Notes: A near-shore belt 50 km in length was surveyed parallel to the shoreline of Lake Constance, central Europe, with a single-beam echosounder five times between July 1993 and February 1994. The species and age composition of fish in the survey area was investigated by gillnet fishing and SCUBA-diving. In summer, the horizontal distribution of perch was patchy. Population density declined from east to west, and highest densities occurred in one shallow bay and close to ports and jetties at steeper shores. During daytime, perch stayed in the sublittoral zone between 3 and 15 m depth and between 2 and 6 m above the thermocline. Within this layer age classes were separated spatially: the relative number of young-of-the-year perch declined with depth whereas the relative number of adult perch (2+ and older) increased with depth. At dusk the fish migrated to the littoral zone, where they spent the night resting on the bottom. In winter, under almost homothermal conditions, perch of all ages were located between the 35 and 70 m depth contours, where they performed pronounced diel vertical migrations. They rested on, or close to, the bottom during daytime and ascended up to 20 m below the surface at night. During this season, horizontal distribution of perch was much more homogeneous than in summer.

Notes: Abstract– The importance of overwinter mortality of 0+ perch (Perca fluviatilis L.) in central European lakes was estimated using a stochastic simulation model. The probability of overwinter mortality of a perch cohort was calculated by using a model developed for yellow perch (P. flavescens Mitchill). Winter duration from a long-term data set and the length of perch at the end of the first year from five lakes were used as input data. After 1000 simulation runs, the total extinction of a cohort in the lakes studied was never predicted. Mortality rates of more than 0.5 were only predicted in two of the five lakes, and rates of more than 0.3 in these two lakes were predicted in approximately 10% of all cases. For two consecutive winters differing in duration, the length-frequency distributions of 0+ perch in the autumn and following spring were compared by a graphical method. No significant size-dependent mortality of smaller individuals could be detected in any of the populations studied. Simulated spring length-frequencies were derived from observed autumn length distributions by the same model that was used for the stochastic simulation. These simulated and the empirical spring length-frequency distributions were not identical. The differences between the two distributions were attributed to growth, which occurred between the sampling dates. The results from the simulation and the analysis of the empirical data suggest that high overwinter mortality caused by starvation is rare in central European lakes.

Notes: Fish species richness was assessed by electrofishing and gillnetting in 16 lakes of the northeastern lowland in Germany (the Schorfheide biosphere reserve). The lakes range from 0.03 to 10.55 km2 and support between 5 and 14 fish species. Species richness is significantly correlated with lake area in an exponential and a power model. Richness is also correlated with shoreline development and total dissolved solids. This supports the hypothesis that larger areas contain more species within a taxonomic group due to increased habitat diversity. The slope of the species-area curve is low compared with most other studies of fish species richness in lakes, and the intercept value is high. This is interpreted as the result of high habitat and food diversity, lack of stress from abiotic factors, and the small regional species pool from which these lakes can be colonized. Two species inventories, one from the beginning of this century and one from the 1950s, are available for comparison. Average species richness did not change during the last decades. Species turnover rates were not related to the degree of anthropogenic eutrophication or to the intensity of fishery exploitation in these lakes. On the species level, however, one effect of accelerated eutrophication is apparent, the disappearance of 4 bottom-living species from one to 6 of the study lakes.

Notes: Abstract  – To study the behaviour of silver eels (Anguilla anguilla L.) during their downstream migration, particularly near a hydroelectric power dam, we tagged nine eels with ultrasonic transmitters and tracked their paths in the River Mosel, Germany. The onset of migration coincided with the first flood event that followed the full moon but was independent of daytime, because migration and turbine passage occurred during both day and night. During migration eels swam actively downstream with a velocity of 0.3–1.2 m · s−1. When migrating eels arrived at the dam, they either passed through the turbines immediately or stayed upstream of the powerhouse for up to 8 days, showing a characteristic circling behaviour. Circling eels repeatedly approached the trashrack, sprinted upstream, and finally passed through the turbines with the next high water discharge. These observations are discussed with regard to the design of appropriate downstream passage facilities.

Notes: Abstract –  Food consumption by Eurasian perch (Perca fluviatilis L.) and ruffe (Gymnocephalus cernuus [L.]) was studied in single and mixed-species treatments in the laboratory, where alternative food resources, chironomids and zooplankton, were offered simultaneously. The effects of structural complexity, which was represented by substrate grain size, and of feeding level on food consumption were analysed. Across all experiments, the outcome of competition between perch and ruffe depended on food abundance and on the structural complexity of the environment. Perch and ruffe both changed their food consumption in the presence of a heterospecific competitor. With high food supply, perch consumed more benthic food than ruffe. With low food supply, the consumption of perch decreased strongly, while that of ruffe remained high on fine sediment. Under all conditions tested, the mechanism of competition appeared to be of interference rather than of exploitative nature. It is suggested that with decreasing lake productivity caused by re-oligotrophication, habitat shifts of both species will occur, which will alleviate interspecific competition. Ruffe will forage over fine sediment and perch over coarse sediment, whereby both species will achieve the highest foraging efficiency under conditions of low food supply.

Notes: The development of the alimentary tract in larvae of Coregonus fera fed with Anemia nauplii has been examined histologically from the 4th to the 24th day after hatching.Buccal cavity and oesophagus are nearly completely developed before the start of feeding. The stomach exists only as an ‘anlage’, but starts to store food during the study period. The intestinal absorptive cells are stuffed with fat vacuoles between the 7th and 13th day after the start of feeding. During this time the rectal absorptive cells contain one large supranuclear body consisting of protein. The occurrence of large vacuoles in the intestinal and rectal absorptive cells may be caused by a partial non-functioning of the larval digestion; after two weeks of feeding this restriction seems to be overcome.

