sophisticated experimental design manipulated salinity (freshwater, brackish water and seawater), temperature (10, 14 and 18°C) and dissolved oxygen (normoxia and hypoxia) in all 18 possible combinations for 6 days using separate trials for three smolt statuses (pre-smolt, smolt and de-smolt). In addition, changes in juvenile behaviour, plasma variables, gill Na+/K+-ATPase activity, body size, body morphology and skin pigmentation supplemented the gene expression responses. We identified biomarkers specific to salinity and temperature that transcended the multiple stressors, smolt status and mortality (live, dead and moribund). Similar biomarkers for dissolved oxygen were not identified. This work demonstrates the unique power of gene expression biomarkers to identify a specific stressor even under multi-stressor conditions, and we discuss our next steps for hypoxia biomarkers using an RNA-seq study.

High risk no gain-metabolic performance of hatchery reared Atlantic salmon smolts, effects of nest emergence time, hypoxia avoidance behaviour and size

When animals are reared for conservational releases it is paramount to avoid reducing genetic and phenotypic variation over time. This requires an understanding of how diverging behavioural and physiological traits affect performance both in captivity and after release. In Atlantic salmon, emergence time from the spawning gravel has been linked to certain behavioural and physiological characteristics and to the concept of stress coping styles. Early emerging fry has for example been shown to be bolder and more aggressive and to have higher standard metabolic rates compared to late emerging fry. The first aim was therefore to examine if emergence latency affect the behavioural stress coping response also beyond the fry and parr stage. This was done using a hypoxia avoidance test, where an active behavioural avoidance response can be related to higher risk taking. No behavioural differences were found between the two emergence fractions either at the parr or pre-smolt stage, instead smaller individuals were more prone to express an “active” hypoxia avoidance response. Further, an individual expressing a “passive” response as parr were also more prone to express this behaviour at the pre-smolt stage. While there are some previous studies showing that early emerging individuals with a bolder personality may be favored within a hatchery setting it is not known to what extent these early differences persist to affect performance after release. The second aim was therefore to compare the physiological performance at the time of release as smolts using the two subgroups; 1) early emerging fish showing active hypoxia avoidance (Early+Bold) and 2) late emerging fish showing a passive hypoxia response (Late+Shy). The Early+Bold group showed a higher red blood cell swelling, suggesting a higher adrenergic output during stress, whereas there was no difference in post-stress plasma cortisol or physiological smolt status. While there was no difference in standard metabolic rate between the groups, the Early+Bold group exhibited a lower maximum metabolic rate and aerobic scope following strenuous swimming. In captivity this may have no clear negative effects, but in the wild, a more risk prone behavioural profile linked to a lower aerobic capacity to escape from e.g. a predator attack, could clearly be disadvantageous.

Food availability in spring affects smolting in brown trout (Salmo trutta)

Prior to out-migration, salmonid fish typically undergo physiological and morphological changes — a process known as smolting. This study indicates that smolting in brown trout (Salmo trutta) is affected by feeding conditions in spring immediately prior to out-migration. This conclusion was reached after experimentally testing the effect of seasonal variation in food availability on smolt status in the spring. A migratory strain of trout was administered either high or low food rations in the autumn, winter, or spring prior to release in the spring. While fish growth or condition could be affected in any season, it was spring rationing that reduced growth and growth-related variables and that caused increased smolting. Our result supports the idea that smoltification and the decision to migrate is affected by spring food availability regardless of conditions in the previous autumn or winter.

Effects of feed quality and quantity on growth, early maturation and smolt development in hatchery-reared landlocked Atlantic salmon Salmo salar.

