Seafloor communities and habitat disturbances in the North Sea

Many different organisms live at the seafloor. These organisms are a crucial component of marine ecosystems, as they play an important role in the food web. Their abundance is strongly regulated by the environment, as each species is adapted to certain conditions. Whereas some species thrive in deep and calm waters, others flourish under more dynamic conditions with waves and currents. The environment of a species also affects the impact of human activities, like demersal fisheries. Demersal fisheries target groundfish, but damage other species and the structure of the seafloor in the catching process. The impacts of this are larger for species in calm environments than for species in dynamic areas. In this thesis, I combined detailed observations of organisms at the seafloor with fisheries data and environmental conditions. I demonstrated that environmental conditions structured the distribution of seafloor-disturbing fisheries, and that such fisheries impact species differently depending on their way of life. The discovery of ross worm reefs in the southern North Sea revealed the importance of seafloor structures as environmental variables in the structuring of both fisheries and organisms. Here, seafloor structures like large sand dunes and smaller sand waves and megaripples create a very heterogeneous landscape. The distribution of the ecological valuable reefs matched the pattern created by megaripples. The troughs of different seafloor structures had higher organism abundancies, which may also explain the higher fishing intensity in the sand dunes troughs. I therefore argue that management should account for such local environmental differences

Experimental bottom trawling finds resilience in large-bodied infauna but vulnerability for epifauna and juveniles in the Frisian Front

In this study, we analysed the benthic effects of two in situ fisheries disturbance experiments using a combination of side-scan sonar, high definition underwater video, sediment profile imagery, and box core sampling techniques after conventional beam trawling and box core sampling after electric pulse trawling in a southern North Sea habitat. Acoustic and optical methods visualised the morphological changes induced by experimental beam trawling, showing the flattening and homogenisation of surface sediments. Video transects found a 94% decrease in epibenthos in beam trawled sediments compared to an untrawled control site and a 74% decrease in untrawled sediments of the same transect. Box core samples taken 5.5 h, 29 h and 75 h after trawling detected a downward trend in infaunal densities and species richness that continued after the initial impact with small-bodied and juvenile taxa being especially prone to depletion. Data from shallow sediment samples showed trawl resilience in large mud shrimps and evidence of their upward movement towards the sediment surface after disturbance. Both trawl gears induced significant changes to infaunal communities, with no differential effect between the two gears. Our results suggest that in the Frisian Front, trawling may favour the survival of deep burrowers while removing surficial macrofauna

North Sea demersal fisheries prefer specific benthic habitats

<p>Introduction The future protection of marine biodiversity through good conservation planning requires both the identification of key habitats with unique ecological characteristics and detailed knowledge of their human utilization through fisheries. Demersal fisheries are important disturbers of benthic habitats. They often have a heterogeneous spatial distribution, pressurizing particular habitats with high abundances of target species. For the North Sea, we quantified the commonness/rarity of habitats in relation to the environmental determinants of so-called fishing hotspots, to support better-informed conservation planning of benthic habitats in this intensively used continental shelf. Methods We first distinguished 9 main seascapes in the study area based on seabed morphology. Secondly, we determined average fishing intensity and fishing hotspots using VMS-data for the three dominant Dutch fisheries from 2008 to 2015: beam-trawlers targeting sole Solea solea (Beam-Sole), beam-trawlers targeting plaice Pleuronectes platessa (Beam-Plaice), and otter-trawlers targeting Norway lobster Nephrops norvegicus and demersal fish (Otter-Mix). Within the seascapes subjected to &gt;80% of the fishing activity, nineteen environmental factors (summarized by PCA) were used to ecologically characterize fishing hotspot locations using MaxEnt response modelling. Results We found that all three fisheries target highly specific, uncommon habitats. Beam-Sole fishers targeted warmer, shallow, dynamic, nearshore habitats, and within these specifically the depressions between sand ridges. Beam-Plaice fishers mainly targeted the exposed, non-muddy flanks of the Dogger Bank and similar large-scale elevations (50–75 km) where especially the ridges of smaller sand banks are used. Otter-Mix fisheries concentrated in areas with low bed shear stress, located in muddy, relatively deeper areas. Implications This study is the first to provide insight in benthic habitat types that are frequently targeted by fishers in the North Sea. We demonstrated unequal exploitation pressure between seabed habitats, with the majority of hotspots in the less common habitats. Our results hence contribute to a more effective, ecologically informed planning for the protection and monitoring of all seabed habitats and biodiversity of the North Sea.</p

Measuring centimeter-scale sand ripples using multibeam echosounder backscatter data from the Brown Bank area of the Dutch continental shelf

