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... Die larvale Phase ist die heikelste Periode im Leben der Kleinfelchen, da sie genau zum Zeitpunkt der tiefsten Seetemperatur und der geringsten Zooplanktondichte stattfindet. Damit macht es Sinn, die ersten Lebenswochen der Kleinfelchen genauer zu untersuchen und ihre Nahrungsverhältnisse abzuschätzen (Müller & Bia 1998). Aus diesen Überlegungen resultieren dann die in Kapitel 4 genannten Fragestellungen. ...
... Solange die Mindestmaschenweiten so gewählt werden, dass die Albeli bis nach ihren ersten Ablaichen durch die Netze schlüpfen können, ist die Art gesichert. Wachstum und Jahrgangsstärken werden sich wohl auf einem tiefen Niveau stabilisieren (Müller & Bia 1998 ...
... Concomitant with the decrease of nutrient concentration, growth, yield and virtual year class strength of the slow growing form of lake whitefi sh decreased signifi cantly after about 1987 ( Fig. 1; Müller & Bia 1998). Between 1978 and 1998, total P concentration accounted for 47% of variation in year class strength (linear regression). ...
... Between 1978 and 1998, total P concentration accounted for 47% of variation in year class strength (linear regression). Natural reproduction was estimated to contribute more than 70% to the stock (Müller & Bia 1998). It was therefore hypothesised that larval mortality of lake whitefi sh, originating from stocking (newly hatched larvae) and from natural reproduction, had increased during re-oligotrophication because of food shortage during the early larval phase. ...
... Concomitant with the decrease of nutrient concentration, growth, yield and virtual year class strength of the slow growing form of lake whitefi sh decreased signifi cantly after about 1987 ( Fig. 1; Müller & Bia 1998). Between 1978 and 1998, total P concentration accounted for 47% of variation in year class strength (linear regression). ...
... Between 1978 and 1998, total P concentration accounted for 47% of variation in year class strength (linear regression). Natural reproduction was estimated to contribute more than 70% to the stock (Müller & Bia 1998). It was therefore hypothesised that larval mortality of lake whitefi sh, originating from stocking (newly hatched larvae) and from natural reproduction, had increased during re-oligotrophication because of food shortage during the early larval phase. ...
Population size and fishing yield of coregonids has considerably diminished in many central European lakes where nutrient concentration, in particular phosphorus, has fallen to oligotrophic levels. In re-oligotrophicated Lake Lucerne, apart from slower growth, reduction in year class strength of the slow growing form of lake whitefish was identified as the major cause for decreasing yield. Stocking of take whitefish larvae could not counteract this process. It was, therefore, hypothesised that larval mortality of lake whitefish, both from stocking and natural reproduction, had increased during re-oligotrophication because of food shortage during the early larval phase. Feeding experiments in aquaria with newly hatched lake whitefish larvae from Lake Lucerne, and using various concentrations of Artemia salina and zooplankton, showed a clear relationship between food concentration and mortality over the first 34 days. Elevated mortality of 40% or more resulted from food concentration of 20 zooplankton organisms per litre or fewer. Analysis of zooplankton data from Lake Lucerne indicated that concentration of zooplankton organisms usable by the whitefish larvae in late winter and spring was much lower in the years after 1984 than before. This might explain, at least in part, why year class strength and yield of the slow growing form of lake whitefish has decreased during re-oligotrophication of Lake Lucerne.
... Here, the need of continuous monitoring and a flexible decision-making system in fisheries management is obvious. Adaptive management has proved to be effective for whitefish (Müller & Bia 1998). The concept of adaptive management means a flexible adaptation of the fisheries management policy to changing situations in the face of uncertainty (Hilborn & Walters 1992, Müller & Bia 1998). ...
... Adaptive management has proved to be effective for whitefish (Müller & Bia 1998). The concept of adaptive management means a flexible adaptation of the fisheries management policy to changing situations in the face of uncertainty (Hilborn & Walters 1992, Müller & Bia 1998). Passive or active policies may be applied, the former being more conservative and suitable for systems with small uncertainties. ...
