FIS002: Physical impact of demersal fishing gears

Start Date: Jan 2015

End Date: Sep 2015

Main Contractor(s): University of Aberdeen

Other Sponsor(s): European Fisheries Fund

Extended Title: Modelling the physical Impact of demersal fishing gears on the seabed

Main Research Category: Environment / Ecosystem

View Final Report


Project Objectives

Towed demersal fishing gears are used globally by the fishing industry to catch species that live on or close to the seabed. A typical trawl gear comprises the trawl warps, otter doors, sweeps, groundgear, clump weight and net. In addition to the impact to the species caught by these gears, it has long been recognised that there are broader ecological and environmental impacts, such as habitat destruction, benthic mortality, nutrient release, suspension of sediment etc [1 – 7]. To understand these processes and to be able to assess the wider implications on the benthic ecosystem, it is essential to understand the physical interaction of these individual gear components with the seabed.

The project will further develop a numerical model of the interaction of the seabed and fishing gear components and compare and validate the results with full scale experimental data collected during sea trials. The numerical model calculates the penetration into the seabed, the sediment displaced and the associated drag and contact forces.

This research will provide a means to model and predict the physical impact of towed demersal gears on a range of soft sediments from sand to sandy mud. Hence it will permit an evaluation of the physical impact of the main Scottish demersal whitefish and nephrops fleets which will allow (i) the development of predictive methodologies to assess the environmental and ecological impact of fishing, (ii) a means to compare fishing impacts with those due to natural forcing (tides, waves) and (iii) design tools to develop fishing gears of reduced impact.

Specific objectives:

  1. The identification of the correct constitutive stress strain relationship for saturated cohesive (muddy) sediments and their incorporation into the numerical code
  2. Numerical simulations of the physical impact of a range of gear components on sand to muddy sediments.
  3. Experimental sea trials to collect data with which to validate the numerical methodology. These trials will be held in March 2015 during a 5 day trip on the RV Alba na Mara.  (this objective will be sub-contracted to Marine Scotland Science).


Summary of Project Outcomes

Towed demersal fishing gears interact with the water column and the seabed through which and on which they are towed. The initial impact is physical and studies have shown that these physical effects (i) can have broader ecological, environmental and biological implications and (ii) can affect the economic performance of the fishing operation. It has been shown that towed demersal fishing gears damage habitats, cause benthic mortality, release nutrients and resuspend phytoplankton cysts and copepod eggs and studies of the fuel efficiency of fishing trawlers have demonstrated that the combined contact and hydrodynamic drag of a demersal towed gear can account for up to 80% of the fuel consumed. 

To develop predictive methodologies to assess and quantify the ecological and environmental impact of towed fishing gears and to be able to evaluate their economic performance we must first be able to predict their physical impact.  Here we further develop the numerical modelling approach of Ivanović et al. (2011), Esmaeili and Ivanović (2014) and Ivanović and O’Neill (2015) who use finite element methods to model the interaction of the seabed and fishing gear components. These models can calculate the penetration into the seabed, the sediment displaced and the associated drag and contact forces that occur when a fishing gear is towed across the seabed. They have focussed on sandy sediments and here we extend these studies to account for saturated cohesive muddy sediments. This allows us to model the physical impact of towed fishing gears on a range of sediments which is reflective of the range of sediments on which fishing takes place.

The resulting predictions correspond very well with data from experimental trials at sea and underline the potential of using deterministic methodologies to assess and quantify the impacts of towed gears. To develop these types of methodologies the physical impacts need to be related directly to the resulting geochemical, biological and environmental effects such as nutrient enhancement and benthic mortality. The development of such a hierarchy of models would provide management tools to direct fishing effort, identify and establish closed areas and develop environmentally friendly fishing techniques. With sufficient spatially and temporally refined fishing effort data and spatially refined data on sea bed typology it will be possible to assess the impact of fishing at the fleet level and hence permit a comparison with the environmental and ecological impact of other anthropogenic activities or naturally occurring events such as storms and tidal currents.