Backscatter data from multibeam echosounders are commonly used to classify seafloor sediment composition. Previously, it was found that the survey azimuth affects backscatter when small organized seafloor structures, such as sand ripples, are present. These sand ripples are too small to be detected in the multibeam bathymetry. Here, we show that such azimuth effects are time dependent and are useful to examine the orientation of sand ripples in relation to the flow direction of the tide. To this end, multibeam echosounder data at four different frequencies were gathered from the area of the Brown Bank in the North Sea. The acoustic results were compared to video and tide-flow data for validation. The sand ripples affected the backscatter at all frequencies, but for the lowest frequencies the effect was spread over more beam angles. Using the acoustic data made it possible to deduce the orientations of the sand ripples over areas of multiple square kilometers. We found that the top centimeter(s) of the seafloor undergoes a complete transformation every six hours, as the orientation of the sand ripples changes with the changing tide. Our methodology allows for morphology change detection at larger scales and higher resolutions than previously achieved

Industrial fishing near West African Marine Protected Areas and its potential effects on mobile marine predators

Marine Protected Areas (MPAs) are increasingly implemented to facilitate the conservation of marine biodiversity and key-habitats. However, these areas are often less effective to conserve mobile marine species like elasmobranchs (i.e. sharks and rays). Industrial fishing near MPA borders possibly impacts vulnerable species utilizing these protected areas. Hence, we aimed to study spatiotemporal patterns of industrial fisheries near MPAs, in relation to the bycatch of elasmobranchs. Specifically, we analyzed the spatiotemporal fishing effort within the West African region, mapped fishing effort in the direct vicinity of the Parc National du Banc d’Arguin (Mauritania) and the Bijagós Archipelago (Guinea Bissau) and compared the seasonal overlap between elasmobranch bycatch and fishing effort near these MPAs. We combined Automatic Identification System data and local fisheries observer data, and determined fishing effort for each gear type and compared this with bycatch of elasmobranchs. We found that industrial fishing effort was dominated by trawling, drifting longlines and fixed gear types. Although no industrial fishing was observed within both MPAs, 72% and 78% of the buffer zones surrounding the MPAs were fished for the Banc d’Arguin and Bijagós respectively. Within the Banc d’Arguin buffer zone, trawling and drifting longlines dominated, with longlines mainly being deployed in fall. In the Bijagós buffer zone, trawling and fixed gears were most prevalent. Fisheries observer data for Mauritania showed that elasmobranch catches increased during the most recent sampling years (2016 to 2018). Elasmobranch catches within the waters of Guinea Bissau peaked in 2016 and decreased in the following two years. Seasonal patterns in elasmobranch bycatch within the waters of both countries are likely caused by increased catches of migratory species. Catches of rays peaked in May and June for Mauritania, and in October for Guinea Bissau. Shark catches were highest in February and July in Mauritanian waters, and in May and October in the waters of Guinea Bissau. Our study indicates that industrial fisheries near the border of ecologically important MPAs may have potentially major implications for ecosystem functioning by the removal of (migratory) predatory species

How habitat-modifying organisms structure the food web of two coastal ecosystems

Contains fulltext : 158868.pdf (publisher's version ) (Closed access

Linking the morphology and ecology of subtidal soft-bottom marine benthic habitats:A novel multiscale approach

High-resolution surveying techniques of subtidal soft-bottom seafloor habitats show higher small-scale variation in topography and sediment type than previously thought, but the ecological relevance of this variation remains unclear. In addition, high-resolution surveys of benthic fauna show a large spatial variability in community composition, but this has yet poorly been linked to seafloor morphology and sediment composition. For instance, on soft-bottom coastal shelves, hydrodynamic forces from winds and tidal currents can cause nested multiscale morphological features ranging from metre-scale (mega)ripples, to sand waves and kilometre-scale linear sandbanks. This multiscale habitat heterogeneity is generally disregarded in the ecological assessments of benthic habitats. We therefore developed and tested a novel multiscale assessment toolbox that combines standard bathymetry, multibeam backscatter classification, video surveying of epibenthos and box core samples of sediment and macrobenthos. In a study on the Brown Bank, a sandbank in the southern North Sea, we found that these methods are greatly complementary and allow for more detail in the interpretation of benthic surveys. Acoustic and video data characterised the seafloor surface and subsurface, and macrobenthos communities were found to be structured by both sandbank and sand wave topography. We found indications that acoustic techniques can be used to determine the location of epibenthic reefs. The multiscale assessment toolbox furthermore allows formulating recommendations for conservation management related to the impact of sea floor disturbances through dredging and trawling.</p