The effects of feed quality and quantity on growth, early male parr maturation and development of smolt characteristics were studied in hatchery-reared landlocked Atlantic salmon Salmo salar. The fish were subjected to two levels of feed rations and two levels of lipid content from first feeding until release in May of their second year. Salmo salar fed high rations, regardless of lipid content, grew the most and those fed low lipid feed with low rations grew the least. In addition, fish fed low lipid feed had lower body lipid levels than fish fed high lipid feed. Salmo salar from all treatments showed some reduction in condition factor (K) and lipid levels during their second spring. Smolt status was evaluated using both physiological and morphological variables. These results, based on gill Na(+) , K(+) -ATPase (NKA) enzyme activity, saltwater tolerance challenges and visual assessments, were consistent with each other, showing that S. salar from all treatments, except the treatment in which the fish were fed low rations with low lipid content, exhibited characteristics associated with smolting at 2 years of age. Sexually mature male parr from the high ration, high lipid content treatment were also subjected to saltwater challenge tests, and were found to be unable to regulate plasma sodium levels. The proportion of sexually mature male parr was reduced when the fish were fed low feed rations, but was not affected by the lipid content of the feed. Salmo salar fed low rations with low lipid content exhibited the highest degree of severe fin erosion.

Effects of feeding regimes and early maturation on migratory behaviour of landlocked hatchery-reared Atlantic salmon Salmo salar smolts.

The migratory behaviour of hatchery-reared landlocked Atlantic salmon Salmo salar raised under three different feeding regimes was monitored through the lower part of the River Klarälven, Sweden. The smolts were implanted with acoustic transmitters and released into the River Klarälven, 25 km upstream of the outlet in Lake Vänern. Early mature males, which had matured the previous autumn, were also tagged and released. To monitor migration of the fish, acoustic receivers were deployed along the migratory route. The proportion of S. salar that reached Lake Vänern was significantly greater for fish fed fat-reduced feed than for fish given rations with higher fat content, regardless of ration size. Fish from the early mature male group remained in the river to a greater extent than fish from the three feeding regimes. Smolt status (degree of silvering), as visually assessed, did not differ among the feeding regime groups, and moreover, fully-silvered fish, regardless of feeding regime, migrated faster and had a greater migration success than fish with less developed smolt characteristics. Also, successful migrants had a lower condition factor than unsuccessful ones. These results indicate that the migration success of hatchery-reared S. smolts released to the wild can be enhanced by relatively simple changes in feeding regimes and by matching stocking time with smolt development.

The effects of ration size on migration by hatchery‐raised Atlantic salmon (Salmo salar) and brown trout (Salmo trutta)

Abstract – The possibility to increase the proportion of migrating hatchery‐reared smolts by reducing their food ration was studied. Lake‐migrating, hatchery‐reared salmon (Salmo salar) and trout (Salmo trutta) smolts were either fed normal rations, based on recommendations from the fish‐farming industry, or reduced (15–20%) rations. They were released into the River Klarälven, western Sweden, and followed as they swam downstream to Lake Vänern, a distance of around 25 km. For both Atlantic salmon and brown trout, smolts fed a reduced ration migrated faster than fish fed a normal ration. Furthermore, a higher proportion of salmon smolts fed reduced rations migrated to the lake than fish fed normal rations in 2007 but not in 2006. This difference between years corresponded to greater treatment differences in size and smolt status in 2007 than in 2006. For trout, the proportion of migrating individuals and smolt development did not differ with ration size. Trout migrants fed a normal ration had a higher standard metabolic rate (SMR) than nonmigrants, whereas there was no difference in SMR between migrating and nonmigrating salmon. These results show that it is possible to use a reduced food ration to increase the migration speed of both Atlantic salmon and brown trout and to increase the proportion of migrating Atlantic salmon.

Seaward migrating Atlantic salmon smolts with low levels of gill Na+, K+ -ATPase activity; is sea entry delayed?

Two groups of migrating wild Atlantic salmon (Salmo salar) smolts caught within a 1 week interval in the River Alta, northern Norway, were tagged with acoustic transmitters and measured for gill Na+, K+ -ATPase activity in order to compare their smolt status with timing of sea entry. The first group of smolts had low levels of gill Na+, K+ -ATPase activity and resided in the lower part of the river twice as long as the second group that had high levels of gill Na+, K+ -ATPase activity. This indicates that early migrating smolts may not be completely physiologically adapted for salt water and delay their sea entry, thereby also synchronizing their seaward migration with the later migrating smolts.

