Physical fisheries’ impacts on seabed habitats (JAMBAY WP2)

There are two overarching objectives in WP2 in JAMBAY; the first is to analyse and map the activities and ecosystem pressures of the different fisheries in the case study area, and the second is to analyse the connectivity and recruitment dynamics of selected fish and invertebrate species in the case study area and the surrounding waters.

In Task 2.1 fishing pressure data of swept area ratio (SAR), landed catch, fuel-use intensity, and sediment mobilisation were estimated for individual grid cells of 0.001 degrees longitude and latitude (approx. 60x100 meter). This was made possible through the development of a polygon-based approach to the calculations of haul-based swept areas for individual vessels and a hierarchical use of information in Black Box (BB), Automated Information System (AIS), and Vessel Monitoring by Satellite (VMS) monitoring data. The refinement and application of these state-of-the-art methodologies to Danish fisheries monitoring data enabled the estimation of fishing pressure data in an unprecedented quality and resolution, which is more than 1000 times higher than other publicly available SAR maps and data that are currently used for impact and trade off assessments. For the foreign fleet, AIS data were combined with information on gear type and vessel size in the EU vessel register to provide the SAR, fuel-use, and sediment mobilization estimates per grid cell. Because we did not have access to logbook data for the foreign vessels, landings weight and value per grid cell could not be estimated, and the estimates of SAR, fuel-use, and sediment-mobilisation are substantially more uncertain than the estimates for the Danish fleet.

The maps of the gridded fishing pressure from 2015-2022 show that the JAMBAY case study area is intensively fished with mobile bottom-contacting gears, where larger areas of the deeper, more offshore parts of the case study area are fished more than 10 times annually. In the shallower fishing grounds, closer to shore, the fishing intensity is lower, but also here there are hot spot areas that are fished more than 10 times a year, and a large proportion of the seafloor is fished between 5 and 10 times a year. Another clear outcome of mapping the SAR estimates by fleet and gear type is that the DK fleet and the foreign fleet are largely complementary in their choice of fishing grounds. Whereas the Danish fleet is responsible for the largest fishing pressure on the sandy sediments (Danish seines) and soft sediments (otter trawlers), the international fleet (e.g., beam trawlers) exerts the largest pressure on the coarser sediments. The spatial distribution of landed catch, fuel-use intensity, and sediment mobilisation closely follows the distribution of fishing intensity, with the highest values per grid cell being found in the grid cells that also have the highest intensity of fishing. There are some deviations between the maps of the different pressures, which are brought about by the price difference of the species, the difference in the engine-power requirements of the gear types, and the differences in sediment silt-fractions of the habitat types.

In Task 2.2 the aim was to get a better understanding of the interaction of towed fishing gears with boulders. This will help fishers choose what gears can be used on given fishing grounds and will help fisheries managers protect seabed habitats and features. We focused on estimating the snagging probability of different demersal trawls and boulders and carried out a scaled model flume tank study (i) to identify which gears are more likely to snag on boulders, (ii) to evaluate at which part of a given gear snagging is more likely to occur, and (iii) to determine if there are any characteristic features of a boulder that increases the likelihood of snagging. A central outcome of the study is that tickler chain beam trawls had the highest snagging probability across boulder shapes and sizes, indicating that this gear type is the most restricted in terms of fishable substrate composition. It was expected that Danish seines would be the most snagging gear type and that tickler chain beam trawls would be better at passing coarse substrate types than both otter trawls and the seines. An explanation might be that high snagging risk in a seabed habitat does not per se prevent beam trawling as these vessel types generally have high engine power and likely can move and turn over boulders when snagging does occur. The study also demonstrated that the use of chain mats instead of tickler chains substantially reduced the snagging probability of the beam trawls. Even so, our modelling-based results indicate that tickler chain beam trawlers are more restricted in their choice of fishing grounds than generally perceived.

In Task 2.3, a high-resolution hydrographic dataset covering the Jammer Bay and surrounding basins was established to assess hydrographic control of biological processes in the Jammer Bay area. Interfaces to Python and Lagrangian simulation software was developed to enable cross-disciplinary analyses within the project. Biological modules describing plaice larvae and early life stages of habitat-forming invertebrates were added.

The connectivity for hydrographic plaice habitats in the Jammer Bay and surrounding basins was assessed by considering drift from known spawning grounds and settlement at coastal nurseries with suitable habitat conditions. The investigations indicated that the recruitment potential has increased slightly in the period 2014-2022, but spatial and temporal fluctuations in recruitment contributions also increased over the period, lowering the stability of the recruitment potential. The connectivity for representative habitat-forming invertebrates in the Jammer Bay and surrounding basins was assessed. A representative set of four species with contrasting traits was considered. The species with longer pelagic duration of early life stages indicate a slight increase in recruitment potential, but also larger spatial and temporal fluctuations, as was found for plaice larvae. Species with shorter pelagic duration of early life stages did not exhibit similar noticeable increase.

Details of settling plaice larval development and the low scope for escape behaviour of the metamorphosing post-larvae together with stage duration define the vulnerability of this recruitment period to mobile bottom-contacting fishing gear. Studies of gear impact on plaice larvae and small juveniles was not found in the literature, but judged from field biology and experimental studies of predator-prey interactions, vulnerability peaks at a larval length of 12-15 mm and exponentially declines until the juvenile has reached about 25 mm. An index for disturbance of plaice recruitment by mobile bottom-contacting gear was defined and assessed for the Jammer Bay. The index described a spatio-temporal overlap between Danish anchor seine fishery and plaice post-larval nurseries, where the relative trend in this index is less sensitive to the current absence of impact quantification studies. The index showed an unfortunate coincidence with peak fishing effort and arrival of newly settled larvae, which stresses the importance of future impact quantification studies.

The primary aim of Task 2.4 was to understand the "source-sink dynamics" of sandeel larvae and the connectivity between habitats from the Jammer Bay to the north-eastern North Sea and western Skagerrak. The project successfully developed a protocol for measuring sandeel larvae using image processing tools and created two valuable datasets for larvae abundances and lengths. The work involved meticulous remeasurement and recalibration of larvae samples to address initial measurement inconsistencies, ensuring data accuracy. Challenges in completing larval drift and statistical modelling analyses were faced due to time and data readiness constraints. Nonetheless, the project offers significant insights into sandeel recruitment patterns from hatching to settlement and is crucial for effective fisheries management and conservation in marine environments.