Markku Viljanen

The gastric evacuation rate of vendace (Coregonus albula L.) larvae predating on zooplankters in the laboratory

Abstract The influence of type of prey animals on the evacuation rate of newly hatched vendace ( Coregonus albula L.) larvae was examined in tanks at three temperatures (6, 12, 18°C). The larvae were fed with live zooplankters: copepod nauplii and copepodids. The changes in the composition of the gut content indicated a slower evacuation rate for copepodids than for nauplii. The evacuation rate of the vendace larvae was slowest at 6°C, and increased with temperature. For vendace larvae that had recently resorbed the yolk completely, copepod nauplii were more suitable prey than the larger copepodids. The nauplii were evacuated faster, and presumably, were also digested more efficiently.

Effects of mining industry waste waters on a shallow lake ecosystem in Karelia, north-west Russia

The mining company Karelian Pellet in NW Russia extracts iron ore and processes it locally into iron pellets. The production operations have effects on the environment in the form of air pollution (SO2 and dust) and waste water emissions. The waste waters from the process and the mining pits are let out into a dammed basin, formerly the natural Lake Kostomuksha. This basin flows north-east through several small lakes (e.g., Poppalijärvi, Kento) to the larger lake Middle Kuito and the White Sea. For these small lakes an exceptionally high mineral content, high pH value, and high concentrations of potassium, lithium and nitrogen in the water have been characteristic. However, the total phosphorus content (6–9 μg l−1) is slightly lower than in the Lake Upper Kuito (11–13 μg l−1) which acts as a reference lake. Biota from bacteria to fish were quantitatively sampled in August 2000 and 2001. Our results suggest a twice higher biomass of both phytoplankton (mainly picoalgae) and fish (biomass per unit effort) in the lakes downstream from the mine in comparison to the reference lake. Zooplankton and zoobenthos biomass both peak in L. Kento, the lake in the middle of the outflow series. The fish fauna (seven to eight species caught) is the same in all lakes and is dominated by perch and roach. Both grow well and roach show best growth in L. Poppalijärvi, next to the waste basin. Perch gills show some histopathological changes which do not, however, appear to affect either growth or reproduction.

Fluctuation of year-class strength of vendace (Coregonus albula (L.)) in Lake Onkamo, eastern Finland

Vendace ( Cor~gon~s albula (L.)) is the most important fish spec1es m commercial inland fisheries in Finland. Variation in year-class strength is common (HAMRIN & PERSSON 1986, VILJANEN 1986, SALOJARVI 1987, SANDLVND et al. 1991, AUVINEN 1988, 1994, HELMINEN 1994), and there are often distinct oscillations with varying periodicity in different populations: in some lakes the time period does not exceed 2 years, while in others it can be as long ~s 5 years (VILJANEN 1986). Regular 2-year cycles m year-class strength of vendace can persist for ma~y years (V~LJ~EN 1986, HELMINEN et al. 1993a). A h1gh morta!Ity 1s a prerequisite for this oscillation (HAMRIN & PERSSON 1986). In Finnish lakes, the fishing mortality of vendace in lakes with commercial seine net or trawl fishing is usually high enough to maintain this oscillation pattern (HELMINEN et al. 1992, AUVINEN 1994). In this paper, we describe the c?anges in year-class strength, stock density and survival between egg, larval and juvenile phases of vendace in 1979-1997 in Lake Onkamo. Lake Onkamo has been famous for its highly produ~tive venda~e stock for at least a century. The maJOr propornon of the commercial catch has been caught by winter seining (AvviNEN et al. 1993). Recreational gill-net fishing has only a minor impact on the vendace stock in the lake. Efficient fishing methods, e.g. seine-netting, have been banned for shorter or longer periods over the course of time. In the 1960s and 1970s fishing with seine nets was not allowed until 1979. There have been cominuous conflicts between professional and recreational vendace fishermen throughout the 1990s (SALMI & AUVINEN 1998).

Comparison of methods used in zooplankton sampling and counting in the joint Russian-Finnish evaluation of the trophic state of Lake Ladoga

The results of a joint Russian-Finnish investigation on zooplankton in Lake Ladoga are presented, and a comparison is made between two sampling techniques, tube sampler and plankton net, and between two counting methods. The precision of subsampling and sampling is further discussed on the basis of zooplankton data gathered in Lake Saimaa, Finland. The comparison clearly indicates that a tube sampler is required for reliable sampling of small-sized animals, while a plankton net saves time and is a more economical sampler for large, rare or active animals. The comparison between the results obtained by Finnish and Russian workers, using different counting procedures, shows that the main groups of crustacean zooplankton are similarly counted and identified in the two laboratories.

