Mikko Olin

Comparison of gillnet and trawl in diurnal fish community sampling

The fish community in a shallow, eutrophic lake basin in southern Finland was sampled diurnally with gillnets and trawl. The differences in species number, relative abundances and length distributions were considered. The fish density estimations differed notably depending on the gear and diurnal period. The most abundant species in the trawl catch, smelt, was almost totally missing from the gillnet catch. The proportions of perch, roach, white bream and asp were higher in the gillnet catch. Gillnets regularly underestimated the proportion of small (<10 cm) individuals in size distributions. The trawl probably underestimated the density of piscivores. In the two gears, diurnal changes, in both fish catch and species distribution, were considerable. Both trawl and gillnets are needed to get more reliable figure of fish communities in shallow eutrophic lakes.

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.

Development of a fish-based index to assess the eutrophication status of European lakes

The use of the CEN (European Committee for Standardization) standard method for sampling fish in lakes using multi-mesh gillnets allowed the collection of fish assemblages of 445 European lakes in 12 countries. The lakes were additionally characterised by environmental drivers and eutrophication proxies. Following a site-specific approach including a validation procedure, a fish index including two abundance metrics (catch per unit effort expressed as fish number and biomass) and one functional metric of composition (abundance of omnivorous fish) was developed. Correlated with the proxy of eutrophication, this index discriminates between heavily and moderately impacted lakes. Additional analyses on a subset of data from Nordic lakes revealed a stronger correlation between the new fish index and the pressure data. Despite an uneven geographical distribution of the lakes and certain shortcomings in the environmental and pressure data, the fish index proved to be useful for ecological status assessment of lakes applying standardised protocols and thus supports the development of national lake fish assessment tools in line with the European Water Framework Directive.

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.

Recovery of the fish community and changes in the lower trophic levels in a eutrophic lake after a winter kill of fish

A severe oxygen deficit induced a fish kill in the eutrophicated two-basin Lake Äimäjärvi in southern Finland during winter 2002–2003, resulting in cascading effects on the lower trophic levels of the lake. Pikeperch disappeared from the lake and bleak and white bream decreased to low numbers. The recovery of the populations of other species started immediately when strong year-classes of roach and perch appeared in summer 2003 and onwards. A sharp increase in the growth of perch and roach was recorded, and perch became the dominant fish species during 2004–2006. Consequent responses after the fish kill included increased Secchi depth, expansion of submerged macrophytes, decreased nutrient concentrations and reduction of Cyanophyta from the more eutrophic northern basin of the lake, and a temporary increase in the size of Daphnia in the early summer 2003. However, the ecosystem of the lake returned gradually to the earlier structure and level of eutrophication. Bluegreen algae have returned since 2005, the water has become more turbid, macrophytes declined in 2007 and the fish community was again dominated by small cyprinid fishes in 2008.

Development and evaluation of the Finnish fish-based lake classification method

The Finnish fish-based lake assessment system EQR4 was updated using more extensive lake data with tighter reference criteria. The updated EQR4 (EQR4new) had significant and tighter pressure (eutrophication) relationship compared to the previous version (EQR4old). The EQR4new separated impacted and reference lakes with lower misclassification risk compared to the EQR4old. All the variables within the multi-metric index EQR4new (total biomass, total number of individuals, cyprinid biomass %, and indicator species) were found to be relevant. The classification output in relation to eutrophication was higher in the northern part of the country compared to the southern lakes which was probably due to the climate-dependent reduction in productivity. Despite the clear pressure response and the ability to separate reference and impacted lakes, the accuracy of the EQR4new values is relatively modest. Including regional patterns in the composition and variability of lake fish communities in Finland would increase the accuracy of the assessment method and, moreover, contribute to the excellence of fish community ecology of boreal lakes.

Intensive fishing can mediate stronger size-dependent maternal effect in pike (Esox lucius)

In pike E. lucius L., evidence on maternal effect on reproductive output is mixed. We studied whether older and larger pike females produce eggs and larvae of higher quality (weight, starvation resistance) in three forest lakes in southern Finland. Later, the study lakes were subjected to intensive experimental pike fishing, which we assumed would increase resource availability and lead to higher maternal investment (larger egg size). Length of female pike was positively correlated with the dry weight of eggs and larvae but this relation was dependent on female age. In old females, the effect of female length on egg weight was lower or even negative. Survival analysis showed a positive effect of female length on larval survival time indicating that larvae from larger females are less vulnerable to starvation during the early stage of life. After the intensive pike fishing, the positive effect of female length on egg weight was stronger in all age classes probably due to the released resources. Based on the high quality and amount of reproductive products in large (but not very old) females, they are important for the reproduction of pike populations. This should be considered in fisheries management.

Feeding efficiency of planktivores under disturbance, the effect of water colour, predation threat and shoal composition.

The consumption of phantom midge Chaoborus flavicans larvae by Perca fluviatilis showed clear response to water colour, predation threat and shoal composition with the most significant negative effect for water colour. In the case of Rutilus rutilus, no similar combined response was observed and the total prey consumption was significantly negatively affected by predation threat of Esox lucius. The results suggest that differences in life-history traits may result in disparity in species-specific responses to disturbance.

Reversed Diel Horizontal Migration of Fish: Turbidity Versus Plant Structural Complexity as Refuge

ABSTRACT Diel horizontal migration (DHM) by small perch (Perca fluviatilis) between the littoral vegetation zone and open water was studied in the eutrophic Kirkkojärvi basin (southern Finland). Small perch showed reversed DHM by occupying the open water area during daylight and moving into the littoral zone at dusk. They avoided the vegetation zone during daylight because water clarity among the plants was much higher than in the open water, which was attributed to the reductive effect of macrophytes on sediment resuspension. The migration of perch into the littoral zone at dusk was explained by the higher availability of zooplankton among the plants. The results indicated that the importance of structural complexity of vegetation as a refuge decreases with increasing turbidity of the water. The effects of macrophytes on water turbidity may create circumstances where predation threat for small planktivorous fish during daylight is highest among the vegetation, and such circumstances may thus induce reversed DHM.

Responses of zooplankton and fish to restoration in eutrophic Lake Tuusulanjärvi in southern Finland

Lake Tuusulanjãrvi ( surface area 6 km2, mean depth 3.2m, maximum depth !Om) is the largest lake of theVantaa river basin, located in a densely populated area o f southernmost Finland, 30 km north o f H e isinki. Symptoms of eutrophication of this originally slightly dystrophic lake were first recorded in the 1930s, and since the mid-1950s rapid eutrophication has taken place due to increased nutrient loading ftom settlement and agriculture. Winter aeration of the lake was started in 1972, and sewage discharge stopped in 1979, but the lake remained hypertrophic due to high diffuse pollution and interna! loading (PEKKARINEN 1990). After positive experiences from some restoration projects of large Finnish lakes, such as L. Vesijãrvi (HORPPILA et al. 1998) and L. Kõyliõnjãrvi (SARVALA et al. 2000), biomanipulation by means of mass removal of planktivorous fish in Tuusulanjãrvi was started in 1997. For details of methodology, see SAMMALKORPI (2000). The annual mass removal catches during 1997-2003 were 24-ll3metric tons, 40-190 kg ha-1, totalling ei ose to 420 metric tons and 700kg ha-1 (Table l).