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.
- 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.
-
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.
- 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
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