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Home > Resources & Publications > Newsletters & Magazines > Fact Sheets > Bycatch When Fishing

Resources & Publications: Fact Sheets

by: Ken Robert and Jerald Horst

If the commercial and recreational harvest of fish and shellfish took place in a clear fish bowl, harvesters could separate target from nontarget species and take only those fish that were desired and legal. In the real world, however, the number and size of fish present cannot be observed until after the fish are caught. These nontarget, or unwanted, species taken by legal means are referred to as bycatch. A federal law defines bycatch as "…fish that are harvested in a fishery, but which are not sold or kept for personal use."

Bycatch is unwanted because it is of nonmarketable or illegal size or because it is not regarded as good to eat. Economic discards are fish that are unmarketable because they are still too small, and regulatory discards are those that must be returned to the water because of fishery regulations. Desirable fish discarded because they are too small to keep may grow to a larger, marketable size. Fish considered inedible by humans may serve as food for other fish that are wanted for harvest. But when they are discarded as bycatch, their potential use may be lost because many of them die in the release process.

Public awareness of the problems of bycatch has increased and lawmakers have been pressured to institute reduction policies. In 1996, Congress specified that bycatch in all fisheries be reduced and that unavoidable bycatch mortality be minimized. Awareness in Louisiana began a few years ago with the debate over gill nets, which were said to result in unintended catch and fish mortality. Another source of bycatch that has received attention is the shrimp trawl.

A two-year study of shrimp trawlers in the Gulf of Mexico from 1992 to 1994 documented finfish bycatch. Gulfwide, the ratio of finfish poundage to each pound of shrimp was 4 to 1, a decline from 10 to 1 observed in the 1970s. In Louisiana's portion of the gulf, the nearshore ratio was 3.3 pounds of finfish to 1 pound of shrimp and the offshore ratio was 6.9 to 1. Requirements for using bycatch reduction devices (BRDs) are expected to further reduce bycatch in shrimp trawls.

The specific results of the bycatch reduction policy are uncertain. In reducing bycatch in shrimp trawls, for example, three important questions must be asked. (1) What will be the effect on shrimp populations if more fish survive to eat them? (2) What will be the impact on marine species that have come to depend on discarded bycatch as food? (3) What will be the impact on finfish populations as bycatch mortality is reduced?

The answers depend on how well fisheries scientists can measure and forecast. Biologists with the National Marine Fisheries Service have developed an ecosystem model to predict what will happen. Though data problems are evident and it is necessary for assumptions to be made, the model is the best tool available.

Data from the two-year bycatch study showed that of the 161 species found in shrimp bycatch, only 14 were identified as shrimp predators. Because of its large population in the gulf, the white trout was the predator having the most effect on shrimp. With this information, federal scientists used the ecosystem model to evaluate three outcomes.

  1. BRDs release all types of fish in equal numbers fish that eat shrimp and those that don't. Under this scenario, a 50 percent lowering of finfish bycatch would reduce shrimp stocks by 10.7 percent.
  2. BRDs result in an increase in the average size of finfish in the gulf; causing an increase in the number of shrimp eaten. Assuming that larger fish eat more shrimp, a 50 percent increase in predation would reduce shrimp stocks by 16.7 percent.
  3. BRDs result in an increase in the average size of finfish in the gulf; causing a decrease in the number of shrimp eaten. With the assumption that many fish change their diets to include fewer shrimp as they get larger, a 50 percent decrease in predation would increase shrimp stocks by 4.7 percent.

Federal regulations that reduce shrimp trawling bycatch were finalized in late 1997. When implemented they will apply to Gulf waters west of Cape San BIas between 3 and 200 miles offshore. Recent concerns over the shrimp fishery bycatch of juvenile red snapper prompted the regulations. The National Marine Fisheries Service (NMFS) goal is to reduce the bycatch of juvenile red snapper to the extent practicable while at the same time minimizing adverse effects on the shrimp fishery. The targeted decrease in juvenile red snapper bycatch mortality is significant It must be reduced 44 percent from the average mortality of the 1984-88 period. This was determined as necessary for the red snapper stock to recover by the legally set recovery date of 2019. There could be changes because the target date is so far in the future. New scientific findings and lawsuits challenging regulations are among the possible changes. Currently available to shrimpers are NMFS-approved bycatch reduction devices (BRDs), which have openings called fisheyes and funnels sewn into trawls. Fish entering the trawls swim out of the openings along with some shrimp. The size and economic impact of the loss will be a continuing concern to fishermen and regulators.

The number of possible outcomes is large. More analysis will demonstrate that in ecosystem alterations, it is not possible to have increases in all species at the same time forever. Though many species are valued for food and recreation by humans, many more are unharvested. The results of bycatch reduction will be evident when populations of predators and prey are compared before and after bycatch regulations are in place. The mix of species is important but can't be predicted with accuracy. Fisheries scientists evaluating bycatch reduction can know the actual effects on the ecosystem only when BRDs are used.

Download: bycatch.pdf (1.12MB)

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