All sampling methods give selective or biased estimates of fish species abundance, distribution and size structure. This creates problems, e.g. in regard to the Water Framework Directive of the European Union, which demands evaluation of the quality and status of fish stocks in lakes. We compared fish sampling by means of Nordic multimesh gillnets, seining, trawling and hydroacoustics in two Finnish lakes in summer 2007 and 2008. Sampling methods were used 'as such', i.e. no special design was implemented for method comparison. In the shallow eutrophicated lake the species' composition of gillnet sampling and seining were very different. The biomass-% of percids dropped from gillnet (61%) to seining (9%) and that of cyprinids grew from 39 to 90%, respectively. In the deep pelagic area of the oligotrophic lake, vendace and smelt predominated in trawl catches. The number of fish caught by gillnetting in that area was too small to make any conclusions about the species composition. In the eutrophicated lake, the combined length distribution for all fish species differed significantly between gillnetting and seining. In the oligotrophic lake, the gillnet catches were too small for any comparison of fish size. The difference in the length distribution of fish between trawl and echosounding was significant in most analysed depth layers. In upper depth layers acoustics sampled larger fish than trawling, and in deeper layers smaller fish. Using a combined acoustic-trawl method, the pelagic fish biomass was estimated to be approx. 17 kg ha -1 in the deep and oligotrophic lake. We conclude that in large and deep-water areas, the use of active gear is enough in fish sampling to evaluate the quality and status of fish stocks. Gillnetting together with seining is an appropriate method to work out the quality and status of fish stocks in shallow and littoral areas of large lakes. Variation in the catch selectivity of fish sampling gear requires a discrete ecological classification for each type of gear.
Anadromous whitefish [Coregonus Iuvuretus (L.)] yolk-sac larvae were marked with a radioactive strontium isotope, "Sr. Before marking the method was tested in accumulation elimination tests using various strontium concentrations. The first-phase effective half-life of "Sr in yolk-sac larvae was4days. The mean 'ISr radioactivity was 81 Bq per larva at theday of release and the half-life of the slow component was 51 days, so the strontium label was easily detected up to 60 days after marking. The larvae were released into the River Simojoki about 12 km upstream from the river mouth and recaptured from early May to the end of July. Marked larvae were found at the river mouth 12-I3 days after release, while the last marked fish in the river were found at 16 days. It IS likely that all leave the river within 2 to 3 weeks post-hatching. Two months after marking some larvae had attained a distance of 10 km from the river mouth.
The anadromous whitefish, Coregonus lavaretus (L.), is the most numerous fish species stocked in the Gulf of Bothnia, Baltic Sea. One‐summer‐old‐whitefish fingerlings are mostly 8–10 cm long when released annually in September–October, whereas the wild whitefish are 10–12 cm at that time. About 6 million, one‐summer‐old, spray‐marked, whitefish were released in the northern and central parts of the Gulf in 1995–1998. To study the effect of the stocking length on the survival of the marked fish, the length of the recaptured whitefish as 1‐year‐olds was back‐calculated. Altogether 1106 whitefish recaptured in the Gulf of Bothnia were analysed. The back‐calculated length was slightly greater than the stocking length but not as large as the length of the wild fish. In the central part of the Gulf of Bothnia, where the mean stocking length was more than 10 cm, the back‐calculated length was 10.5–11.1 cm. In the northern part of the Gulf the mean stocking length varied between 8.8 and 10.0 cm annually, and the corresponding back‐calculated mean lengths were 9.3–9.7 cm. It also seemed that bigger fingerlings started their feeding migration earlier or they migrated faster than the smaller ones to the southern parts of the Gulf of Bothnia.
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