https://doi.org/10.11583/DTU.23617944

Welcome!

This platform aims to provide accurate information on Danish fisheries with bottom-towed gears, to support science-based management of the Danish waters. It shows the spatial distribution and intensity of all bottom-towed gears used by Danish fishermen and provides quantitative fishing pressure indicators as advised by the International Council for the Exploration of the Sea (ICES). Using the tab 'Map' you can select your ICES area(s), year(s), and fishing gear(s) of interest, for which the spatial distribution is shown in a map, and the fishing pressure indicators are given in a table. Under the tab 'Areas of interest', special attention is given to Natura-2000 areas and off-shore wind farms. Explanations of the outputs, how these are estimated, and the specifications of the different fishing gears are summarized in the 'Information' menu.


This page is currently under development and subject to frequent updates.













This webpage is designed by the National Institute of Aquatic Resources (DTU Aqua), within the EMFF project 'Dansk Fisker-Forsker Netværk 3. Fase', which is a collaboration of DTU Aqua with the Danish Fishermen’s Producer Organization (DFPO).


Please, feel free to contact us if you have questions or comments on this webpage. See the relevant email addresses underneath.
kjova@aqua.dtu.dk
Webpage developer within DTU Aqua.
hl@dkfish.dk
Contact person within DFPO.

Select the area, gear and period of interest and press 'Submit my selection'. The corresponding fishing intensity will then be shown on the map and in the table.


Select the ICES area(s):

Select the gear(s):
Otter trawls (OT)
Dredges
Beam trawls (BT)
Demersal seines

Select the year(s):

Map

This map shows the average annual fishing effort of the selected gears and years in the selected area. It can also show the national waters (EEZ), the official regional areas as defined by ICES (ICES Areas), and the international fishing effort (IFE) of all demersal gears combined over the year 2020. You can choose to co-display the Broad Habitat Types as defined under the Marine Strategy Framework Directive (MSFD), or the water depth (Depth). The legends for these layers can be found by clicking the corresponding button below the map.

Table

This table shows the average annual Swept Area Ratio (SAR) and the Percentage Unfished Area (PUA) for the selected area, for all demersal fishing gears combined and for the selected gears separately. The numbers represent the average of the annual values, and the minimum annual PUA observed for the selected years. Please see the 'Fishing pressure indices' tab for information on their calculations.

International fishing effort

This table shows the average annual Swept Area Ratio (SAR) and the Percentage Unfished Area (PUA) of the international fishing effort for the selected gears and years in the selected area. The data is based on the ICES-VMS call for the OSPAR advice.

Find more detailed fishing information on special areas, like Natura-2000 sites.


Select the year(s):
Select the gear(s):
The table underneath only shows fishing activity indicators for the selected gears and periods inside the selected area. So, the displayed fishing activity outside of the selected site is not included in this table. The table shows the annual average Swept Area Ratio (SAR) and the Percentage Unfished Area (PUA) for the selected years, gears, and areas. Please see the 'Fishing pressure indices' tab for information on their calculations.

Fishing pressure is expressed according to ICES indicators 1 and 3: the Swept Area Ratio (SAR) and the Percentage Unfished Area (PUA). A description of their calculation is given underneath.


We will use the hypothetical example underneath to explain how the fishing pressure indicators are calculated. In the figure you see a gridded area, with four trawling tracks depicted in blue. Some tracks overlap, and most of the area is not trawled.

The Swept Area Ratio (SAR) presents the ratio between the area (m2) swept by fishing gears within a grid cell, and the surface area (m2) of that grid cell (here: 100 x 100 meters). Hence, a SAR of 1 means that an area equally sized as the grid cell is fished. SAR less than 1 means less area is fished, and a SAR larger than 1 means that more area is fished. Note that this indicator displays the average fishing intensity; even with a SAR larger than 1, some spots can remain unfished.

We calculated the swept area ratio for each grid cell in the example. They range from 0 to 1.88. Some cells are thus unfished, while others are fished more than once a year. However, you can see from the actual trawling tracks that no single grid cell has all its surface area fished; not even the grid cells with the highest SAR.

On average, the grid cells of the full area have a fishing intensity (SAR) of 0.37.

When only looking at the subset with trawled grid cells, the average fishing intensity (SAR) is 0.61.


