Marine turtles arose in the oceans over 100 million years ago, yet over the past 100 years numbers have dropped dramatically. Hundreds of thousands of these ancient reptiles are caught each year as bycatch, especially on longlines, in shrimp trawlers, gill nets and purse-seine nets. The sea turtle population has now six of the seven marine turtle species currently categorized as "Vulnerable," "Endangered" or "Critically Endangered" globally by the IUCN Red List.
While there are uncertainties about the precise contribution of bycatch to global marine turtle declines, it is beyond any doubt that turtles cannot sustain current bycatch rates.
A report published in 2010 "Global Patterns of Marine Turtle Bycatch", by Dr. Bryan Wallace of CI, and Dr. Rebecca Lewison of San Diego State University, investigated the impact of bycatch on sea turtles around the globe from 1990-2008. Their findings show that tens of thousands (~85,000) of marine turtles have been reported as bycatch in the past twenty years, but Wallace and colleagues stress that actual numbers are likely to be significantly higher.
"The good-will and expertise of fishermen are part of the solution to the bycatch problem. These gentle giants need fishermen to be part of the collective effort to save them for our grandchildren to see."
Carlos Drews, Regional Marine Programme and Species Coordinator for WWF's Latin America and the Caribbean Programme
With over 300,000 small whales, dolphins, and porpoises dying each year from entanglement in fishing gears, bycatch is causing one death every two minutes. It is the single-largest cause of mortality for small cetaceans.
While large whales can usually break free, entanglement can nevertheless cause debilitating injuries and eventual death.
Bycatch reduction may make the critical difference in the fight to save nine of the fifteen cetacean species or populations classified as endangered or critically endangered. These include New Zealand’s maui’s dolphin, Mexico’s vaquita, North America’s north Atlantic right whale, the Philippine’s irrawaddy dolphin, Europe’s harbour porpoise, and South America’s franciscana dolphin.
Vast numbers of juvenile fish, including the young of commercially valuable species, are caught in many fisheries around the world, only to be discarded as they are under-size. Most do not survive. Even those that are small enough to escape through the net can die as a result of injuries/stress.
This loss of fish contributes to over-fishing and hampers the recovery of already depleted fish stocks. For example, the North Atlantic cod fisheries, once some of the most productive on the planet and which played a pivotal role in the economic and social development of the northern hemisphere are now being pushed to the brink because cod juveniles and spawning adults are killed in nets that target other fish.
Many seabird species are victims of bycatch, particularly albatrosses, petrels, shearwaters, and penguins.
Such deaths threaten 26 species, including 18 species of the albatross family, with extinction, according to the latest figures. Tens of thousands more seabirds die in drift nets, trawls nets, and gillnets. Seabirds also die after hitting trawl warps (the rope that attaches the net to the boat) often while feeding on fish offal discharged from on-board fish processing.
Bycatch accounts for about half of global shark catches. Longlines are mostly responsible, but bycatch in nets is also important. In the Pacific Ocean alone, 3.3 million sharks are caught each year as bycatch on longlines. Indeed, in terms of numbers, sharks are the most significant bycatch species in the world’s major high seas fisheries. They are also particularly vulnerable to over-fishing due to their relatively slow reproductive rate, with several species showing recent drastic declines.
Blue, hammerhead, shortfin, mako, thresher, and great white sharks are among the species most affected by bycatch. In the north east Atlantic Ocean, for example, 89% of hammerhead sharks and 80% of thresher and white sharks have disappeared in the last two decades as a result of bycatch. Deep-water sharks are also increasingly at risk.
Bottom trawling is responsible for most coral bycatch. Such bycatch particularly threatens cold-water coral reefs which are extremely slow growing and so cannot recover easily from even one trawler pass.
Large amounts of such corals are caught / destroyed: 40 tonnes per year in the North Pacific Ocean, for example, with over 2,000 tonnes taken around the Aleutian Islands alone between 1990 and 2002.
Even though the technology to trawl on cold-water corals has only existed for a couple of decades, the damage is already widespread. On heavily fished seamounts off southern Australia, 90% of the sea bed where cold-water coral used to grow are now bare rock, while 30-50% of cold-water coral reefs off Norway have been damaged or destroyed, most likely by trawling equipment.
Enormous numbers of starfish, crabs, sea urchins, brittle stars, mollusks, sponges, worms, and other bottom dwelling creatures are caught by bottom trawlers and dredgers targeting shrimp, Nephrops (Norway lobster), scallops and fish such as plaice and sole.
North Sea bottom trawl fisheries, for example, have been estimated to discard up to 150,000 tonnes of invertebrates annually. This is having a dramatic affect on the ecosystem of the seafloor and altering this balance may well negatively impact commercial species which prey on bottom living invertebrates.