Bror Jonsson

Life History Patterns of Freshwater Resident and Sea-Run Migrant Brown Trout in Norway

Abstract Brown trout Salmo trutta in Vangsvatnet Lake, Norway, include freshwater residents that mature sexually without any sea run and migrants that smoltify and migrate between fresh water and the sea one or more years before maturing. Migrants leave Vangsvatnet Lake during spring and move in coastal waters up to 100 km from the lake outlet before they return to their home river in summer or autumn; older migrants return earlier than younger migrants, sexually mature fish before immatures. Both residents and migrants spawn together in tributaries to Vangsvatnet Lake. Young brown trout (parr) grow in the tributaries and lake; lake dwellers grow faster than stream dwellers. Parr that become migrants at age 2 grow faster than parr that become residents, but parr that become migrants at age 4 and older grow more slowly than those that become residents. Parr that smoltify or mature at a young age grow faster than parr that do so later. Resident males are 2–8 years of age, and 13–47 cm in tip length, residen...

Use of space and food by resident and migrant brown trout, Salmo trutta

SynopsisParr and resident forms of brown trout,Salmo trutta, from Vangsvatnet Lake, Norway live in freshwater, while migrant forms live in coastal waters during summer and in freshwater during winter. About 80% of parr and residents live at depths <5 m, smolts and migrants are more confined to near-surface water. Brown trout partly segregate by size, age and sex from spring through autumn. More than 90% of parr and residents in the tributaries are 0–2 years old, 2–14 cm in length, in the littoral zone 0–3 years old, 7–24 cm in length, and in the pelagic zone 2–6 years old, 18–32 cm in length. The mean body length of equal-aged fish increases from tributaries through littoral to pelagic zones in the lake. Smolts (2–7 years, 14–29 cm) leave the lake from April through August and return during September–October. Migrants (2–11 years, 23–67 cm) leave the take in April–May and return during August–September; sexually mature fish return earlier than immatures. Female brown trout are less stream-dwelling, but more migrant and pelagic than males. Most individuals in the lake population spend the winter in the littoral zone. In the tributaries, diet differs significantly between age-groups of parr; young fish feed on smaller food items than do older fish. In the lake, parr and residents living in the same habitats feed on the same food items. Littoral brown trout feed mainly on insect larvae and chironomid pupae, pelagic brown trout feed on zooplankton and surface insects. Migrants feed little while staying in freshwater, except for matures which feed on young salmonids and surface arthropods during the 2 first months after they had returned from coastal waters. The results are discussed in relation to growth possibilities and mortality risks of the different habitats.

Environmental factors and life histories of isolated river stocks of brown trout (Salmo trutta m. fario) in Søre Osa river system, Norway

SynopsisThe brown trout in the Søre Osa river system are isolated from the lake above by a dam. They are generally of a small size, but their condition coefficient is high. They reach sexual maturity at an early age and have a short life span. The substratum in the Søre Osa mainly consists of large stones covered with a dense mossy vegetation that creates good fish cover. A high biomass of zoobenthos gives an adequate food supply throughout the year. The trout spawn mainly in tributaries, where the water level is unpredictable, some years being too low to permit successful spawning for the migrating individuals in the stock. The life history of these fish depends on the amount of food available in the habitat and on reproduction in unstable environments where density-independent mortality factors appear important. Many young migrate to the main stream during their first year. In the tributaries, there is an excess of resident males and it is hypothesized that a population structure with small resident males and large migratory males is maintained by partial inbreeding and kin selection. In the main stream the trout are larger and they have higher growth rates in the upper part than farther downstream. This is probably so because the food particles that are flushed down from the lake above provide a better supply of zoobenthic food.

Age and Sex Segregation in Habitat Utilization by Brown Trout in a Norwegian Lake

Abstract The resident brown trout Salmo trutta in Lake Myrkdalsvatnet utilized benthic and pelagic areas down to a depth of 40 m. Most 1- and 2-year-old fish were littoral; with increasing age, pelagic and deeper benthic habitats were utilized. It is hypothesized that the significant habitat segregation is caused by differential foraging profitabilities and social interactions among age groups. The proportion of females was significantly higher in pelagic than in benthic areas. Among mature fish, the males, but not the females, congregated in onshore areas in September before entering the tributaries to spawn in October-November. Age groups 1–2 fed mainly on littoral zoobenthos. In August, when zooplankton and surface insects were abundant, age-2 fish also were numerous offshore. Older benthic fish fed largely on chironomid larvae and pupae. Surface insects and zooplankton were important food for pelagic brown trout. Adult chironomids mainly were eaten during early summer, swarming ants during late summer...

Demographic Strategy in a Brown Trout Population in Western Norway

(1) The individuals in the densely populated stock of brown trout, Salmo trutta L., living in Lake Lane and Stranda River, are small, mature at an early age, and have a relatively short life span. Such a demographic strategy appears favourable for populations having a large, but physically unstable spawning and nursery area, intense competition for food, and little vertebrate predation except for human exploitation. (2) For females, the age of sexual maturation appears largely dependent on growth‐rate, size, and survival‐rate of the fish. Males do not appear to have the advantage of a corresponding large body size. They mature when smaller and at more varying ages than the females, although the two sexes have almost identical growth‐rates. (3) Sexually mature fish have higher condition coefficients than immature ones, which is probably an adaptation to ensure gonadal development and enhance the prospect of reproductive success. (4) Mortality appears to be largely dependent on the age when the fish achieve sexual maturation. Most individuals seem to die after their first spawning season. Females show a delay of one year in their sexual maturation relative to males. This adaptation gives protection to the females.

A SURVEY OF TULAREMIA IN WILD MAMMALS FROM FENNOSCANDIA

A total of 2696 wild mammals from Fennoscandia were surveyed for tularemia. Francisella tularensis was not detected in livers/spleens or kidneys from any of the 1992 small rodents captured in Norway and Denmark as judged by one or more of the following methods: cultivation, immunofluorescence microscopy and inoculation in laboratory mice. Serologic examination of 704 wild mammals from Norway, Finland and Sweden demonstrated 11 cases of antibody titers. Agglutinating antibodies were demonstrated in 2 of 565 small rodents (titer 1:160), 2 of 26 wild rabbits (titer 1:80) and in 7 of 60 red deer (titer 1:20-1:40). The titers in red deer were low and could be due to cross reactions. No agglutinating antibodies were demonstrated in the sera of 53 domestic reindeer.