Bycatch and discards

The problem

Common skate bycatch, © Mark Tasker, JNCC

Most fisheries are at least partially non-selective – in other words they catch organisms that were not originally targeted. This extra catch is known as bycatch. Of the organisms not targeted, some have a commercial value and are landed by fishers to the market. However, a greater proportion is unwanted and is usually discarded (i.e. thrown back over the side). The discarded proportion of the catch will also include commercially important fish species that are not wanted: this might be because the fish are under-sized, over quota or not of sufficiently high value to the fisher.

The unwanted bycatch is one of the main environmental effects of fishing. On a global basis the scale of the effect is large. In 1996, UN Food and Agriculture Organization (FAO) estimated annual global discards at approximately 27 million metric tonnes, equivalent to about 35% of reported annual production from fisheries. However, these data have been updated more recently and the figure is thought to be 7.3 million metric tonnes. This large reduction in bycatch is thought to be the result of use of more selective gear, introduction of bycatch and discard regulations and their enforcement, and increased utilisation of the catch with improved processing technologies and expanding market opportunities.

The amount of bycatch varies considerably between different fisheries. Demersal finfish fisheries account for 36% of global discards and shrimp trawl fisheries for 27%, whilst together these fisheries represent 22% of total landings. In contrast, relatively low bycatch levels are associated with purse seines, handlines, jig, trap and pot fisheries.

Bycatch rates are high in ground fisheries
targeting species such as cod
© Simon Jennings/CEFAS

Bycatch rates are low in many pelagic
fisheries such as those targeting herring
Simon Jennings/CEFAS

In the North Sea fishery in 1990, an estimated total of almost 800,000 tonnes of fish, invertebrates and offal were discarded, compared with total landings estimated at about 2.7 million tonnes. More than 70% of discards were demersal roundfish (e.g. cod and haddock) and flatfish. Approximately half of the discards occurred during beam-trawling, with demersal otter trawlers also being implicated in high discard rates (except during industrial fishing).

Bycatch can affect nearly all marine species including animals living on the seabed, marine mammals, reptiles such as turtles and seabirds.

Solutions

The first step towards finding a solution to the problem of bycatch is determining the scale of the bycatch. With some notable exceptions (especially in the Scottish demersal fishery), there are very few long-term studies of levels of bycatch. Not only has this lack of research meant that we have no clear picture of the true scale of environmental effects, it has also meant that fisheries scientists have had only a sketchy idea of the impact of bycatch on commercial fish stocks. A concerted effort has been made in very recent years to remedy this situation, but in most cases results of this monitoring have yet to be published.

The most obvious solution to limiting bycatch of under-sized target species is to increase the mesh size of fishing nets. Mesh sizes have progressively increased in demersal fleets over the past twenty years. Eventually though, there is a limit to the usefulness of this approach in mixed fisheries as marketable fish of some species are smaller than the allowable size of other species. Secondly, the effective mesh size may be substantially smaller than the actual mesh size as the traditionally diamond shaped mesh becomes compressed during towing. This effect has been overcome partially in some UK fleets by the inclusion of a panel of 'square-mesh' in the roof of nets to allow easier escapes for small fish.

In Norwegian waters, two further mechanisms have been introduced:

under-sized fish of commercial species are not allowed to be discarded (those caught are apparently landed for processing into fish meal)
fisheries can be closed very rapidly if an area is found to have large numbers of young species.

Both of these measures will be examined in future to see how useful they might be in European Community waters.

Although there are measures associated with management of commercial stocks that influence bycatch levels, no specific measures have yet to be introduced to address bycatch issues in relation to non-target species in European waters. The exception to this is the 2004 regulation introducing measures to reduce the bycatch of marine mammals in bottom-set gill and tangle net fisheries, which come into force in 2005-2007 depending on the regional sea area.