... Most of the studies of coregonid population dynamics claimed that anthropogenic eutrophication was the main cause of the acceleration in whitefish growth (Grimås et al. 1972;Nümann 1972). The decrease in whitefish growth from many European lakes towards the end of the 20th century was generally attributed to reoligotrophication (Kirchhofer 1995;Müller and Mbwenembo Bia 1998). Thus, it was an accepted hypothesis that changes of a lake's trophic state, through changes of primary and secondary production, was the main, if not the only, mechanism that controlled fish growth (Downing et al. 1990). ...
... The earlier studies focussed exclusively on lake nutrient content and considered increasing lake P loads and their bottom-up effects on the pelagic food chain to be the ultimate and only cause of faster fish growth (Downing et al. 1990). Growth deceleration towards the end of the 20th century was attributed exclusively to lake reoligotrophication (Kirchhofer 1995;Eckmann and Rösch 1998;Müller and Mbwenembo Bia 1998). The similar time course of P concentration and whitefish growth, e.g., in ULC (Fig. 4), lends supports to this general concept. ...
Accelerated growth of freshwater fish during anthropogenic eutrophication has been attributed almost exclusively to the increased nutrient content, while density-dependent effects have been largely neglected. We evaluated the relative importance of these factors by studying the growth of 43 consecutive year classes of common whitefish (Coregonus lavaretus) from Upper Lake Constance. This prealpine lake underwent eutrophication from the 1950s to 1970s, followed by reoligotrophication. Because whitefish are harvested with gill nets in a strongly size-selective way, we used back-calculated lengths of average fast-growing fish to compare growth among cohorts. Standing stock biomass was estimated based upon virtual year-class strengths. Multiple linear regression analysis revealed that growth of whitefish during their second year was most strongly related to standing stock biomass followed by PO4-P content during spring turnover and by calendar year, which was incorporated as a third independent variable (adjusted R2 = 0.84). The negative correlation between whitefish growth rate and calendar year is interpreted as evidence of an evolutionary response to the highly size-selective fishery during at least four decades. We conclude that density-dependent effects on whitefish growth are more important than had been realized previously and that the impact of eutro- phication on growthof whitefish needs to be reconsidered.
... Dass grundsätzlich in Phosphor-limitierten Gewässern ein rückläufiger Phosphorgehalt zu sinkenden Fischbiomassen führt, ist seit Jahrzehnten bekannt (Downing et al., 1990;Mills, 1985;Mills & Chalanchuk, 1987). Ebenso ist belegt, dass ein Absenken unter einen Phosphorgehalt von 10 µg L -1 (hier: PO 4 -P) zu besonders dramatischen Ertragseinbrüchen bei Felchen führt (Müller, 1992;Müller & Bia, 1998;Müller et al., 2007). Daher wurde schon vor 10 Jahren in den Gremien, die über die Vorgehensweise in der Bodenseefischerei entscheiden, darüber diskutiert, wie diese Entwicklung in das Fischereimanagement integriert werden könnte (IBKF, Url1). ...
The professional fishermen of Lake Constance are facing challenging times. Due to decreasing nutrient levels their yields are constantly declining. In comparison to former mesotrophic times during 1990 to 2005 with TPmix-levels from 35 to 10 μg L-1, the yield of whitefish, their main target species, has reduced during recent years (2006 until today) with TPmix-levels < 10 μg L-1 by half. In addition, due to the pelagial invasion of the non-native three-spined stickleback in 2013 and its competition for food with whitefish, a further decrease in growth and yield was observed. Since 2013 only 25 % of the amount of whitefish formerly caught during the last mesotrophic phase was generated by local fishermen. This yield is nearly 100 t below the mean value of the last oligotrophic phase during 1910 and 1955 and therefore 100 t below the expected yield for recent days. Thus, the number of active fishermen at Lake Constance are constantly declining and actual fisheries management decisions could not stop this trend. Due to the fact that in 2016 the invasive quagga mussel was introduced in the lake, further decreases in nutrient level and yield are expected. Secondary, the local stock of cormorants is increasing dramatically, leading to increasing predation pressure on fish species targeted by fishermen. All in all, one of the most environmentally sustainable forms of animal food production, namely inland fisheries of Lake Constance, is shortly before extinction. New management decisions with the help of all involved stakeholders at Lake Constance are needed to reverse this trend.
