Ultrastructural features of chloride cells in the gill epithelium of the Atlantic salmon, Salmo salar, and their modifications during smoltification

To elucidate the ultrastructural modifications of the gill epithelium during smoltification, gills of the Atlantic salmon (Salmo salar) were examined by electron microscopy at three stages of this process, which were defined as follows: "parrs" were freshwater fish that had not yet started their transformation; "freshwater smolts" were freshwater fish that were ready to enter seawater; and "seawater smolts" were smolts that had been transferred from fresh water and maintained for 4 days in seawater (35%). In the gill epithelium of parrs, there were two types of chloride cells. The large chloride cells contained deeply stained mitochondria and numerous apical, irregular, dense, membrane-bound bodies that formed 77% of the chloride cell population and were distinguished easily from small chloride cells that have distinctly paler mitochondria and no dense bodies in their apical cytoplasm. In freshwater smolts, the large chloride cells formed 95% of the chloride-cell population. In contrast to the small chloride cells that were not modified, they almost doubled in size. Their tubular system developed extensively to form a tight network with regular meshes significantly smaller than those observed in parr chloride cells. Forty percent of the large chloride cells were associated with a new type of cell, the accessory cell, to which they were bound by shallow apical junctions. Half of these accessory cells were not seen to be in contact with the external medium. In seawater smolts, 80% of the large chloride cells were associated with accessory cells. Most accessory cells reached the external medium and sent numerous cytoplasmic interdigitations within the apical portion of the adjacent chloride cells. As a result, a section through the apical portion of the chloride cells and their associated accessory cells revealed a mosaic of interlocked cell processes bound together by an extended, shallow apical junction. It was concluded that the Atlantic salmon develops in fresh water most of the ultrastructural modifications of the gill epithelium which in most euryhaline fish are triggered by exposure to seawater. The effective transfer into seawater would act only as a final stimulus to achieve some adequacy between the freshwater smolt and its new environment.     

The influence of three unchanging photoperiods on growth and parr-smolt transformation in Atlantic salmon, Salmo salar L.

Groups of pre-smolt Atlantic salmon were reared under three experimental photoperiods. Growth rate was significantly enhanced under 24 h of light: 0 h of darkness per diem (24 L: 0 D) compared with 16 L: 8 D and 8 L: 16 D from early January to early May. From the beginning of May until the termination of the experiment on 27 May, growth rate was highest under 8 L: 16 D.
All groups developed bimodal length-frequency distributions during the experiment. The proportion of the population in each of the two growth modes was significantly affected by photoperiod treatment.
The level of plasma cortisol increased significantly from February to May. There were no differences in levels of plasma cortisol among photoperiod treatments.
Judged by development in plasma cortisol, changes in condition factor and external appearance, the parr-smolt transformation was not completed under any of the experimental photoperiods.     

Gonadotropic cells in the Atlantic salmon,Salmo salar

Summary The gonadotropin-producing cells (GTH-cells) in the Atlantic salmon were studied light and electron microscopically before, during and after spawning, and after injections of luteinizing hormone-releasing hormone (LH-RH). The double immunofluorescent technique was applied using rabbit anti-carp GTH as the first antibody. Numerous immunofluorescent cells were observed throughout the pars distalis, but very few in the pars intermedia. These cells are basophilic and PAS-positive, and ultrastructurally classified as globular gonadotropes. Only one gonadotropic cell type could be identified; its size, morphology and fine structure vary considerably. In the same specimen the GTH-cells can be predominantly globular or vesicular in appearance, depending on the reproductive phase of the fish. At spawning and after LH-RH injection, many GTH-cells reach a vacuolar stage; the content of the vacuoles is not immunofluorescent. Another cell type, which resembles GTH-cells in semithin sections, did not show gonadotropic properties; its nature and functional significance are unknown. In addition, the present study revealed an increase in the synthetic and exocytotic activity of prolactin cells after LH-RH injections. It is suggested that LH-RH mediates this effect via LH and eventually via estradiol.The authors are indebted to Dr. N. Johansson and superintendent N. Steffner for generously providing facilities at their institutes in Älvkarleby. They also wish to thank Mrs. Y. Lilliemarck and Mrs. V. Lundin for their skillful technical assistance. The grants made available by the Swedish Natural Science Research Council (No. 2124-037) are also gratefully acknowledged. Thanks are also due to Mrs. S.M. McNab for linguistic assistance     

Changes in polyamines during embryonic development of Atlantic salmon, Salmo salar.

