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Browsing by Subject "reproduction area"

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  • Pursiainen, Annie (2020)
    Introduction: High anthropogenic pressure exercised on the Baltic Sea causes a decline in valuable reproduction areas of coastal fish, such as the northern pike (Esox lucius L.). Hence, the conservation and restoration of the reproduction areas of pike are increasingly of importance. The Kvarken Archipelago, located in the Gulf of Bothnia, presents valuable reproduction habitats for fish. The pre-flads, flads, glo-flads and glo-lakes provide warm and sheltered areas that are preferred by spring-spawning fish, such as pike. However, the precise environmental factors influencing pike reproduction in the region have not yet been identified. In this study, different environmental factors influencing pike reproduction in the Kvarken Archipelago were assessed. Furthermore, as technology advances, there is a need to evaluate the use of new techniques alongside the generally time consuming and expensive traditional research methods applied for surveying the pike reproductive areas. Henceforth, the present study assessed the suitability of the high-resolution aerial photography for identifying potential reproduction areas of northern pike. Methods: The data of pike fry density, the vegetation, the abiotic and biotic environmental factors, as well as the aerial images of the 45 sampling sites were collected by Natural Resources Institute Finland and Metsähallitus Parks and Wildlife Finland as part of the Kvarken Flada -project. Pike fry were sampled with a flat dipnet amongst vegetation along a 100 m sampling line during spring. Concurrently, the vegetation, the occurrence of sticklebacks (Gasterosteidae) and the abiotic factors affecting water quality, including salinity, pH, oxygen content and turbidity were determined. The temperature was recorded with temperature loggers from early spring until late summer. The temperature sum for June showed the greatest effect on pike fry densities and was thus used in the analyses. The archipelago zones, which are based on previously modelled temperature sums, were defined for the sampling sites in the geographic information system (QGIS). Additionally, factors describing the morphology, the vegetation and bottom substrate coverage of the sampling sites were utilized in the analyses. The presence of pike fry was analysed with binary logistic regression. The pike fry densities were studied using non-parametric analyses due to the high number of zero observations in the data. The aerial images were taken in late summer of 2017 and 2018, when the vegetation was fully developed. The vegetation types, i.e. reed, fallen reed, wetland grasses, submerged vegetation and filamentous algae, identified in the images were compared to the vegetation data gathered during field survey in QGIS. The performance of the aerial images was assessed by calculating the accuracy, sensitivity and specificity for the different vegetation classes, first for the whole dataset and then for the sampling points with pike fry observations. Results and conclusions: This study determines two key factors defining suitable reproduction areas of pike, the temperature sum for June and the morphology of the sampling site. The results indicate that pike prefer warm and sheltered areas with suitable vegetation, such as reed and wetland grasses. In particular, glo-lakes provide favorable conditions for pike and the temperature sum for June was significantly higher in the glo-lakes compared to the pre-flads. The temperatures of the glo-lakes were constant throughout the archipelago, providing suitable conditions for pike reproduction also in the outermost archipelago. In all of the glo-lakes pike fry were observed and generally the highest pike fry densities were found in glo-lakes. However, the year class strength of pike can be affected by the possibility of the juvenile pike to emigrate from the enclosed glo-lakes back to the sea. Salinity reflects the connectivity of the site to the surrounding sea and had a significant negative effect on pike fry density. Additionally, the presence of sticklebacks affected negatively the presence of pike fry, which might be traced back to the different use of the archipelago zones and site morphologies of these species. Increasing pH negatively influenced the pike fry density. However, pike fry were observed also in the highest pH conditions and therefore pH alone is not sufficient to explain the presence and density of pike fry. Due to the small sample size of the dataset, it can be assumed that not all of the factors influencing pike reproduction were identified and thus the use of a larger dataset is recommendable in future studies. This study provides new methodological information on reproduction area surveying of pike with high-resolution aerial photography. The different vegetation classes were successfully identified from the aerial images. The seasonal differences were noticeable in the results, as in the aerial photographs taken in late summer, the vegetation was fully developed compared to the field study in early spring. Nevertheless, aerial photography proved to be an effective method for identifying reproduction areas of pike based on the vegetation. Furthermore, reproduction areas in need of restoration could be assessed with aerial photography.