... Accordingly, in zooplanktivorous fishes such as vendace (Viljanen 1983, Hamrin 1986, Helminen & Sarvala, 1994, zooplankton production largely determines somatic growth potential. A positive relationship between somatic growth and lake productivity was shown for several coregonids (Nümann 1972, Kirchhofer 1995, Müller & Mbwenembo 1998. However, per capita food availability is not only a function of food production, but also depends further on competition. ...
Vendace is commonly assumed at low risk to recruitment overfishing. This assumption has been confirmed for boreal stocks but might not apply at lower latitudes. We evaluated the risk of recruitment overfishing at the southernmost extent of vendace populations by comparing fecundity, natural mortality, and fishing mortality of two stocks differing in growth with those of a population not subject to fishing. Although in one study lake vendace grew faster owing to high lake productivity and low stock density, and had substantially higher age-specific fecundity, intense harvest before first spawning negatively affected the stock's recruitment potential by reducing spawner abundance. In contrast, in the slow-growing stock, vendace entered the fishery after first spawning, and egg production per recruit was similar to that of the population not subject to fishing. We conclude that vendace stocks characterized by fast somatic growth may be at higher risk of recruitment overfishing, which can be reduced by protecting first-time spawners.
... Variations in nutrient concentrations are known to impact resident whitefish (Eckmann, 2013). It has been demonstrated in various studies that decreasing P concentrations lead to lower fish production and lower yields for fisheries (Eckmann & Rösch, 1998;Müller & Bia, 1998). The recently reported slowdown in whitefish growth (Schubert & Härth, 2017) together with the dropping yields of fisheries in pre-alpine lakes (Koops, 2016) further underline the relevance of this topic. ...
• The combination of increasing atmospheric depositions of reactive nitrogen (N) and the highly effective diminishing of external phosphorus (P) loadings can change key nutrient ratios in lake ecosystems. Consequently, ratios of dissolved inorganic N (DIN) to dissolved P (DP) in lakes are increasing. However, potential consequences for aquatic organisms are as yet far from understood.
• We formulated three hypotheses on the potential effects of rising DIN:DP ratios on a lake food web: (1) increasing DIN:DP ratios intensify the P limitation of phytoplankton communities and lower their food quality; (2) densities of P rich zooplankters (e.g. cladocerans) will be negatively affected by P‐limited food algae; (3) as result, planktivorous fish will experience a reduction of their main prey (especially Daphnia species) and respond with lowered growth.
• These hypotheses were tested in a mesocosm experiment conducted in a pre‐alpine lake in southern Germany, Lake Brunnensee. For 76 days, the natural phytoplankton and zooplankton communities were exposed to a wide gradient of DIN:DP ratios. At the end of the experiment, juvenile planktivorous whitefish (Coregonus macrophthalmus) were released into the mesocosms and allowed to feed on zooplankton communities for 72 hr.
• Along the gradient of DIN:DP ratios, we found evidence for a rising P limitation of autotroph growth, which was indicated by increasing ratios of N:P (15:1–157:1) and C:P (91:1–797:1) in seston biomass. The rising P limitation in algae reduced the nutritional food quality for the majority of herbivorous zooplankton. Both the total zooplankton biomass and the Daphnia biomass declined substantially with increasing DIN:DP ratios. In contrast, increasing DIN:DP ratios favoured rotifer species, showing strong positive correlation with rotifer biomass. Whitefish weights decreased with increasing rotifer biomass and increased with rising Daphnia biomass in zooplankton communities.
• In summary, our results provide an experimental demonstration that increasing DIN:DP ratios can cause stoichiometric shifts in the biomass of primary producers towards higher N:P and C:P ratios. Effects on zooplankton were changes in the taxonomical community composition towards lower cladoceran biomass (mainly Daphnia spp.). The reduction in Daphnia biomass in turn caused significantly reduced growth rates of whitefish in our experiment. Our experimental results therefore support the assumption that stoichiometric effects can travel up the food chain. General consequences of such multi‐trophic effects induced by altered nutrient ratios could be potentially visible during re‐oligotrophication of water bodies, often resulting in high N:P ratios. Further empirical studies could look for signatures of these effects on the yield of economically important species.
