Søren Berg

Pike (Esox lucius L.) stocking as a biomanipulation tool 1. Effects on the fish population in Lake Lyng, Denmark

From 1990–1993 juvenile pike (Esox lucius)were stocked each spring in the eutrophic Lake Lyng (9.9 ha, max. depth 7.6 m, mean depth 2.4 m) in densities between 515 and 3616 pike ha–1. In 1989-90 the fish population consisted mainly of roach (Rutilus rutilus),rudd (Scardinius erythrophthalmus),perch (Perca fluviatilis)and ruffe (Gymnocephalus cernuus),and total fish biomass was estimated at 477 kg ha–1. Prior to stocking pike was not present in the lake. Following the first year of stocking, the density of roach, rudd and ruffe fry expressed as catch per unit effort decreased significantly by 64 to 97%. In 1991 ruffe disappeared completely. The pike stocking did not affect the density of perch significantly. The growth of pike was high and also the growth of perch increased significantly from 1990 to 1991 (p 0.4). Pike survival was low in late August/early September. We suggest that growth of the piscivorous perch increased due to increased Secchi depth and a continuous high density of 0+ planktivores in the pelagic zone of the lake during the years of pike stocking, possibly caused by behaviourial changes and the forcing of the 0+ planktivores into the pelagic zone.

Interactions between piscivores, zooplanktivores and zooplankton in submerged macrophytes: preliminary observations from enclosure and pond experiments

The effects of piscivores upon zooplanktivore behaviour anddistribution and the impact of zooplanktivores on the abundance anddistribution of zooplankton are well documented. However, thepotential indirect effect of piscivores reducing the predationpressure upon grazing zooplankton through behavioural changes ofzooplanktivores has received little attention, even though this maybe an important mechanism in enhancing the stability of submergedmacrophytes in shallow lakes. Preliminary observations from anunreplicated large-scale field enclosure experiment and areplicated pond experiment suggest that this mechanism is plausiblewith the set of piscivores (pike Esox lucius and perch Perca fluviatilis) and the zooplanktivores (0+ roach Rutilusrutilus and perch) common in temperate Europe. The presence ofpiscivores typically changed the habitat use and the activity levelof zooplanktivores and the presence of zooplanktivores typicallychanged the habitat selection of cladoceran zooplankton. In thecase of piscivore/zooplanktivore interactions, the risk ofpredation was enough to generate clear responses even where thelosses to predation were low. However, only in the enclosureexperiment was an indirect impact of the presence of piscivores,enabling Daphnia spp. to utilise open water in the presenceof a high density of zooplanktivorous fish observed. Whether themagnitude and direction of the effect of piscivores is sufficientto benefit zooplankton may depend on the functional group (capableof foraging within structured habitats) of the predator (bothpiscivore and zooplanktivore), absolute and relative densities ofpredator and prey and predator dietary choice.

Pike (Esox lucius L.) stocking as a biomanipulation tool 2. Effects on lower trophic levels in Lake Lyng, Denmark

In order to study how pike stocking affects trophic structure pike fingerlings (0–3600 ha–1) were stocked during six years in eutrophic Lake Lyng (lake area 10 ha), Denmark. Subsequently, marked changes were recorded in the abundance of zooplanktivorous fish, catch per unit effort of roach, which was the dominant fish species, thus varied from 17 to 272. Simultaneously, marked changes were recorded in the abundance and relative composition of zooplankton. Daphnia abundance ranged from 7 to 36 ind. 1_1 and Bosminafrom 2 to 99 ind. 1–1 mean summer abundance of Daphnia being highest and Bosminalowest in years with low fish density. Additionally, cyclopoid copepod and rotifer abundance correlated significantly (p<0.05, r 2 = 0.70 and 0.83, respectively) and positively with fish abundance. Mean summer chlorophyll acorrelated significantly (p = 0.04, r 2=0.70) with the abundance of roach, suggesting an impact on phytoplankton level also, disregarding that the phytoplankton biomass was dominated by filamentous cyanobacteria. Total phosphorus concentration was lowest in years with low zooplanktivorous fish abundance, which is hypothesized to be due to lower sedimentation of organic material and lower phosphorus release from the sediment in years with high zooplankton grazing on phytoplankton. It is concluded that pike stocking can be used as a lake restoration tool to increase lake water transparency by creating a trophic cascade. The effect of stocking, however, seems to last only during the season in which it has been undertaken, the impact being most significant at high stocking densities. The method is therefore regarded to be most useful in shallow, turbid lakes in which the nutrient loading has been sufficiently reduced to allow a substantial and permanent macrophyte coverage if clearwater conditions are established.

