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.     

Relationships between smolt size, postsmolt growth and sea age at maturity in Atlantic salmon ranched in the Baltic Sea

Tagging data were used to examine the relationships between smolt size, post-smolt growth and sea age at first maturity for the short-migrating Neva strain of Atlantic salmon ( Salmo salar L.) ranched in the Bothnian Sea and the Gulf of Finland. The results provided evidence that post-smolt growth was influenced by both relative and absolute smolt size. For both sea-areas, 2-year smolts of small relative size within a release group grew more rapidly in the sea than did smolts of higher relative, but equivalent absolute size. The negative influence of increasing relative smolt size on marine growth was, however, outweighed by the stronger positive influence of increasing absolute smolt size. A 160-mm increase in smolt size (140–300 mm) resulted in an overall growth advantage of about 1 year. In the Bothnian Sea, the predicted mean length after 1 year in the sea was 288 ± 25 mm for 140-mm smolts and 560 ± 16 mm for 300-mm smolts. Under the more favourable conditions of the Gulf of Finland, the respective mean lengths were 369 ± 15 mm and 613 ± 12 mm. The sea age at first maturity was inversely related to both freshwater and marine growth rates. For both sea areas, large smolts yielded proportionately more grilse than did small ones. Smolt years with good post-smolt growth rates yielded more grilse than did years with poor growth rates. The overall level of grilsing was higher in the Gulf of Finland than in the Bothnian Sea. These results suggest that the relationships between smolt size, post-smolt growth and age at first maturity in the sea are influenced by the environmental conditions of the respective sea area. A framework explaining the links between smolt size, marine growth, survival and sea age at maturity in Neva salmon is presented for the Gulf of Finland and the Bothnian Sea.     

Marine post-smolt growth and age at maturity of Atlantic salmon

The annual variation in sea-age of maturation for a hatchery dependent stock of Atlantic salmon was compared to variation in post-smolt growth as evidenced by circuli spacing patterns. The proportion of returns of 1-seawinter (1 SW) and 2 SW salmon and the fraction of the smolt year class or cohort that maturated as 1 SW fish, were compared to seasonal growth indices determined from circuli spacing on the scales of smolt class survivors returning as 1 SW and 2 SW spawners. Using image processing techniques, we extracted inter-circuli distances from scales from 2244 recaptured fish. Spacing data for the first year at sea were collected and then expressed as seasonal growth indices for the spring period, when post-smolts first enter the ocean; the summer, when growth appears maximal; and winter, when growth appears to be at a minimum. In general, circuli spacings were wider for 1 SW than for the 2 SW returns of the same smolt cohort. The 1 SW fraction was significantly and positively correlated with late summer growth, suggesting that growth during this season is pivotal in determining the proportion of a smolt class that matures early.     

Parental influence on growth rate, smolting rate and survival in hatchery reared juvenile Atlantic salmon, Salmo salar

The proportion of potential 1-year smolts, their mean length, the mean length of potential 2-year smolts, and the mortality rate in four half-sib families of Atlantic salmon, reared under four contrasted conditions of overhead cover, is shown to be inffuenced primarily by genetic factors (89.9%, 86.1 %, 82.7% and 80.2% of total variance respectively). Variation between families in smolting rate and mortality rate is influenced by both parents, but more by the male than the female. Variation in mean length is influenced almost entirely by the female parent. These results are discussed in relation to previous findings on bimodality of size distribution and inheritance of growth and mortality characteristics in Atlantic salmon.     

Population dynamics of Salmincola salmoneus on Atlantic salmon in a northern Norwegian river

Atlantic salmon Salmo salar are often heavily infected by the gill maggot Salmincola salmoneus, but little information exists on the population dynamics of this parasite. Through a combination of in vivo field examination and laboratory analysis of gills from the Alta River S. salar population in northern Norway, we describe the population dynamics of the parasite and suggest a model for the host-parasite interactions. S. salar did not become infected with S. salmoneus until they returned to the river as first-time spawners. The infection increased rapidly until autumn, and just after spawning 96% of the spent fish (kelts) were infected with a mean intensity of 53 parasites per fish. In May, the prevalence of S. salmoneus on the descending kelts had increased to 100%, but the intensity exhibited little change. A small proportion of the adult S. salar population returned as immature to the river during autumn and had lower parasite intensities than the kelts the following spring. When the fish that had spawned previously (repeat spawners) returned from their second (or more) sea migration, they had an average infection rate of 36 S. salmoneus individuals per fish. The kelts seemed to be the main habitat for the parasite during winter and spring, and they stay long enough in the river to pass the infection to maiden S. salar that enter the river early in summer. These fish then became a source of infection for the maiden fish entering the river later. However, in years that have a possible mismatch between the opposite migration of kelts and maiden S. salar, the immature fall-running and returning repeat spawners will be crucial for maintaining the parasite population. We hypothesize that heavily infected S. salar may suffer reduced growth and survival at sea, potentially reducing the abundance of repeat spawners.     

