A positive relationship between occupancy and average local abundance of species is found in a variety of taxa, yet the mechanisms driving this association between abundance and occupancy are still enigmatic. Here we show that freshwater fishes exhibit a positive abundance-occupancy relationship across 125 Swedish lakes. For a subset of 9 species from 11 lakes, we estimated species-specific diet breadth from stable isotopes, within-lake habitat breadth from catch data for littoral and pelagic nets, adaptive potential from genetic diversity, abiotic niche position, and dispersal capacity. Average local abundance was mainly positively associated with both within-lake habitat and diet breadth, that is, species with larger intraspecific variation in niche space had higher abundances. No measure was a good predictor of occupancy, indicating that occupancy may be more directly related to abundance or abiotic conditions than to niche breadth per se. This study suggests a link between intraspecific niche variation and a positive abundance-occupancy relationship and implies that management of freshwater fish communities, whether to conserve threatened or control invasive species, should initially be aimed at niche processes.
Movements, site fidelity, exploitation rate and total length (TL) increment of pikeperch, Sander lucioperca L., were studied by tagging 3977 pikeperch, of 334 mm mean TL, in Lake M€ alaren in 1994. Fish were recaptured in gillnets, trap nets and to a lesser extent with rod and line. Total recapture rate was 30.4%, with 50% of the recaptures made within 8.4 km of the release site. Movements were restricted during summer, whereas movements from shallow to deeper basins were frequent in autumn. The yearly TL increment was estimated at 99 mm for fish 300 mm long at release and 42 mm for fish of 400 mm at release. The results have direct implications for future management, as the mortality of undersized fish in the fishery; the strong site fidelity and fast growth indicate that fishery closures may be an effective measure to attain a larger stock of fish above the size limit.
K E Y W O R D S :fishery closures, inland fisheries, length increment, migration, pikeperch, recapture.
We analyse trends and dynamics in time-series of recruitment in Smelt and Vendace in the Swedish large Lakes Vänern and Vättern, with particular emphasis on the importance of climate variability. Climate-related predictor variables in our analyses were air temperature, water temperature, ice coverage and duration of ice winters. The variation between years with successful recruitment and years with low recruitment was considerable in both species and especially for Vendace. Ultra-oligotrophic Lake Vättern was characterized by strong pulses in Vendace recruitment, occurring mainly in four year intervals. Smelt recruitment was positively related to recruitment pulses of Vendace. In meso-oligotrophic Lake Vänern, Vendace pursuit was more successful in years with long ice winters that lasted until the middle of March or longer. Smelt recruitment in Lake Vänern was positively correlated to the air temperature in April. There was no obvious synchrony in recruitment patterns between the two lakes. Recruitment of Smelt in Lake Vänern’s two main basins was, however, positively correlated. The results indicated that climate factors may influence the recruitment of important pelagic species, but that the effects may differ between lakes, even though the systems are adjacent and situated at the same latitude.
Large zooplankton has an important role for the functioning of the ecosystem in many lakes. Most are predatory, which makes them both competitors as well as food for many planktivorous fish species. In general, it has been elusive to measure the abundance of this group of zooplankton with traditional sampling methods, particularly in large volumes of water. In this study we tested the potential and adequacy of multi-frequency hydroacoustics as a method to map the spatial patterns in abundance of the large zooplankton in Lake V€ anern, Lake V€ attern and Lake M€ alaren. In addition, we used a plankton net to groundtruth biovolume estimates vertically, and a Tucker trawl for the horizontal distribution. Two frequencies were used to separate the acoustic backscattering from fish, mysids and plankton. The main target species were Leptodora kindtii, Bythotrephes longimanus and Limnocalanus macrurus.The plankton communities were very heterogeneous both within and among the lakes. Lake V€ anern and Lake M€ alaren were dominated by Leptodora, whereas Lake V€ attern was dominated by Limnocalanus. Bottom depth at the sample site was the most important community structuring factor.Our results indicate that the biomass of large predatory zooplankton is comparably high and in most surveyed areas even higher than the biomass of planktivorous fish. Consequently, they are more important for the dynamics of lake food webs than previously assumed. Using multi-frequency hydroacoustics show promising results and with some alterations to the sampling design this would be a valuable addition to the traditional zooplankton monitoring in large lakes.
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