Spatial and temporal distribution of fish and zooplankton in a shallow lake

1. We performed both a large- and a small-scale echo sounding study on the spatial and temporal distribution of fish (mainly roach, Rutilus rutilus and perch, Perca fluviatlis), as well as a small-scale study of zooplankton distribution in the small, shallow and eutrophic Lake Hanebjerg in Denmark. In the small-scale study, sampling was conducted in open water as well as in the edge zone immediately outside two different types of vegetation. 2. Fish daytime abundances differed between the northern and the southern parts of the lake and, on a small scale, small fish aggregated in the edge zones during day, preferably outside dense emergent vegetation. Copepods avoided emergent vegetation, while cladocerans showed no habitat preference. Both small fish and cladoceran numbers were found to be higher during night than day. 3. The relative abundance (number per sample) of cladocerans in the edge zone immediately outside vegetation was negatively correlated with the relative abundance of fish in that zone. There was no correlation between cladoceran and fish abundance in open water, or between the relative abundance of copepods and fish. 4. The presence of pelagic piscivores in combination with avoidance behaviour of both fish and zooplankton is a likely explanation for the observed distribution of small fish and cladocerans in Lake Hanebjerg. Both small- and large-scale distribution patterns may be dependent on the type and distribution of complex structure in the lake. Even in a small lake, large-scale patterns may affect the interpretation of small-scale data

Consequences of fish predation, migration and juvenile ontogeny on zooplankton spring dynamics

In order to disentangle if and when resource supply and adult and young-of-the-year (0+) fish predation affect zooplankton dynamics during spring, we monitored zooplankton during three consecutive years in a lake in southern Sweden. We also experimentally assessed 0+ fish predation rates and estimated changes in predation rates of adult fish on zooplankton. Decline in abundances of large-sized zooplankters in early spring was not caused by 0+ fish predation. Instead, this decline was most likely a combined result of size-selective predation from adult fish (stationary in the lake and from those returning from surrounding streams) and competition for diminishing algal food resources. On the other hand, the decline in medium-sized zooplankton in the lake during spring was strongly affected by 0+ fish. Hence, during spring, zooplankton are facing predation both from adult fish selecting large prey and from 0+ fish, which start feeding on small-sized prey and eventually switch to larger. Neither predation by different ontogenetic stages of fish (adult and 0+) nor resource supply shape the zooplankton spring dynamics, but rather they affect the timing and strength of these events. 0+ cyprinids tend to have stronger effect on zooplankton dynamics than other taxa of 0+ fish. A combination of predation from adult and 0+ fish during spring is the main mechanism behind the crash of the zooplankton community, which in many lakes leads to the termination of the clear-water phase

Perchloroethylene: Effects on body and organ weights and plasma buturylcholinesterase activity in mice

The effects of continuous and intermittent inhalation of perchloroethylene (PCE) on plasma butyrylcholinesterase (BuChE) activity, organ weights, liver morphology and motor activity in mice (strain NMRI) were tested. PCE exposure increased plasma BuChE activity in a time- and concentration dependent manner in both sexes. The increase was statistically significant at 37 p.p.m. in animals continuously exposed for 30 days. BuChE increased approximately 1.5 times in females and 2.5 times in males after 120 days exposure to 150 p.p.m. After rehabilitation of animals exposed for 30 days to 150 p.p.m., BuChE levels returned to normal. Liver weight also increased in a time and concentration dependent manner. Both sexes exhibited significant liver enlargement at 9 p.p.m. The increase was about 2.3 in females and 1.9 in males after continuous exposure to 150 p.p.m. for 120 days. After rehabilitation (120 days) of animals exposed to 150 p.p.m. for 30 days, a 10% increase still remained. A decrease in body weight gain was seen in both sexes after exposure to concentrations above 75 p.p.m. Female kidney weight was slightly increased. No clear effect on spleen weight could be detected. When the same time-weighted average concentration was used, intermittent exposure for 30 days had similar effects on liver weight and BuChE activity as continuous exposure, even when exposures lasted for only one hour per day. Liver cell morphology was changed after PCE exposure. The alterations could be observed already at 9 p.p.m. but disappeared after rehabilitation

Trichloroethylene: Further studies of the effect on body and organ weights and plasma buturylcholinesterase activity in mice

