What is bottom trawling and why is it conducted?

Bottom trawling (also called dragging) is a non-selective method of mobile fishing. It involves a fishing boat towing a heavy fishing nets and gear along the sea floor in order to catch deep water fish.  It is often conducted by large fishing vessels using nets which can sometimes be as tall as 3 storey buildings and as wide as a football pitch. Deep sea fishing has produced 800,000 – 1,000,000 tons per year globally since 1964 thus supporting jobs in the fish industry worldwide. With large amounts of money and the number of people relying on it for employment it makes bottom trawling a fiercely debated issue [1].

A diagram of how bottom trawling is conducted,                     Author: mister bijou

Although most studies on the effect of bottom trawling have been carried out in coastal and shelf sea environments, it is thought that the damage is very similar in the deep sea.

Fishing fleets which deep sea bottom trawling normally aim to catch species like Cardinal fish, orange roughy and grenadiers. But in catching these target species they scoop up many other non-target fish species as bycatch which will ultimately die [1].

 

How did it come about?

Deep sea fishing including deep sea bottom trawling began in meaningful numbers in the 1960s and 1970s. This was due to the failure of shallow water fishing ground combined with advances in fishing technology. Such as larger vessels which possessed stronger cables, more powerful winches and rock hopper trawls [2].

A large trawling vessel off loading its catch in harbor. Author: skagman

Deep water fishing was further encouraged by governments offering subsides and grants. This was done in order to counter the negative effects of the collapse of shallow water fish stocks which was occurring at the time [2]. These grants and subsides caused a rush from the declining shallow fisheries to their deep sea counterparts in order to exploit these new resources. Today 40% of the worlds trawling grounds are in waters deeper than the continental shelf [3].

 

Why does it matter?

As with the devastation  of logging on rainforest ecosystems, deep sea bottom trawling has large direct and indirect effects on benthic organisms (sea bottom animals) and fish populations in areas in which it is conducted. With the deep sea still relatively unexplored bottom trawling threatens to destroy areas before it is known fully what is there [4]. Plus many of the areas exploited are isolated and relatively unique such as sea mounts, which are easily impacted by the effect of bottom trawling.

 

Why is exploitation of the deep sea worse than the shelf sea?

Targeted species over 500m deep when fished are often more severely affected than their shallower shelf sea counterparts’, due to them having different life cycle characteristics. They tend to live much longer lives, with them maturing at a later age, growing at a slower rate while also being less fertile [5].

Many organisms are also located in topographically restricted features such as sea mounts. This makes them less productive and extremely vulnerable to over fishing with very little resilience against over exploitation.

 

Seamounts

The locations where deep sea bottom trawling can be especially harmful are seamounts, where organisms are isolated on the sea mount.

A topographical image of Bear Seamount.                    Author: National Oceanic and Atmospheric Administration

Seamounts are submerged mountains which rise up from the ocean floor, often formed by seismic activity. Oceanographers define them as separate features which rise 1000m or more from the ocean floor [6].

In one study sampling small seamounts off south Australia, seamounts which had not being subject to heavy fishing were found to have 106% greater biomass than those which had. The number of species were also found to be 46% greater than the heavily fished seamounts [6].

 

Can We Prevent It?

There are three main ways to protect the deep sea and reduce the impact of bottom trawling. These are changing the equipment used, creating marine protected and legislation restricting fishing practices.

Changing the equipment

The main damage to the sea floor from bottom trawling is caused by the contact made with the sea floor by the leading equipment. Some of this damage could be reduced by reducing the pressure of this equipment on the sea floor by reducing the weight of the trawl doors, using lighter gear components with lifting capabilities to raise the leading cable off the bottom and reducing the warp length to depth ratio [7].

Restricting Fishing Practices

Restricting fishing practices can take many forms from banning fishing below a certain depth, introducing quotas and restricting the use of certain equipment in certain areas. An example of this; the General Fisheries Commission of the Mediterranean has prohibited bottom trawling below 1000m.

Marine protected Areas

Marine protected Areas (MPAs) are areas where fishing is banned or there is legislation in place restricting the equipment and techniques which can be used in that area. One such location is Rosemary bank seamount, which is located off western Scotland, which was designated an MPA in the Marine and Coastal Access Act 2009 [8].

 

Summary

Deep sea habitats are less able to cope with the impacts of heavy deep sea bottom trawling due to their different biology and the locations where the organisms live. The environment’s physical processes, such as those in the sediment, are also easily changed and can affect the organisms living there. But there are a number of workable solutions available to reduce the effect of bottom trawling on these unique unexplored places.

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