1999, Radke, Robert J., Eckmann, Reiner

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.

1998, Eckmann, Reiner, Rösch, Roland

This paper summarizes changes in the fish species list and in fishery yields in Upper Lake Consiance during the last century. Species richness has remained nearly unchanged since the end of the last century, although three species and one form of whitefish have been lost from the lake probably as a result of eutrophication and four other species have been introduced either accidentally or intentionally. Fisheries yields have increased since the 1950s because eutrophication enhanced fish production and fish stocks were exploited more intensively with highly efficient nylon nets. With increasing lake trophy during the 1960s and 1970s, the proportion of coregonids in commercial catches decreased, whereas the proportion of perch and cyprinids rose. The recent oligotrophication of the lake is reflected in an augmenting proportion of whitefish in commercial catches and an increasing age-at-capture. Lake whitefish (Coregonus lavaretus) and Eurasian perch (Perca fluviatilis) are commercially the most important fish species in Lake Constance. Their ecology has been studied intensively during the last decade, while most recently the littoral fish community has been included in our research activities. The effects of newly introduced and rapidly spreading ruffe (Gymnocephalus cernuus) on the native fish fauna are of special concern to fishery managers and will be a central topic of future research.

1997, Fischer, Philipp, Eckmann, Reiner

The spatial distribution and patterns of habitat use of littoral fish species in Lake Constance were studied using electric fishing and trammel net sampling to assess fish abundance and biomass in three depth strata at six different sampling sites within the littoral zone. Distribution differences among depth strata were found to be often more pronounced than among different sampling sites. Juvenile chub (Leuciscus cephalus) and dace (Leuciscus leuciscus) were most abundant in the shallow areas of

1996, Imbrock, Frauke, Appenzeller, Alfred, Eckmann, Reiner

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.

1999, Eckmann, Reiner

In larval and juvenile whitefish (Coregonus lavaretus L.) from Lake Constance, Germany, the otolith increments are deposited daily, whereas daily deposition could not be confirmed in larval whitefish from Lake Pyhäselkä, Finland. The calcium concentration in Lake Constance is high (around 1.3 m m), while calcium deficiency is typical for Finnish lakes (around 0.15 m m). Therefore, the hypothesis that the distinctness of daily otolith increments in whitefish is related to water calcium content was tested by rearing three groups of Lake Constance whitefish in water of 0.2, 1.3 and 4.7 m m Ca. The eggs were incubated in lake water (1.3 m m Ca), and the larvae were acclimated to the experimental calcium concentrations on the day of hatching. After 39 days of ad libitum-feeding with Artemia nauplii, the three groups did not differ significantly in total length, wet and dry weight, and otolith length and width. The daily increments were easily recognizable, and contrast between dark (D)- and light (L)-zones was the same in the fish of all test groups. For the experimental set-up of this study, and particularly the range of calcium concentrations tested, the hypothesis that water calcium content influences the distinctness of daily otolith increments was rejected.

1998, Eckmann, Reiner, Czerkies, Piotr, Helms, Christian, Kleibs, Karin

Lake Werbellin (804 ha. 18 m mean depth), a mesotrophic lake in north-eastern Germany yielded between

1996, Radke, Robert J., Eckmann, Reiner

Research on predator-prey relationships in the littoral zone of Lake Constance showed that eels (Anguilla anguilla (L.)) were the most numerous piscivorous predators in the shallow water zones up to 3-metres depth in 1992. From July on fish was the most important component of the diet of eels. Perch (Ferca fluviatilis L.), burbot (Lota Iota (L.)) and bream (Abramis brama (L.)) were the most frequently consumed fish. As 61% of all identifiable fish the eels had consumed were perch, an attempt was made to estimate the impact of eel predation on the young-of-the-year (y-o-y) of the perch population. Consumption by the total eel population never exceeded the amount of perch fry consumed by adult, cannibalistic perch estimated in other studies, but it seems possible that eel predation could have an adverse influence on weak year classes. Further reoligotrophication of the lake might lead to even higher fish consumption by the eels due to declining benthic production and consequent increased predation pressure.