1. Polyamine composition of eggs and alevins from cultured and wild Atlantic salmon, Salmo salar, was compared during early development. 2. Eggs collected from wild stock had higher amounts of bound and free putrescine, spermidine and spermine than those collected from cultured stock. 3. Cadaverine was present in bound form in cultured and wild stocks before and after fertilization, respectively. However, free cadaverine was detected in both stocks throughout development. 4. Atlantic salmon embryos and alevins can synthesize polyamines which could be associated with their growth and development.     

Comparing upriver spawning migration of Atlantic salmon Salmo salar and sea trout Salmo trutta

Radio tagged wild Atlantic salmon Salmo salar(n = 30) and sea trout Salmo trutta(n = 19) were simultaneously released from a sea pen outside the mouth of the River Lærdalselva and their migration to spawning areas was recorded. The distance from the river mouth to a position held at spawning ranged from 2 to 24 km and did not differ between the species (mean ± s .d . 15·9 ± 4·3 and 14·9 ± 5·2 km for Atlantic salmon and sea trout, respectively). The duration of the migration phase, however, was significantly shorter for Atlantic salmon than for sea trout (8–12 days, respectively). All Atlantic salmon migrated straight to an area near the spawning ground, whereas 50% of the sea trout had a stepwise progression with one or more periods with erratic movements before reaching the spawning area. After the migration phase, a distinct search phase with repeated movements up‐ and downstream at or close to the position held at spawning was identified for the majority of the fishes (75%, both species). This search phase was significantly shorter for Atlantic salmon than for sea trout (mean 13–31 days, respectively). Mean ± s .d . length of the river stretch used during the search phase was larger for sea trout (3·3 ± 2·5 km) than for Atlantic salmon (1·2 ± 0·9 km). A distinct holding phase, with no movements until spawning, was also observed in the majority of the Atlantic salmon (80%, mean duration 22 days) and sea trout (65%, mean duration 12 days). For both species, a weak, non‐significant trend was observed in the relationship between time spent on the migration phase, and time spent on the search (r² = 0·43) and holding phase (r² = 0·24). There was a highly significant decrease, however, in the duration of the holding phase with an increase in the time spent on the search phase (r² = 0·67).     

Flumequine in Atlantic salmon Salmo salar: disposition in fish held in sea water versus fresh water

14C-labeled flumequine was administered as a single oral (5 mg kg(-1), 86 microCi kg(-1)) or intravenous (5 mg kg(-1), 82 microCi kg(-1)) dose to Atlantic salmon Salmo salar held in sea water or in fresh water. The absorption, tissue distribution and elimination were determined by means of liquid scintillation counting and whole-body autoradiography. The drug was rapidly absorbed and extensively distributed in all groups of fish. Radiolabeled compound was present in blood and muscle for more than 8 wk in the freshwater groups. In the seawater groups, however, no radioactivity was detected in the blood and muscle after 4 d and 2 wk, respectively. It was concluded that flumequine was eliminated at a substantially higher rate from Atlantic salmon in sea water than in fresh water.     

Peripheral blood and head kidney leucocyte populations during out-of-season (0+) parr-smolt transformation and seawater transfer of Atlantic salmon (Salmo salar L.)

Using monoclonal antibodies (MAb) and flow cytometry, Atlantic salmon neutrophils and Ig+ cells in blood and head kidney were studied in under-yearling out-of-season (0+) smolts, and 2 and 4 weeks after transfer to seawater. The parr-smolt transformation was induced using a phase advanced simulated natural photoperiod regime, and sampling of four fish was performed at regular intervals, starting on the date of the photoperiod initiation. During the freshwater period the proportion of neutrophils in the head kidney leucocytes (HKL) remained quite stable and only gradual changes in Ig+ cells were observed. In the peripheral blood leucocytes (PBL), the proportion of neutrophils markedly increased during the last month prior to seawater transfer. The most notable changes in the proportions of MAb+ leucocytes were observed in PBL after seawater transfer, with a significant increase in Ig+ cells and a significant decrease in neutrophils after two weeks in seawater. In the freshwater samples, although there were fluctuations, a decrease in the numbers of total leucocytes per millilitre blood and per gram head kidney during parr-smolt transformation was observed. The number of MAb+ cells in blood appeared to be relatively stable, while the number in head kidney tended to decrease. Following seawater transfer, the numbers of total and MAb+ leucocytes in both blood and head kidney increased markedly. The results suggest that changes in both distribution and numbers of leucocytes in peripheral blood and head kidney take place during parr-smolt transformation, and that marked changes are associated with seawater transfer. Some mechanisms possibly involved are indicated.     

