CE11015

In the Celtic Sea and VIIj, herring acoustic surveys have been carried out since 1989, and this survey is the 20th in the overall acoustic series or the seventh in the modified time series (i.e. conducted in October). The geographical confines of the annual 21 day survey have been modified in recent years to include areas to the south of the main winter spawning grounds in an effort to identify the whereabouts of winter spawning fish before the annual inshore spawning migration. Spatial resolution of acoustic transects has been increased over the entire south coast survey area. The acoustic component of the survey has been further complimented since 2004 by detailed hydrographic and marine mammal and seabird surveys

Effects of experimentally increased near-bottom flow on meiofauna diversity in the deep Arctic Ocean

Effects of increased near-bottom flow velocities on the sedimentary environment and its associated small biota were studied in a long-term in situ experiment at 2,500 m water depth at the Deep-Sea Observatory HAUSGARTEN in the eastern Fram Strait. In 2003, the Remotely Operated Vehicle (ROV) "Victor 6000" was used to install a stainless steel flume of about 8.5 m in length, consisting out of a 6 m long passageway with a cross-section of 50 x 50 cm and 3-4 m wide funnel-like doorways to increase bottom currents by a factor of approximately 6. Sediment sampling for biochemical sediment analyses, bacterial studies and meiofaunal investigations (with special focus on the nematode communities) was carried out four years after the installation of the flume using the ROV "Quest 4000". The data showed clearly reduced values for parameters indicating organic matter (food) availability in the sediments, and corresponding lower bacterial and meiofaunal densities inside the flume, compared to control sites outside the channel. Results suggest that increased near-bottom currents and food deficiency not only diminish sediment-inhabiting meiofaunal assemblages but also alter the meiobenthic composition. Compared to background sediments, the nematode community inside the flume evidently showed adaptations to the overall reduced food availability and a more heterogeneous environment due to generally increased and more turbulent flow velocities. The variable environmental conditions inside the flume have an effect not only on the number of genera present, but also on the identities of the genera and the functional composition of the nematode community

Fine-scale spatial association between baleen whales and forage fish in the celtic sea1

Baleen whales can be regularly observed in the Celtic Sea; however, little is known about their local foraging behaviour. The study objective was to determine whether or not baleen whales selectively prey upon particular forage fish species or, on the contrary, is predation on the Celtic Sea plateau driven by random encounters between prey and predator? Concurrent sighting surveys for fin (Balaenoptera physalus), minke (Balaenoptera acutorostrata), and humpback (Megaptera novaeangliae) whales were carried out simultaneously from 2007 to 2013 during dedicated fisheries acoustic surveys assessing the abundance and distribution of forage fish. Probabilities of spatial overlap between baleen whales and forage fish were analysed and compared with the probability of a random encounter. For estimations of foraging threshold and prey selectivity, mean fish biomass and fish length were calculated when baleen whales and forage fish co-occurred. Whales were dominantly observed in areas with herring (Clupea harengus) and sprat (Sprattus sprattus), while areas with mackerel (Scomber scombrus) were not targeted. A prey detection range of up to 8 km was found, which enables baleen whales to track their prey to minimize search effort. Fish densities within the defined foraging distance ranged from 0.001 to 3 kg . m(-2) and were correlated to total fish abundance. No prey size selectivity according to fish length was found. By linking baleen whale distribution to high-density herring and sprat areas, it was possible to identify the Celtic Sea as a prey hot spot for baleen whales during autumn

Fine-scale spatial association between baleen whales and forage fish in the celtic sea1

Baleen whales can be regularly observed in the Celtic Sea; however, little is known about their local foraging behaviour. The study objective was to determine whether or not baleen whales selectively prey upon particular forage fish species or, on the contrary, is predation on the Celtic Sea plateau driven by random encounters between prey and predator? Concurrent sighting surveys for fin (Balaenoptera physalus), minke (Balaenoptera acutorostrata), and humpback (Megaptera novaeangliae) whales were carried out simultaneously from 2007 to 2013 during dedicated fisheries acoustic surveys assessing the abundance and distribution of forage fish. Probabilities of spatial overlap between baleen whales and forage fish were analysed and compared with the probability of a random encounter. For estimations of foraging threshold and prey selectivity, mean fish biomass and fish length were calculated when baleen whales and forage fish co-occurred. Whales were dominantly observed in areas with herring (Clupea harengus) and sprat (Sprattus sprattus), while areas with mackerel (Scomber scombrus) were not targeted. A prey detection range of up to 8 km was found, which enables baleen whales to track their prey to minimize search effort. Fish densities within the defined foraging distance ranged from 0.001 to 3 kg . m(-2) and were correlated to total fish abundance. No prey size selectivity according to fish length was found. By linking baleen whale distribution to high-density herring and sprat areas, it was possible to identify the Celtic Sea as a prey hot spot for baleen whales during autumn

Prespawning herring distribution in the Irish Celtic Sea between 2005 and 2012

Knowledge of the spatial distribution of species is fundamental to understanding trophic interactions and ecosystem structure. Intraspecies-specific dynamics and environmental factors shape species distribution within an ecosystem. Distribution patterns and the realized habitat of Atlantic herring (Clupea harengus), a key fish species in the Celtic Sea, were examined using distribution data collected during annual acoustic stock assessment surveys during 2005-2012. Distribution patterns during migration to spawning grounds were analysed using geostatistical methods, including modelling of variograms and comparing the centre of gravity (CG). Distribution patterns were further linked to oceanographic variables collected with the acoustic data to describe habitat. Herring density was greatest inshore along the spawning grounds and lower farther offshore. Herring shoals were clustered and randomly distributed during spawning migration. Variograms of fish densities described the global structure, with high local variability of the same order of magnitude as variability at a regional scale, indicating that no continuous structure can be found within the study area. The CG values showed that the average position of the population was located northeast. The realized habitat for herring encompassed a wide range of temperatures and salinities; therefore, oceanographic features were not a limiting factor for herring distribution during spawning migration. The present study changes the perception of the spatial distribution of the Celtic Sea herring stock from a more continuous distribution to a discrete model, with implications for trophic ecosystem modelling on local scales. Future studies of herring distribution and its influence within the Celtic Sea ecosystem would benefit from using the approach employed in the present study for analysing aggregation patterns