The feeding behaviour of large perch Perca fluviatilis (L.) in relation to food availability: a comparative study

The aim of the present study was to analyse and compare the feeding behaviour of large perch (>25 cm total length) in two lakes of different state and corresponding differences in food availability. Investigations were conducted in the Bautzen Reservoir, Germany, (biomanipulated, macrophyte cover <1%) and in Lake Ring, Denmark (non-biomanipulated, macrophyte cover ca. 20%). Fish were sampled regularly using trawl nets and gill nets. We focused our study on seasonal diet patterns. Further, we investigated size-related feeding and predation on both the juvenile fish and invertebrate communities. The availability of Chironominae was high in both lakes (e.g. ca. 1500 ind. m−2 in Lake Ring), but age-0 fish abundance was most probably higher in Bautzen Reservoir (6.4 individuals m−3 in the beginning of June 1998). Invertebrates and age-0 fish were the main food components of large perch of both populations. Mean lengths of prey fish were consistently smaller than those in situ. Large perch in Bautzen Reservoir started earlier to feed on age-0 fish, which was not related to prey fish sizes. Predation on the age-0 fish community was distinctly higher in Bautzen Reservoir than in Lake Ring. Generally, invertebrates represented the most important food of large perch in Lake Ring, whereas the by far dominant food resource of large perch in Bautzen Reservoir was the age-0 fish stock. In conclusion, it can be stated that the prey fish availability was an important factor in determining the feeding behaviour of large perch.

Piscivory of 0+ pike (Esox lucius L.) in a small eutrophic lake and its implication for biomanipulation

Stocking 0+ pike in shallow eutrophic lakes as an indirect way to facilitate clear water by increased phytoplankton grazing by zooplankton, has been used in several studies with varying success. Lack of piscivory could be one reason for poor impact of the pike stockings, on reducing the 0+ cyprinid fish population. In relation to this, we present data on food and feeding habits of 936 0+ pike (Esox lucius L.) (2–18 cm) sampled during five years in a small eutrophic lake undergoing biomanipulation. Overall, the food of 0+ pike followed a sequence of diet shifts including microcrustacea, insects, macrocrustacea and finally vertebrates (fish) with increase in pike length. Despite a high abundance of potential prey fish in the lake, in general less than 50% of the pike were piscivorous until a length of 10.1–13.7 cm. As piscivory increased, the principal prey fish (0+ roach) apparently displayed antipredatorial behaviour which decreased their availability for 0+ pike. We conclude that lack of piscivory caused by e.g. anti predator tactics by prey fish, may reduce the impact of 0+ pike stockings in lake restoration projects.

Utilization of natural and artificial habitats by YOY pike in a biomanipulated lake

Biomanipulation by stocking with YOY pike Esox luciushas been shown to be a useful tool in the restoration of shallow hypertrophic lakes. The success of such stocking depends partly on the standing stock of the YOY pike in the first 2–3 months following stocking, this in turn depending on habitat type and abundance in the lake. The present study evaluated the general habitat use and preferences of YOY pike and the suitability of artificial submerged habitats formed by small laid out spruce trees Picea omoricausing Point Abundance Sampling by Electrofishing (PASE). This sampling method was assessed as being suitable for catching and quantifying small pike. While very few pike were present in either the dominant emergent vegetation Phragmites australisor the subdominant vegetation Typhaspp. in June and July, they were present in significantly greater numbers in the artificial habitats. This suggests that the use of spruce trees as artificial habitats in biomanipulation projects could potentially enhance the standing stock of YOY pike. In August, in contrast, habitat utilization by the YOY pike did not differ from that expected if they used the habitats randomly. This suggests that YOY pike change their habitat utilization as a function of their size. Thus they use habitats with a complex structure in the early summer, but in late summer when they are larger, they use habitats with a less complex structure such as Phragmitesand Typha.

Piscivory and trophic position of Anguilla anguilla in two lakes: importance of macrozoobenthos density.

