Effect of restricted feeding on early maturation in female and male amago salmon, Oncorhynchus mason ishikawae

Incidence of early maturation of female amago salmon was reduced by >32% by feed restriction beginning in May, 5 months prior to spawning. Male early maturation was unaffected by this feed restriction regime.     

Migration of Atlantic Salmon, Salmo salar, Smolt through the Estuary Area of River Ellidaar in Iceland

Synopsis We tagged both wild and hatchery Atlantic salmon, Salmo salar, smolts from River Ellidaar (64 ° 08′ N, 21 ° 50′ W) with ultrasonic tags. We caught the wild smolts in a smolt trap and selected the largest individuals from the run. We implanted the transmitters in the abdominal cavity of the fish and then released them in River Ellidaar close to the estuary. We used four ultrasonic receivers; one in the river’s estuary, one outside the estuary and two further away on both sides of an island in the estuary zone. The receivers recorded all transmitters within a 600 m radius. The hatchery smolts were larger than the wild smolts. Some of the smolts were lost on the way through the estuary. Only 4 out of 9 wild smolts and 14 of 17 hatchery smolts were recorded all the way through. The tags and the tagging likely affected the survival of the smolts especially the smaller fish. There were no differences in the smolt migration between the 2 years of study and no differences in the migration behavior between the wild and the hatchery smolts. After being released the smolts stayed on average for 10 h in the river then migrated into the estuary were they stayed for 54 h on average. Then they migrated straight through the area to the sea at approximately 0.2 fish lengths per second. We recorded large differences in the migration.     

Evolutionary-physiological aspects of adaptation of the Pacific salmon fry of the oncorhynchus genus to migration to the sea water

A complex of adaptive changes occurring in the Pacific salmon fry in the process of migration to the sea is described, including behavior, ion content in carcasses, and morphological changes in Stannius bodies, gill epithelium, and nephron tubular epithelium. Participating in experiments with transfer from fresh water into a two-layer aquarium (the lower layer—sea water, the upper layer—fresh water) were smolts of chum salmon and underyearlings of cherry salmon as well as the trachurus and leiurus forms of the three-spined stickleback Casterosteus aculeatus. All fish, regardless of their salt preference, at once after placement into the two-layer aquarium, occupied the sea water zone, at the very bottom of the aquarium. After 1 h, there started brief excursions of cherry salmon and chum salmon to the upper, fresh water layer; however, both forms of the three-spined stickleback did not participate in these excursions. After 12 h, the chum salmon settled down in the lower, sea water layer, while the cherry salmon—in the upper, fresh water layer. Both forms of the three-spined stickleback never left the sea water layer and felt quite comfortably on the aquarium bottom. It seems that the high tolerance of the both stickleback forms to wide salinity limits allows them to choose the convenient position regardless of the water salt composition. By analyzing the material obtained for three years (2001–2003) on structure and functions of the gill epithelium chloride cells (CC), we have come to the conclusion that the fresh water fry of two salmon species, chum and cherry salmons, caught at the same time and practically in the same water reservoirs can be divided into three groups. The underyearlings of the cherry salmon as a rule are characterized by the thickened epithelium of secondary gill lamellae, but by a very small number of CC. In smolts of chum salmon, on the contrary, the epithelium is sufficiently thin, but enriched in the CC that demonstrate an active structure in the very beginning of migration to sea. However, with approaching the sea (and with an increase of terms of migration) the CC activity drops, but their amount does not change. And only after migration to the sea the CC activity rises again, although their amount seems to remain unchanged. The described peculiarities of behavior and of the ion composition regulation in the migrating salmon fry confirm the hypothesis that the salmons evolutionized in fresh water, that the Oncorhynchus genus appeared in large spaces of saltish waters, such as the Japan sea at the period of the early Pleistocene, and that learning of fry of the Oncorhynchus genus (for instance, of O. gorbuscha and O. keta) is the most specialized in the salmons migration to the sea, whereas the fresh water species of chars (Salvelinus) and of trouts (Salmo) are more primitive.     

Bimodal size distributions in wild juvenile Atlantic salmon populations and their relationship with age at smolt migration

A bimodal length distribution developed in an Atlantic salmon population planted as start-fed fry in a Norwegian stream. The earliest fish to smolt were from the upper modal group.     

