Fishing net. Photo: DTU Aqua

Reservations and limitations of the mapping tool “The seafloor footprint of Danish fishing”

The interactive mapping tool “The seafloor footprint of Danish fishing” displays two main types of results for the area, fisheries, and time period in question: 1) a map of fishing distribution and intensity, and 2) table(s) presenting numerical indicators of the fishing distribution and intensity.

We describe the most important reservations and limitations of the data below. 

Limitations in methodology and data

In general, the results present a (slightly) simplified overview of fishing activity. It is important to be aware that the processing and presentation of data has its limitations. Consequently, the results on the map cannot be compared directly to the results in the tables, with the tables being more accurate.

We have adopted a method that divides the selected area into small squares (100 x 100 meter). Fishing activity is then determined for each square, based on satellite recordings of the fishing vessels. Although we take the amount of fishing activity into account, all fished squares will be colored in the map, even when only a small proportion of the square is affected by fisheries.  

In addition, due to the amount of data, we have aggregated the map to a resolution with squares of 500 x 500 meter, while the quantitative indicators shown in the tables are calculated for squares of 100 x 100 meter. This difference in scale contributes to the higher accuracy of the numeric indicators in the tables.

The Percentage Unfished Area (PUA) in Danish fisheries may be overestimated

The table value for the Percentage Unfished Area (PUA) takes into account that a very large number of squares is only partially fished. Moreover, it is determined at a finer spatial scale than what is shown on the map. This indicator is therefore more accurate than the map.

However, it is important to be aware that the PUA estimate for periods spanning multiple years is determined as the minimum annual PUA observed. As such, the presented PUA could represent an overestimation of the actual PUA.

The Percentage Unfished Area (PUA) in international fisheries may be underestimated

Due to insufficient access to other countries' data for fishing within the mapping area, the indicators for the international fishing effort (Danish + foreign vessels) are calculated from publicly available data from the International Council for the Exploration of the Sea (ICES). This data is much less detailed on the spatial scale (rectangles of approx. 3000 x 5000 meters), which means that the indicator 'Percentage Unfished Area' (PUA) for international fisheries is significantly underestimated

 

 

Contact

Ole Ritzau Eigaard
Senior Researcher
DTU Aqua
+45 35 88 34 66