Unmanned aerial vehicle (UAV) images and structure-from-motion (SfM) methodology allow very high resolution surface reconstruction. This fairly low-cost and time-saving method is used for northern bog hydrology characterisation. The primary interest of this study lies in seasonally changing bog pool water levels. The study site is located in a potential conflict zone where water extraction from an underground oil shale mine threatens protected wetlands. The study involves two test areas that were surveyed three times, allowing spatio-temporal quantification of seasonal changes of the bog surface. Aerial lidar data and manual measurements were used to assess the quality of SfM surfaces and discrepancies between the different methods were analysed. Overall, there is a good fit between the methods used, validating the use of SfM methodology in a natural wetland environment. This study demonstrates that the nearby mine is influencing the seasonal water-level fluctuations of the protected wetland.
Pikeperch (Sander lucioperca) is a European fresh and brackish water piscivorous fish, important as both a key predator and a valuable commercial and recreational fisheries species. There are concerns that some stocks are depleted due to overfishing and environmental changes. We review data collection and population assessments currently used for nine pikeperch stocks across six European countries and apply a unified assessment framework to evaluate population status and trends. For this we first standardised commercial, scientific, and recreational catch-per-unit-effort (CPUE) and catch time series and then applied Bayesian surplus production models. Our results showed that three stocks (including two in the Baltic Sea) were strongly depleted, with estimated biomasses considerably lower than the biomass at maximum sustainable yield (Bmsy). Other stocks were either close or higher than their estimated Bmsy. Looking at the trends, we find that four stocks (Lake Oulujarvi, Kvadofjarden, Lake Peipsi and Lipno) showed increasing biomass trends and two (Curonian Lagoon, Galtfjarden) had a strong decline in biomass. In most cases the stocks with clear signs of recovery were also those for which strong management strategies have been implemented. We find that, despite pikeperch being one of the most valuable inland fisheries, formalised stock assessments and regular surveys remain rare. Importantly, although most stocks are strongly targeted by recreational fishing, estimates of recreational catch are highly uncertain. We conclude that data limited stock assessment methods are useful for assessing fish population status and highlight an urgent need to improve pikeperch scientific monitoring and assessment of recreational catches.
Cod is usually monitored for scientific purposes using bottom trawl surveys, although its regular pelagic occurrence is well documented. Here we analysed, using Generalized Additive Models, the spatio-temporal changes in the Eastern Baltic cod adult population using pelagic catches from an acoustic survey covering 37 years and the whole central Baltic Sea. Our analysis shows that in the northern areas cod catch per unit effort (CPUE, kg h-1) was high in the early 1980s whereas it dropped and remained very low thereafter. Conversely, in the southernmost area CPUE largely oscillated after the early 1990s. Our model was able to capture key ecological features of the Baltic cod such as preferred depth of occurrence and response to hypoxic conditions. The model also revealed a clear daily cycle of CPUEs, indicating diel vertical migrations at the population level. The temporal trends of pelagic CPUEs generally followed those from the bottom trawl surveys, although differences were observed especially in the recent years with a relative decline in the cod occurring in the pelagic waters. Our results point to the great potential of acoustic survey trawl catches to complement bottom trawl surveys for investigating the spatio-temporal population dynamics and behaviour of the Baltic cod.
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