Long-Term Changes in Brown Trout and Rainbow Trout Populations in Great Lake, Tasmania

Abstract

Great Lake, central Tasmania, was first stocked with brown trout Salmo trutta in 1870 and with rainbow trout Oncorhynchus mykiss (formerly Salmo gairdnert) in 1910; both species established self‐supporting populations. Angling statistics revealed a general decline in mean weight of both species from 1892 (brown trout) and 1912 (rainbow trout) to 1950. During 1950–1985, the annual mean size of trout in anglers' catch and spawning migrations stabilized; overall mean weights of brown trout and rainbow trout were, respectively, 1.2 and 1.0 kg in anglers' catch, and 1.4 and 1.5 kg in spawning migrations. Four stages were evident in the fishery: initial dominance of brown trout prior to 1920, rainbow trout dominance in anglers' catch and spawning migrations during 1920–1940, declining numbers of rainbow trout and mean weight in catch from 1940 to 1950, and equal representation of both species in angler catches with a predominance of brown trout in spawning migrations from 1950 to 1985. Various correlations between lake levels, the size of trout in angler catches and spawning migrations, and the species composition of catch and spawning migrations are explained in terms of inundation of new ground, location of feeding zones, and bias due to angling method. Mean weight of fish in the catch was inversely correlated with minimum lake depth during the period 1945 to 1985. Characteristics of the recreational fishery were evaluated from license form census data from 1945–1946 to 1957–1958 and from postal questionnaire returns for the 1985–1986 and 1986–1987 seasons for full‐season license holders. A significant decrease in catch per angler day since the 1945–1957 period has been accompanied by an increase in the total harvest, an increase in days fished per angler, and an increase in the number of anglers fishing the lake. These changes have not been accompanied by significant changes in the mean catch weights or the length‐frequencies in anglers' catches or in the spawning migrations of either trout species. Two management strategies, removal of spawning brown trout and stocking with juvenile rainbow trout, failed to significantly increase the proportion of rainbow trout in the catch or in the spawning migrations. Man‐made environmental changes and lake‐level fluctuations associated with management for hydroelectric power generation are considered the dominant influences on the dynamics of the trout populations in Great Lake.

Comparison of the performance of brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), in a put-and-take fishery

“…O'Grady (1987) has reviewed the performance of brown and rainbow trout in stocked fisheries in Britain, and the results of the present study support his general conclusions. It has not previously been demonstrated, however, that brown trout are consistently less vulnerable than rainbows to fly fishing, other than in circumstances where differential accessibility is inferred (Oliver 1968;Fleming Jones 1974;Coles 1981;Davies & Sloane 1988). The topography and fishing pattern at Whittle Lake provide no opportunity for trout tobephysicallyout of reach of anglers.…”

“…This is not a problem in put-and-take fisheries, where it is expected that the trout population is maintained solely with reared fish. Most stocking, however, is with the rainbow trout, Oncorhynchus mykiss (Walbaum), which has been shown to be well suited to put-and-take fisheries; it is easily and cheaply cultivated, adapts to a wide range of water quality and, importantly, is generally considered to be relatively easier to catch than brown trout (Fleming Jones & Stent 1975;Taylor 1978;Davies & Sloane 1988). This latter attribute and the cause of its variability are of particular interest, because the proportion of a stock taken by a unit of angling effort, the trouts' 'catchability', is a prinicipal element ofthe management strategy of any fishery (Beverton & Holt 1957).…”

Relative lake level fluctuations and their influence on productivity and resilience in tropical lakes and reservoirs

“…Moreau (1980) applied the hydrodynamic floodplain model of Welcomme and Hagborg (1977) to the fishery in lake Alaotra, Madagascar, and predicted a decrease in catches as a result of water extraction for agricultural irrigation. In a study from Australia, Davies and Sloane (1988) considered variations in water levels of a Tasmanian lake the principal factor dictating the dynamics of trout populations. Tweddle and Magasa (1989) demonstrated a significant relationship between the difference of actual and predicted catch and the changes in the lake level 3 years previously in a cichlid multispecies trawl fishery of Lake Malawi.…”

The Influence of Lake Level Fluctuations on Fisheries in Lake Van

“…Furse et al (1979) providing no statistical results noted that species composition and magnitude of catches in the shallow Lake Chilwa in Malawi were related to the level of the lake. Davies and Sloane (1988) documented the impact of variations in lake water level on the population dynamics of two introduced trout species (Salmo trutta and Oncorhynchus mykiss) in the Great Lake in Tasmania. Tweddle and Magasa (1989) examined the catch data of three cichlid species (Oreochromis spp.)…”