The feeding habits of the European eel Anguilla anguilla (>300 mm total length, L(T)) were compared in two lakes of different environmental state: Lake Grosser Vätersee (LGV), Germany (clear water, mesotrophic and submerged macrophytes), and Lake Vallum (LV), Denmark (turbid, eutrophic and no submerged macrophytes). The density of macrozoobenthos was higher in LV (3500 individuals m(-2)) than in LGV (1500 individuals m(-2)). The abundance of small prey fishes (40-99 mm L(T)) was highest in LV. In LV, A. anguilla fed on macrozoobenthos, in particular, chironomid larvae. In LGV, A. anguilla used fishes as the main food component. Stable isotope analyses confirmed the stomach contents dietary results. The estimated mean +/-s.d. trophic positions of A. anguilla in LGV (3.7 +/- 0.2) was one level higher than those of fish in LV (2.7 +/- 0.2). Based on these results, it is concluded that piscivory among A. anguilla was generally controlled by the density of macrozoobenthos. Stable isotope analysis further indicated that A. anguilla may act as integrators between benthic and pelagic food webs when density of insect larvae is low.

Non-indigenous signal crayfish Pacifastacus leniusculus are now common in Danish streams: preliminary status for national distribution and protective actions

North American signal crayfish (Pacifastacus leniusculus) are invasive in Europe and pose a serious threat to indigenous European crayfish such as the noble crayfish (Astacus astacus). This is mainly because signal crayfish is the carrier of crayfish plague agent, Aphanomyces astaci, which freshwater crayfish from all other continents are highly susceptible to. Until recently, the distribution of signal crayfish in Danish streams has been considered local and restricted to a small geographical area. Here we present data demonstrating that signal crayfish are now widespread in Denmark, including the largest Danish river, River Gudenå. For one of the rivers where co-existing signal crayfish and indigenous noble crayfish were documented, sensitive molecular tests could not detect the crayfish plague agent Aphanomyces astaci in either species. Hence, it seems that not all signal crayfish are chronic carriers of the disease. For the remaining freshwater systems with the introduced signal crayfish, the infection status is presently unknown. Large areas of the freshwater systems in Denmark also remain unexplored with respect to presence/absence of signal crayfish and noble crayfish. Nevertheless, our preliminary data that covers about 14% of the Danish rivers, strongly suggests that signal crayfish should be considered as a common invader that poses an increased threat to the biota in Danish streams, in particular for the indigenous noble crayfish.

Environmental effects of introducing whitefish, Coregonus lavaretus (L.), in Lake Ring

The impact of whitefish (Coregonus lavaretus (L.)) on the trophic structure of eutrophic lakes was studied in Lake Ring, a small eutrophic Danish lake (22.5 ha, mean depth 2.9 m) in which the natural fish fauna is dominated by pike (Esox lucius L.), perch (Perca fluviatilis L.), and eel (Anguilla anguilla (L.)), roach (Rutilus rutilus (L.)) and burbot (Lota lota (L.)) being the only other fish species present. A total of 10993 0+ whitefish were stocked in the lake from October 1989 to July 1990 and the structure of the fish, zooplankton and benthic invertebrate communities studied during the period 1989–1991. Stomach contents analysis revealed that the whitefish mainly ate Daphnia and copepods in 1990–1991, the proportion of copepods decreasing with increasing size of the fish and Daphnia being the overall most important food source. The density of Daphnia in the lake decreased from 72 ind. 1-1 in 1989 to 9 ind. 1-1 in 1991; concomitantly the large species Daphnia magna and D. pulex almost disappeared and the density of cyclopoid copepods increased from 72 to 101 ind. 1-1, presumably because of improved food conditions, while that of calanoid copepods remained virtually unchanged. As a result chl-a increased from 19 to 47 µg 1-1 and Secchi depth decreased from 2.4 m to 1.7 m, despite there being no change in total P and total N (0.6 mg P 1-1 and 1.3 mg N 1-1, respectively). Changes were also observed in the benthic invertebrates; Chaoborus, oligochaetes, and chironomids all decreased, whereas Pisidium increased. It is concluded that the stocking of whitefish in eutrophic lakes for commercial purposes may delay their recovery, or even lead to enhanced eutrophication.

Phenotypic variation in metabolism and morphology correlating with animal swimming activity in the wild: relevance for the OCLTT (oxygen- and capacity-limitation of thermal tolerance), allocation and performance models

Combining physiological and morphological measures in the laboratory with registrations of detailed measures of field activity, we tested the hypothesis that individual activity patterns correlate with individual metabolism and morphology as proposed by several conceptual models. We found no evidence indicating an effect of metabolism, whereas morphology correlated with several activity measures.