Exposure to continuous light disrupts retinal innervation of the preoptic nucleus during parr–smolt transformation in Atlantic salmon

High quality salmon smolts are essential for aquaculture, enhancement programs and wild populations. However, intensification of aquaculture smolt production and changes in natural habitats can cause sub-optimal environmental conditions, which may result in poor smolt quality. The salmon brain, as the integrator of environmental information, plays a focal role in relaying this information through the light–brain–pituitary axis, which includes retinal and pineal innervation of the hypothalamus. Here we investigated the effect of rearing juvenile Atlantic salmon, Salmo salar, under constant light (LL) on optic nerve fiber growth into the hypothalamus. This was compared with the normal increased fiber growth in fish reared under a simulated-natural photoperiod (LDN). Parr were sampled from the LDN group in February and from the LDN and LL groups in May (peak smolt status for the LDN group). Retinohypothalamic projections to the preoptic area were traced using 1, 1′-dioctadecyl-3,3, 3′,3′-tetramethylindocarbocyanine perchlorate (DiI) and confocal laser scanning microscopy. Data showed that parr exposed to LL did not develop the same extensive retinal innervation to the preoptic nucleus (NPO) observed in control salmon smolts raised under LDN. Since the cells in NPO are central pituitary regulatory neurones, the increased retinohypothalamic innervation during normal smoltification may be responsible for the increased endocrine response to photoperiod information. The deprivation of photoperiod information, during continuous light exposure, may inhibit the natural developmental program to proceed during the parr–smolt transformation.

Comparison of physiological smolt status in descending and nondescending wild brown trout (Salmo trutta) in a Danish stream

Abstract – Downstream movement of a wild population of brown trout was examined in a small Danish stream in relation to morphological and physiological smolt status from March to May. Downstream movement was monitored in a Wolf‐type trap covering all possible passage routes in the stream. Trout caught in the trap were classified as parr, pre‐smolt or smolt based on morphological criteria and compared with trout randomly caught by electrofishing upstream of the trap. Representative gill samples from trap‐caught and electrofished trout were analysed for gill Na+, K+‐ATPase activity and used as a measure of physiological smolt status. Only a few parr occurred in the trap. Few pre‐smolts occurred in the trap evenly in March and early April. In late April, pre‐smolt movement peaked. By comparison, the main downstream movement of smolts occurred in distinct peaks through late March and April. The majority of fish caught in the trap were judged as pre‐smolts or smolts based on morphological criteria's and they were characterised by relatively high gill Na+, K+‐ATPase activity compared with trout judged as parr. Trout caught by electrofishing upstream the trap, were classified as parr, pre‐smolts and smolts early in the season (March). During and after the main smolt‐run in April the distribution of the remaining trout in the brook became skewed in favour of pre‐smolt and parr. The study suggests that smolting trout initiate downstream movement once having reached a certain physiological smolt condition (judged by increased gill Na+, K+‐ATPase activity).

Individual‐level analysis of pre‐ and post first‐feed growth and development in Atlantic salmon

Atlantic salmon Salmo salar were followed from egg to smoltification using genetic analysis to identify individuals and to link observations from pre‐ and post first feeding. Egg size and hatch timing significantly influenced alevin size at first feed but neither egg size, hatch timing or alevin sizes were correlated to size, condition factor or smolt status post first feed. In a hatchery environment the potential advantage gained by early hatching, larger alevin does not persist after first feeding. Different physiological and genetic complexes appear to influence growth in these two distinct phases of the Atlantic salmon's life‐cycle.

The influence of dietary lipid inclusion and daily ration on growth and smoltification in 1+ Atlantic salmon ( Salmo salar) parr

