Determination of body shape variation in Irish hatchery‐reared and wild Atlantic salmon

Discriminant function analysis was used to distinguish morphologically between samples of parr, smolts and adult Atlantic salmon Salmo salar from several hatchery and river systems in Ireland. The effect of habitat shift was investigated in Atlantic salmon parr. Parr grown from the eyed‐egg stage with a non‐sibling group in a hatchery environment, came to resemble the mean body shape of their host hatchery Atlantic salmon stock more closely than that of a full sibling group grown at their natal hatchery. Wild Atlantic salmon smolts differed in shape from hatchery‐reared smolts. This difference was less pronounced, but still statistically significant when wild adults were compared with hatchery‐reared adults captured in the coastal drift‐net fishery after a year spent at sea. Rearing conditions had a significant impact on the production and growth of fish body shape. This in turn may have affected adaptability and survivorship of ranched Atlantic salmon in the marine environment.     

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

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

Variations in juvenile growth, energy allocation and life-history strategies of two populations of Arctic charr in North Norway

Within the populations of Arctic charr Salvelinus alpinus in Storvatn and Rungavatn, Norway, fish that attained the largest size in the late parr stage migrated as 4+ smolts, fish that attained a medium size became 5+ smolts, and the smallest parr became lake residents. Within the last 2 years of the parr stage, those that became anadromous had a lower growth rate during the winter and early summer than those that became resident, and vice versa in late summer. Thus, anadromous fish grew faster in the parr stage, but in years prior to migration, their growth pattern seemed to differ from that of parr that became resident. Due to early size differences between parr that became anadromous or resident, it is suggested that some of the basis for the decision to smoltify or not in charr depends on the growth rate through the whole parr stage, but with significant modification by the local environment. Rungavatn parr had a significantly higher population density (5 ×), higher lipid content and mortality, an earlier maturation, a lower rate of growth, and only one-third the degree of anadromy than those from Storvatn. It is suggested that the two charr populations have evolved different life-history adaptations due to unequal growth, energy allocation and competition opportunities in the lakes.     

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

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

Individual growth and phase differentiation of lacustrine masu salmon, Oncorhynchus masou, under artificial rearing conditions

 In lacustrine masu salmon, Oncorhynchus masou, originated from anadromous fish and inhabiting an artificial lake (Shumarinai Lake), we examined the relationship between individual growth during the juvenile stage and phase differentiation under artificial rearing conditions. In females, the mean fork length of potential (subsequent) 1+ smolts and potential 1+ parr (1+ parr show fish that neither smolted nor matured until 1+ autumn) always differed after their first summer (0+ summer), the former being larger. In males, the juveniles that grew faster during their first spring became 0+ mature male parr. After their first summer, potential 1+ smolts were always larger in body length than potential 1+ mature parr and potential 1+ parr. These results were similar to previous reports of anadromous masu salmon, suggesting that the lacustrine masu salmon studied seems to have maintained the phase differentiation as in the original fish, likely because only 60 years have passed since the formation of the lacustrine population.     

The relationships between anadromy, sex ratio and parr growth of Arctic charr in a lake in North Norway

Relationships between the degree of anadromy, sex ratio and parr growth of Arctic charr Salvelinus alpinus were investigated in Lake Storvatn in Hammerfest town, by estimating the densities of resident and anadromous charr in the lake, and by comparing the smolt characteristics of fish captured in the outlet river with those of fish of the same age in the lake. About two-thirds of the charr older than 5 years were anadromous. The sex ratio of smolts was approximately 1 : 1 and the frequency of mature male parr in the lake was low. As females tended to dominate the mature portion of the anadromous population, there was probably a relatively higher mortality among sea-run male smolts. The youngest smolts (3+ and 4+) were probably recruited from fast-growing parr in the littoral zone, while older smolts (>4+) may have been recruited from all habitats in the lake.     

