Lauri Urho - Academia.edu (original) (raw)

Papers by Lauri Urho

Along with increasing eutrophication the pike (Esox lucius) has decreased in numbers in some part... more Along with increasing eutrophication the pike (Esox lucius) has decreased in numbers in some parts of the Baltic Sea. We undertook a field study to identify the environmental factors affecting condition of pike larvae around the southwestern coast of Finland. We also sampled the natural zooplankton community to reveal food preferences of pike larvae. Our mixed model showed that larval condition decreased with increasing turbidity, whereas temperature, salinity and stomach content were non-significant factors. One third of the variation in larval condition was explained by sampling site, indicating that site characteristics influenced the condition index significantly. The prey selection index showed that pike larvae preferred adult copepods and cladocerans, whereas they ignored rotifers and seemingly copepod nauplii as well. Together, our data show that food availability and several environmental factors are crucial for the condition of pike larvae, and probably also survival and recruitment.

Fisheries Research, Sep 1, 2001

Size and abundance of age-0 perch and pikeperch were studied in three adjacent bays in the Gulf o... more Size and abundance of age-0 perch and pikeperch were studied in three adjacent bays in the Gulf of Finland, Baltic Sea. The age-0 ®sh were sampled with a beach seine in littoral habitats from June to August in 1982±1993. The results showed that once the studied percids were recruited to the seine, at about the size of 10 mm, the lengths were positively correlated with water temperature, measured as degree-day sums over a threshold of 108C. The lengths differed among the three bays with time, but this was due to spatial differences in water temperatures. In June, the correlations between abundance and water temperature were positive, indicating an earlier recruitment to the seine in warmer years. In late July, abundance correlated negatively with the accumulated temperature, indicating an earlier outward migration in warmer years. Due to this temperature-dependent abundance pattern there was no single time period when the age-0 abundances were comparable between years. Only when the accumulated temperature at the time of seining was taken into account annual differences in summer abundance could be compared, and age-0 perch and pikeperch were progressively smaller and less abundant the colder the summer.

Estuarine Coastal and Shelf Science, 2009

The assemblages of young-of-the-year fish were studied in coastal lagoons in an archipelago with ... more The assemblages of young-of-the-year fish were studied in coastal lagoons in an archipelago with post-glacial land-uplift, which affects environmental gradients at local and regional scale, i.e. lagoon habitat isolation and archipelago position, respectively. The categorisation of 40 undisturbed lagoons into nine habitat types based on habitat isolation and archipelago position was supported by clear relationships with spring temperature and total

Estuarine Coastal and Shelf Science, Dec 1, 2016

Rapidly warming shallow archipelago areas have the best energetic options for high ecological pro... more Rapidly warming shallow archipelago areas have the best energetic options for high ecological production. We analyzed and visualized the spring and summer temperature development in the coastal areas of the Northern Baltic Sea. Typical for the Baltic is a high annual periodicity and variability in water temperatures. The maximum difference between a single day average temperatures across the study area was 28.3 °C. During wintertime the littoral water temperature can decrease below zero in outer archipelago or open water areas when the protective ice cover is not present and the lowest observed value was-0.5 °C. The depth and exposition are the most important variables explaining the coastal temperature gradients from the innermost to the outermost areas in springtime when water is heated by increasing solar radiation. Temperature differs more within coastal area than between the basins. Water temperature sum was highest in innermost areas, lowest in open water areas and the variation in daily averages was highest in the middle region. At the end of the warming period, the difference in surface water temperatures between the innermost and outermost areas had diminished at the time when the cooling began in August-September. These clear temperature gradients enabled us use the cumulative water temperature to classify the coastal zones in a biologically sensible manner into five regions. Our study shows a novel approach to study detailed spatial variations in water temperatures. The results can further be used, for example, to model and predict the spatial distribution of aquatic biota and to determine appropriate spatio-temporal designs for aquatic biota surveys. The new spatial knowledge of temperature regions will also help the evaluation of possible causes of larger scale climatological changes in a biological context including productivity.