The effects of diet regime on growth and smoltification in 1+ Atlantic salmon parr were studied. Two groups of approximately 400 salmon parr in triplicate were fed diets containing either 25% or 12.5% lipid from first feeding. Two further treatments were fed, either the 25% or 12.5% lipid diet for 98 days, after which they were fed the alternate diet. In a second experiment, three groups of 550 parr in duplicate were fed at full, two-thirds or one-third of the manufacturers' recommended ration from first feeding. All groups were maintained on their respective diet regimes until smoltification, approximately 1 year after first feeding. In experiment 1, lipid level had a significant effect on whole body lipid content. However, growth and the incidence of smoltification were not affected by dietary lipid inclusion, with the upper modal group (UMG) fish from each treatment achieving a similar smolt status (in terms of condition factor [CF] and Na +, K + ATPase) at seawater transfer. In experiment 2, growth and the incidence of smolt transformation increased with ration. Full and two-thirds ration fish maintained similar body lipid contents throughout the experiment, with that of the one-third ration fish lower only during the early development, indicating that growth was controlled by the maintenance of a distinct lipid level. It is concluded that ration and not dietary lipid inclusion has a significant effect on growth and the decision to undergo smoltification in salmon parr.

Osmoregulation, feed intake, growth and growth hormone levels in 0+ Atlantic salmon (Salmo salar L.) transferred to seawater at different stages of smolt development

We studied the physiology and endocrinology of seawater acclimation in two size-groups (body weight [b.wt.] at transfer 38 and 53 g, respectively) of 0+ Atlantic salmon (Salmo salar L.) smolts transferred to seawater (33‰) in autumn at three different times following the completion of the photoperiod-induced smoltification process. Smoltification was monitored by use of 24-h seawater challenge tests, and measurements of gill Na+, K+-ATPase activity and condition factor in freshwater acclimated fish. Parr–smolt transformation in large fish was slightly delayed compared to small fish. In late October 1999, when both groups displayed full smolt characteristics, each size group was transferred to four tanks (trial I) supplied with either running fresh water (FW) or running seawater (SW), both at 8.8 °C. In all groups of fish, individual feed intake, body weight, gill Na+, K+-ATPase activity, and plasma levels of chloride and growth hormone (GH) were monitored on days 3 and 30 following transfer. The same protocol was repeated with starting dates December 3 (trial II) and January 11, 2000 (trial III). At the start of trial III, both size-groups displayed reduced hypoosmoregulatory ability (seawater challenge tests) and decreasing gill Na+, K+-ATPase activity. Mortality was low (≤4.4%) in all groups during the trials. For the small SW-smolts, the highest rates of feed intake and specific growth (SGR) were recorded in trial I (SGR: 1.2% b.wt. day−1), whereas feed intake and growth of the larger SW-smolts were highest during trials II and III (SGR: 0.9% b.wt. day−1). Plasma chloride levels in small SW-fish on day 3 increased slightly with date of transfer (from 143 mM in trial I to 154 mM in trial III), but were within the normal range by day 30 across all size-groups and trials (140–144 mM). Following seawater transfer, plasma GH levels increased transiently, whereas gill Na+, K+-ATPase activity increased permanently in all groups. The different patterns of growth displayed by the two size-groups of fish following seawater transfer suggest that the optimal time for seawater transfer may depend on the body size of the fish. Fish transferred past their peak in smolt status showed high hypoosmoregulatory capacity and retained, at least in part, their ability to increase plasma GH levels and gill Na+, K+-ATPase activity following exposure to seawater. Growth rates in seawater may indicate that in commercial farming of underyearling Atlantic salmon, seawater transfer can be delayed for a considerable period of time after smolting, provided that seawater temperature is favourable.

Photoperiod control and influence of body size on off-season parr–smolt transformation and post-smolt growth

To examine the combined effects of photoperiod and body size on off-season parr–smolt transformation and post-smolt growth, groups of small and large graded potential Atlantic salmon smolts (initial size 10 and 23 g) were raised under four different photoperiods between 26 July and 14 November. Photoperiod treatments included a 10 months phase-advance of simulated natural photoperiod (SNPa, from LD12:12 to LD24:0 over 16 weeks); two square wave photoperiod regimes; which were continuously light interrupted by LD12:12 for 9 (9) or 12 (12) weeks, respectively; and continuous light (LD24:0). Photoperiod manipulation significantly influenced growth and hypo-osmoregulatory ability. Between July and November, smolts raised on SNPa and LD12:12 (9) showed an increase in seawater tolerance, concurrent with an increase in Na +, K +-ATPase activity and a decrease in condition factor (CF). No differences in plasma chloride levels were seen among the small and large graded smolts (SG and LG), whereas a higher Na +, K +-ATPase activity was recorded in the small smolts at weeks 12, 14, and 16. Higher growth rates were seen in the small smolts during the freshwater period. Continuous light increased growth in freshwater, but interfered with smolt development. Following transfer to seawater, both SG and LG previously held under SNPa and LD12:12 (9) grew better than the fish reared under LD24:0, which showed a decrease in growth. The high growth rates in seawater in the SNPa and LD12:12 (9) groups concur with the good smolt status prior to transfer. Our findings indicate that both SNPa and LD12:12 (9) can be recommended in the production of off-season salmon smolts and among the smolt parameters monitored, gill Na +, K +-ATPase activity appears to be the most reliable indicator of smolt status.

