Mika Vinni

Diet shifts and food selection of perch Perca fluviatilis and roach Rutilus rutilus in humic lakes of varying water colour

The field data from four humic lakes suggested that water colour may have both direct and indirect effects on inter- and intra-specific interactions of perch Perca fluviatilis and roach Rutilus rutilus. The results agree with suggestions that, compared with R. rutilus, P. fluviatilis may be an inferior forager on zooplankton in highly coloured water. As an indirect effect, water colour decreases the coverage of macrophytes and limits suitable littoral habitats, benefiting R. rutilus over P. fluviatilis. Perca fluviatilis benefiting from complex habitats does not have the advantage in macrophyte-poor highly coloured water.

A metalimnetic oxygen minimum indirectly contributing to the low biomass of cladocerans in Lake Hiidenvesi - a diurnal study on the refuge effect

The diurnal vertical migrations of smelt (Osmerus eperlanus), larvae of phantom midge (Chaoborus flavicans) and cladoceran zooplankton in eutrophic Lake Hiidenvesi were studied in order to clarify the factors behind the low zooplankton biomass. In the study area, an oxygen minimum occurred in the metalimnion in the 10–15 m depth. No diurnal fluctuations in the position of the minimum were observed. Cladocerans inhabited the epilimnion throughout the study period and their vertical movements were restricted to above the thermocline and above the oxygen minimum. C. flavicansconducted a diurnal migration. During the day, the majority of the population inhabited the 12 – 15 m depth just in the oxygen minimum, while during darkness they were found in the uppermost 8 m. Smelts started ascending towards the water surface before sunset and reached the uppermost 3 m around 23:00. During daytime, the majority of smelts inhabited the depth of 7–9 m, where the water temperature was unfavourably high for them (18 °C). Smelts thus probably avoided the steep oxygen gradient in the metalimnion, whereas Chaoborusused the oxygen minimum as a refuge against predation. Those smelts that were found in the same water layers as Chaoborusused the larvae as their main prey. The metalimnetic oxygen minimum thus seemed to favour the coexistence of vertebrate and invertebrate predators, leading to a depression of cladoceran zooplankton.

The food, growth and abundance of five co-existing cyprinids in lake basins of different morphometry and water quality

The food, growth and abundance of five co-existing cyprinid fish species in the eutrophic Lake Hiidenvesi were studied. The diet overlaps within the cyprinid community in shallow lake basins were compared with those in a deep basin, where littoral resources are less available. Roach, bleak and white bream inhabited both the shallow and the deep parts of the lake. Their growth rate was slow, probably due to the low availability of animal food, indicated by the increasing proportion of detritus and plant material in the diets towards the end of the summer. In the deep basin, roach and bleak, contrary to white bream, did not forage on the very abundant invertebrate Chaoborus flavicans, explained by the migration behaviour of C. flavicans. Blue bream, unlike other cyprinids, utilized copepods and had a relatively fast growth rate, but was mostly restricted to the shallow areas. The condition of the bream stock was weak both in terms of growth and abundance. The availability of zoobenthos was low and bream was not able to compete for zooplankton with the more specialized planktivores.

The supremacy of invertebrate predators over fish: factors behind the unconventional seasonal dynamics of cladocerans in Lake Hiidenvesi

In the eutrophic Lake Hiidenvesi (30.3 km 2 ), the early summer biomass maximum of cladoceran zooplankton is missing and the highest biomass is found unusually in mid summer. The contribution of the phantom midge Chaoborus flavicans to the unconventional seasonal dynamics of cladocerans was studied. The seasonal development of the C. flavicans population and the zooplankton community was studied together with food consumption by chaoborids and the most common vertebrate predator, smelt. The population density of Chaoborus peaked in June and collapsed in mid summer. Thereafter, another peak occurred at the end of August. The maximum density of chaoborids in the water column varied between 5,500 and 8.700 inc./m 2 . In early summer, the most common food items of Chaoborus were bosminids while later in summer the share of daphnids increased. Food consumption by the Chaoborus population peaked at 26-46 μg Cl -1 d -1 in June and clearly exceeded the production rate of cladocerans. Cladoceran consumption by smelt population reached its maximum, 1.13 μg Cl -1 d -1 , in early July. Cladocerans showed a 20-40 μg C/l biomass peak in July-August, coincidently with the minimum density of chaoborids. When predation pressure decreased, the mean size of cladocerans increased at 10-15 m, the depth occupied by chaoborids during daytime. The results suggest that in early summer the high predation pressure prevents cladocerans from reaching a sufficient body length to defend them against Chaoborus and that the timing of the cladoceran biomass peak is determined by the relaxation of chaoborid predation.

