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Abstract Catches of anadromous whitefish in the Gulf of Bothnia have declined since theearly 1990s. It is generally assumed that the cause is overfishing. The professional fishermeninterviewed in the present study were united in the opinion that it is necessary to set limits onthe fishing of whitefish, but had differing views about the means of achieving this. Thesedifferent arguments involved separate whitefish stocks, various types of gear and differentmotives for fishing. Most of the professional fishermen accepted a minimum mesh size for gillnets. Other means proposed included setting limits on non-professional fishing and sales ofcatches. There were conflicting attitudes about seasonal restrictions because their experienceswith fishing restrictions for salmon had been negative. On the whole, decision-making requiresmore regional information on whitefish stocks and fishing, but also on the social and economicflexibility of fishermen. 相似文献
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Stocking and fishing effort are two important potentially conflicting factors in fish stock management that require appropriate assessment to ensure a sustainable fishery. In the River Tornionjoki, which discharges into the northern Baltic Sea, a summer‐ascending whitefish, Coregonus lavaretus L., stock has long been a target by traditional dipnet fishing. Enhancement stocking of young whitefish started in the River Tornionjoki in the 1970s after a collapse in catches, with millions of age‐0 whitefish stocked annually in the river, but after about three decades, the stocking rates were considerably reduced. As a result, dipnet catches of whitefish in the Kukkolankoski Rapids rose simultaneously, peaking in the 1980s and 1990s, and then subsequently decreased. There was a significant positive correlation between stocking intensity and catch, both in weight and in numbers, revealing a strong relationship between whitefish releases and dipnet catch. Changes in gillnet fishing effort in the sea affected dipnet catches in weight as well as in mean size of captured whitefish. When the combined effect of stocking and gillnet effort was evaluated, only stocking significantly affected dipnet catches. 相似文献
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Whole otolith elemental analysis reveals feeding migration patterns causing growth rate differences in anadromous whitefish from the Baltic Sea 
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下载免费PDF全文 Henry Hägerstrand Yvette Heimbrand Mikael von Numers Jan‐Olof Lill Erkki Jokikokko Alpo Huhmarniemi 《Ecology of Freshwater Fish》2017,26(3):456-461
Impact of feeding migration pattern on growth rate of whitefish (Coregonus lavaretus L.) from River Kemijoki was investigated. River Kemijoki is located in the northernmost Gulf of Bothnia (Baltic Sea). Whitefish ascending the river to spawn show major growth rate variations. To study whether these variations are related to different feeding areas (migratory connectivity), whole otoliths from fast‐growing (FASTG) whitefish (n = 9, age 4 years, average weight 790 g) and slowly growing (SLOWLYG) whitefish (n = 9, age 8 years, average weight 600 g) were analysed for elemental (Ba, Sr, Mg, Zn, Mn, Ca) concentrations by inductively coupled plasma‐optical emission spectrometry. Results showed that the FASTG whitefish had much lower otolith Ba concentration (~45%, p ≤ .01) than SLOWLYG whitefish, revealing that FASTG whitefish have spent more time in water of higher salinity/lower latitude, that is at feeding grounds in the southern Gulf of Bothnia. Otoliths of the FASTG whitefish had also higher Mg (~25%, p ≤ .001) and Zn (~20%, p ≤ .05) concentrations, but lower Ca (~5%, p ≤ .01) concentrations, than the otoliths from SLOWLYG whitefish. These results further reveal that FASTG and SLOWLYG whitefish have been spatially separated. In conclusion, bulk otolith elemental analysis revealed spatial separation and migration differences in river‐spawning whitefish subpopulations in the Gulf of Bothnia, which apparently lead to differences in fish growth rate. Our results increase the understanding of river‐spawning whitefish biology and its population structure and dynamics and may contribute to a sustainable management of river‐spawning whitefish stocks. 相似文献
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We studied the effects of seed origin and sowing time on height development and timing of height growth cessation of first-year silver birch (Betula pendula Roth) seedlings in a greenhouse experiment. Seeds of seven origins ranging in latitudes from 58 degrees to 67 degrees N were sown at 1-2-week intervals eight times from May 21 to July 30, 2001. The day/night temperature in the greenhouse was set at 20/10 degrees C, but lighting was natural and day length varied accordingly. Seedling height was measured twice a week. The interaction term between seed origin and sowing date was significant, but the pattern of height development and timing of growth cessation depended systematically on latitude of seed origin and sowing date. As seed origin became increasingly northern, growth cessation began earlier and resulted in shorter growth periods. Later sowing dates delayed growth cessation but also shortened the growth period. Final seedling height systematically decreased with increasingly northern origins and with later sowings. Linear regression analysis predicted timing of growth cessation, night length at growth cessation, length of growth period and final seedling height with high precision when the latitude of seed origin and sowing time were predictor variables. The timing of height growth cessation was determined by the seed origin, night length and developmental stage of the seedlings. 相似文献
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Alpo Ranta-Maunus 《Wood Science and Technology》1975,9(3):189-205
Wood is regarded as a viscoelastic material. Creep deformations that arise from variations in the moisture content are described by a theory of hydroviscoelasticity developed by the author. Two different types of behaviour have been apparent: one, arising from a continuously increasing strain with periodic variation in the moisture content, and another with no cumulative effect. The theory has been applied to previously published experimental results concerned with beech, pine, hoop pine, klinki pine, along with birch and spruce plywood. Birch and spruce plywood have been used for experiments concerned with periodically-cycling bending moment and moisture content. The results obtained have been compared with the theory presented. Glue-laminated beams have been subjected to long-term outdoor loading extending for five years. A brief discussion is given of the results obtained. 相似文献
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The main problem in machine strength grading of wood lies in the grade determining parameters and machine grading parameters, which are all stochastic, and the correlation between them is of critical importance. Therefore it is difficult to reliably determine the grade of a particular piece. In order to understand the effectiveness and accuracy of machine grading a large number of experiments should be performed in which the non-destructive machine grading parameters as well as destructive grade determining parameters should be determined. So far we have had access to only a limited number of such experiments. The basic idea of this paper is to use experimental data only for the determination of statistical parameters—mean vector and variance—covariance matrix—and afterwards use these statistical parameters in numerical simulation of machine grading. This numerical procedure gave some valuable data on the average values and the variability of grading results which now make it easier to decide what sample size should be used in determining machine settings. 相似文献
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