The Percentage Unfished Area (PUA) represents the minimum extent of the area that is not fished. For this, the total swept area is determined over all grid cells, in which the swept area of a grid cell can never exceed the surface of that grid cell. This total swept area depicts the maximum fished area, because it does not control for overlapping fishing tracks (which result in a SAR of 1 in a grid cell, but could leave some unfished spots in that grid cell that are not taken into account). Hence, the PUA is a conservative estimate of the area that is unfished.

In this example, a minimum of 66 % of the area is not fished.

That is a much higher percentage than if this indicator is calculated at the spatial resolution of full grid cells, which would result in an estimate of 39 % of the area being not fished.

This website shows all the bottom-towed gears that are used by Danish fishermen in Danish waters. They have in common that the fishing gear (parts of it) are actively towed over the bottom. Their design and dimensions, however, span a wide array of different gear types ranging from beam trawlers to otter trawlers to Scottish and Danish seines. We have grouped certain gears together based on their similarities. You can find more information on these gear groups here.


Source: drawings from SEAFISH, photo from Fiskerforum.
Otter trawls are named after the so-called otter boards (trawl doors) that maintain the horizontal opening of the net. These boards are attached to the vessel with ropes, and to the fishing nets with sweeps. In the picture of the fishing vessel on the left, you can see the (blue) otter boards hanging on the side of the vessel. These boards are designed in such a way they remain in contact with the floor, while the water ‘pushes’ the boards outwards during the towing, keeping the trawl net open horizontally. The vertical opening of the trawl net is obtained by equipping the head rope with floats, while the ground rope (fishing line) is made heavier to ensure bottom contact. The ground rope is often also equipped with (rubber) discs, chains, or bobbins, to prevent the net from being damaged by contact with the seafloor. Their exact shape and size depends on the target species and the habitat type of the fished area. (Lower drawings; left, middle, and right respectively).

The otter trawls are divided into 6 different groups based on their main target species:

Otter trawls specifically targeting Norway lobster (Nephrops norvegicus) are linked to specific “Nephrops fishing grounds”, where the gear is adjusted for these typically muddy areas, for instance by the usage of light ground gear. This “pure” nephrops fishery is mostly located in Kattegat. In addition, a multi-rig gear is often used to increase gear width without using more engine power. Vessels deploying multi-rig gears have multiple trawl nets between the otter boards. In between the different nets, a clump weight is added to ensure bottom contact. Often the Nephrops fishery additionally targets a mix of demersal fish such as cod, plaice, lemon sole and monkfish. This is mostly the case in the North Sea and Skagerrak and here the fishery is less confined to muddy grounds and the ground gears more heterogenous.
Otter trawl fishing for deep-water shrimp (Pandalus borealis) takes place almost exclusively in the deep part of the Skagerrak and the easternmost part of the North Sea. Despite a relatively low effort, the intensity of fishing is therefore relatively high in the limited area where fishing takes place. The trawling takes place on soft bottom in deep water with large and heavy otter boards and relatively light ground gear.
Otter trawls targeting demersal fish are widespread and can be used in single-species fisheries targeting plaice or cod, but most frequently fishermen catch a mix of several demersal fish species, depending on location and season. This type of otter trawl is often fished on different sediment types and the ground gear adjusted accordingly with e.g., the use of bobbins or rockhoppers.
The otter trawl fishery targeting Norway pout (NOP) takes place on even, mostly soft bottom and is primarily localized to Fladen ground in the North Sea. The trawl gear is designed so that both otter boards, sweeps, and ground gear have bottom contact.
Otter trawls targeting sandeel (Ammodytes marinus) are linked to specific sandbanks, mainly in the North Sea, where this species can be caught in dense shoals off the bottom during the 2nd quarter of the year. The trawl gear is often designed so that only the ground gear and parts of the sweeps are in contact with the seabed, while the otter boards are towed slightly above the seabed.
Otter trawls targeting small pelagic fish like sprat and herring are widespread and are deployed on different types of generally smooth sediments. The trawls are often designed so that only the ground gear, which is generally relatively light, and parts of the sweeps are in contact with the seabed, while the otter boards are towed slightly above the seabed.