Triploid Atlantic salmon Salmo salar have a higher dietary phosphorus requirement for bone mineralization during early development

The effect of a dietary phosphorus regime in freshwater on vertebra bone mineralization was assessed in diploid and triploid Atlantic salmon, Salmo salar. Fish were fed either a low phosphorus (LP) diet containing 10.5 g kg⁻¹ total phosphorus or a normal phosphorus (NP) diet containing 17.4 g kg⁻¹ total phosphorus from ∼3 to ∼65 g (day 126) in body weight. Two further groups were fed the NP diet from ∼3 g in body weight, but were then switched to the LP diet after 38 (∼10 g in body weight) or 77 (∼30 g in body weight) days. Growth, vertebral ash content (% ash) and radiologically detectable vertebra pathologies were assessed. Triploids were initially smaller than diploids, and again on day 77, but there was no ploidy effect on days 38 or 126. Vertebral ash content increased with increasing body size and those fish fed the NP diet had higher vertebral ash content than those groups fed the LP diet during the intervening time period, but this diet effect became less apparent as fish grew, with all groups having relatively equal vertebral ash content at termination. In general, triploids had lower vertebral ash content than diploids on day 38 and this was most evident in the group fed the LP diet. On day 77, those triploids fed the LP diet during the intervening time period had lower vertebral ash content than diploids. At termination on day 126, the triploids had the same vertebral ash content as diploids, irrespective of diet. There was a ploidy × diet interaction on vertebral deformities, with triploids having higher prevalences of fish with ≥1 deformed vertebra in all dietary groups except continuous NP. In conclusion, between days 0 and 77 (3–30 g body size), triploids required more dietary phosphorus than diploids in order to maintain similar vertebral ash content. A possible link between phosphorus feeding history and phosphorus demand is also discussed.     

Feeding ecology of early marine phase Atlantic salmon Salmo salar post-smolts

Dietary analyses of Atlantic salmon Salmo salar post-smolt stomachs collected from 2001 to 2005 in Penobscot Bay, Maine, U.S.A., have yielded insights into the feeding ecology of early marine phase post-smolts from different rearing origins. Most stomachs contained only one or two prey types, suggesting active prey selection. Post-smolts that lived in the river longer (i.e. from naturally reared and parr-stocked origins) were smaller and consumed more fishes than invertebrates compared to larger post-smolts that emigrated immediately post-stocking (i.e. from smolt-stocked origins). Naturally reared S. salar consumed c. 84% fishes and 16% crustaceans and parr-stocked S. salar consumed 64% fishes and 34% crustaceans. Stocked smolts consumed 48% fishes and 40% crustaceans. Differences in the type and quantity of consumed prey may be indicative of behavioural differences among rearing origins that influence post-smolt survival.     

Somatolactin mRNA expression during the parr–smolt transformation in hatchery-reared Atlantic salmon Salmo salar smolts

Expression of somatolactin (SL) mRNA was quantified during smoltification at 2 week intervals during January and February and weekly during March, April and May 2004 in a line-bred ranching stock of Atlantic salmon Salmo salar . Gene expression of SL (and a splice variant termed SL₁₈₀) was lower in fish sampled in the later weeks of smoltification than in those at the beginning, a pattern opposite to that of gill Na⁺ K⁺-ATPase activity.     

Genetic assessment of Atlantic salmon Salmo salar and sea trout Salmo trutta stocking in a Baltic Sea river

Microsatellite DNA variation was used to assess the outcome of stocking Atlantic salmon Salmo salar and migratory trout Salmo trutta in River Sävarå, N Sweden. No information on pre‐stocking genetic composition of S. salar and S. trutta in River Sävarå was available. In 2 year‐classes of S. salar smolt, microsatellite data indicated that post‐stocking genetic composition differed markedly (F_ST= 0·048) from the main donor strain, Byskeälven S. salar, and from other Gulf of Bothnia S. salar stocks (F_ST 0·047 and 0·132). The STRUCTURE programme failed to detect any substructuring within Sävarå salmon. It was concluded that only minor introgression estimated to a proportion of 0·11 (95% CI 0·07–0·16) has occurred in S. salar. Salmo trutta showed overall low differentiation among populations with maximum F_ST of 0·03 making analysis more cumbersome than in S. salar. Still, the SävaråS. trutta deviated significantly from potential donor populations, and STRUCTURE software supported that majority of trout in Sävarå formed a distinct genetic population. Admixture was more extensive in S. trutta and estimated to 0·17 (95% CI 0·10–0·25).     

