Martin-A. Svenning

Takvatn through 20 years: long-term effects of an experimental mass removal of Arctic charr, Salvelinus alpinus, from a subarctic lake

Between 1984 and 1989, the experimental removal of 31 tons (666 000 fish) of stunted Arctic charr, Salvelinus alpinus, from Takvatn in northern Norway, had strong effects on the populations of Arctic charr, brown trout, Salmo trutta, and three-spined sticklebacks, Gasterosteus aculeatus. The littoral catch per unit effort (CPUE) of charr had decreased by 90% in 1990 and then increased to about 50% of the initial level by 1994 while the pelagic CPUE had decreased to zero. Growth in both charr and trout greatly improved when the charr density had decreased, and large fish of both species appeared in the catches. These large fish became predators on small charr in the littoral zone. The incidence of trout increased from below 1% to 15% from 1988 to 1999 after a brief peak at 30% in 1992 and 1993. The charr population attained a bimodal size distribution and did not return to the stunted state during the 10 years following the intensive fishing period. The mass removal experiment showed that it is possible to change the structure of a charr population by intensive fishing. Predation on small charr from cannibals and large trout was probably essential for maintaining the new population structure. An increase in the growth of young charr from 1995 to 1997 was related to a high consumption of Daphnia and Eurycercus. Rapid changes in the growth of charr followed the density fluctuations in sticklebacks, which show large annual variations in this system; the rapid changes in charr growth were probably caused by variations in the competition intensity for cladoceran prey between young charr and sticklebacks. Twenty years of data has provided important information, but even more time is needed to follow the long-term trends in northern lakes such as Takvatn.

Development of a species-specific fractionation equation for Arctic charr (Salvelinus alpinus (L.)): an experimental approach

A species-specific fractionation equation for Arctic charr (Salvelinus alpinus (L.)) was developed experimentally for use in ecological studies of temperature-driven phenologies for the species. Juvenile Arctic charr were reared in controlled conditions at different temperatures (2–14°C), with three replicates of each temperature. Otoliths from the fish and water samples from the chambers were analysed for oxygen isotope composition and used to estimate temperature-dependent fractionation equations relating the isotopic ratio to rearing temperature. A linear and a second order polynomial relationship were estimated and validated using comparable Arctic charr data from another study. Temperatures predicted using the polynomial equation were not significantly different from recorded experimental temperatures, whereas with the linear equation there were significant differences between the predicted and measured temperatures. The polynomial equation also showed the least bias as measured by mean predictive error. Statistical comparisons of the polynomial fractionation equation to a similarly estimated equation for brook charr (Salvelinus fontinalis (Mitchill)) indicated significant differences. Results imply the need for species-specific fractionation equations, even for closely related fish. Results further suggest the polynomial form of the fractionation equation will facilitate more accurate characterisation of water temperatures suitable for use in ecological studies of temperature-driven phenologies of Arctic charr.

Stable isotope-based determinations of the average temperatures experienced by young-of-the-year Svalbard Arctic charr (Salvelinus alpinus (L.))

Mean ambient water temperatures experienced by individual young-of-the-year (YOY) Arctic charr, Salvelinus alpinus (L.), from a Svalbard lake were estimated using measurements of oxygen stable isotopes (δ18O) derived from fish otoliths. Otolith-derived water temperatures differed significantly from temperatures recorded at the outlet river of the Dieset watercourse but were consistent with temperatures previously recorded in shallow littoral areas of other Svalbard lakes where YOY charr are commonly found. This indicates that fixed-point monitoring does not necessarily represent the temperatures and thermal habitats used by individual fish. Otolith-derived water temperatures were also positively related to fish length-at-capture and otolith size, although much of the variation remained unexplained. Differences among individuals could be related to variability in food availability and food intake as well as variation in the initial fish size at hatching. Implications for subsequent investigations into how YOY charr respond and adapt to future climate change are discussed.

First records of three-spined stickleback Gasterosteus aculeatus in Svalbard freshwaters: An effect of climate change?

Several fish species, including three-spined stickleback, Gasterosteus aculeatus, have shown a more northerly distribution in the Norwegian Sea into the eastern part of the Arctic Ocean during the last decades. Three-spined stickleback has however never been recorded in freshwaters on Svalbard, whereas Arctic charr, Salvelinus alpinus, has been the only reproducing fish with anadromous, resident, and landlocked populations. On this background, we sampled fish by gillnetting, electrofishing, and trapping in two watercourses on the island of Spitsbergen, Svalbard, during the summers 2001 and 2006. During these surveys, three-spined sticklebacks were captured in both localities. To our knowledge, this represents the first records of the species in Svalbard freshwaters. The presence of a few adult sticklebacks does not provide evidence that breeding populations are established. However, increased temperatures associated with climate change in recent years in both Arctic marine and freshwater environments may have facilitated the appearance of the species in these localities and thus may be a first indication of the potential establishment of this species in Svalbard freshwaters.

