A.P. Gonczi and N.-A. Nilsson
The Fishery Superintendency
Harnosand, Sweden
ABSTRACT
Lake trout have been introduced into Swedish lakes since 1959 with very variable results. The percentage recapture ranges between 0 and 60 percent. Growth rates also vary considerably, depending on the availability of suitable prey, which mainly consists of stunted whitefish, cisco, smelt and Mysis relicta. No harm to the native fish populations has been observed. On the other hand many valuable introductions have been recorded.
RESUME
L'introduction depuis 1959 de truites (Salvelinus namaycush) dans les lacs suédois a donné des résultats très variables, le pourcentage de recapture étant compris entre 0 et 60 pour cent. Le taux de croissance de ces poissons est également très variable; il dépend avant tout de la présence de proies appropriées, lesquelles se composent essentiellement de formes naines de poisson blanc, de cisco, d'éperlan et de Mysis relicta. Ces introductions ne semblent pas avoir été préjudiciables aux populations de poissons indigènes; dans de nombreux cas, elles ont été bénéfiques.
Lake trout (Salvelinus namaycush) have been introduced into some 70 Swedish waters since 1959 (Nilsson and Svardson, 1968) and before that introductions had been made into Switzerland and Finland.
The primary aim of these introductions was to enhance fisheries in lakes subjected to the adverse influences of hydro-electric development, but has since also been seen as an attempt to fill a vacant niche in large lakes with cold-water hypolimnia.
Lake trout were chosen as a possible candidate for these purposes, because:
It is a cold-stenothermal species, adapted to the Arctic-temperate conditions prevailing in the waters to be stocked;
It is known as the most piscivorous of Salvelinus species, which would be desirable in reservoirs where dwarfed coregonids often dominate the fish communities;
It does not depend on streams or shallow waters for spawning;
It is an esteemed food and game fish in North America.
The introductions have in most cases not been successful; recaptures of released fish vary from 0 to 60 percent. This variation is supposed to depend mainly on biotic variables, such as predation on young released fish and competition from native fish species.
Four lakes have been chosen to illustrate in some detail the reactions of lake trout to different habitats and fish communities.
Lake Storsjon: a large (456 km2) oligotrophic lake in the boreal region of the northern Swedish highland, with a complex fish community, was first stocked with S. namaycush in 1962, and since 1964 has been continuously stocked with eggs and fingerlings. 500 to 1 000 fish per year were tagged to secure data on recapture and growth rates. Captures of young wild fish have possibly been results of natural reproduction or of fertilized eggs introduced in boxes at presumed future spawning sites.
The growth rate (Fig. 1) was initially very good, probably because of a super-abundance of food (ten-spined stickleback, stunted whitefish (mainly Coregonus wartmanni) and the introduced Mysis relicta). Later growth rate declined, possibly because of overgrazing of prey species (Fig. 2). After the introduction of smelt (Osmerus eperlanus), this species became the most important food of lake trout as well as of Arctic charr (Fig. 2), which probably explains the subsequent increase in growth rate (Fig. 1). Figs. 1 and 2 thus suggest three phases in interaction between the three exotic species and the indigenous fish community:
During the first phase lake trout mainly fed on suitable native species, whitefish (Coregonus wartmanni) and ten-spined stickleback (Pungitius pungitius), with a consequent high rate of growth.
After having grazed down the stickleback population, which until 1970 was very dense, the introduced Mysis relicta became an important food item for lake trout, as well as for Arctic charr. The growth rate of lake trout, however, decreased, possibly because of over-exploitation of the stunted Coregonus wartmanni.
After 1977 the introduced smelt (Osmerus eperlanus) became more and more important as food for both lake trout and Arctic charr. The growth rate of lake trout consequently increased.
The recaptures of lake trout have been good, ranging between 11 and 67 percent with yields of between 60 and 460 kg per 1 000 released fingerlings.
Lake Kallsjon: a large (155 km2) oligotrophic lake in the sub-Arctic region upstream of Lake Storsjon, with a simple fish community, was first stocked in 1964, and since then has been continuously stocked with fingerlings.
The growth rate of tagged fish has been very poor as compared to Lake Storsjon (Fig. 1) probably because the one species of whitefish found in Lake Kallsjon has not been available in sizes suitable for food. The predominant food has been the introduced Mysis relicta even for relatively big specimens of lake trout.
Natural reproduction has not been observed for certain, although some wild fish with deviant colour and behaviour have been caught (biggest fish recorded - 7kg). In Lake Kallsjon recaptures of tagged fish were calculated to range between 50 and 60 percent. Most of the recaptured fish, however, were very small in size and in poor condition.
Lake Stora Tjultrasket: an oligotrophic lake in the sub-Arctic region of the north Swedish highland, with a very simple fish community, consisting of brown trout, Arctic charr and burbot) was stocked with lake trout in 1966 (6 000 one-year old fingerlings) and in 1972 (2 000 two-year old fingerlings). The growth rate characteristically has been poor for the first few years after introduction. However, since then has increased to around 2 kg after five years (biggest fish on record - 5 kg). The main food has appeared to be burbot and Arctic charr (up to 20 cm in length). Interestingly enough the growth rate of the native Arctic charr has also increased, very probably because of the predation of lake trout on young charr (Fig. 3). Natural reproduction has probably occurred, judging from the capture of young wild fish (Filipsson, pers.comm.).
Lake Ivosjon: a eutrophic lake in south Sweden, with very complicated fish community, was stocked yearly since April 1972 with 1 000 fingerlings. The introduced fish have grown well (Fig. 4), feeding mainly on small smelt. Recaptures have been good, up to around 300 kg per 1 000 released fingerlings (Almer, 1978).
It may be assumed that introductions of lake trout into Sweden have not been harmful to the native fish community. On the contrary, the species has added to the general value of the fishery in several lakes.
Almer, G., 1978 Fish in the offshore region of Lake Ivosjon. Inf.Inst.Freshwat.Res., Drottningholm, 1978:49 p.
Nilsson, N.-A. and G. Svardson, 1968 Some results of the introduction of lake trout (Salvelinus alpinus Walbaum) into Swedish lakes. Rep.Inst.Freshwat.Res., Drottningholm, (48):5–16
Fig. 1 The growth of tagged lake trout in the Lakes Storsjön and Kallsjön
Fig. 2 The food of lake trout in Lake Storsjön
Fig. 3 The growth rate of Arctic charr in Lake Tjulträsket, before and after lake trout predation (from Filipsson, 1982)
Fig. 4 The growth of lake trout in Lake Ivösjön as compared to some other lakes (after Almer, 1978)
