100 millions sharks, 300,000 whales and dolphins, and 100,000 albatrosses – all caught and killed every year in nets or on fishing lines. It’s called bycatch and it is killing ocean life.

Millions of healthy mammals, birds, fish and other ocean life are scooped up in nets or hooked on lines stretching many kilometres across the ocean every year. All too often most of these are not wanted by the fishermen – who are usually only interested in one or two types of fish – so they are thrown back overboard, dead or dying.

Some fishing techniques are worse than other for bycatch. Up to 90% of the catch in tropical shrimp trawling is thrown away. Discarded bycatch may include perfectly good fish that could feed people and provide income to others; it could be juvenile fish that now can’t help re-populate species; it can also be protected andendangered species. 

Different types of fishing practices result in different species being killed as bycatch: gill nets commonly kill dolphins, porpoises and whales; longline fishing is a particular problem for birds which dive on the baited hooks and are pulled underwater and drowned; and bottom trawling devastates corals and sponges growing on the seabed.

Bottom trawling 'strip-mines' the ocean floor – a single pass of huge metal trawl net doors, ropes, and rollers leaves deep gouges in the seabed and scoops up or crushes any creatures in its path, including centuries-old corals that give life, food and shelter to vast numbers of species. 

The scale of the problem is hard to measure, as fishing fleets often don’t report what they don’t land. The most recent estimate of fisheries discards is that over 6.8 million tonnes of sealife get thrown over the side. This figure is mainly for fish, and doesn’t include things like seabirds, mammals and turtles or chunks of coral. Bycatch is widely recognised as one of the most serious impacts of modern fishing. It is also largely unnecessary.

What can be done to reduce the death toll?

There are many technical fixes that can reduce bycatch. Turtle exclusion devices are used in some shrimp trawl fisheries. Bird-scaring devices can be used on longline vessels  and lines weighted properly to sink fast so birds can’t grab the baited hooks.

But technical fixes are only ever useful if they are used well and often, reviewed, revised and regularly monitored. Technical fixes also do not solve the main problem of large-scale bycatch, which is large-scale fishing. Ocean sanctuaries that protect specific populations from being over-fished will also protect the other marine life from becoming collateral damage in the same ocean.