Notes: Larvae of Coregonus spp. were reared in the laboratory under different temperature (4, 6 and 8°C) and feeding conditions (ad libitum, limited, with starvation intervals). Their RNA/DNA ratios were determined with a highly sensitive fluorescence technique. After resorption of their yolk reserves (about 2 weeks after hatching), well fed larvae (RNA/DNA 〉2.5) could be significantly distinguished from larvae reared under limited food supply (RNA/DNA 〈 2.5), both at the 6 and 8°C levels. At 4°C, no differences due to the feeding regime were found. During a second series of experiments, larvae were affected by an intestinal disease, which was caused by the ingestion of unsuitable copepod plankton. This disease provoked high mortality, decreased growth and RNA/DNA ratios which were almost as low as in temporarily starved larvae from the first series. Coregonid larvae sampled in Lake Constance during spring 1990 showed RNA/DNA ratios which were unexpectedly low when judged on the basis of mean body length and average ambient temperature. It was obvious from macroscopic observations that some of these wild larvae were severely attacked by the intestinal disease. The low RNA/DNA ratios in field samples are, therefore, interpreted as a sublethal result of this disease.

Notes: The body condition, lipid reserves and mortality of 0 and 1 year-old perch Perca fluviatilis and ruffe Gymnocephalus cernuus, sampled during the winter in Lake Constance, Germany, were compared. Length-frequency analyses did not provide evidence for overwinter mortality in either species. The fresh and dry mass of perch as well as their lipid contents decreased during winter, while ruffe were heavier and contained more lipid at the end of the winter. The superior performance of ruffe was mainly attributed to its sensory capabilities, which allowed it to ingest zoobenthos throughout the winter, while the zooplankton feeding of perch was constrained by low light levels. In lakes that undergo a process of re-oligotrophication, this advantage of ruffe over perch may be even more pronounced, since lower food supply during the growth season and thus lower fish lipid content at the start of winter is probably better tolerated by ruffe than by perch.

Notes: In a small, 12 ha, mesotropic lake, roach Rutilus rutilus performed diel habitat shifts that clearly influenced the composition of their diet. During daytime, roach stayed in the littoral zone and concentrated on littoral prey. At night they were found in the pelagic zone, and pelagic prey items such as Daphnia spp. or Chaoborus flaricans dominated their food. On a seasonal scale, there were shifts in the importance of different food items and in the diel pattern of feeding intensity. Bioenergetics modelling in combination with an evacuation rate method for estimating daily rations allowed for changes in feeding modes to be taken into account, and so food item specific daily rations over the season could be determined. With the evacuation rate method applied on selected days, diel changes in diet compositions and feeding intensities could be quantitatively accounted for. When the 24 h integrated diet proportions were then used as an input parameter for bioenergetics modelling, food item specific consumption could be determined over the entire sampling season. The consideration of the diel diet shifts proved to be essential for the model output. If only the daytime or the night-time diet composition (derived from one single daily sampling) was taken into account for bioenergetics modelling, severe under- or overestimations of daily rations for specific food items resulted.

Notes: In einem vergleichenden Fütterungsversuch wurden Coregonenlarven mit Nauplien des Artemia-Stammes Great Salt Lake/Utah beziehungsweise San Francisco Bay gefüttert. Trotz sehr geringer Mortalitäten in beiden Fütterungsansätzen konnten die Gruppen anhand der Ultrastruktur des Darmepithels eindeutig unterschieden werden. Coregonenlarven, die mit Nauplien des Artemut-Stammes Great Salt Lake gefüttert worden waren, hatten große Mengen Bakterien im Darm und zeigten Schädigungen der Darmschleimhaut von unterschiedlichem Schweregrad. Die Ergebnisse wurden im Vergleich zu Darmepithelveränderungen bei Fischen diskutiert, die anderweitig verursacht wurden, sowie zu bakteriellen Darminfektionen bei Säugetieren.〈section xml:id="abs1-2"〉〈title type="main"〉SummaryDamage to the intestinal epithelium of coregonid larvae (Coregonus fera) due to Artemia feeding: A bacterial infectionIn a comparative feeding experiment coregonid larvae were fed with nauplii of Artemia strain Great Salt Lake/Utah and San Francisco Bay, respectively. Though very low mortalities were observed the different feeding groups can be distinguished by means of the ultrastructure of the intestinal epithelium. Coregonid larvae fed with nauplii of Artemia strain Great Salt Lake/Utah had a lot of bacteria in the intestine and showed damage to the intestinal epithelium in different extent. The results were discussed in comparison with alterations of the intestinal epithelium of fish caused by other factors as well as with bacteria! intestinal infections in mammals.〈section xml:id="abs1-3"〉〈title type="main"〉Résumé Lésion de l'épithélium intestinal des larves de corégones (Coregonus fera) causée par une alimentation sà base d'artémia: une infection bactérielleDans une expérience comparative des larves de corégones ont été nourries avec des nauplius de la souche Great Salt Lake/Utah et San Francisco Bay d'artémia, respectivement. En dépit d'une faible mortalité les deux groupes d'alimentation pouvaient être clairement différenciés sà l'aide de l'ultrastructure de l'épithélium intestinal. Les larves de corégones nourris avec des nauplius de la souche Great Salt Lake/Utah d'artémia avaient beaucoup de bactéries dans l'intestin et montraient des lésions sur la muqueuse intestinale sà des degrés différents. Les résultats ont été discutés et comparés avec les altérations de l'épithélium intestinal chez les poissons causées par d'autres facteurs et aussi avec les infections bactérielles intestinales chez les mammifères.