Estimating and Mitigating Perinatal Mortality in the Endangered Saimaa Ringed Seal (Phoca hispida saimensis) in a Changing Climate

The ringed seal (Phoca hispida) is dependent on sufficient ice and snow cover for a breeding habitat. Therefore, climate change has a negative effect on pup survival. We developed methods to estimate perinatal mortality and to mitigate the effects of mild winters on the critically endangered subspecies (P. h. saimensis). Underwater surveys were used for collecting pup carcasses and placentas. Lanugo pup mortality was 13.5% and Brucella sp. was not found in the placentas. Camera traps showed sporadic human and medium-sized carnivore activity in the breeding habitat. Although carnivore activity was most intensive at lair sites, no penetration of the lairs was observed. We developed a method to mitigate the effects of poor snow cover by piling up snowdrifts (n = 117) at potential lair sites. Seals subsequently occupied 62% of these snow drifts. We suggest that the methods developed in this study should be implemented to conserve the Saimaa ringed seal.

Experimental estimation of the food intake of larval vendace ( Coregonus albula L.) under field conditions

We estimated the food intake of larval vendace (Coregonus albula L.) directly in an experimental chamber under field conditions. The larvae were stocked into net enclosures and reared without additional feeding for 6 weeks immediately after the ice melted in Lake Yla-Enonvesi, Eastern Finland. The food intake was estimated on the basis of the decrease in zooplankters due to larval predation in the transparent plastic chambers anchored in the littoral zone. The experiments were carried out twice in 1988 and three times in 1989 (May–June). The numbers of zooplankters inside the chambers were counted from the subsamples using an inverted microscope. The relative daily ingestion rate (mg dry mass of prey animals mg dry mass offish-1 24 h-1) of the larvae of different sizes was: 1.09 and 0.84 for 9–11 mm larvae, 1.27 for 11–13 mm larvae and 0.34 for 17–20 mm larvae. The measured ingestion rate is presumed to be near the maximum consumption of the larvae. This method could provide accurate estimates of food consumption in the field or in the laboratory.

Horizontal distribution of vendace larvae in Lake Pyhäjärvi (SW Finland): effect of wind-induced currents

The high fecundity of many fish species ensures a large number of offspring even if fertilization rate and survival during the embryonic period are low. Vendace (Coregonus albula (L.)) is an example of a species with high reproductive capacity and variable survival of embryos and larvae; both are factors that together result in large year-class fluctuations. Due to their high number, vendace larvae have the potential to control the population development of the main prey species (KAR)ALAINEN 1992, SARVALA et al. 1994), which in turn may lead to the decreased survival of the larvae. Therefore, the study of spatial distribution may prove to be useful in understanding the population dynamics of vendace. Vendace larvae usually hatch synchronously within 2 weeks and maximum abundance is observed soon after ice-off (KARJALAINEN et al. 2000). After hatching, positively phototactic larvae concentrate primarily in the upper 2 m of the water column, where their distribution may be influenced by wind-driven surface flow. In marine environments, the effect of wind-driven currents on the distribution of larval and juvenile fish has been a subject of numerous studies (e.g. TAGGART & LEGGETI 1987, FECHHELM et al. 1994), whereas much less has been published on freshwater fish. The horizontal distribution pattern of newly hatched vendace larvae differs in Finnish lakes: in certain lakes most of the larvae were in the littoral zone, while in others a large proportion utilized the pelagic zone or larvae were more evenly distributed between both zones (KARJALAINEN et al. 200 l). It has been suggested in several papers that the spatial distribution of vendace larvae may be related to prevailing water currents after hatching (BAGGE & HAKKARI 1987, SARVALA et al. 1994, KAR)ALAINEN et al. 2001), but, to date, such an analysis has not been performed. The objective here was to study the density and spatial distribution of vendace larvae in Lake Pyhajarvi (SW Finland) in different years and to examine whether wind has an effect on the distribution of the larvae. This analysis was carried out by d ual sampling o f larval coregonids in five areas o f the lake in 1992-2000, using simultaneous wind data to simulate prevailing water currents. The distribution of larvae in the first sampling immediately after iceoff was thought to be unaffected by any post-hatch biotic or abiotic factors, and hence roughly represents the distribution of spawning grounds in that particular year. The second sampling took place l week later, and if there were differences in the larval distribution pattern between sampling occasions an d the main sampling areas they were caused either by different site-specific mortalities, temperatures or movement of the larvae.