Seasonal growth of wild Atlantic salmon juveniles and implications on age at smoltification

The seasonal growth trajectories of wild Atlantic salmon Salmo salar juveniles by age group within the Margaree River, Canada, are described. Circuli counts from scales were used to infer growth rates at different ages and these were used to predict the proportions of age 2‐ and 3‐year old smolts from different portions of the watershed. In the wild Atlantic salmon juveniles from the Margaree River, there was no bimodality in fork length frequencies and no 1 year old smolts were produced. Water temperature differences during the growing season were insufficient to explain the differences in growth rates and size at age among the sites sampled. There was a positive association between the growth rate in the first year and the subsequent age at smoltification. In the Margaree River, differences in tributary specific growth rates and size at age were expected to produce important differences in the relative ages at smoltification.     

Heart rate as an indicator of metabolic rate and activity in adult Atlantic salmon, Salmo salar

Telemetered heart rate (f_H) was examined as an indicator of activity and oxygen consumption rate (VO₂) in adult, cultivated, Atlantic salmon, Salmo salar L. Heart rate was measured during sustained swimming in a flume for six fish at 10° C [mean weight, 1114 g; mean fork length (f. l.), 50·6 cm] and seven fish at 15° C (mean weight, 1119 g; mean f. l., 50·7 cm) at speeds of up to 2·2 body lengths/s. Semi–logarithmic relationships between heart rate and swimming speed were obtained at both temperatures. Spontaneously swimming fish in still water exhibited characteristic heart rate increases associated with activity. Heart rate and Vo₂ were monitored simultaneously in a 575–1 circular respirometer for six fish (three male, three female) at 4° C (mean weight, 1804 g; mean F. L., 62· cm) and six fish (three male, three female) at 10° C (mean weight, 2045 g; mean f. l., 63·2 cm) during spontaneous but unquantified activity. Linear regressions were obtained by transforming data for both f_H and Vo₂ to log values. At each temperature, slopes of the regressions between f_H and Vo₂ for individual fishes were not significantly different, but in some cases elevations were. All differences in elevation were between male and female fish. There were no significant differences in regression slope or elevation for fish of the same sex at the two temperatures and so regressions were calculated for the sexes, pooling data from 4 and 10° C. There was no significant difference in the mean ± S. D. Vo₂ between the sexes at 4° C (male, 66·0 ± 59·6 mgO₂ kg^?1 h^?1; female, 88·0 ± 60·1 mgO₂ kg^?1 h^?1) or 10° C (male, 166·2 ± 115·4 mgO₂ kg^?1 h^?1; female, 169·2 ± 111–1 mgO₂ kg^?1h^?1). Resting Vo₂ (x?± s. d.) at 4°C was 36·7 ± 8.4 mgO₂ kg^?1 h^?1, and 10° C was 72·8 ± 11·9 mgO₂ kg^?1 h^?1. Maximum Vo₂ (x?± S. D.) at 4° C was 250·6 ± 40·2 mgO₂ kg^?1 h^?1, and at 10° C was 423·6 ± 25·2 mgO₂ kg^?1 h^?1. Heart rate appears to be a useful indicator of metabolic rate over the temperature range examined, for the cultivated fish studied, but it is possible that the relationship for wild fish may differ.     

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.     

The effect of domestication on some life history traits of sea trout and Atlantic salmon

During the period 1968–1991, certain morphological traits of Atlantic salmon Salmo salar and sea trout S. trutta have been recorded regularly at the hatchery at Älvkarleby, central Sweden. Total body length, weight (for females both before and after stripping), number of eggs, egg size and date of ovulation. A smaller data set for fish marked and released as smolts, providing information about total body length of released smolts, time spent at sea and body size of the recovered adults, was also available for analysis. According to theory and empirical data, the process of artificial breeding results in an evolutionary divergence of the cultured strain from the wild phenotypic norm. The reason for such a divergence is that both natural and sexual selection pressures are altered or relaxed during the process of artificial breeding, as well as random genetic processes, such as founder effects and in- and outbreeding. A path analysis of both species and sexes revealed that the size of the released smolts had increased during the study period. Time spent in sea has decreased for both female and male sea trout, but not for Atlantic salmon. Adult body size for female and male trout have increased as well as female trout condition factor. The increase found in egg size of both species was greater for the salmon than for the trout, indicating that female salmon invest more in egg size with increasing body size. These results support the view that domestication probably has a significant and selective impact on the life history traits of the two salmonid species studied.     

