Reiner Eckmann

Distribution of perch (Perca fluviatilis L.) during their first year of life in Lake Constance

The distribution and behaviour of larval and juvenile perch (Perca fluviatilis L.) were studied for two years in large, deep Lake Constance. After hatching larvae were transported by water currents to the open water. The majority of larvae remained in the pelagic zone for about one month. In both years, their return to the littoral zone coincided with the decline of pelagic zooplankton abundance. After returning to the littoral zone, juveniles stayed among submerged macrophytes within 5 m depth and lived apart from larger perch which lived at depths of 6–20 m. By late summer, juveniles changed their distribution pattern: during the day they stayed intensively close to piers and ports, but increased their swimming activity at dusk, cruising among shallow and deep waters and feeding on zooplankton, and rested on the bottom at night. This behaviour appears to be related to the decrease of inshore food resources and to the presence of predators in deeper water. 0+ perch left the littoral zone and moved into deep waters when autumnal mixing began in late October. They overwintered near the bottom at depths of more than 30 m. During most of the year, juvenile and adult perch were separated from each other. But as soon as they occupied the same habitat, the occurrence of cannibalism increased.

Kin-structured subpopulations in Eurasian perch ( Perca fluviatilis L.)

Based on ecological and behavioural studies it has been assumed that Eurasian perch (Perca fluviatilis) within one lake may not represent one panmictic population, but that they are subdivided into subpopulations. In order to investigate the genetic substructuring of populations, we used gene frequencies of five microsatellite loci to compare perch from six different sites from Lake Constance, Germany, and as outgroups perch from the lake Grosser Vätersee, Berlin, and two Swiss lakes, Lake Zurich and Lake Walensee. We examined whether homing behaviour of subadults to the spawning sites of their parents occurs and whether philopatric behaviour of adults results in significant population genetic substructuring. The distribution of genetic variation revealed two major, genetically distinct populations in Lake Constance: one in the eastern part of the lake and another in the western part (GST=0.07). Within each of these two populations, no further genetic substructuring, nor any indication of inbreeding could be detected, either because genetic exchange was sufficiently high or because the time since separation has been too short. Homing behaviour of subadults to parental spawning sites after having spent several weeks of their life cycle in the pelagic zone could not be detected. Instead, subadults stay within either the western or the eastern region of the lake. There is evidence that some shoals contain full- and half-sibs. Despite females spawning in close proximity to each other, some siblings stay together. This might suggest that perch possess kin preferences and kin recognition.

Kin and population recognition in sympatric Lake Constance perch (Perca fluviatilis L.) : can assortative shoaling drive population divergence?

Prior studies have shown that perch (Perca fluviatilis L.) of Lake Constance belong to two genetically different but sympatric populations and that local aggregations of juveniles and adults contain closely related kin. In this study, we analysed the genetic structure of pelagic perch larvae to investigate if kin-structured shoals already exist during early ontogenetic development or might be the result of homing to natal sites. Analysis of the gene frequencies at five microsatellite loci revealed that three out of five pelagic aggregations of larvae showed significant accumulation of kin. To investigate possible mechanisms of shoal formation, we tested if perch use olfactory cues to recognize their kin. Choice tests in a fluviarium showed preference for odours of unfamiliar kin vs unfamiliar non-kin. Additionally, we showed that perch could differentiate between the odours of the two sympatric populations and significantly preferred unfamiliar and unrelated conspecifics of their own over the foreign population. Our results present a behavioural mechanism that can lead to the observed formation of kin-structured shoals in perch. We further discuss if the ability to discriminate between their own and a foreign population can result in assortative mating within populations and thus form the basis of “socially mediated speciation” in perch.

Daily increments on the otoliths of larval and juvenile Coregonus spp., and their modification by environmental factors

The sagittal otoliths of pre- and posthatch embryos, larval, and juvenile coregonids (Coregonus spp.) were examined for growth increments. Under laboratory conditions, a check is formed on the day of hatching and subsequently one growth increment per day is deposited during at least 265 days. Under the experimental conditions of this study, the age of young coregonids can therefore be determined with high accuracy.In starving larvae, both increment width and ring contrast decrease during the first 10 days after hatching until daily increments are no longer recognizable. A change from one diet to another, alteration of the water temperature, or a short starvation period lead to the formation of characteristic ring patterns which appear on the otoliths within 1 to 3 days. These patterns are highly reproducible among all specimens of each treatment group and can therefore be used as intrinsic marks. They could be applied to hatchery-reared coregonids, thus providing a basis for judging the efficacy of stocking operations.

Effects of temperature and food density on egg development, larval survival and growth of perch (Perca fluviatilis L.)

Abstract Egg development of perch ( Perca fluviatilis L.) at a series of constant temperatures, and larval survival and growth in intensive culture systems at different temperatures and food densities were compared. Hatching was observed within the temperature range of 6–22°C, with the highest survival between 12 and 20°C. The longer larvae at mass hatching were found at 12 to 18°C. In one trial, 4-day-old larvae were fed at 15 and 20°C with 0, 500, 3000, 6000, 9000 and 12 000 rotifers·l −1 over a 9-day period. A high mortality was observed during the second week after hatching. Survival and growth of larvae at different temperatures and food densities were significantly different. The best survival and growth were observed at 20°C and 6000 rotifers·l −1 . In another trial, 12-day-old larvae were fed at 15 and 20°C with 0, 50, 100, 200, 400, 600, 800 and 1000 copepodites·l −1 over a 12-day period. Survival rates were similar between both temperatures but growth was significantly better in the warmer water. Survival rates at both temperatures increased with copepodite densities from 50 to 200·l −1 . At 200 or more per liter, survival rates were similar, but growth rates were highest at 400–1000 copepodites·l −1 and 20°C.