... In many Swiss lakes, whitefish are the most important fish in terms of yield and biomass (Mü ller & Bia, 1998). Due to their substantial economic importance, these populations are subjected to supportive breeding programmes to compensate for insufficient natural reproduction which is caused presumably by eutrophication (Ryman & Laikre, 1991). ...
... Whitefi sh populations which are exploited by commercial gillnet fi sheries are often studied to determine the effects of fi sheries and fi sheries management on the population structure and other biological factors in Austrian (e.g. Wanzenböck & Jagsch 1998, Wanzenböck et al. 2002) and in other European (e.g. Müller & Bia 1998, Turunen et al. 1998) lakes. Contrary to the various investigations in gillnet-fi shed lakes, less is known about the effects of intensive angling on whitefi sh populations. ...
The whitefish population of Irrsee, a typical Austrian prealpine lake, is exploited exclusively by anglers, which is an unusual situation for many lakes dominated by Coregonids. Because of a significant decrease in the whitefish harvest, fishery regulations were changed between 1996 and 2002. The object of this study was to investigate whether the changed harvest regulations influence the proportion of catchable whitefish, to test the effectiveness of the size limit in force, and to investigate whether the practice of releasing undersized and hooked whitefish affected condition factors. The results from our study show a significant effect of changed harvest regulations on the number of catchable whitefish, on the proportion of hooked whitefish with typical injuries, and on the population size structure. Until now the effect of intensive angling on a whitefish population was largely underestimated in the Austrian lakes, but more attention should be paid to these effects in the future.
... In Canada, the lake whitefish Coregonus clupeaformis is one of the most valuable commercial freshwater fish (Spangler 1970;Scott and Crossman 1973). A similar whitefish species, European whitefish C. lavaretus, has also been noted for its importance in commercial catch (Heikinheimo and Raitaniemi 1998;Mü ller and Bia 1998;Salonen et al. 1998;Turunen et al. 1998;Wolos et al. 1998). ...
To determine the more efficient stocking strategy for lake whitefish Coregonus clupeaformis in Lake Simcoe, Ontario, we compared growth and survival of fish stocked as fall fingerlings and as spring yearlings. Paired lots were stocked in April and October from the 1986 to 1991 year-classes. Survival and growth of fish from the two rearing practices were indexed as the relative abundance and length of 6-year-old lake whitefish captured with trap nets during fall spawning runs from 1992 to 1997. No difference in relative abundance or length between lake whitefish stocked at 6 months of age and those stocked at 12 months of age was detected. This contradicts results reported for many other fish species, which have exhibited greater survival rates when stocked as yearlings rather than fingerlings. As a result of these findings, all lake whitefish stocked into Lake Simcoe are stocked in the fall when the fish are 6 months of age, thus freeing up limited hatchery space for other purposes.
... In many Swiss lakes, whitefish are the most important fish in terms of yield and biomass (Mü ller & Bia, 1998). Due to their substantial economic importance, these populations are subjected to supportive breeding programmes to compensate for insufficient natural reproduction which is caused presumably by eutrophication (Ryman & Laikre, 1991). ...
The sequence variation of three exons of the major histocompatibility complex (Mhc) was examined in a lake whitefish Coregonus sp., population from the Swiss lake of Hallwil. DNA sequences from the Mhc class I A1, A2 and class II B1 exons, corresponding to the 1, 2 and β1 domains of the Mhc glycoproteins, were obtained by the polymerase chain reaction followed by cloning and sequencing. The numbers of variable sequences detected for each exon were 15 (A1), 11 (A2) and 20 (B1). Levels of nucleotide similarity ranged from 82 to 99% for the A1 exon, 58–96% for the A2 and 88–99% for the B1 exon. At the A1 and B1 exons, the nonsynonymous substitution rates (dn) exceeded synonymous substitution rates (ds) greatly within the peptide binding regions, indicating the effect of balancing selection. Sequence diversity at the A2 exon did not seem to be maintained by balancing selection (ds > dn). Phylogenetic comparison of whitefish Mhc sequences with sequences from other salmonid species and more distantly related teleosts indicated shared ancestral (trans-species) polymorphism.
