Abstract
The number of cormorants has rapidly increased in the northernmost Baltic Sea. In 2018, 50 km × 50 km ICES catch rectangle 55H1 had 3140 breeding pairs. To estimate the predation effect of ...cormorants on perch populations, we Passive Integrated Tags tagged 1977 perch and 9.9% of tags were found. The median instantaneous cormorant-induced mortality during the breeding time, with consumption by non-breeding individuals, was estimated at 0.23 and at 0.35 during the whole residing period. We estimated with a yeild-per-recruit model that the long-term maximum loss of perch yield of tagged sub-population would be at 80% probability interval 32–67%, and when extended to the entire 55H1, 10–33%, respectively. The cormorants’ share of the >2-year-old perch biomass and production would be 8%, while that of other natural mortality would be 63% and that of fishing 29% in 55H1. The yield-per-recruit-results should be interpreted as an estimate of maximum cormorant effect because the dependence of predation rate on prey density was not accounted for, and density-dependence of growth, mortality, and reproduction of perch could partly compensate the loss. The results indicate that high density of cormorants can reduce the perch stocks and catches locally.
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 Finnish ...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.
The pikeperch (
Sander lucioperca (L.)) is an economically important fish species occurring in the fresh and brackish waters of Europe. To evaluate the distribution and extent of the reproduction ...areas in the northern Baltic Sea, a field survey was carried out in two separate coastal areas. Presence/absence data were used to develop a geographic information system (GIS)-based predictive spatial distribution model, where high resolution raster maps of the focal environmental variables and a logistic regression equation were used to predict the probability of larval occurrence. The results indicated that the pikeperch reproduction areas are located in the innermost archipelago zone where high water turbidity best explained their presence. Turbidity was related to several other variables such as fetch and depth. Contrary to our preliminary hypothesis, surface water temperatures measured during the survey had no significant effect in the model due to the low spatial variation in the measured values. Since turbidity is possible to determine by remote sensing methods, the probability maps can be cost-effectively extended to more extensive coastal areas with proper validation.
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► First study that characterizes the pikeperch larval areas and constructs a geographical model. ► Larvae were found in innermost archipelago areas. ► Turbidity explains the presence of larvae. ► The turbidity based model can be extended to cover other coastal areas in Northern Baltic Sea.
Summary
We explore the mortality rate of disentangled sea trout in whitefish fishery using gillnets with a 35–43 mm bar length. The study was conducted during the main fishing seasons in the Gulf of ...Bothnia in the northern Baltic Sea. Overall 59.5% of the sea trout were alive at the end of a 2–7 day observation period following release from the gillnets. Altogether, 12.1% of the captured fish were found dead in gillnets and 28.4% died due to injuries during an extended observation period. The average length of the captured sea trout was 435 mm, indicating that the majority were spending their first or second year at sea. The proportion of the survived and not injured (no observed damage) fish was highest in larger fish, >450 mm. The injured and not injured fish died equally frequently. Post capture survival was not connected to the removal time from gillnets or type of observed injuries but to the type of entanglement. Most of the fish were entangled by a mesh around the body, which caused extensive scale loss and open sores on the skin. The smallest fish may have had internal wounds that were not registered in this study. These results can be used in fisheries management to estimate the mortality of multi‐species gillnet fishing to sea trout populations in relation to management actions.
We evaluated the distribution and the extent of sea-spawning whitefish Coregonus lavaretus (L.) s.l. and vendace Coregonus albula larval areas in the Gulf of Bothnia, northern Baltic Sea, and suggest ...that the distribution of the reproduction areas could be an indicator of the health of the Baltic Sea shores. Our Geographic Information System (GIS) based predictive spatial model of habitat selection covers nearly the whole distribution area of both species. Extensive sampling data on larval occurrence were combined with GIS raster layers on environmental variables and used in a Gaussian process model, which predicts the spatial probability of larval occurrence. Out of 22 studied variables, shore profile, distance to sandy shallow shore, distance to 20 m depth contour line and ice break-up week were the most important for describing larval areas of both species. The earliest larval stages of sea-spawning whitefish can be found in various habitats close to the shoreline, but the highest densities of larvae were observed along gently sloping, shallow sandy shores. Vendace reproduction occurs in the northernmost and less saline areas of the Bothnian Bay and larval stages use the shallow areas. Compared to previous studies from 1990s, the extent of whitefish larval areas has decreased. We discuss the possibility that long-term changes in the environment, such as more frequent iceless winters and increasing eutrophication, have reduced the reproductive success of sea-spawning coregonids. Larval distribution maps can be used to focus conservation measures in the most appropriate places. We propose to use this method as a monitoring tool, and produce maps to assist integrated coastal zone management and environmental protection
Spatial distribution of brown trout (Salmo trutta) was studied on the Finnish coast of the northern Baltic Sea in 1998–2010 based on smolt tagging. The studied trout stocks were hatchery reared, and ...smolts were tagged with Carlin tags before release into the rivers. The distance between the release and recapture sites as well as location of the recapture site in relation to the release site (north, south, west, east) were analysed, taking the stock and sea age of the trout into account. The most important tag recovery areas at sea were the estuaries of the spawning rivers and coastal areas surrounding them. The natural direction of movement was along the coast line, north or south on the western coast and east or west in the Gulf of Finland. The release site and age affected migration direction. The distance of recoveries from the release sites varied for the same genetic stock released at different sites. The longest median recapture distances were recorded during the second sea year. The stocked brown trout (80%–95%) were mainly caught during their first two years in the sea before they become mature. Knowledge of spatial dispersal of sea trout is important for the management of the stocks and fisheries.
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