Drought, mutualism breakdown, and landscape-scale degradation of seagrass beds

In many marine ecosystems, biodiversity critically depends on foundation species such as corals and seagrasses that engage in mutualistic interactions [1-3]. Concerns grow that environmental disruption of marine mutualisms exacerbates ecosystem degradation, with breakdown of the obligate coral mutualism ("coral bleaching") being an iconic example [2, 4, 5]. However, as these mutualisms are mostly facultative rather than obligate, it remains unclear whether mutualism breakdown is a common risk in marine ecosystems, and thus a potential accelerator of ecosystem degradation. Here, we provide evidence that drought triggered landscape-scale seagrass degradation and show the consequent failure of a facultative mutualistic feedback between seagrass and sulfide-consuming lucinid bivalves that in turn appeared to exacerbate the observed collapse. Local climate and remote sensing analyses revealed seagrass collapse after a summer with intense low-tide drought stress. Potential analysis-a novel approach to detect feedback-mediated state shifts-revealed two attractors (healthy and degraded states) during the collapse, suggesting that the drought disrupted internal feedbacks to cause abrupt, patch-wise degradation. Field measurements comparing degraded patches that were healthy before the collapse with patches that remained healthy demonstrated that bivalves declined dramatically in degrading patches with associated high sediment sulfide concentrations, confirming the breakdown of the mutualistic seagrass-lucinid feedback. Our findings indicate that drought triggered mutualism breakdown, resulting in toxic sulfide concentrations that aggravated seagrass degradation. We conclude that external disturbances can cause sudden breakdown of facultative marine mutualistic feedbacks. As this may amplify ecosystem degradation, we suggest including mutualisms in marine conservation and restoration approaches

An opportunistic comparison of a pulse and a traditional beam trawl fishing gear

Beam trawls are criticized for their negative impact on benthic ecosystems. Pulse trawls may be an environmental more friendly alternative. In addition to the environmental benefits, the pulse trawl is expected to improve the catch efficiency for the main target species (sole Solea solea). Here, we report on an opportunistic comparison between a pulse and traditional tickler chain beam trawl (TCBT). Catch efficiency for sole and plaice was determined in 39 paired hauls, during which both vessels fished parallel, and by comparing average landings during commercial hauls in which the vessels fished in close proximity of each other. Additionally, total catch quantity and composition and benthos composition were compared between both gears. Unfortunately, the TCBT fished with a smaller mesh size than the pulse trawl (67mm versus 80mm). To take account of the differences in mesh-size, plaice and sole catches for the comparative hauls were corrected using selection ogives of recent mesh-size selection experiments. The current study found a 23% higher catch efficiency for market sized sole and a non-significant 3% lower catch efficiency in market sized plaice. The improved size selectivity observed in a similar experiment in 2011, when pulse fishing was just introduced, could not be corroborated. With the exception of Norway lobster and spider crabs, all benthic invertebrate species showed lower catch rates in the pulse-trawl. Due to the difference in cod-end mesh-size, the results of the experiment should be interpreted with caution

Multiyear footprint of the Danish bottom-towed fisheries in the Danish EEZ for 2010-2022.

These raster layers (.tif) represents the fishing footprint of Danish bottom-towed fishing gears in the Danish Economic Exclusive Zone (EEZ). We have provided annual footprints for 2010-2022, and the multiyear footprints for the periods 2010-2016, 2017-2022, and 2010-2022, similar to to data shown in the online mapping tool, under the "multiyear footprint" page (https://doi.org/10.11583/DTU.23617944.v1). The multiyear footprints represent the cumulative fishing footprint over that period, showing how much of the seabed has been affected by physical abrasion of bottom-towed fishing gears (from Danish vessels).Data from the Vessel Monitoring System (VMS), the Automatic Identification System (AIS), and the Black Box (BB) system were combined with electronic logbook data to determine fishing activity location. (see van der Reijden et al (2023) DOI:10.1016/j.marenvres.2023.105935 for a more detailed description). From that, overall fishing intensity was determined as the Swept Area Ratio (SAR), which should be interpreted as the number of times the raster cell is fished per year. A SAR-value lower than 1 means that the raster cell is not completely fished; a value higher than 1 means that the raster cell has been fished completely (and even more frequently) than once a year. Find more information, including a visual example of the SAR-calculation here: https://ono.dtuaqua.dk/DDFAM/ (under the tab "Information"). More information on how the multiyear footprint is determined can be found at the same location.The fishing intensity is presented in 8 classes:>> 0 - 0.5*>> 0.5 - 1>> 1 - 2>> 2 - 5>> 5 - 10>> 10 - 15>> 15 - 20>> 20+* grid cells with SAR values ≤ 0.05 are set to 0.Note that we strongly advice to not fully color grid cells with the lowest fishing intensity class in footprint maps, as that will cause a visual mismatch between maps and actual footprint numbers.The raster layers have a spatial resolution of 0.001 x 0.001 longitude x latitude degree (ca 100 x 60 m) and are saved in the ETRS89 UTM32 projection (EPSG: 25832). Note that the fishing data in this raster layer has been restricted to the Danish EEZ; it does not mean that Danish fisheries are not active in foreign waters.</p