Feeding of wild and hatchery reared Atlantic salmon (<Emphasis Type="Italic">Salmo salar</Emphasis> L.) smolts during downstream migration

In general, hatchery salmonid smolts experience higher mortality during migration than wild smolts, which is suggested to be due to domestication effects and that hatchery fish lack experience of the natural environment. However, possible differences in feeding during smolt migration between hatchery and wild smolts have rarely been addressed. We compared the number of feeding smolts and stomach fullness among wild Atlantic salmon smolts, hatchery-reared smolts released as 1-year-old parr, and hatchery-reared smolts released as 2-year-old smolts during their descent to sea in River Tornionjoki. In addition, estimations of prey selection among the smolt groups were conducted. A high proportion of wild smolts and smolts stocked as parr actively fed during the smolt migration. A lower proportion of smolts stocked as smolts was feeding and their stomach fullness were much reduced in comparison with the two other groups. The study also indicated that the feeding of migrating smolts is selective rather than opportunistic. In conclusion, this study suggests that stocked 2-year-old smolts may enter sea with an inferior foraging behaviour and it is a possibility that this may contribute to the observed low post-smolt survival in the Baltic Sea.     

Differential lifetime success and performance of native and non‐native Atlantic salmon examined under communal natural conditions

The lifetime success and performance characteristics of communally reared offspring of wild native Burrishoole (native), ranched native (ranched) and non‐native (non‐native) Atlantic salmon Salmo salar from the adjacent Owenmore River were compared. Non‐native 0+ year parr showed a substantial downstream migration, which was not shown by native and ranched parr. This appears to have been an active migration rather than competitive displacement and may reflect an adaptation to environmental or physiographic conditions within the Owenmore River catchment where the main nursery habitat is downstream of the spawning area. There were no differences between native and ranched in smolt output or adult return. Both of these measures, however, were significantly lower for the non‐native group. A greater proportion of the non‐native Atlantic salmon was taken in the coastal drift nets compared to the return to the Burrishoole system, probably as a result of the greater size of the non‐native fish. The overall lifetime success of the non‐native group, from fertilized egg to returning adult, was some 35% of native and ranched. The ranched group showed a significantly greater male parr maturity, a greater proportion of 1+ year smolts, and differences in sex ratio and timing of freshwater entry of returning adults compared to native, which may have fitness implications under specific conditions.     

Polymorphism of smolts of pink salmon <Emphasis Type="Italic">Oncorhynchs gorbuscha</Emphasis> in the Indera River (Kola Peninsula)

Pink salmon introduced into the White Sea started to exploit as spawning grounds middle and upper reaches of the river 20 years after its appearance in the Indera River. As a result of this, the migration pathway of smolts and late smolts appeared in addition to early smolts. The intraspecies polymorphism of smolts is confirmed by differences of early and late smolts by body length and weight, migration dates, food spectrum, and indices of stomach fullness. The food spectra of late juveniles of pink salmon coincide with those of parr of Atlantic salmon Salmo salar and of brown trout S. trutta. Greater abundance of late migrants of pink salmon may cause competition of these species for food.     

Effect of actinomycin-D on gill Na+-K+ ATPase of juvenile chinook salmon, Oncorhynchus tshawytscha (Walbaum)

Actinomycin-D reduced gill Na⁺-K⁺ ATPase activity of chinook salmon Oncorhynchus ishawytswha (Walbaum), smolts and saltwater-adapted juveniles but had no significant effect on the enzyme activity of parr in fresh water. The similarity in response suggests that even though smolts are found in fresh water, their enzyme system is more characteristic of saltwater-adapted juveniles than freshwater-dwelling parr. A greater reduction in enzyme activity was found when fish were treated with actinomycin-D in salt water than in fresh water, suggesting that the enzyme degradation rate wasgreater in salt water than in fresh water.     