Alternative developmental pathways and the propensity to migrate: a case study in the Atlantic salmon

Migratory behaviour with its associated phenotypic changes is generally viewed as an adaptive strategy because it incurs survival or reproductive advantages to migrants. The development of a migrant phenotype is believed to be controlled by threshold mechanisms, where individuals emigrate only after surpassing a particular body size but delay migration if below. For such a strategy to respond to natural selection, part of the phenotypic variance in the propensity to migrate must be explained by variation in additive genetic effects. Here, we use data gathered in the field and from a common rearing experiment to test for a genetic basis associated with seaward migration in Atlantic salmon (Salmo salar L.). We document a high heritability of the liability trait underlying the propensity to emigrate in juvenile salmon, and significant differences between offspring grouped according to their sires in body-size threshold values above which emigration takes place. The presence of additive genetic variance in both the liability and thresholds makes the onset of migration a process sensitive to selection and may therefore constitute an important explanatory mechanism for the interpopulation differences in the size at seaward migration observed in this species.     

Net ground speed of downstream migrating radio-tagged Atlantic salmon (Salmo salar L.) and brown trout (Salmo trutta L.) smolts in relation to environmental factors

The downstream migration of Atlantic salmon (Salmo salarL.) and sea trout smolt (S. trutta L.) was investigated using radio telemetry in the spring of 1999 and 2000. Forty wild sea trout smolts, 20 F1 sea trout smolts, 20 hatchery salmon smolts and 20 salmon smolts from river stockings were radio tagged and released in the Danish River Lilleaa. The downstream migration of the different groups of fish was monitored by manual tracking and by three automatic listening stations. The downstream migration of radio tagged smolts of both species occurred concurrently with their untagged counterparts. The diel migration pattern of the radio tagged smolts was predominantly nocturnal in both species. Wild sea trout smolt migrated significantly faster than both the F1 trout and the introduced salmon. There was no correlation between net ground speed, gill Na⁺,K⁺-ATPase activity or fish length in any of the different groups. The migration speed of wild sea trout smolts was positively correlated with water discharge in both years. In F1 sea trout smolts, migration speed was positively correlated with temperature in 1999. The migration speed of salmon smolts did not correlate to any of the investigated parameters.     

The seaward movement of Atlantic salmon smolts in the Usk estuary, Wales, as inferred from power station catches

The greatest numbers of Atlantic salmon smolts were caught at Uskmouth power station during the day on the flood tide and the least on an ebbing night tide. Catch rates during the day on an ebb tide and at night on a flood tide represented an intermediate state. The results, together with tracking data, suggest that salmon smolts prefer to migrate seaward through the lower Usk estuary during the night Oncc an ebbing tide.     

Experimental measurement of thermal preferenda in sea trout smolts

Experiments were conducted to estimate the threshold water temperature rise (Δt) for avoidance by smolts of sea trout Salmo trutta . In three out of four experiments, the median avoidance threshold was found to be c . +6° C (+5·4 to +6·4° C). In the fourth experiment, the fish were able to detect and avoid temperature rises of < +1° C.     

Avoidance responses to low frequency sound in downstream migrating Atlantic salmon smolt, Salmo salar

The possibility of using intense sound as an acoustic barrier for downstream migrating smolt of the Atlantic salmon ( Salmo salar ) was studied by observing, the reactions of smolt to 10 and 150 Hz sounds in a small river. At the observation site the river branched into a main course and a minor channel, the latter rejoining the main stream after 30 m. The sound sources were positioned at the lower end of the channel. The number of smolt re-entering the mam stream at the lower end of the channel was recorded during alternating periods with and without sound. Intense 150 Hz sound had no observable effects on the smolt, even at intensities 114 dB above the hearing threshold at this frequency. At intensities above 1.0. 10⁻²ms⁻² the 10 Hz sound was an effective deterrent for the smolt, which turned and left the channel at the upstream branching point.     

Effects of size and early sexual maturity on downstream migration during smolting in Baltic samon (Salmo salar)

The effects of size and previous sexual maturity on downstream migration in two-summer-old Baltic salmon were studied during their normal seaward migratory period in spring. We used an automatic PIT (Passive Integrated Transponder) tag monitoring system for individual recognition of tagged fish. Size affected the probability of migration among both previously immature and previously mature males while there was no such relationship among females. Nearly all females and 63.6% of the previously immature males migrated during the smolting period, while only 24.5% of the previously mature males did. In migrating fish the individual activity level increased from almost zero in May to about 450 antenna passages per day in mid June, coinciding with a pronounced increase in ambient water temperature. During the period of increasing activity in early June the fish changed from night activity to day activity and finally activity all day and night. The fish moved downstream in schools but no specific school size dominated. This paper demonstrates the strong effect of body size and previous sexual maturity on the probability of migration.