Application of coded microtags to study growth rates of stocked sympatric whitefish (Coregonus lavaretus (L.)) forms

Abstract Five ecologically different whitefish forms were stocked in Lake Vuokalanjarvi in eastern Finland to establish dense populations and to analyse competition between different forms. Altogether, 213 253 whitefish were tagged with coded wire tags (microtags) injected into their snouts in 1987–1988. The morphologically highly similar whitefish forms have different gill-raker count distributions, but distributions overlap. In 1989–1992, 12 755 tagged whitefish were recaptured and the codes of the microtags were identified. In this study a growth model was applied to analyse and compare growth rates of different whitefish forms. The model was based on annual length increments. Differences in both the length increments in the youngest age-group and also the annual decline in length increments of different forms were statistically significant (F-test, p

Stunted growth of pikeperch Sander lucioperca in Lake Sahajärvi, Finland

Growth of pikeperch Sander lucioperca in the eutrophic and clay-turbid Lake Sahajärvi, Southern Finland, was extremely slow in comparison with other lakes at similar latitudes. The most important food item in July was phantom midge larvae Chaoborus flavicans for all sizes of S. lucioperca (239-423 mm total length L(T)), while later, in August and September, the diet of S. lucioperca (149-407 mm L(T)) consisted of small (30-100 mm L(T)) perch Perca fluviatilis, ruffe Gymnocephalus cernuus and roach Rutilus rutilus.

Mechanism of hybridization between bream Abramis brama and roach Rutilus rutilus in their native range

Mechanisms of hybridization between bream Abramis brama and roach Rutilus rutilus were studied within the native range of the species in a lake in southern Finland. Through the genetic analysis of A. brama, R. rutilus and putative hybrids, hybridization is shown to have occurred between female A. brama and male R. rutilus. These results match with previous findings from introduced habitats, suggesting that mating between female A. brama and male R. rutilus is the predominant mechanism through which the two species hybridize.

First Record of Natural Hybridization and Introgression between Pikeperch (Sander lucioperca) and Perch (Perca fluviatilis)

A case of natural hybridization between pikeperch (Sander lucioperca) and perch (Perca fluviatilis) was confirmed based on the intermediate morphological, anatomical and genetic characteristics of the hybrid. Micro satellite analyses ruled out the possibility that the focal individual was of either pure species, and indicated it was a S. lucioperca backcross.

Living in the edge: the fate of individually marked pike (Esox lucius) stocked in a hyper-eutrophic lake with frequent winter hypoxia

ABSTRACT Eutrophication of lakes has increased the risk of fish kills during winter under ice cover. If such a lake is isolated, the fish assemblage consists of only oxygen-tolerant species without predatory species. Here, our main aim was to analyze the fate of 99 marked pike (Esox lucius) stocked to a pikeless lake that is known to have low oxygen concentrations under ice cover. These pike (total lengths 33.5–76.9 cm; weights 260–3050 g) were stocked in May 2008 and were followed 7 years. The first marked pike was caught in August 2008 and the last one in September 2012. Between these dates, 21 pike were caught at least once, five pike were caught twice, and two pike were caught three times. Relative condition was based on the length–weight relationship. Both length and weight affected significantly the recapture probability, whereas relative condition had no effect. The growth of marked pike was estimated based on length or weight increments in two separate models with either starting length or weight together with the number of days between observations. In these models, the number of days had positive, and size, either length or weight, had a negative relationship with growth. According to marginal r2, length and the number of days explained 65.5% of variation in length increments, whereas the marginal r2 for weight and the number of days was 49.3% for weight increments. The length increment model was used to compare pike growth in Lake Savijärvi with other Finnish lakes. The comparison showed that during the first four growing seasons after stocking, the pike growth in Lake Savijärvi was faster than in the other lakes studied.