A method for tracking the behaviour of mature and immature salmon parr around nests during spawning

A remote monitoring system was developed to provide information on the behaviour of mature and immature Atlantic salmon Salmo salar parr at nests during the spawning season. An octagonal passive integrated transponder (PIT) detector (0·865 m maximum diameter) designed to surround nests of Atlantic salmon was used to identify individual salmon parr present at 38 spawning events in three circular spawning channels. The range of the detector for PIT tags presented in the optimum orientation was 2·4 cm (range between tags 1·7–3·0 cm). Using a sub-sample of 20 spawnings, the mean efficiency of the detector (number of fish passes registered relative to number of passes observed on video) was 70·5% (range 32-100%). There were no significant effects of time from spawning, total number of registrations, body size or maturity status (mature or immature) on efficiency. However, fish were more likely to be detected entering nests than leaving, as departures were more rapid and higher in the water column. The PIT detector did not affect the numbers of parr at spawnings or between spawning intervals of females, and allowed for the individual identification of 65 of the 72 parr observed in nests during spawning. In all cases where certain identifications were not possible and the video was of satisfactory quality, this was due to obstruction of the camera view by anadromous fish. The remote monitoring system was thus effective in identifying behavioural differences, and only one of 20 immature parr was ever detected during the period encompassing 10 min before and after spawning compared with 30 or 40 mature parr.     

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

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

Divergence in smolt production from the stocking of 1-summer-old and 1-year-old Atlantic salmon parr in a northern Baltic river

One‐year‐old and 1‐summer‐old salmon parr have been stocked in the Simojoki, a northern Baltic river, to protect the natural salmon stock from extinction. This enhancement practice was studied during a 5‐year stocking period, when an average of 14.3 1‐summer‐old parr and 5.4 1‐year‐old parr was needed to produce one sea running smolt. The mean yield from stocking was 70 smolts per 1000 stocked 1‐summer‐old parr and 186 smolts per 1000 stocked 1‐year‐old parr. The mean cost of smolts produced by stocking 1‐year‐old and 1‐summer‐old parr was 1.2 and 2.6 times the cost of producing one 50 g smolt in the hatchery, respectively. There was a negative relationship between density of wild parr and length of smolts stocked as 1‐summer‐old parr in the river, but no relationship between wild parr density and length of smolts stocked as 1‐year‐old parr. This indicated that 1‐summer‐old parr were more susceptible than the older parr to competition with wild parr. Considering production costs and smolt yield, stocking of 1‐year‐old parr appeared to be more profitable than that of 1‐summer‐old parr in the enhancement of the endangered salmon stock.     

The importance of stocking age in the enhancement of River Kymijoki salmon (Salmo salar)

Hatchery‐reared Atlantic salmon parr and smolts are regularly released into the River Kymijoki, Southern Finland, to enhance the local river and coastal fisheries. In 1988–1994, a series of micro‐ and Carlin‐tagging experiments were carried out to examine the influence of stocking age on the stocking results. Five age groups were compared: 1‐year and 2‐year‐old smolts, and 1‐summer, 1‐year and 2‐summer‐old parr. The aim was to determine the most profitable way of producing salmon for fishing in the River Kymijoki. Stocking age had a strong influence on the stocking results, measured as the proportion of adult recoveries. The most favourable results were obtained using 2‐year‐old smolts (survival index 100), followed by 1‐year‐old smolts (52), 2‐summer‐old parr (51), 1‐year‐old parr (37) and 1‐summer‐old parr (24). Available data on rearing costs suggest that 2‐year‐old smolts were also the most economically profitable choice, followed by 1‐year‐old parr, 1‐year‐old smolts and 1‐summer‐old parr. The most expensive way of producing salmon was by stocking 2‐summer‐old parr.     

Age composition of masu salmon smolts in northern Japan

Smolts of anadromous masu salmon Oncorhynchus masou aged 3+ years were found in a northern Japanese river. This is the first recording of 3+ year smolts in Japan. These fish appeared to originate in the cold upper river where 2+ year parr were found during autumn.     