Comprehensive microsatellite baseline for genetic stock identification of Atlantic salmon (Salmo salar L.) in northernmost Europe

Mikhail Ozerov, Juha-Pekka V€ah€a*, Vidar Wennevik, Eero Niemel€a, Martin-A. Svenning, Sergey Prusov, Rogelio Diaz Fernandez, Laila Unneland, Anti Vasem€agi, Morten Falkegård, Tiia Kalske, and Bente Christiansen Kevo Subarctic Research Institute, University of Turku, Turku FI-20014, Finland Department of Biology, University of Turku, Turku FI-20014, Finland Association for Water and Environment of Western Uusimaa, P.O. Box 51, Lohja FI-08101, Finland Institute of Marine Research (IMR), P.O. Box 1870 Nordnes, Bergen N-5817, Norway River Teno Fisheries Research Station, Natural Resources Institute Finland (Luke), Utsjoki FI-99980, Finland Arctic Ecology Department, Fram Centre, Norwegian Institute for Nature Research (NINA), P.O. Box 6600 Langnes, Tromsø N-9296, Norway The Knipovitch Polar Research Institute of Marine Fisheries and Oceanography (PINRO), Murmansk 183038, Russia Department of Aquaculture, Estonian University of Life Sciences, Tartu 51014, Estonia County Governor of Finnmark (FMFI), Vadsø N-9815, Norway *Corresponding author: tel: þ358 19 5682 940; fax: þ358 19 325 697; e-mail: juha-pekka.vaha@luvy.fi These authors contributed equally to this work. Present address: Department of Biology, University of Turku, FI-20014 Turku, Finland; Juha-Pekka V€ah€a, Association for Water and Environment of Western Uusimaa, POB 51, FI-08101 Lohja, Finland

Change in relative abundance of Atlantic salmon and Arctic charr in Veidnes River, Northern Norway: a possible effect of climate change?

Temperature changes affect salmonids across a hierarchy of spatial and temporal scales, so that shifts in thermal river regimes may influence interspecific interactions in sympatric species. In Northern Norway, anadromous populations of Arctic charr (Salvelinus alpinus) and Atlantic salmon (Salmo salar) coexist, but in recent years landings of Arctic charr have decreased, while those of Atlantic salmon have been stable or even increased. Here we studied relative abundance, habitat use and growth rate of sympatric stream-living juveniles of both species in Veidnes River, where they are the only fish species present. In 1998/2000, juvenile Arctic charr dominated, especially in the upper and colder part of the river. In 2010, however, Atlantic salmon juveniles were now prominent in all habitat types, whereas nearly all Arctic charr were captured in slow-flowing water near the river bank. Summer air temperature has increased in the region during the last decade. Positive correlations between summer temperatures and back-calculated growth rates were documented in both species, but the growth response was significantly higher in Atlantic salmon. Accordingly, we suggest that juvenile Atlantic salmon may benefit from a warmer climate in northernmost Norway, at the expense of the more cold-water-adapted Arctic charr.

Recorded bimodal length frequency distributions of Arctic charr, Salvelinus alpinus (L.), and brown trout, Salmo trutta L.: an effect of both population structure and sampling bias

The length distributions of gillnet catches obtained by fishing on unexploited or slightly exploited Arctic charr, Salvelinus alpinus (L.), and brown trout, Salmo trutta L., populations are frequently bimodal. To test the hypothesis that these length distributions may not only be due to sampling bias, but also an effect of population structure, fleets of gillnets were used in sampling from allopatric populations of Arctic charr and brown trout, where the population numbers had been estimated by mark–recapture experiments. In both populations, the length distributions of the populations and the corresponding gillnet catches appeared as bimodal. The second mode of the length frequency distributions consisted of individuals from many age classes accumulated within a restricted length range, which is in turn a consequence of stagnation or low annual growth in old fish. Our results show that the gillnet catches by the use of fleets of gillnets described the overall bimodal length frequency distributions of the populations, although the fraction of larger fish may be significantly overestimated due to a strong increase in gillnet catchability by increasing fish size.