Notes on the behaviour of lumpfish in sea pens with and without Atlantic salmon present

The behaviour of lumpfish, Cyclopterus lumpus L., in sea pens, with and without Atlantic salmon, Salmo salar L., present, was assessed by underwater camera technology. Behaviour was classified by recording the principal activity of individual fish for 30-s intervals. The majority of daylight time was spent actively foraging for food. Antagonistic behaviour between Atlantic salmon and lumpfish was not observed during the whole experimental period and no mortality was seen in either species. Cleaning behaviour, but at low frequency, was observed as lumpfish cleaned sea lice off Atlantic salmon. Significantly lower sea lice infection levels were seen on Atlantic salmon when reared together with lumpfish compared to the control group without lumpfish. Feeding behaviour can be classified as strongly opportunistic.     

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 .     

Residence time of the sea louse, Lepeophtheirus salmonis K., on Atlantic salmon, Salmo salar L., after immersion in fresh water

Returning adult salmon caught at the mouth of the River Dee, Aberdeenshire, were transferred to tanks in the laboratory. For fish placed in fresh water, sea lice remained attached for up to 6 days, though most lice were lost in the first 48 hours. Few lice were lost from salmon maintained in sea water. The experiments were conducted in water within a temperature range of 12·8 to 16° C, equivalent to summer river temperatures in the Aberdeenshire Dee.     

Effect of age, sex, and smoking on serum catalase activity

In selected hospitalized patients (9294) with normal serum catalase activity, 7.58 per cent higher (p less than 0.001) serum catalase activity was detected in males than in females. With age, serum catalase activity decreased (slope-0.0683) in males, increased (slope 0.101) in females and did not change in all patients (slope 0.049). Cigarette smoking did not influence serum catalase activity (p greater than 0.05).     

Halastatic demineralization in the vertebrae of Atlantic salmon, during their spawning migration

There was a significant decrease ( c . 10%) in the mineralization of the vertebrae, determined by analysis of quantitative microradiographs, of both sexes of Atlantic salmon Salmo salar during their ascending spawning migration. The decrease was hypothesized to be the result of halastatic demineralization, i.e. the removal of mineral substance with no degradation of the organic matrix of the bone.     

Functional models for growth and food consumption of Atlantic salmon parr, Salmo salar, from a Norwegian river

1. The chief objectives were to analyse and model experimental data for maximum growth and food consumption of Atlantic salmon parr (Salmo salar) collected from a cold glacier fed river in western Norway. The growth and feeding models were also applied to groups of Atlantic salmon growing and feeding at rates below the maximum. The growth models were validated by comparing their predictions with observed growth in the river supplying the experimental fish.
2. Two different models were fitted, one originally developed for British salmon and the other based on a model for bacterial growth. Both gave estimates for optimum temperature for growth at 18–19 °C, somewhat higher than for Atlantic salmon from Britain. Higher optimal temperature for growth in salmon from a cold Norwegian river than from British rivers does not concur with predictions from the thermal adaptation hypothesis.
3. Model parameter estimates differed among growth groups in that the lower critical temperature for growth increased from fast to slow growing individuals. In contrast to findings for brown trout (Salmo trutta), the optimum temperature for growth did not decrease with decreasing levels of food consumption.
4. A new and simple model showed that food consumption (expressed in energy terms) peaked at 19.5–19.8 °C, which is similar to the optimal temperature for growth. Feeding began at a temperature 1.5 °C below the lower temperature for growth and ended about 1 °C above the maximum temperature for growth. Model parameter estimates for consumption differed among growth groups in a manner similar to the growth models. Maximum consumption was lower for Atlantic salmon than for brown trout, except at temperatures above 18 °C.
5. By combining the growth and food consumption models, growth efficiency was estimated and reached a maximum at about 14 °C for fast growing individuals, increasing to nearly 17 °C for slow growing ones, although it was lower overall for the latter group. Efficiency also declined with increasing fish size. Growth efficiency was generally higher for Atlantic salmon than for brown trout, particularly at high temperature.     

Linking models of animal behaviour and habitat management: Atlantic salmon parr and river discharge

Understanding preferences of animals is of fundamental importance for modelling habitat quality and quantity. Important theoretical developments, for example using ideal free and ideal despotic distributions (IDD), have enabled biologists to build conceptual frameworks for relating habitat preferences of individual animals to distributions and dynamics of populations. At the same time, managers of natural resources have established predictive empirical models as a basis for understanding habitat quality. For example, the Physical Habitat Simulation model (PHABSIM) has been widely applied for managing river flows. The aim of this study was to conduct experiments, using Atlantic salmon parr, to test whether observed population distributions could be predicted using simple behaviour theory and PHABSIM. We show that predictions from PHABSIM depend crucially on population density, discharge and the interaction between density and discharge at the time when the model is parameterized. These findings can, in part, be explained by consideration and application of the IDD. However, the results of the experiments also suggest that models derived from first behavioural principles may need to be unexpectedly complex and species‐specific if they are to capture the population response to variations in water discharge effectively.