Relationship between gill Na+,K+-ATPase activity and downstream movement in domesticated and first-generation offspring of wild anadromous brown trout (Salmo trutta)

The relationship between smolt status and downstream movement following release was investigated in two stocks of hatchery-reared anadromous brown trout (Salmo trutta). Yearlings from a domesticated stock (DS) and first-generation offspring (F1) of wild anadromous trout were held under identical conditions from August 1997 until the following spring, where they developed smolt characteristics as judged from increasing gill Na+,K+-ATPase activity. Presmolts (low Na+,K+-ATPase activity), smolts (high Na+,K+-ATPase activity), and desmolts (regressed Na+,K+-ATPase activity) were released on three occasions into the River Salten. Using both dye-marked and radiotagged fish, downstream movement was monitored by either trapping 3 km downstream (dye-marked fish) or radiotracking on a daily basis. The experiments showed a positive correlation between smolt status (gill Na+,K+-ATPase activity) and downstream movement. Gill Na+,K+-ATPase activity may therefore be used as an indicator of migratory readiness in brown trout. F1 and DS trout had the highest migration frequency when released as presmolts and smolts, respectively. Despite smaller size, F1 trout had similar or better survival than DS trout after release. Our data suggest that initiation of downstream movement is influenced by an interaction between the previous physiological development of the fish and a discrete level of water discharge or water temperature.

Relationship between gill Na<sup>+</sup>,K<sup>+</sup>-ATPase activity and downstream movement in domesticated and first-generation offspring of wild anadromous brown trout (<i>Salmo trutta</i>)

The relationship between smolt status and downstream movement following release was investigated in two stocks of hatchery-reared anadromous brown trout (Salmo trutta). Yearlings from a domesticate...

Smolt development in growth hormone transgenic Atlantic salmon

Growth hormone transgenic Atlantic salmon ( Salmo salar) produced using a gene construct comprised of an antifreeze protein (AFP) gene promoter from ocean pout ( Macrozoarces americanus) and the growth hormone (GH) gene from chinook salmon ( Oncorhynchus tshawytscha) were used for this study of smolt development. An F 2 generation of these transgenic salmon was initiated in November 1995 using milt from a transgenic F 1 male and eggs from a normal female, hatching in January and first feeding in February 1996. With elevated temperature (16°C) initially and various combinations of temperature–photoperiod manipulation, the transgenic individuals approached smolt size (16 cm) in June; their normal siblings were <10 cm. Most transgenic individuals survived >96 h following direct transfer to 35‰ seawater in June. Normal individuals survived less than 24 h. Transgenic individuals had high gill Na +–K + ATPase activity levels and declining condition factor in late June, indicative of smolt status. Following transfer to seawater in early July, transgenic fish from the various temperature–photoperiod treatments grew rapidly, with few mortalities, until observations were discontinued in October 1996. High temperature (16°C) in one treatment, which inhibits development or maintenance of elevated gill Na +–K + ATPase activity in non-transgenic Atlantic salmon, resulted in only slightly lower gill ATPase levels in transgenic individuals. These survived and grew well during 4 months in seawater. Similarly, constant light (LD:24) in another treatment, which inhibits normal smolt development in non-transgenic salmon, did not prevent smolt development or adversely affect post-smolt survival and growth in seawater. It is concluded that transgenic salmon from all treatments completed smolting at age 6 months with satisfactory survival and growth in seawater. It appears that GH transgenic Atlantic salmon can be reared under temperature and photoperiod regimes which optimize growth, but which would inhibit normal smolt development and post-smolt performance of non-transgenic salmon.

