Bycatch - Mitigation

Mitigation

Concern about bycatch has led fishermen and scientists to seek ways of reducing unwanted catch. There are two main approaches.

One approach is to ban fishing in areas where bycatch is unacceptably high. Such area closures can be permanent, seasonal, or for a specific period when a bycatch problem is registered. Temporary area closures are common in some bottom-trawl fisheries where under-sized fish or non-target species are caught unpredictably. In some cases fishermen are required to relocate when a bycatch problem occurs.

The other approach is alternative fishing gear. A technically simple solution is to use nets with a larger mesh size, allowing smaller species and smaller individuals to escape. However, this usually requires replacing the existing gear. In other cases, it is possible to modify gear. The "Bycatch Reduction Device" (BRD) and the Nordmore grate are net modifications that help fish escape from shrimp nets.

BRDs allow many commercial finfish species to escape. The US government has approved BRDs that reduce finfish bycatch by 30%. Spanish mackerel and weakfish bycatch in the South Atlantic was reduced by 40%. However, recent surveys suggest BRDs may be less effective than previously thought. A rock shrimp fishery off Florida found the devices did not exclude 166 species of fish, 37 crustacean species, and 29 species of other invertebrates.

In 1978, the National Marine Fisheries Service (NMFS) started to develop turtle excluder devices (TEDs). A TED uses a grid which deflects turtles and other big animals, so they exit from the trawl net through an opening above the grid. US shrimp trawlers and foreign fleets which market shrimp in the US are required to use TEDs. Not all nations enforce the use of TEDs.

For the most part, when they are used, TEDs have been successful reducing sea turtle bycatch. However, they are not completely effective, and some turtles are still captured. NMFS certifies TED designs if they are 97% effective. In heavily trawled areas, the same sea turtle may pass repeatedly through TEDs. Recent studies indicate recapture rates of twenty percent or more, but it is not clear how many turtles survive the escape process.

The size selectivity of trawl nets is controlled by the size of the net openings, especially in the "cod end". The larger the openings, the more easily small fish can escape. The development and testing of modifications to fishing gear to improve selectivity and decrease impact is called "conservation engineering."

Longline fishing is controversial in some areas because of by-catch. Mitigation methods have been successfully implemented in some fisheries. These include:

• weights to sink the lines quickly
• streamer lines to scare birds away from baited hooks while deploying the lines
• setting lines only at night with minimal ship lighting (to avoid attracting birds)
• limiting fishing seasons to the southern winter (when most seabirds are not feeding young)
• and not discharging offal while setting lines.

However, gear modifications do not eliminate by-catch of many species. In March 2006, the Hawaiʻi longline swordfish fishing season was closed due to excessive loggerhead sea turtle by-catch after being open only a few months, despite using modified circle hooks.