... The terminology used to describe the process of nutrient reduction in lakes varies (Mü ller & Bia, 1998;Jeppesen et al., 2002;Anderson et al., 2005;Anders & Ashley, 2007;Brinker & Hamers, 2007). We use the term 'oligotrophication' sensu Edmondson & Lehman (1981), who considered Lake Washington as '. . . ...
Between 1951 and 1979, total phosphorous concentrations in Lake Constance increased from 7 to 87 μg L−1. Following wastewater treatment, phosphorus levels were brought under control, returning to 7.6 μg L−1 by spring 2007. The biological and chemical data from 1980 to 2004 were first modelled by seasonal time series analyses and then used to create a general model. Excluding collinear variables allowed the data set to be condensed to six variables that could be fitted into a general linear model that explained ∼75% of the observed annual variation in chlorophyll a. A clear seasonal influence was apparent, with chlorophyll a tracking trends in temperature and the progress of spring. A nonseasonal influence was also observed in the interaction of two biological components, the proportion of phytoplankton biomass available to Daphnia (i.e. the percentage of ingestible size <30 μm) and the grazing intensity. In combination, these biotic variables had a negative impact on chlorophyll a levels. In contrast, the concentration of soluble reactive phosphorus (SRP) correlated positively with chlorophyll a. The effect of SRP showed a significant seasonal component, as it was more abundant in spring than at other times of year. In general, the model predicts a negative exponential response of chlorophyll a to further depletion of SRP in Lake Constance, while the temperature trends predicted by current global warming scenarios will result in a moderate increase in productivity. Data from 2005 to 2007 were used to verify the model. The modelled chlorophyll a values were nonbiased and showed a close match to the measured values (r2: 75%). Thus the applicability, reliability, and informative value of the model for pelagic Lake Constance was confirmed. The approach might easily be applied to other waters.
... The general pattern observed was growth acceleration coupled with higher yields. The effects of re-oligotrophication on fish communities have received less attention so far (Muïler & Bia 1998; Mills, Casselman, Dermott, Fitzsimons, Gal, Holeck, Hoyle, Johannsson, Lantry, Makarewicz, Millard, Munawar, Munawar, O'Gorman, Owens, Rudstam, Schaner & Stewart 2003; Jeppesen, Jensen, Sondergaard & Lauridsen 2005b), which might be because of the general relief that in many lakes recovery targets have been met and the negative consequences of eutrophication abated. During lake re-oligotrophication, however, fish biocoenoses, food web interactions and yields will not simply return to the pre-eutrophication situation (cf. ...
Abstract Commercial yields of perch, Perca fluviatilis (L.), from Upper Lake Constance changed markedly during the 20th century. The perch boom in the 1950s and 1960s is attributed to higher fishing intensity and lake eutrophication. Decreasing yields in recent decades are mainly because of slower growth of perch, while a gradual decrease in year-class strength, and an overall reduction of fishing intensity, might also have contributed to yield reduction. Slower growth is mainly attributed to lake re-oligotrophication and infestation of perch with the pike tapeworm Triaenophorus nodulosus (Pallas). Competition for food with non-native ruffe, Gymnocephalus cernua (L.) is likely of minor importance, while the invasion of the lake by alien macrozoobenthos species has the potential to reduce the food supply for benthivorous perch. Perch yields are assumed to remain at the low level attained at the beginning of the 21st century; however, under changing climate conditions, fluctuations around the long-term mean will probably be wider than in the past.
... The final stage: adaptive management Fish biomass and fish community composition have to be controlled annually, and the management (stocking measures and harvest regulations) be adapted to the prevailing situation and the management targets. This kind of continuous evaluation and adaptation of management decisions is called Ôadaptive managementÕ ( Walters & Hilborn 1976), and has been applied successfully in inland fisheries systems to enhance and stabilise fisheriesÕ and angler's yields ( Johnson & Staggs 1992;Garvey, Dingledine, Donovan & Stein 1998;Muïler & Bia 1998). According to ecosystem theory, strong and intense perturbations are required to shift a system into another stable state, whereas pulsed perturbations are of too little impact ( Persson, Johansson, Andersson, Diehl & Hamrin 1993;Perrow et al. 1997). ...