1998, Eckmann, Reiner

In acoustic sampling for fish, thresholding is normally applied to eliminate the unwanted contribution of noise to the integrator output. Since thresholding discriminates against small targets, this technique cannot be used for the quantitative study of these small targets in the presence of larger ones. When the integrator output (area backscattering coefficient, sa) due to one size class of targets is plotted vs. decreasing sv-threshold (volume backscattering strength) in 1 dB steps, the resulting curve can be described by an asymptotic Bertalanffy growth function. If the slope of the resulting curve decreases at some intermediate threshold level and then increases again before the final plateau is reached, the total integrator output can be allocated to two groups of targets according to the linearity principle in acoustics. The maximum sa value for the larger targets (sa-large) is estimated by fitting a Bertalanffy function to the lower part of the curve. The echo integral that corresponds to the smaller targets is then found by subtracting sa-large from the sa value that is reached at the lower threshold level. In a shallow (max. depth 8.5 m) mesotrophic lake of 24 ha surface area in northeastern Germany, this new technique was tested to estimate the abundance of insect (Chaoborus) larvae in spring. Hydroacoustic data were sampled during night, when juvenile roach occurred together with midge larvae in the pelagic zone. Chaoborus abundance was estimated by replicate vertical tows of a 1 m2 plankton net of 0.5 mm mesh size. The sa values that were allocated to Chaoborus larvae were significantly correlated to mean Chaoborus abundance. This relationship was then used to map the lakewide distribution of Chaoborus larvae on two dates in spring.

1997, Fischer, Philipp, Eckmann, Reiner

Seasonal changes in the abundance, biomass and species richness of fish in the littoral zone of Lake Constance were examined. Electric fishing was conducted in three depth strata at six different sampling sites within the littoral zone from November 1991 to Februar 1993 during daytime. Minimal abundance and biomass values were observed in February with 3.9 (SD ±. 4.2) ind 100 m[-2] and 12.7 (SD ± 13.8) g-wet . 100m[-2]. Maximal abundance values were observed in August with 119.5 (SD ± 88.5) ind . 100 m[-2], maximal biomass values in May with 903.9 (SD ± 1942.6) g-wet . 100 m[-2]. The influence of individual habitat variables on the littoral fish community was examined by canonical correlation analysis. Seasonal changes in total fish abundance, biomass and species richness were mainly explained by seasonal changes in water temperature followed by lake level. For the individual species, different substrate characteristics such as sediment type or the amount of large stones lying on the bottom providing shelter were also found to be important. However, these variables often influence fish abundance and biomass only within a range that is predefined by water temperature. Different patterns were found for the two benthic species. burbot (Lota lota) and stone loach (Noemacheilus harbatulus). The distribution of both species was mainly correlated with the availability of gravel substrate and shelter and mostly independent of littoral water temperature. Burbot larger than >5 cm TL emigrated from the littoral into deeper adjacent areas during summer when water temperature increased above a certain threshold, indicating an upper thermal threshold for this cold stenothermal species.

1996, Eckmann, Reiner, Imbrock, Frauke

The distribution and migration patterns of overwintering perch were studied by hydroacoustics and gill netting in Lake Constance, central Europe. In autumn, perch left the littoral and moved to deeper waters for overwintering. They lived at temperatures below 6°C for about six months. In January, perch were present between the 42 and 69 m depth contours. Their distribution then shifted to increasingly shallower depths by 0.25 m per day, and they returned to the littoral in early May where adults spawned in late May. During winter, perch performed diel vertical migrations, resting on the bottom during day and swimming up to 25 m off the bottom during night. Light was the proximate cause of this regular vertical migration. Perch did not gain an energetic advantage by migrating in a homothermal water column, but they may have gained access to copepods in addition to their main diet of benthic invertebrates. It is hypothesized that predator avoidance is the ultimate cause of diel migration in overwintering perch which avoid piscivorous birds by resting on the bottom during day and avoid burbot by swimming off the bottom during night.