Damage limitation on the seabed 

Kaiser M.J. & Horwood J.W. 
from Forum, New Scientist 
15th October 1997 

"Bottom fishing in the world's oceans has been compared to clear-felling of forest habitats.  This implies that fishermen systematically sweep entire areas of the seabed in order to maximise their catch.  If this were true, fishermen would have become extinct years ago. 

Bottom dwelling animals, such as flatfishes and clams, are pursued using a variety of trawls and dredges fished in close contact with the seabed.  An unavoidable consequence of this type of fishing is a relatively large non-fish by-catch comprising bottom fauna such as sponges, soft corals, 
crabs, urchins, starfish and molluscs.  Many of these animals are also killed on the seabed where they become food for scavengers.  This might create concern that fishing is causing major changes to seabed life, especially when viewed in conjunction with statistics that imply that entire areas of the seabed are fished seven times per year.  However, these figures give an unintentionally misleading picture of the extent and ecological significance of bottom fishing disturbance. 

These statistics are based on fishing effort data collected for large areas of the seabed covering 30' latitude x 30' longitude (approximately 3800 km²).  Within these areas fishing effort is very patchily distributed and Adriaan Rijnsdorp's (The Netherlands Institute for Fisheries Research) study of the Dutch beam trawl fleet has shown that it only becomes randomly distributed at a scale of 1 km x 1 km.   This means that while some small areas of the seabed may be fished in excess of 100 times per year, others are never fished. 

This is hardly surprising especially when the harvesting strategy used by bottom fishermen is considered.  Unless forced to behave differently by legislation, fishermen are creatures of habit, tending to return to known fishing grounds year after year.  Knowledge of grounds and specific tows, selected for their lack of bottom snags or proximity to specific fish or shellfish habitats, is passed between generations and individuals.  The aggregation of fishing effort has important implications for the resulting magnitude of disturbance experienced by seabed communities. 

In a pristine environment, the first ever passage of a fishing gear will proportionately have the greatest effect on the community within its path, whether considered in terms of individuals captured or killed.  Thereafter, successive passages of the trawl will cause proportionately fewer changes. Moreover, the first ever tow may have occurred years or decades ago. 

A useful analogy is to think of a tractor repeatedly ploughing the same track across a meadow.  While the disturbance along the track will be intense, the proportionate effects on the entire meadow community will be relatively small. The tendency of fishermen to distribute their effort 
patchily may inadvertently act as an effective conservation method, minimising the areal extent of physical disturbance.  That fishermen continue to return to the same grounds is testament to the fact that the target species are able to recolonise these areas and continue to be attracted to them. 

It is also important to consider fishing disturbance in the context of the background of natural disturbances that occur at a variety of spatial and temporal scales.  Animal communities found in coastal sandy seabeds tend to be adapted to frequent physical disturbance by wave and tidal action. These habitats are usually dominated by short-lived species (e.g. small worms) who's populations are unlikely to be affected by trawling disturbance. 

Long-lived animals with slow recolonisation and growth rates tend to be found in sheltered waters or at depths below 100 m where physical disturbances caused by natural phenomena are less frequent.  Where these animals still exist there may be merit in considering restricting the use 
of the most intrusive fishing gears, especially as the functional significance for the ecosystem and fisheries of many of these communities is poorly understood. 

However, in some cases habitat or community changes may have already occurred, such that excluding fishing gears from these areas would achieve little.  Clearly a rocky reef community is unlikely to re-establish itself if the reef has been removed by fishing gears. Furthermore, change is not necessarily deliterious.  Agriculturalisation of the land has enabled the development of civilisation.  In the North Sea the growth of young sole has increased dramatically since the 1960s, as has the fecundity of the plaice.  These changes have been attributed to an increased food supply for these flatfish, possibly associated with a change in the fauna due to trawling. 

These globally important and scientifically challenging issues are currently the focus of  major research initiatives worldwide, some of which can be traced back to the early 1970's.  The Ministry of Agriculture, Fisheries & Food CEFAS Laboratories currently are involved in 5 related research programmes in the United Kingdom and Europe. As a result, we now have a much better understanding of the ecological significance of fishing disturbance, both in the context of protecting the environment and maintaining sustainable fisheries."

Link to gear effects directory