Source: SEAFISH.
Otter trawls targeting sandeel (Ammodytes marinus) are linked to specific sandbanks, mainly in the North Sea, where this species can be caught in dense shoals off the bottom during the 2nd quarter of the year. The trawl gear is often designed so that only the ground gear and parts of the sweeps are in contact with the seabed, while the otter boards are towed slightly above the seabed.

Otter trawl fishing for deep-water shrimp (Pandalus borealis) takes place almost exclusively in the deep part of the Skagerrak and the easternmost part of the North Sea. Despite a relatively low effort, the intensity of fishing is therefore relatively high in the limited area where fishing takes place. The trawling takes place on soft bottom in deep water with large and heavy otter boards and relatively light ground gear.

Beam trawls targeting brown shrimp are used in coastal waters, as that is where the brown shrimp are dominantly present. This fishing gear is adjusted to the relatively small crustaceans, with shorter beams (around 4 m), smaller trawl nets, light-weight ground gears, and smaller mesh sizes than the beam trawls used for demersal fish.
Beam trawls targeting demersal fish often catch a variety of species, but primarily target flatfishes such as plaice, sole, turbot and lemon sole.

Source: drawing from IFREMER, photo from Dansk Skaldyrscenter.
Dredges are fishing gear specifically aimed to target bottom-dwelling shellfish, such as cockles, blue mussels, and oysters. It consists of a heavy steel frame, with a trawl net made partly of chain meshes and partly of very thick and strong nylon netting. The dredge is towed over the seafloor and collects the targeted shellfish in the net.

Source: drawings from SEAFISH, photo from Fiskerforum.
Demersal seine gears are typically designed with relatively small trawl (seine) nets with very long seine-ropes (sweeps, often 3000-4000 meter) on both sides. From a buoy, the fishing vessel sets the gear in a large circular or triangular shape, starting with a sweep, followed by the net and the second sweep. This results in the seine net being located opposite of the vessel, with both sweeps in between. Then, the vessel starts hauling the sweeps, which will herd the fish inside the circle/triangle, and eventually into the fishing net. The ground rope of the gear is often equipped with small discs to protect the net from damage.

The demersal seines are divided into 2 different types based on slight differences in haul methodologies:

Danish seines are also referred to as anchor seines, because the first sweep is attached to an anchored buoy. The remainder of the gear (the seine net and the second sweep) is set in a circular or triangular shape, which is ended by the same buoy. There, the first sweep is picked up and the hauling of the net is started. During hauling, the fishing vessel remains near the anchored buoy.
Scottish seine is relatively like Danish seine, except for that the first sweep is attached to a floating buoy, and that the fishing vessel is sailing forwards while hauling.

This page contains information with respect to data acquisition and processing, and keeps a versions log.

Data acquisition and processing

The activity of Danish fishing vessels is monitored by several systems. The Vessel Monitoring System (VMS) is a satellite-based tracking system that has been mandatory for all fishing vessels over 12 m, since 2012. The system provides the location, speed and course of the vessel at a given time, with an interval rate of around an hour. The Automatic Identification System (AIS) is also a tracking system, primarily intended to prevent collisions between vessels. This system is mandatory for all fishing vessels over 15 m since 2015 and registers the same information as the VMS. AIS has a much shorter recording interval (in the order of seconds) than VMS, but a lower spatial coverage, especially offshore. A third tracking system is the Black Box (BB) system, which is mandatory for Danish fishing vessels targeting mussels and oysters. This system combines continuous positional data with vessel speed and measurements of winch activity to exactly determine fishing activity location. Lastly, each fishing vessel over 12 meter is obliged to report on their daily landings and the fishing gear used in their online logbook.
AIS and VMS data are interpolated to provide information for every second, and are merged with the logbook data based on fishing vessel ID and timestamp. Then, fishing activity (gear deployment) is determined from gear-specific speed profiles and the exact seafloor footprint is calculated from the fished locations and the gear width. The total fishing footprint is determined in a hierarchical method that prioritizes the most detailed information available for each fishing event: first BB, then AIS, and finally VMS. This total fishing footprint is available as the total area swept (m2) in individual grid cells with a size of 100 x 100 meter.

Version log

Data processing and dissemination is a constant work-in-progress, which sometimes results in the discovery of mistakes. If a new version of the data is uploaded, this will be reported here, including the main changes it will cause.