Effects of beam and pulse trawling on the benthic ecosystem
Here we study the effects of fishing trawl gear on the seabed and benthic organisms. A BACI-design experiment was used to examine the effects of a traditional beam trawl gear and the pulse trawl gear.
The pulse trawl gear is gaining popularity amongst Dutch fishers in recent years due to reduced fuel costs and good sole catches.
The research was carried out in the northern part of the Dutch Voordelta (southern North Sea coastal zone area, 15 – 22m deep, sandy habitat) in June 2013. In this experimental area, fishing disturbance was considered to be low so interference with the experiment was minimalized. Within the experimental area 3 sub-areas (150m x 1000m) were established for the BACI experiment: 1) a pulse trawl area; 2) a beam trawl area; 3) a reference area. The pulse and beam trawl areas of the BACI experiment were trawled by a commercial vessel (pulse) and a research vessel (beam) for one day. Before trawling, the areas were sampled with a multibeam, a sediment profile imaging (SPI) camera and a benthic sledge. The same sampling was carried out after the trawling, as well as boxcore sampling.
An fourth area was set aside for measuring sediment resuspensions from the two types of trawls. This fourth area was located in the experimental area as well.
The key findings for each of the different aspects sampled are:
- The multibeam showed that trawl tracks of fishing disturbance (most likely beam trawls) prior to experimental fishing were most evident in the area set aside for sediment resuspension measurements (mainly southern section), and in the eastern section of the pulse site.
- Significant differences inside and outside the trawl marks (hereafter called ‘penetration depth’) were found for the pulse an beam trawl treatments and beam trawls were shown to penetrate deeper into the sediment than the pulse trawl.
- The benthic data revealed large variability between stations and it was difficult to detect an effect from fishing. Whilst overall biomass did decrease following (beam trawl and pulse trawl) fishing, this reduction was also found in the reference area.
- No obvious consistent patterns of fish trawling effect on the densities of individual species was noted.
- Based on various life history traits, the recorded benthic species were assigned to one of three categories describing their vulnerability to trawling: resistant to trawling, vulnerbale to trawling, or intermediate. Of the three categories, the highest densities were found in the resistant category, which is likely to explain the lack of trawling effect observed.
- The pulse and conventional beam trawl were shown to mobilise similar quantities of sediment, but the pulse trawl had higher values for more particle size bins. Total concentrations of resuspended particles were also higher for the pulse at 25 and 45m behind the beam.
- The reduction of dissolved oxygen in the water column following a the trawl of a pulse and that of a conventional beam trawls is similar, and oxygen levels appear to revert back towards the baseline levels soon after trawling.
Analysis of boxcore data (infauna and sediment characteristics) is still undergoing. Sediment profile image analysis is also undergoing pending software development for image analysis.