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Skurdal J.,Oslo University College | Hessen D.O.,University of Oslo | Garnas E.,Buskerud County Environmental Administration | Vollestad L.A.,University of Oslo
Freshwater Biology | Year: 2011

1.The striking variation in fecundity of the noble crayfish (Astacus astacus) was assessed from 1981 to 2009 in the lake Steinsfjorden, Norway.2.Egg size showed no distinct variation with crayfish size or between years and seems to be a fairly fixed property. Ovarian egg numbers increased linearly with body size, from a mean of 70 eggs at maturity to nearly 250 eggs in the largest individuals. There were strong and significant interannual differences in the size-specific fecundity, and variation in size and among years explained 60% of the observed variance in fecundity.3.The proportion of mature females in the population ranged from 0.068 to 0.817, with extreme year-to-year fluctuations. The proportion of mature females increased significantly over the years, in parallel with increased temperature, but we were unable to link maturity with ambient temperature either in the same year or in the previous year.4.While several indices point to increased crayfish productivity with higher temperature in this population, there is also a complex and apparently chaotic interplay between life-cycle attributes, density-dependent parameters and ambient conditions that lead to high variability in annual recruitment. This probably reflects the slow and delayed life-cycle responses to a changing environment. © 2010 Blackwell Publishing Ltd.

Skurdal J.,Norwegian Institute for Nature Research | Garnas E.,Buskerud County Environmental Administration | Johnsen S.I.,Norwegian Institute for Nature Research
Freshwater Crayfish | Year: 2013

During 1979-1981, a simple and non-expensive monitoring programme for crayfish was developed in Lake Steinsfjorden. From the programme, which includes data on total trapping effort, catch statistics and population composition from test fishing before and after the fishing season, we have estimated yield and monitored population development since 1979. Based on data from the monitoring programme, an adaptive management scheme was introduced for the crayfish population. The results from the monitoring programme have been the basis for authorities when adopting changes in the management scheme and revealing the effects. High exploitation, catching non-legal sized crayfish, low fraction of mature females and the spread of Canadian pondweed have all challenged crayfish management regulations in Lake Steinsfjorden. Management regulations have changed several times during the study period. The major changes included reducing the trapping season from 147 to 7 days, a measure that is easy to enforce, and increasing the mesh size of traps from 17.5 mm to 23 mm. In spite of these measures, the harvest declined by 75%, mainly due to the Canadian pondweed invasion, as this plant now occupies a large part of the shallow areas in the littoral zone. Copyright © 2013 International Association of Astacology.

Johnsen S.I.,Norwegian Institute for Nature Research | Skurdal J.,Norwegian Institute for Nature Research | Taugbol T.,GLB Water Management Association | Garnas E.,Buskerud County Environmental Administration
Knowledge and Management of Aquatic Ecosystems | Year: 2014

Minimum size is a common management regulation in noble crayfish (Astacus astacus) fisheries. The most used catching method baited traps, catch crayfish down to 50 mm total length (TL). In Norway the minimum size is 95 mm TL and this regulation allow female crayfish to spawn a least once before being harvested. To reduce the fraction of non-legal sized crayfish in trap catches, mesh-size were increased from 17.5 to 21 mm (knot to knot) in Lake Steinsfjorden (1983) and nationwide (2001). We compared catch per unit effort data in traps with different mesh sizes from the two most important noble crayfish fisheries in Norway to evaluate the effects of the regulation. Traps with mesh size 21 mm catch significantly fewer crayfish compared to traps with 17.5 mm and 12 mm. As intended, traps with 21 mm caught significantly less non-legal sized crayfish compared to 17.5 mm. Increased mesh size thus reduce the catch-induced mortality of non-legal sized crayfish, resulting in a larger spawning population. Mesh size regulations is an important management tool, especially for recruitment limited crayfish population, and is rather easy to enforce compared to minimum size. © 2014 ONEMA.

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