Canadian Journal of Fisheries and Aquatic Sciences, Dec 1, 2008

The roach ( Rutilus rutilus ) is a common freshwater fish species in the brackish coastal areas o... more The roach ( Rutilus rutilus ) is a common freshwater fish species in the brackish coastal areas of the northern Baltic Sea. In this study, surveys of roach larvae were carried out at reed-belt shores encompassing the entire archipelago zone in the northwestern Gulf of Finland. A logistic regression model was constructed and then used to spatially predict and map potential roach reproduction areas using a geographic information system (GIS). The results indicate that low spring salinity (<4‰) is the most important factor determining the success of roach reproduction. Reed-belt shores in the inner archipelago with large freshwater inputs in the spring constitute the key reproduction areas. Spring runoff peaks caused by melting snow together with the effects of ice cover on the spreading of freshwater runoff enable roach to reproduce relatively far from river mouths. Nevertheless, 68% of the reed-belt shores in the study area are presently beyond the 4‰ salinity limit and thus unsuitable for reproduction. In the future, climate change is predicted to reduce the salinity of the Baltic Sea, but the potential climate change impacts on roach are partly contradictory. The most likely outcome, however, is a spatial increase in the extent of roach reproduction areas in the northern Baltic Sea.

Environmental Biology of Fishes, Apr 1, 1990

Several freshwater species use the Kyronjoki River estuary as a spawning and nursery area. The ma... more Several freshwater species use the Kyronjoki River estuary as a spawning and nursery area. The main reasons for this seem to be the morphology of the estuary, the abundance of shelter provided by aquatic macrophytes, high food production and favourable temperature conditions. Acidification of the estuary due to drainage from acidic soils has made part of the estuary unsuitable for fish reproduction. In addition, year to year fluctuations in the acidity of the estuarine water have affected the reproductive success of several species. The severity of the effects of the acidification at the population level is determined by the spatial and temporal distribution of the larvae and juveniles.

Estuarine Coastal and Shelf Science, Nov 1, 2011

Advances in limnology, Oct 30, 2013

ABSTRACT Catches of sea-spawning whitefish (Coregonus lavaretus (L.)) have decreased and stocks o... more ABSTRACT Catches of sea-spawning whitefish (Coregonus lavaretus (L.)) have decreased and stocks of this diverse species have become vulnerable in the Baltic Sea area. The objectives of this study were 1) to find out whether whitefish can still hatch in the wild in the Archipelago Sea of the southern Gulf of Bothnia, and 2) to define the differences in spawning and hatching times as well as hatching success between areas at different latitudes with diverging temperature loads. To estimate the spawning and hatching temperatures and times of the sea-pawning whitefish, both field and laboratory experiments were carried out in 2007 and 2008. Fertilised whitefish eggs were incubated in spawning areas and at two different temperatures in the laboratory. Naturally-spawned whitefish larvae were also sampled in the vicinity of these test areas and from a more northern reference area. The study indicated that the spawning of whitefish in the northern Baltic Sea starts in late autumn when the day length decreases to approximately 8 h 15 min and the seawater temperature decreases to a level of 7.2 to 3.5 °C, regardless of latitude. The hatching period of sea-spawning whitefish larvae is not only dependent on temperature day-degrees but also on the temperature increment. Whitefish larvae hatch in spring after the ice breakup when the water temperature reaches a level of 2 to 4 °C. The hatching experiment revealed that some natural reproduction can still take place in the southern parts of the Gulf of Bothnia in the Baltic Sea, specifically the Archipelago Sea and the Bothnian Sea, although whitefish larvae are more frequent in the northern areas.

Diversity, Apr 22, 2023

This article is an open access article distributed under the terms and conditions of the Creative... more This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY

About 100 fish species (98 teleosteans, 1 cartilaginous fish and 3 lampreys) were found to be liv... more About 100 fish species (98 teleosteans, 1 cartilaginous fish and 3 lampreys) were found to be living in Finnish waters. This publication includes a checklist of Finnish fish species which lists the scientific, Finnish, Swedish and English names in a systematic table, and the occurrence of species in fresh and different brackish water areas in Finland is also recorded. Fifty-eight fish species can be considered to be native and resident. Annually, it is possible to find 67 bony fish and two lamprey species in Finnish waters. There are twenty-two marine fish species. More than one third of the species (24) have fresh and brackish water populations and also anadromous ones. The variable conditions (mainly temperature and salinity) have not made it easy for new species to naturalize into Finnish waters. The fish fauna was basically established about 4 000 years ago when the present Baltic Sea era started. Only four species were added to the species list during the last century but two new species were recognised in 2005. Fourteen new fish species have been imported and introduced into Finnish waters. Four of these species have naturalized, i.e. they have established reproductive and self-sustaining stocks after introductions into some water bodies, and four other species are maintained by repeated introductions, though there has been some reproduction success in a few exceptional cases. Two lamprey species and 59 fish species are known to reproduce more or less every year in Finnish natural waters. One third of all species are either escapers or have been released from fish farms in neighbouring countries, or explorers that have migrated from the southern Baltic Sea or further away. Three species are extinct and anthropogenic changes affect fish fauna far more than all natural events. Dredging and damming of rivers have had the most significant impact on our fish stocks, mainly on anadromous species. At least 30 or maybe even as many as 47 salmon stocks have been lost and only six native stocks have survived. Similarly, only nine original sea trout stocks out of 62 rivers running from Finland to the Baltic Sea are viable. Acidification and eutrophication have also changed the structure of fish fauna in many water areas. Eleven species or their forms have been put into different classes of conservation depending on their vulnerability. Thirty-one fish species are considered to be commercially exploited. The estimated commercial and recreational fish catch in Finland annually is about 150 million kg. Scenarios on climate change and recent events predict significant changes in the structure of fish assemblages and the arrival of new fish species into Finland, e.g. it may be possible to catch the Chinese sleeper (Perccottus glenii) or some explorer species migrating from the South or East in the near future.

Boreal Environment Research, 2008

The coastal reproduction areas of pike in the western Gulf of Finland were surveyed during spring... more The coastal reproduction areas of pike in the western Gulf of Finland were surveyed during spring 2004 and 2005 using a new approach. The locations of reed belts, the most important spawning substratum, were first identified using aerial photos, and 36 sites in three archipelago zones were selected for field sampling. The occurrence of pike larvae at each 100 m long site was observed with the aid of a white plate and a scoop. The majority of pike larvae were found in a habitat formed by the previous season's flattened reeds at a water depth of 20-80 cm. The comparison between archipelago zones revealed that reed belt shores in the innermost archipelago and bay area, substantially influenced by freshwater inputs in the spring, are the key reproduction areas of pike. Pike larvae were abundant in these areas, in contrast to the intermediate and outer archipelago, where pike larvae were found only sporadically. It is likely that productive habitats in the inner archipelago serve as a source and the outer archipelago as a sink, the latter maintaining pike population with the aid of juvenile immigrants from the main reproduction areas. The results also indicate that pike can not take full advantage of the slowly increasing spatial coverage of reed belts in the intermediate and outer archipelago of the western Gulf of Finland.

Perch (Perca fluviatilis L.) spawn in lakes, rivers and estuaries and even in the brackish water ... more Perch (Perca fluviatilis L.) spawn in lakes, rivers and estuaries and even in the brackish water of the Baltic Sea. In the outer archipelago of the Baltic the selection of unexposed spawning sites can be explained by the genetic component in larval dispersal and the return to inshore waters. The larvae were found to be active, at least in initiating the dispersal. Nevertheless, currents probably aid in the dispersal process. Although the shift back inshore has been documented in all lakes studied, it was here noted to be a gradual process. The timing of the return to littoral areas varies somewhat from one lake to another, and it may also depend on the size and trophic state of the lake. Perch thus appears to be adapted to variable environments. It is suggested that the shift may be a sensitive period in the formation of year-class strength, since the 0+ perch are then more exposed to predator pressure. The dispersal strategies of roach and perch larvae differed, and only after perch returned to the littoral did the distribution area of 0+ perch overlap with that of 0+ roach. when and why these changes take place. Previous hypotheses were based on data often collected from only a few inshore and offshore stations or on samples taken only at certain depths. These shortcomings were not always recognised when the hypotheses were formulated. Observations made on the habitat shifts of two closely related species (perch and yellow perch) manifest very similar, but still not quite identical, patterns. This does not mean that the species behave differently; rather that several different surroundings are involved. A number of papers have been published on the subject of habitat changes in lakes, but data on rivers, estuaries and sea areas are scarce. Here I present data on habitat shifts in the early life history of perch in a small lake and review earlier publications.