Changes in seawater tolerance and gill Na +,K +-ATPase activity during desmoltification in Atlantic salmon kept in freshwater at different temperatures

Atlantic salmon ( Salmo salar L.) smolts were maintained in freshwater past their normal time of seawater transfer (early June) and reared at temperatures of 6, 10, 12 and 14°C until late July. Reduction in gill Na +,K +-ATPase activity was temperature dependent, with a significant decrease at 12 and 14°C towards the end of the study. The duration of the period of good smolt status, as determined by gill Na +,K +-ATPase activity, was approximately 280–350°C at 12 and 10°C, with an extended period at 14°C (450°C). All groups maintained good hypo-osmoregulatory ability despite a reduction in gill Na +,K +-ATPase activity in freshwater. Gill Na +,K +-ATPase activity was higher in seawater challenged smolts at 12 and 14°C towards the end of the study. Desmoltification in Atlantic salmon is not a synchronised process as normally observed during smolting, nor is it a parr-reversion as seen among some Pacific salmonids ( Oncorhynchus spp.).

Susceptibility to furunculosis, cold water vibriosis, and infectious pancreatic necrosis (IPN) in post-smolt Atlantic salmon ( Salmo salar L.) as a function of smolt status by seawater transfer

A possible correlation between smolt quality and post-smolt mortalities in Atlantic salmon provoked by the infectious agents Aeromonas salmonicida, Vibrio salmonicida, or infectious pancreatic necrosis virus (IPNV) was investigated. Smolt groups of different qualities were produced by shifting the photoperiod from 8 h light per day to continuous light, followed by elevation of the water temperature from 4 to 7 °C, 10, 6, and 2 weeks prior to seawater transfer of the smolt groups A (‘early smolts’), B (‘optimal smolts’), and C (‘late smolts’), respectively. All smolt groups were vaccinated at the same time with an oil-emulsified injected vaccine protective against A. salmonicida and V. salmonicida several weeks before the start of smoltification. One group of fish (smoltified like group B ‘optimal smolts’) was left unvaccinated. At the time of seawater transfer, the three smolt groups were shown to be at different stages in the smoltification process based on the ‘chloride ion test’, but all were seawater tolerant, as judged by survival and growth in seawater. Immediately after transfer to seawater, fish from all smolt groups were challenged with IPNV or A. salmonicida. After being reared for 29 weeks in a seacage, fish were also challenged with V. salmonicida. No significant differences in mortalities between the vaccinated smolt groups A, B, and C were found after challenge with A. Salmonicida, V. salmonicida or IPNV, and differences in IPNV titres were also non-significant. Protection against A. salmonicida and V. salmonicida was clearly obtained with the oil-emulsified injected vaccine. In addition, protection against IPNV (18.8% mortality in the unvaccinated, compared to 0% in the vaccinated group) occurred, possibly as a result of a stimulation of non-specific immunological defence mechanisms by certain components in the vaccine. The results indicate that timing of seawater transfer in relation to the smoltification process is not critical for the survival, growth, and disease resistance in the post-smolt, provided that the fish have been vaccinated with an injected oil-emulsified vaccine and have an acceptable seawater tolerance at transfer.

A Nonlethal Measure of Smolt Status of Juvenile Steelhead Based on Body Morphology

A nonlethal morphometric method to assess smolt status of juvenile steelhead Oncorhynchus mykiss was validated. Fish were collected from hatcheries before release and during their seaward migration in a large river system. Fifteen anatomical landmarks were digitized from photographs of each fish, resulting in 34 morphometric characters based on a truss network. Principal component and canonical discriminant function analyses were used to assess smolt status. A single canonical variate was significantly correlated with gill Na+,K+-ATPase activity, a commonly used measure of smolt status. This nonlethal method may be a useful measure of smolt status when sacrificing fish is not desirable or possible; it requires little training to perform, but it does require a larger sample size than some other methods.