Remodeling of the notochord during development of vertebral fusions in Atlantic salmon (Salmo salar)

Histological characterization of spinal fusions in Atlantic salmon (Salmo salar) has demonstrated shape alterations of vertebral body endplates, a reduced intervertebral space, and replacement of intervertebral cells by ectopic bone. However, the significance of the notochord during the fusion process has not been addressed. We have therefore investigated structural and cellular events in the notochord during the development of vertebral fusions. In order to induce vertebral fusions, Atlantic salmon were exposed to elevated temperatures from fertilization until they attained a size of 15 g. Based on results from radiography, intermediate and terminal stages of the fusion process were investigated by immunohistochemistry and real-time quantitative polymerase chain reaction. Examination of structural extracellular matrix proteins such as Perlecan, Aggrecan, Elastin, and Laminin revealed reduced activity and reorganization at early stages in the pathology. Staining for elastic fibers visualized a thinner elastic membrane surrounding the notochord of developing fusions, and immunohistochemistry for Perlecan showed that the notochordal sheath was stretched during fusion. These findings in the outer notochord correlated with the loss of Aggrecan- and Substance-P-positive signals and the further loss of vacuoles from the chordocytes in the central notochord. At more progressed stages of fusion, chordocytes condensed, and the expression of Aggrecan and Substance P reappeared. The hyperdense regions seem to be of importance for the formation of notochordal tissue into bone. Thus, the remodeling of notochord integrity by reduced elasticity, structural alterations, and cellular changes is probably involved in the development of vertebral fusions.     

Movement rhythms in juvenile Atlantic salmon, Salmo salar L.

Nocturnal downstrean migration of juvenile Atlantic salmon is usually interpreted as increased locomotor activity. The frequency of downstream passages of 0–1 + salmon in an endless stream channel was greater by night than by day in both smoking and non-smolting fish in autumn and spring. Movement increased at dusk, and decreased after dawn. Mature male 1 + fish moved slightly less than immatures in October, but significantly more in November. Total movement frequency was lower at full moon than at other moon phases, and movement was reduced when the moon was up. Under turbid conditions by day, the threshold water velocity inducing nett downstream movement was 8.2 cm s⁻¹, and the relative velocity of fish swimming downstream was never more than one third that of fish holding station at the normal maximal flow of 25–30 cm s⁻¹.
At the end of their first growing season in October, fish which had been offered food continuously through 24 h did not differ in size from those fed by day only, but the latter were significantly larger than those offered food only at night.
We conclude that: (1) the fish fed actively by day, and not by night; (2) station-holding represented activity, and downstream nocturnal movement represented relative inactivity (displacement) which occurred on loss of visual orientation, hence migration resulted from reduced activity; (3) lack of displacement in early autumn has adaptive value for maturing fish, but not for non-spawners.     

Observations of the biology and parasites of postsmolt Atlantic salmon, Salmo salar, from the Norwegian sea

The main food items of postsmolt Atlantic salmon caught in the Norwegian sea during summer 1991 were Parathemisto spp. and herring ( Chipca harengus ). The most prevalent parasite species were Chloromyxion sp., Apatemon sp., H. adunction, A simplex and I. salmonis .     

Chromosome polymorphism and constitutive heterochromatin in the Atlantic salmon,Salmo salar

Chromosome preparations from lymphocyte cultures of 30 Atlantic salmon were studied. Robertsonian polymorphisms were observed with different individuals having diploid numbers of 56, 57 and 58 but a constant arm number of 74. C-banding shows constitutive heterochromatin at the centromeres, associated with the satellites on a small pair of metacentric chromosomes and in the interstitial regions of the long arms of a number of chromosomes.     

Sperm traits in farmed and wild Atlantic salmon Salmo salar

Differences in sperm metabolism and morphology between wild and non‐local farmed Atlantic salmon Salmo salar were assessed by measuring metabolic enzyme activities and length of sperm flagella. No differences were observed between wild and farmed S. salar sperm with regards to cell counts or any of the biochemical variables assessed. Flagella of sperm cells were significantly longer in wild than farmed S. salar; however, this did not result in higher energy levels or different fertilization rates.     

The production of gynogenetic Atlantic salmon,Salmo salar L.

The production of Atlantic salmon gynogenomes by the combined use of a novel method for sperm irradiation and differently timed high hydrostatic pressure shocks is described. Sperm solutions were exposed to UV irradiation in a temperature-controlled flow-through device. Eggs fertilised with such sperm were exposed to shocks of 9500 psi at 30 min or approximately 7 h after fertilisation in order to produce meiotic and mitotic gynogenomes respectively. Yields of meiotic gynogenomes were generally high (up to 95%); those of mitotic gynogenomes were lower (range 2–20%). Analyses of the offspring by ploidy status and fingerprinting confirmed their gynogenetic origin. Small numbers of mitotic gynogenetic fish were grown on for 2 years in fresh and salt water. S1/S2 ratios were lower in gynogenetic fish and mean age at maturity was greater. Of the presumptive gynogenetic fish subjected to destructive sampling (n = 87) all were female.