Biomanipulation, the reduction of planktivorous fish to enhance filter-feeding zooplankton, has been used to rehabilitate eutrophied lakes. However, efficacy and long-term success were dependent on nutrient load, lake morphometry and biomanipulation measures. The ongoing focus on sustainable use of aquatic resources offers the chance to perform lake rehabilitation using a combined strategy of nutrient load reduction and traditional inland fisheries management techniques. Particularly in Central and Western Europe where piscivorous fish are the target species of most commercial and recreational fisheries, an enhancement of the piscivores by stocking and harvest regulations may act successfully in the co-management of ecosystem and fisheries. Guidelines are presented on how biomanipulation can be used as in lake rehabilitation by considering the objectives and constraints of traditional fisheries management. Alternatives in the decision tree are elucidated by examples from biomanipulations and lake management programmes in the temperate zone of Europe and North America. It is suggested that biomanipulation may support many lake rehabilitation programmes where fisheries’ stakeholders are the principal user groups.
... In general, eutrophic lakes support much higher fish production and fishing yield than oligotrophic lakes. Conversely, decreasing trophic state, a process referred to as oligotrophica-tion, leads to lower fish production and yield (Müller and Mbwenemo Bia, 1998;Eckmann and Rçsch, 1998). While a decrease in total phosphorus concentration from very high (>150 mg L -1 ) to moderately high values (30 -60 mg L -1 ) has rarely caused a significant drop in fishing yield (Müller and Stadelmann, 2004), fish production and yield tend to decrease quite rapidly at the lower end of the phosphorus scale, i.e. below 10 -15 mg L -1 (Jeppesen et al., 2005;Rellstab et al., 2004). ...
Lake Brienz, an oligotrophic pre-alpine Swiss lake, went through a mesotrophic period between around 1960 and 1990. The lake
is moderately turbid caused by fine suspended solids from glaciers. In 1999, yield of the economically important whitefish
collapsed to about 10% of preceding years. Age and growth analysis of the two whitefish types examined – small and large type
– revealed an almost complete halt of growth from 1999 until June 2000, paralleled by poor condition. Zooplankton data showed
that cladocerans, the preferred food of whitefish, were rare from January 1999 until June 2000. In order to elucidate the
trophic relationships between zooplankton and fish, the «Wisconsin Bioenergetics Model 3.0» was applied. The analysis showed
that poor growth and condition of whitefish in 1999 and 2000 were caused by the scarcity of primary food organisms. The relatively
small and slender fish could not be caught by legal gillnets, which resulted in poor fishing yield. Evidence is presented
that cladoceran biomass governs food consumption by the fish («bottom- up effect»), while the effect of fish predation on
cladocerans was found to be negligible, most likely also during the period of poor growth. Turbidity did not appear to significantly
interfere with the feeding of whitefish. Growth, condition and commercial yield of whitefish partly increased again after
2000, but due to the very low productivity of Lake Brienz, fishing yield will remain low. Food chains in such oligotrophic
systems are fragile. It is likely that a future collapse of the cladoceran population and, thus, the whitefish fishery will
happen again.
... Most of the studies of coregonid population dynamics claimed that anthropogenic eutrophication was the main cause of the acceleration in whitefish growth (Nümann 1962, Grimås et al. 1972. The decrease in whitefish growth from many European lakes towards the end of the 20 th century was generally attributed to re-oligotrophication (Kirchhofer 1995, Müller andMbwenembo Bia 1998). Thus, it was an accepted hypothesis that changes of a lake's trophic state, through changes of primary and secondary production, was the main, if not the only, mechanism that controlled fish growth (Downing et al. 1990). ...
This thesis deals with anthropogenic effects on the whitefish stock of Lake Constance in the time period 1950-2000. Eutrophication, size selective-fishery as well as stocking affects different whitefish traits such as somatic growth, reproductive investment as well as vertical distribution. Most of the analysis are based on fisheries data documented during the last 50 years. This thesis shows that beside lake trophy, the practice of fishery itself could be a factor for ongoing growth depletion. Strong fluctuations in fish biomass acts strongly on growth rate as well. Diese Arbeit befasst sich mit anthropogenen Langzeiteffekten auf die Felchenpopulation des Bodensees im Zeitraum zwischen 1950-2000. Dabei konnte gezeigt werden, dass Eutrophierung, Besatz und die größenselektive Fischerei auf verschiedene Weise auf die Felchen wirken. Es konnten Zusammenhänge dieser Umweltfaktoren mit dem Wachstum, dem Reproduktionsinvestment, sowie der vertikalen Verteilung gezeigt werden. Der Großteil der Auswertung beruht auf der Datenanalyse von Fischereiaufzeichnungen der letzten Dekaden.