Trophic Characterization, Pelagic Food Web Structure and Comparison of Two Mesotrophic Lakes in Brandenburg (Germany)

We conducted comprehensive field investigations to characterise the trophic status and food web structure of Grosser Vatersee (GV) and Kleiner Vatersee (KV), two stratified lakes in the Baltic lake region of northeastern Germany. The lakes will be used as manipulation (GV) and reference (KV) ecosystems in food web studies. Both lakes are mesotrophic to slightly eu rophic with mean epilimnetic total phosphorus (P) concentrations of 24 and 21 μg L -1 in GV and KV, respectively. Mean phytoplankton biomass was 0.9 and 0.5 cm 3 m - in GV and KV. During summer, the algal community of both lakes was dominated by cyanobacterial picoplankton and Ceratium spp. Filamentous cyanobacteria (mostly Planktothrix sp.) prevailed in a deep chlorophyll maximum found within the chemocline of both lakes. Bacterial biomass in the epilimnion of GV usually ranged between 1 and 3 cm 3 m 3 . Mean ciliate abundance was 13 ind. mL -1 in GV and 14 ind. mL -1 in KV. While Oligotricha dominated in GV, Scuticociliata were found most frequently in KV. Mean annual crustacean biomass was high in both lakes (GV 2.5 cm 3 m -3 , KV 1.2 cm 3 m -3 ); Eudiaptomus gracilis, E, graciloides and Daphnia spp. were the most prominent species. Total fish biomass of both lakes was dominated by roach (GV 52%; KV 57%) and perch (GV 24%; KV 34%) Vaucheria dichotoma and Chara spp. dominated the macrophyte community in GV; Chara spp. dominated in KV. The slightly more eutrophic character of GV was most likely a result of greater P accumulation in the sediments due to external loading, which resulted in greater hypolimnetic P concentrations throughout the summer in GV. However, on an annual basis these differences were only occasionally reflected at higher trophic levels. The differences in fish biomass, however, might have been a consequence of higher structural diversity in GV provided by submersed macrophytes rather than its more eutrophic character. We conclude that the trophic characteristics and food web structure of Grosser Vatersee and Kleiner Vatersee are similar enough to be suitable as manipulation and reference lakes in food web experiments.

Allocation of echo integrator output to small larval insect (Chaoborus sp.) and medium-sized (juvenile fish) targets

Abstract 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, s a ) due to one size class of targets is plotted vs. decreasing s v -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 s a value for the larger targets ( s a -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 s a -large from the s a 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 m 2 plankton net of 0.5 mm mesh size. The s a 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.

Postglacial colonization shows evidence for sympatric population splitting of Eurasian perch (Perca fluviatilis L) in Lake Constance.

Previous microsatellite analysis showed that two subpopulations of perch (Perca fluviatilis L.) exist in Lake Constance. This raises questions of whether (i) Lake Constance was colonized by two populations that diverged in allopatry, or (ii) the two subpopulations diverged in sympatry. Sequence analysis of a 365 bp mtDNA fragment (5′‐end of the D‐loop) of perch from Lake Constance and adjacent waters revealed 10 haplotypes. We suggest colonization via the Danube river, based on the frequency and dispersion of haplotypes, and knowledge of the lakes palaeohydrological development. Pairwise FST‐values using mitochondrial DNA sequences showed no significant population subdivision. Our study provides strong evidence that subpopulations of perch in Lake Constance have diverged in sympatry.

Growth and body composition of juvenile Colossoma macropomum Cuvier 1818 (Characoidei) feeding on artificial diets

Abstract Juvenile Colossoma macropomum were fed for 56 days with six pelleted feeds which contained fish meal and blood meal as sources of animal protein in varying proportions. Blood meal can be incorporated in a diet together with fish meal but it was inadequate as the only source of animal protein for young C. macropomum . Growth rates from 1.1 to 2.5% dry weight/day were proportional to dietary crude protein levels (25–37%) while dry matter content of whole fish bodies (35-26%) was inversely proportional to growth rate. Body composition on a dry matter basis shows highest protein (53%) and lowest fat level (26%) for the fastest growing fish.

Respiration Rates of Eurasian Perch Perca fluviatilis and Ruffe: Lower Energy Costs in Groups

The effect of group size on the routine metabolic rate and activity of the two shoaling percids, Eurasian perch Perca fluviatilis and ruffe Gymnocephalus cernuus, was studied by using twin-flow intermittent respirometry and time-lapse video techniques. In both species, we found a clear group effect. In isolated fish, oxygen consumption was as much as twice that in groups of eight fish, with intermediate values in groups of four fish. The routine metabolic rate was highest during twilight in both species, irrespective of group size. Eurasian perch consumed more oxygen and were more active during the day than during the night, whereas the oxygen consumption and activity of ruffe were higher during the night than during the day. With increasing group size, the differences between day and night decreased and the diel cycle was less pronounced. Individual fish may benefit from the presence of conspecifics through a calming effect that reduces their energetic costs. We advise that the social behavior of a species be more thoroughly considered when planning behavioral, growth, and respiration experiments. Because bioenergetic model parameters for many species are based on data gained from isolated fish, we conclude that without considering group size the results of bioenergetic modeling may be seriously biased.