Temporal and spatial variation in pelagic phytoplankton and zooplankton in the central basin of a large lake, Finland

Spatial heterogeneity is a common feature of ecosystems and is the product of many interacting physical, chemical, and biological processes (PINEL-ALLOUL 1995). The spatial distribution of zooplankton is usually highly aggregated (GEORGE 1981, MALONE & McQuEEN 1983, URABE 1990, VILJANEN & KARJALAINEN 1993, CARTER et al. 1995, KARJALAINEN et al. 1996a), with densities being mainly controlled by the feeding regimes of planktivorous fish (SARVALA et al. 1998). Physical processes such as wind (JONES et al. 1995), wind-induced currents (THACKERAY et al. 2004, RINKE et al. 2007), water temperature and stability (PINEL-ALLOUL et al. 1999), and inshore-offshore temperature gradients (JoHANSsoN et al. 1991) are thought to have a dominant influence on the large-scale spatial distribution of organisms. On smaller scales, however, there are various biological processes that serve to decouple organisms from the direct effects o f many physical processes, notably predation, competition, swarming and avoidance behaviour, reproductive behaviour, size of plankton, food resources (PINEL-ALLOUL et al. 1988, JoHANSSON et al. 1991, VILJANEN & KARJALAINEN 1993, LACROIX & LESCHER-MouTouE 1995). The combined action of wind-induced water movements and organism behaviour can result in large-scale spatial heterogeneity (PINEL-ALLOUL 1995, TuACKERAY et al. 2004). We examined spatial and temporal heterogeneity in the plankton community of a moderately large 1ake, Lake Pyhãselkã, in July of 2 years and revealed and verified short-interval day-to-day temporal and spatial distribution pattems among pelagic plankton. We also analysed the influence of the short time interval, place, depth, temperature, and water currents on phytoplankton and zooplankton.

Diel vertical movements: warm water does not prevent vendace (Coregonus albula (L.)) from tracking zooplankton

We observed small schools of vendace (Coregonus albula (L.)) which occasionally crossed the thermocline and migrated into the exceptionally warm (22 to 23 °C) epilimnion of a large Finnish lake in August 2010. This is against previous observations regarding diel vertical migration (DVM) in this species. The ascending schools were detected for a short period of time about an hour after sunset. The fi ndings were based on data gathered with a 120 kHz echo sounder, an Acoustic Doppler Current Profi ler, a Laser Optical Plankton Counter, a MultiNet plankton sampler and a mid-water trawl for the experimental sampling of pelagic fi sh. The study emphasises the importance of examining DVM with different kinds of samplers simultaneously. We are unable to state the ultimate reason why some vendace migrated from cool to warm water, but we can state that there was a lot of food for them in the epilimnion around sunset. It would have been impossible to detect the phenomenon using only mean or maximum abundances of fi sh, and we had to use very fi ne scales to detect the phenomenon on echograms. The ascension of macrozooplankton, mostly Chaoborus fl avicans (Meigen), followed a rhythm described by a third order polynomial function. Vendace were feeding in the evening near sunset within the ascending anti-predation window in which there was enough light for detecting zooplankton and shelter against predators, while during daytime the majority of stomachs of vendace (63%) and smelt (Osmerus erperlanus (L.)) (50%) were empty in mid-water depth layers of the lake where the main pelagic fi sh populations were detected. Based on our observations on fi sh movements during thermal stratifi cation in August, we conclude that vendace has a potential to tolerate by behavioural means warm water up to 23 °C.

Estimation of zooplankton abundance, distribution patterns and patchiness in a large humic lake using an Optical Plankton Counter

Zooplankton usually behaves in complex and dynamic ways on various temporal and spatial scales and its spatial distribution is usually highly aggregated, as it possesses a large-scale spatial variability similar to that observed in the physical environment in general. We assess here the suitability of the Optical Plankton Counter for estimating zooplankton abundance, as well as the day-to-day temporal and spatial patterns in its distribution during the open water season. The influence of certain environmental variables on zooplankton abundances is also examined. Abundances of mesozooplankton-sized particles were measured with the Optical Plankton Counter monthly at five stations in a large mesohumic lake from May to September 2005 and agreed rather well with zooplankton abundances counted using a microscope. The poorest agreement was in autumn, when the Counter overestimated the zooplankton abundance, and for some samples in July, when it underestimated the abundance. Fluorescence and chlorophyll a had a significant effect on the Counter readings. Both the intercept and the slope of chlorophyll a varied randomly between depths, because the chlorophyll a decreased much more markedly in deeper waters than did the Counter particle counts. This emphasized the stratified nature of sampling. In addition to seasonal variations, there were also marked short-interval variations (day-to-day) in zooplankton abundances in all the sampling periods. These patterns are highly dynamic and can on some occasions change rapidly in response to fluctuations in the weather. There were no clear differences in zooplankton abundance between the sampling stations. The horizontal heterogeneity was less pronounced than the seasonal and short-interval heterogeneity, possibly because there were no trophic gradients in the basin. While the Optical Plankton Counter provided a rapid assessment of temporal and spatial patterns of zooplankton abundances, it does have limitations. At times it either overestimated abundances due to a large contribution from non-zooplankton particles, or underestimated abundances due to coincidence.