Investigations on recreational fisheries have developed slowly In Europe. There is an obvious need for an integrated management approach to promote sustainable inland fisheries management systems taking into account the multifaceted nature of inland water uses. Besides the high social importance recreational fisheries have a number of ecological impacts. Therefore, sustainable inland fisheries management systems constitute both a challenge and a duty.
Keywords:
Binnenfischereimanagement;
Europa;
Fischerei;
Industrielander;
Europe;
fisheries;
industrialized countries;
inland fisheries;
inland fisheries management;
sustainable inland fisheries
Forecasting potential yield may be important for adequately managing intensive commercial fisheries in highly productive lakes. Between 1986 and 1995, a small midwater trawl was used in November/December at night to assess population density and age structure of Lake Hallwil whitefish (Coregonus suidteri) in eutrophic Lake Hallwil, Switzerland. Acoustic data were recorded on the same day and night using dual beam technology. Year-class strength was calculated by virtual population analysis, based on catch statistics and age data of the commercial catch. Year-class strength varied highly between years. The number of young-of-the-year Lake Hallwil whitefish caught by standardized one-night trawling effort showed close correlation with year-class strength. Acoustic data corresponded well with pelagic fish abundance but not so well with size frequency distribution and biomass. Potential Lake Hallwil whitefish yield in subsequent years can be inferred from year-class strength assessed by the two methods, and using data on growth and size-at-harvest. Altogether, midwater trawling, combined with acoustics, was found to be an effective tool for forecasting year-class strength and yield of coregonids in small to middle-sized eutrophic lakes.
Water turbidity impacts the distribution and feeding success of European whitefish (Coregonus lavaretus L.) as shown during intensive research programs at Lake Ammersee (4660 ha, Bavaria, Germany). Vertical and horizontal distribution of European whitefish and population density were assessed with hydroacoustic methods. Whitefish were sampled for analysis of stomach contents and growth data. Laboratory experiments were conducted to quantify the relationship between turbidity and feeding activity at different zooplankton concentrations. Whitefish showed clear avoidance in horizontal and vertical directions of areas with high turbidity. Vertical displacement of whitefish into deeper water layers, obviously caused by turbidity, leads to worse feeding conditions for whitefish. This effect is the result of poor visual conditions on the one hand and lower prey density on the other. This pattern was found in the laboratory where declines in ration were related to increased turbidity. Cladocerans, the preferred prey of the whitefish, mainly inhabit the epilimnion of Lake Ammersee. During periods of elevated turbidity, caused by high flows of the main tributary, the stomachs of whitefish were found to be nearly empty. Depending on the intensity and frequency of turbidity events, feeding may stop for several weeks during the growing season. In addition to the effects of oligotrophication and density-dependent factors, this is an important reason for the comparatively poor growth of Lake Ammersee whitefish.
The population dynamics of coregonids in European alpine lakes have been studied intensively during recent decades. Many of these studies have focussed on the impacts of anthropogenic eutrophication and subsequent re-oligotrophication. These changes in productivity represent the single most important perturbation of coregonid habitats since the mid-twentieth century. While the problem of eutrophication has been successfully addressed in many European alpine lakes, new forms of anthropogenic challenge such as climate change and the colonization of lakes by non-native aquatic species continue to impact on coregonid populations and demand research. The picture that emerges from a compilation of primary research papers suggests that abiotic and biotic factors are most influential on year-class strength during the larval phase, exerting more moderate effects during the embryonic and reproductive phases, and only a weak influence during the growth phase. In contrast, individual biomass is mainly determined during the growth phase, but moderate effects on growth may also occur during the larval and reproductive phases. Changes in lake productivity influence the various life-history phases of coregonids in both positive and negative ways, for example by improving feeding conditions and via deterioration of spawning habitat quality. Climate change, on the other hand, seems to influence mainly the embryonic and larval phases, with the available studies suggesting that higher temperatures and earlier onset of stratification in spring tend to boost year-class strength. These effects, however, appear insufficient to balance the marked effects of re-oligotrophication, which has caused and continues to cause a marked depression of growth in many European alpine lakes. The bumper fish harvests obtained during the peak of anthropogenic eutrophication are thus a thing of the past.
