Environmental impacts of trawling and longline fishing

This is the second article in a series that I’m writing for a Chinese magazine targeting wildlife conservation. As you may

Photo courtesy of the NOAA photo library (www.photolib.noaa.gov) Photographer: Robert K. Brigham

Photo courtesy of the NOAA photo library (www.photolib.noaa.gov)
Photographer: Robert K. Brigham

guess, they started with Panda conservation, so the magazine is called Giant Panda, but they are running a series on exploitation of natural resources. In the first I covered overfishing. This current article delves a bit deeper into some of the bigger impacts (certainly not all of them!) of trawling and longline fishing. This is an abridged version.

In the previous article I discussed why overfishing is such a harmful and global issue and how it is leading to negative changes in marine ecosystems. It is not only this overall effect that we should be concerned about, however, because some fishing practices can have large negative impacts on other species, such as sea birds and turtles, or the environment, even if they are not overfished.

What many people don’t realise is that many fishing techniques have some level of unintended negative impacts. The two biggest impacts are 1) the destruction of benthic habitats and 2) bycatch. Benthic habitats are the habitats on the sea floor such as kelp forests or coral reefs which support many other species and are essential to functioning of the ecosystem. Bycatch is the unintentional catch of species that are not of commercial value, not of interest to the fishermen, cannot be sold under their fishing licence or because it is a protected species. It is estimated that approximately 40% of the global fisheries catch is bycatch and discarded back into the ocean. The major issues with bycatch are that it is discarded back into the water, usually dead, contributing to the decline of the ecosystem. Below, I discuss two globally common fishing techniques and some of their impacts.

Longline fishing

Longline fishing is a technique where long fishing lines, up to 10 km long, and containing thousands of baited fishing hooks, are floated along the surface of the ocean to catch pelagic fish species such as tuna or marlin. These lines are set in place for many hours to days and left to drift on the ocean to catch their prey. Over this time, however, many other species are both exposed to and attracted to the baited hooks, meaning that longline fisheries are responsible for a large amount of unwanted bycatch. Unlike trawl fisheries which catch big and small species (explained below), longline fisheries have the dubious record of killing larger animals such as seabirds, turtles, sharks and whales. For example, it has been estimated that global longline fisheries kill somewhere between 160,000 – 320,000 seabirds annually. Unfortunately, many of these species are protected or endangered and such bycatch only serves to increase the pace at which their populations decline.

Another impact that some fisheries, including longline fisheries, have on marine ecosystems is a phenomenon known as ghost fishing. This is when the fishing gear is lost and not collected by the fishermen, allowing it to float around the world’s oceans indiscriminately catching and killing marine life. For longlines it is easy to see how this can occur, as the lines are kilometres long and can be lost when other ships run over the lines, cutting them and separating them from the marker buoys so the fishermen cannot find them again. Unlike bycatch which is pulled from the ocean within a matter of hours, however, such “ghost gear” will continue to kill animals until it degrades and breaks up, usually several years after it is lost.

Trawl fishing

Much of the world’s seafood is caught by trawling, where fishing vessels (trawlers) drag large nets through the water to catch the target species. There are broadly two main types of trawling, pelagic, where the net is dragged through the water column, and benthic, where the net is dragged along the bottom. While a common practice and quite cost-effective for the fishing industry, trawling has two large negatives, 1) a very large bycatch and, 2) for benthic trawls, damage to the seabed.

While effective in catching the target species, the nets used for trawling are not selective and catch many of the animals which are in their path. Imagine a net that can be as much as 100 m wide at the mouth, travelling faster than most fish can swim and it is easy to see that most organisms cannot escape the net, becoming bycatch. The extent of this bycatch can be astonishing. In some regions of the world up to 64 % of the catch is discarded back into the ocean, dead. In the trawl fisheries of the Gulf of Mexico alone, for example, bycatch is estimated to be the equivalent of 1 billion meals a year3! In addition to this wastage, large species such as dolphins, sharks, turtles and seals are often caught in the nets and drown, severely impacting their populations and causing them great suffering. For example, a single prawn fishery that does not employ Turtle Exclusion Devices (TEDs) can catch more than 50,000 turtles per year. Not only are some of these species in decline and protected by law but, as discussed in my previous article, they perform important roles in regulating the function of marine ecosystems on which we depend.

The other major negative impact of trawling is damage to the seabed. When nets are dragged along the seabed they not only catch the species that is being targeted, but they also rip up the seabed itself. Indeed, many fisheries, such as prawn or shrimp trawl fisheries, use chains on the bottom of the net to drag along the seabed and scare the prawns up into the net to be caught. Unfortunately, this type of trawling is now very common and in some regions the seabed is highly impacted. An example is the North Sea, much of which is turned over every year, some areas up to three times per year. Such intense disturbance corresponds with a decline in faunal abundance and species diversity, meaning that over 100 years of intense trawl activity in the North Sea has led to marked declines in species diversity.

This type of physical disturbance of the seabed also, leads to dramatic changes in benthic habitats – larger structures are gradually removed or broken leading to homogenous habitats which are less suitable for most species. Biological habitats such as seagrasses or deep sea sponge beds are destroyed. Unfortunately, many of these types of biological habitats are extremely slow-growing and can take 100’s – 1000’s of years to regrow, assuming that they are not disturbed again in that time.

Are there solutions?

Bycatch is a problem of massive scale which requires a global effort to improve. Thankfully, there are emerging fishing techniques, practices and gear which will start the process of limiting some bycatch. One of the best examples for trawl nets is the inclusion of a device known as a Turtle Exclusion Device, or TED, which also work for excluding other large animals like dolphins and sharks. These devices are large metal bars which cross the opening of the net allowing small species, like prawns or shrimp, into the net so they are caught but larger animals, like turtles, are allowed to escape and are free to swim away. In fisheries where TEDs are now compulsory the bycatch of turtles has deceased by up to 100 %. But this is only one species being excluded.

There are also changes to longline fisheries that can be implemented to reduce bycatch, in particular of seabirds. For instance, setting the baited hooks deeper in the water rather than on the surface and only setting lines at night when the birds aren’t feeding have shown to be effective to some degree.

Limiting the damage of benthic habitats by trawling is more difficult. Changing some practices, such as not using chain on the bottom of the net, can reduce the impact a little, but it is unlikely that these techniques will be broadly implemented as they also reduce the catch. As such, one way to limit damage to benthic habitats is to reduce the frequency with which an area is trawled to allow habitats to recover, but even this will not be effective when geological features or long-lived biological habitats are destroyed. This means that the only truly effective way to ensure the protection of these habitats is by implementing systems of Marine Protected Areas in international waters, currently a very difficult task.

As with all fishing activities, however, regulating and policing these techniques is extremely difficult, especially in international waters. In addition, some fishermen, especially in developing nations, incorrectly think that using these techniques (such as TEDs) will reduce their catch. These people are often already poor and desperate to feed their families and therefore afraid to make any changes that could further harm their families’ health. Until we move towards complete implementation of these techniques, or even improvements on them, the impact of fishing will go beyond what we see on the target fish stock and continue to degrade marine ecosystems. One effective way that you can help towards implementing these changes is by only eating or purchasing seafood from restaurants and shops that support sustainably managed fisheries. Making this choice is becoming more and more viable (see and example guidebook here) and by doing so this will only continue to improve, meaning that you too can help improve the health of our oceans.