I. Berglund

The effect of time, size and sex on recapture rates and yield after river releases of Salmo salar smolts

Abstract In each of the years 1988 and 1989, about 15 000 individually tagged Baltic salmon juveniles were released into the Umealven (Ume river) in northern Sweden. Two release groups, sexually immature fish and previously mature males, of varying sizes were released at five different times during the natural period for seaward migration for Baltic salmon smolts in this area. Analyses of 1069 adult recoveries caught in the commercial sea fishery from 29 863 releases in 1988 and 1989 were undertaken. An average 7–8 times higher total recapture rate for releases of immature fish than releases of previously mature males was found, suggesting a post-release mortality related to the maturity type of fish after river release. In 1988, the highest recapture rates were obtained for immature fish (18.7%) and previously mature males (3.0%) released in late May. Larger fish had a higher recapture rate at all release dates. Fish released in 1989 showed about the same overall ratio in total recapture rate between immature fish and early mature males although the total number of recoveries was lower in 1989. The two maturity types of released fish had a similar growth pattern in the sea. Average weight at catch for releases of immature smolts and previously mature males was 4.0 kg. There was no relationship between early sexual maturation and grilsing (fish returning after 1 sea-winter). At optimal time of release (27 May), the yields (kg/1000 smolts released) from releases of immature smolts and previously mature males were 748 kg and 120 kg, respectively. This difference in yield between releases of the two maturity types of fish is suggested to be due to a low smolting rate among the previously mature males. If the ultimate goal of river stockings of Baltic salmon is a high survival to adulthood and a maximal yield, the proportion of early mature males in the stocking material has to be controlled. The annual loss in production value by releasing a large proportion of non-migratory previously mature males in Swedish rivers is suggested to be 400 tonnes having a present catch value of c. 16 million Swedish krona (at a salmon price of 40 Kr/kg). Alternative smolt management strategies are discussed.

Thermal growth performance of juvenile brown trout Salmo trutta: no support for thermal adaptation hypotheses

Using thermal growth data from eight populations of anadromous and lake-feeding brown trout Salmo trutta, hypotheses of adaptation to local optima and countergradient variation in growth were tested. The adaptation to local optima hypothesis suggests that natural selection can shift optimal performance temperatures to match the prevailing temperature in a new or changed thermal niche. In contradiction, the countergradient variation hypothesis suggests that populations from hostile environments perform better than conspecifics from benign environments at all temperatures. In this study, growth capacity varied between populations but there was no significant correlation between any of the estimated thermal performance parameters (e.g. lower and upper thermal growth limits, optimal temperature for growth and maximum growth capacity) and natural climatic conditions among populations. Hence, S. trutta growth response to temperature lends no support for either of the two suggested thermal adaptation hypotheses. Instead, growth capacity among populations tended to correlate positively with female size at maturity.

Effects of castration and androgen treatment on aromatase activity in the brain of mature male Atlantic salmon (Salmo salar L.) parr.

Brain homogenates from male Atlantic salmon parr aromatized tritiated androstenedione to estrogens. The aromatase activity in homogenates of whole brains from castrated male parr was lower than that in homogenates from sham-operated male parr in autumn. This was also found in homogenates of the telencephalon and diencephalon, but not in homogenates of the tectum opticum. Treatment of castrated males with testosterone (T) and 11-ketoandrostenedione (OA) increased the aromatase activity in whole brains. T, but not OA, also significantly increased aromatase activity in all brain parts. In spring, half a year after the peak breeding period, mature parr males had a lower aromatase activity in telencephalon, diencephalon and tectum than that found in castrated males in autumn, whereas immature males displayed an even lower level of activity.

Downstream migration of immature salmon (Salmo salar) smolts blocked by implantation of the androgen 11-ketoandrostenedione

Abstract The effect of androgen treatment on individual downstream migratory activity of 2-year-old Baltic salmon ( Salmo salar ) smolts was studied in a circular stream channel during the natural smolt migration period. Fish were tagged with passive integrated transponder (PIT) tags and implanted with silastic capsules containing the androgen 11-ketoandrostenedione or empty capsules. 11-ketoandrostenedione gives increased blood plasma levels of 11-ketotestosterone through peripheral conversion. The migratory activity of individual fish was monitored automatically by means of a PIT-tag system. In androgen-treated fish, the number of downstream passages per day during peak migration was reduced to about 10% of that of control fish. It is concluded that androgens seem to prevent downstream migration, the development of hypo-osmoregulatory ability, silvering, and the decrease in condition factor associated with smolting. Rather, androgens seem to promote retention of freshwater adaptations and increased growth in weight during spring.

Effects of aromatase inhibitors on sexual maturation in Atlantic salmon, Salmo salar, male parr

Atlantic salmon, Salmo salar, male parr were implanted with Silastic capsules filled with different aromatase inhibitors: 1,4,6-androstatriene-3,17-dione (ATD), 4-hydroxy-4-androstene-3,17-dione (4OH), and the non-steroidal CGS16949 A, 4-benzonitrile monohydrochloride (CGS). Aromatization in brain homogenates were lower in salmon implanted with CGS and ATD than in controls. This was not the case for 4OH, but administration of 4OH to brain homogenates reduced the aromatase activity. All three aromatase inhibitors had effected gonadal weights in fish sampled in the summer, but the effects were markedly different among inhibitors. Plasma levels of the androgen 11-ketotestosterone (11KT) and the progestin 17α, 20β-dihydroxy-4-pregnen-3-one (17,20P) were measured by means of radioimmunoassay. CGS and ATD, but not 4OH, significantly decreased the plasma 17,20P levels in the autumn. Plasma levels of 11 KT were not influenced by ATD or CGS treatment, but 4OH had a lowering effect in one autumn sampling. ATD and 4OH (CGS not tested) increased the proportion of maturing males.These findings suggest that aromatization is of physiological importance in different mechanisms controlling reproduction in salmon.

Growth hormone response to seawater challenge in Atlantic salmon, Salmo salar, during parr-smolt transformation

Abstract The relation between plasma growth hormone (GH) levels and hypo-osmoregulatory ability during parr-smolt transformation was studied in 2-year-old immature parr and previously mature male parr of two different Baltic stocks of Atlantic salmon ( Salmo salar ). At several times between early May and the end of June fish were transferred to seawater (25‰ salinity) and sampled after 24 h. Among freshwater controls plasma GH levels increased only slightly in one stock during the smoltification period, whereas GH levels were significantly elevated in mid-June in the other stock. Seawater adaptability reached an optimum by the end of May to early June in one stock and 2 weeks later in the other one. Plasma GH levels in fish exposed to seawater were similar to those of freshwater fish until mid-May, but increased to significantly higher levels at the time of peak seawater adaptability. Changes in plasma sodium levels during seawater exposure were negatively correlated to plasma GH levels. At the time of peak seawater adaptability two groups were detectable among previously mature males: one group had plasma sodium levels comparable to those of immature fish, while the other had significantly higher sodium levels. In one stock plasma GH showed a similar bimodal distribution, where the group with high GH levels corresponded to low plasma sodium levels and the group with low GH levels had significantly higher sodium levels. The results indicate that some Baltic stocks of Atlantic salmon develop an ability to respond to seawater exposure with a rapid increase in GH levels during the parr-smolt transformation. It is suggested that increasing environmental salinity triggers the final development of hypo-osmoregulatory ability.