From 1991 to 1997 the gonadosomatic index (GSI) of pelagic spawning female whitefish (Coregonus lavaretus, local name “Blaufelchen”) in Lake Constance-Obersee was determined. Average GSI increased from low values in summer to a maximum of about 24% immediately before spawning (end of November/beginning of December). The coefficient of variation of GSI ranged from 14–35% of the mean in summer and autumn and decreased to values between 9 and 16% immediately before spawning. The seasonal increase in GSI was similar for all years investigated, except in 1995, when after an exceptionally mild autumn spawning took place at a water temperature of about 9.4°C. In this year, the post-August development of GSI was retarded compared to the other years of the investigation, but immediately before spawning GSI was at a value of about 24% body weight, i. e. in the same range as in the other years of the study. Despite continuing re-oligotrophication of Lake Constance no decrease in GSI was found during the study period.
IntroductionStudy sitesMethods
ResultsDiscussionAcknowledgements
In the early 1990s whitefish (Coregonus lavaretus L.) of Ammersee, a prealpine bavarian lake, showed extremely poor growth. The average weight of 3+ whitefish was lower than 150 g in 1994. But in 1994 the trend changed. Growth of whitefish doubled between 1994 and 1999. The average stomach contents also showed a significant increase in the course of recent years. Parallel to the weight increase a distinct decrease in population density could be seen, monitored with a SIMRAD EY 500 echo sounder. In the first half of 1999, there were about 350 fish per hectar (F ha−1) in Ammersee. That was only about a fifth of the number at the end of 1995. In 1996/97 fish density in a nearby prealpine lake, oligotrophic Walchensee, was clearly lower with fewer than 350 F ha−1 than in Ammersee (mean value 1996: approx. 850 F ha−1, 1997: approx. 770 F ha−1). Despite the lower trophic status, Walchensee whitefish showed clearly better growth: the average weight of 3+ whitefish was 308 g. At the same time the weight of 3+ whitefish of prealpine Kochelsee was 260 g on average. There was an extended food resource for Kochelsee whitefish with the additional possibility of feeding on benthic prey. In comparison to Ammersee this could explain the significantly better growth of Kochelsee whitefish despite an even higher fish density (approx. 1300 F ha−1 in 1996/97). Chiemsee, another prealpine lake, is very similar to Ammersee in relation to fundamental limnological parameters and was studied in 1993 and 1994. In 1994 fish density of Chiemsee was at a maximum of approx. a fifth of that of Ammersee, and whitefish growth was significantly better. 3+ whitefish weighed above 300 g, about double the weight of Ammersee whitefish at that time. A comparison of mean stomach contents of whitefish in both lakes points to intraspecific food competition in Ammersee. Those results indicate a causal connection between whitefish density and growth.
In northern industrialized countries, the inland fisheries sector has long been dominated by recreational fisheries, which normally exploit fish for leisure or subsistence and provide many (poorly investigated) benefits to society. Various factors constrain the development and existence of inland fisheries, such as local user conflicts, low social priority and inadequate research and funding. In many cases, however, degradation of the environment and loss of aquatic habitat are the predominant concerns for the sustainability of inland fisheries. The need for concerted effort to prevent and reduce environmental degradation, as well as conservation of freshwater fish and fisheries as renewable common pool resources or entities in their own right is the greatest challenge facing sustainable development of inland waters. In inland fisheries management, the declining quality of the aquatic environment coupled with long-term inadequate and often inappropriate fisheries management has led to an emphasis on enhancement practices, such as stocking, to mitigate anthropogenic stress. However, this is not always the most appropriate management approach. Therefore, there is an urgent need to alter many traditional inland fisheries management practices and systems to focus on sustainable development.
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