The effects of continuous and intermittent inhalation of trichloroethylene (TCE) were studied in male and female mice. Plasma butyrylcholinesterase (BuChE) activity, body, liver, kidney and spleen weights were measured. The liver was studied histologically and motor activity measured with doppler radar. Continuous TCE-exposure (37–300 p.p.m.) increased plasma BuChE activity in the males in a time and concentration dependent manner. After 30 days at 37 p.p.m. the increase was about 25%. Exposure to 300 p.p.m. for 30 days increased the activity three times. BuChE activity in females was only slightly influenced even at 300 p.p.m. Liver weight was increased in a time and concentration dependent manner in both sexes. In animals continuously exposed for 30 days to 300 p.p.m., liver weight was roughly twice that of the air-exposed controls. Morphological changes were observed in the liver of TCE-exposed animals. Above 150 p.p.m. kidney weight in both sexes was significantly increased. This effect was more pronounced in the males than in the females. Spleen weight was not influenced by the exposure. Body weight increase was slightly lower in exposed animals. Plasma BuChE activity and liver weight returned to normal when exposure was terminated. Intermittent exposure to short pulses of high concentration of TCE had roughly the same effect on BuChE, body and organ weights as continuous exposure to the same time-weighted average. Motor activity was affected by the intermittent exposure schedules. At 900 p.p.m. decrease in activity was observed. At 3600 p.p.m. motor activity was considerably increased

Effects of geographic origin on growth and food intake in Eurasian perch (Perca fluviatilis L.) juveniles under intensive culture conditions

Survival, growth, and food intake of Eurasian perch (Perca fluviatilis) juveniles from different stocks originating from various geographic regions of Europe were compared under the same conditions of laboratory-scale intensive culture. In Experiment 1, four stocks originating from Italy (1), southwest (SF) and northwest (NF) France, and Belgium (B) were examined at larval and early juvenile (initial body weight, IBW = 0.53 g) stages. In Experiment 2, B stock was compared to a Finnish (F) one in two trials, including small (IBW = 1.26 g) and large (IBW = 32 g) juveniles. In Experiment 3, small (IBW = 1.29 g) and large (IBW = 7.33 g) juveniles from Polish (P), F, and B stocks were examined. In Experiment 1, body weight means at hatching and survival at the end of the larval stage were significantly lower in the I and SF stocks than in the B and NF stocks. In the early juvenile stage, survival and growth rates were significantly lower in the I and SF stocks than in the B and NF ones. In Experiment 2, {early juvenile stage} survival in the F stock was significantly lower than in the B stock, partly due to a higher incidence of cannibalism. During this stage, growth rates and food intake or feed efficiency in the F and B stocks were comparable, but at the end of the juvenile stage, the F stock outperformed the B one. In Experiment 3, survival in all the three stocks was comparable both in small and large juveniles. In contrast to the higher performance of F juveniles in Experiment 2, growth rates were comparable between the B and F stocks, and fish from the P stock had the highest growth rates. The results indicate a high level of variation within and between hatchery stocks in survival rates, growth rates, and food intake. These variations depended on the geographic origin of the fish, with the lowest survival and growth potentials being in the stocks originating from the southern regions. These findings highlight the interest in evaluating growth and food consumption of different Eurasian perch stocks. Such evaluation is a necessary tool for genetic selection in improving performance in perch aquaculture. (C) 2004 Elsevier Science B.V. All rights reserved

Consumption patterns, complexity and enrichment in aquatic food chains

The interactions between consumers and prey, and their impact on biomass distribution among trophic levels, are central issues in both empirical and theoretical ecology. In a long-term experiment, where all organisms, including the top predator, were allowed to respond to environmental conditions by reproduction, we tested predictions from 'prey-dependent' and 'ratio-dependent' models. Prey-dependent models made correct predictions only in the presence of strong interactors in simple food chains, but failed to predict patterns in more complex situations. Processes such as omnivory, consumer excretion, and unsuitable prey-size windows (invulnerable prey) increased the complexity and created patterns resembling ratio-dependent consumption. However, whereas the prey-dependent patterns were created by the mechanisms predicted by the model, ratio-dependent patterns were not, suggesting that they may be 'right for the wrong reason'. We show here that despite the enormous complexity of ecosystems, it is possible to identify and disentangle mechanisms responsible for observed patterns in community structure, as well as in biomass development of organisms ranging in size from bacteria to fish