Epsilon Open Archive

Sweden’s northernmost char lake of significance, Rostujávri (Råstojaure), has for more than half a century, been regarded as one of the national prime sport fishery sites for large sized Arctic char (Salvelinus alpinus) and grayling (Thymallus thymallus). The lake is 3.531 hectare and very shallow, draining both west into the Atlantic and east into the Bal-tic Sea.

Already, in the mid 1960s, a complete shift from dominance of Arctic char to domi-nance of grayling was observed in the commercial fishery and during repeated test fishing efforts. In August 2003 standardised gillnetting with multi-mesh sized experimental gillnets using the Nordic protocol, indicated a fish biomass consisting of 80.2 % grayling, 12.3 % brown trout (Salmo trutta), 4.9 % burbot (Lota lota) and only 2.6 % Arctic char measured in terms of weight.

Gillnet fisheries and angling focusing on Arctic char seems to have contributed to a long term shift in the species balance, with the age structure of the Arctic char also indicating a climate induced short term shift in its habitat use. Comprehensive analyses of demography, growth, diet and parasites of the four fish species revealed concordant evidence of a fish community dominated ecologically and numerically of fast growing benthic grayling, boosted by an increase in summer air temperature for northern Sweden, and also repeated rich year class production during years with record low precipitation and high summer temperatures. Whereas large-sized burbot, grayling and trout had shifted from benthos to piscivory, the remains of a population of young and small-sized Arctic char demonstrated a compressed dietary niche of zoo-plankton in the narrow pelagic habitat.

As no sexually mature char were caught in the lake in mid August, a temporary down stream migratory escape from increasing water temperature, inter-specific competition and predation is suggested. Samples from the Sámi ice fishery in May 2004 revealed the missing size- and age groups of Arctic char in August 2003 to have returned to the lake the next spring. Their otoliths, which were used for age analyses, also demonstrated yearly check-marks from reduced growth from the third summer, indicating corresponding habitat shifts into environments with lower temperature. Thermal enhancement has most likely favoured the grayling population intensifying the inter-specific interactions between grayling and Arctic char, causing both long-term and short-term shortcomings for the latter population. The report suggests further sampling during other seasons.