Reconstructing the lengths of juvenile Atlantic salmon from atlas bones: estimating the parr to smolt ratio from regression analyses revisited

Analyses of regressions of atlas bone width against fish length (mixed regressions) of Atlantic salmon Salmo salar can be used to infer the parr:smolt ratio of the source populations. Analyses of mixed regressions have been used to infer that, contrary to previous reports, sawbill ducks Mergus spp. eat only a few smolts. Here a Monte Carlo simulation model is presented which quantifies the effect of variation in the smolt:parr ratio on the form of the mixed regression. Based on published mixed regressions, the model predicts 95% CI of 1–36% and 1–42% smolts in diets of sawbill ducks, which are broadly consistent with results of earlier studies. However, other large inconsistencies still exist between results of previous studies and these are briefly discussed.     

Seasonal foraging and piscivory by sympatric wild and hatchery-reared steelhead from an integrated hatchery program

We compared the diet of hatchery-reared steelhead produced from an integrated hatchery program as emigrating spring smolts and non-migrating hatchery residuals to their sympatric wild counterparts. Our results suggest that there is a potential for hatchery fish to affect wild steelhead populations due to dietary overlap and subyearling salmonid predation; however, relative ecological risk did not increase as steelhead delayed or forwent emigration. Predation by hatchery smolts was related to release timing, but not experience with native fish. Diet composition appears to be more strongly affected by seasonal and yearly differences in prey abundance and presence rather than differences in rearing environments. Hatchery and wild steelhead showed small but important foraging differences. Hatchery smolts did not consume as many salmonids as wild fish and hatchery residuals showed relatively stronger surface oriented feeding behavior than wild parr. Because most hatchery smolts emigrated shortly after release and the overall number of residuals in the study creek was low, we speculate that in this case there is low dietary and predatory-based risk of hatchery steelhead in Abernathy Creek negatively impacting wild salmonids.     

Growth and development of a bimodal length-frequency distribution during smoking in a wild population of white-spotted charr in northern Japan

Smolt age and size, and individual growth during smoking were examined in a wild population of white-spotted charr, Salvelinus leucomaenis , in a small stream in northern Japan. Smolt size (110–210 mm F.L.) differed neither between sexes nor among age cohorts of 2–7 years. Back-calculation of riverine growth of smolts using otoliths showed a gradually increasing discrepancy in fork lengths of these age cohorts in both males and females, the faster-growing individuals smolting at an earlier age. A mark—recapture study on individually-identified fish revealed that the initial (spring-summer) length-frequency distribution of parr was unimodal, but became slightly skewed to the larger side in autumn, resulting in a bimodal distribution being evident by the following spring, with the upper modal group primarily composed of smolts. However, closer examination revealed that both male and female potential (subsequent) smolts were larger than the potential parr in the previous spring, the minimum lengths of the former being around 90–100 mm f.l. Specific growth rates of the potential smolts remained relatively high after summer through to the following spring, whereas those of the potential parr decreased after summer, despite having been similar between the two groups during the previous spring and summer. These observations revealed that the developmental triggering of smelting possibly occurs in or following the summer season.     

Population structure of the masu salmon <Emphasis Type="Italic">Oncorhynchus masou</Emphasis> from the Kol River (Western Kamchatka) and geographic variation in the species area

According to materials from original investigations in Kamchatka as well as published data, the population structure and geographical variation of the masu salmon Oncorhynchus masou are considered in the species area. The intraspecies structure, dates of their run to rivers, dates of spawning, traits of spawning, size-age and sex composition, fecundity, biological traits of parr, downstream migration of smolts are investigated. The relationship is shown between the manifestations of geographical variation of masu salmon and the factor of temperature. It is assumed that the global warming of climate would contribute to the advancement of this species to northern areas and to the increase in its abundance.