Invertebrates: The Other Food Source
The problem of over fishing is not limited to fish. We catch and eat
many other marine animals including shrimp,
crabs, clams, lobsters, oysters, and scallops. Where do these creatures
live? What do they eat? What eats them? How many are there? How are
they affected by coastal development?
Lets Concentrate On One Class:
Shrimp. Shrimp remained the top choice for seafood in the United States
at 4.1 pounds per person in 2007 according to NOAA.
Commercial Shrimp Species Gulf of Mexico
Shrimp live part of their life in lagoons, bays, and mangrove areas and
in deeper water offshore. In Texas, the commercial shrimp species are
Brown Shrimp, White Shrimp, and Pink Shrimp.
Brown Shrimp: Sexual maturity
for brown shrimp is near 4.5 inches for males and 6.5 inches for female.
Brown shrimp spawning takes place in Gulf waters ranging from 151 to
299 feet. There is some indication that during the
winter, brown shrimp post-larvae may remain offshore
in the Gulf for some period before moving
into estuaries in early spring. Peak influx of post-larval
brown shrimp to estuarine waters is February–April. After entering
bays, brown shrimp tend to be found in significantly higher densities
in vegetated marsh areas. As brown shrimp grow from juvenile to
sub-adults, they begin to feed more on detritus and
benthic organisms such as polychaete worms and
amphipods. During the juvenile to sub-adult phase,
brown shrimp enter deeper bay waters and eventually
emigrate from bays to the Gulf. This emigration is generally in association
with a full moon and
strong tidal cycles from May through August with
peaks from May to July.
White Shrimp: Size at sexual maturity
for white shrimp is 6.0 inches for males and 6.5 inches for females.
White shrimp spawning occurs offshore in depths ranging from
23 to 108 feet from March to September. Peak influx
of post-larval white shrimp to estuarine bay waters is
during summer. As white shrimp grow they begin to
move into deeper open bay waters, preferring soft
mud or peat bottoms. Decreasing water temperature
accelerates emigration of white shrimp to the Gulf.
Offshore movements peak from September to
December. Along the south-central Texas coast,
shrimp move southward during cool months and
northward during spring.
Pink Shrimp: Minimum size at sexual
maturity for male pink shrimp is 2.9 inches and 3.3 inches for females.
Pink shrimp spawning occurs offshore in depths ranging
from about 13 to 164 feet. Although spawning occurs
all year, activity increases as water temperature rises.
Peaks in spawning occur in late spring, summer and
early fall. Peaks of immigration of pink shrimp post-larvae
into nursery areas occur in the spring and fall.
Dense sea grass beds appear to be important to both
post-larval and juvenile pink shrimp. Emigration of
pink shrimp occurs throughout the spring, summer,
and fall and seems to be correlated with full moon
ebb tides. Some pink shrimp over-winter in Texas
bays, residing in estuaries for up to nine months.
Quotes from The Texas
Shrimp Fishery: A report to the Governor and the 77th Legislature
of Texas. The link will download a 5 MByte pdf file
published by the Texas Parks and Wildlife Department (2002).
Images from Florida Fish and Wildlife Commission.
The life cycles of brown, white and pink shrimp are similar.
part of their life in estuaries, bays and the Gulf of Mexico. Spawning
occurs in the Gulf of Mexico (a on the map below). One female shrimp
releases 100,000 to 1,000,000 eggs that hatch within 24 hours. The
young shrimp develop through several larval stages as they are carried
shoreward by winds and currents (b, c, and d). By the time the young
shrimp (post-larvae) reach the gulf passes and enter the bays, they
are one-fourth inch long, transparent and have a shrimp-like appearance
Post-larvae drift or migrate to nursery areas within
shallow bays, tidal creeks, and marshes where food and protection necessary
for growth and survival are available (f). There they acquire color
and become bottom dwellers. If conditions are favorable in nursery
areas, the young shrimp grow rapidly and soon move to the deeper water
of the bays (g).
When shrimp reach juvenile and sub-adult
stages (3-5 inches long) they usually migrate from the bays to the
Gulf of Mexico where they mature and complete their life cycles (h).
Most shrimp will spend the rest of their life in the Gulf. The fishery
for them begins when the shrimp are two to four months old and continues
for the rest of their lives. If not caught by anglers or eaten by
fish, they may live to be two years old.
Quotes from Texas
Parks and Wildlife Department.
Shrimp Life Cycle
Left: Life cycle of shrimp
showing distribution with depth. From Shrimp
News International. Right:
Life cycle of shrimp showing distribution in the horizontal off the
Texas Coast. a) shrimp eggs. b) nauplius larva. c) protozoea. d)
mysis. e) postmysis. f) juvenile shrimp. g) adolescent shrimp. h)
mature adult shrimp. From The
Texas Shrimp Fishery: A report to the Governor and the 77th Legislature
of Texas. The link will download a 5 MByte pdf file published
by the Texas Parks and Wildlife Department (2002): page 3.
Commercial shrimping is an important industry along the Gulf coast and
the east coast of the USA from the Carolinas southward. Demand for shrimp
eventually lead to over fishing, which caused environmental problems.
- Over fishing of shrimp reduced the numbers of shrimp.
- Reduced shrimp populations reduced the number of fish. Many fish
eat juvenile and adult shrimp.
The truth about fishing in 1963 is, it is going down so fast [on the
Texas coast] that it's time for somebody to alert those who are interested
in its future to learn what is causing this decline ...
The decline in fishing is caused by several things. The principal reason
is the scarcity of food. Everything from mites to elephants and from
larvae to whales must have food to survive. For us, bread is the staff
of life. For fish, it is shrimp.
- Shrimp nets catch many fish that are discarded. This is called by-catch.
Four to eight pounds of fish are killed and discarded for each pound
of shrimp caught.
- Shrimp nets pulled along the bottom disrupt the ecology of the bottom,
reducing the population of bottom fish and other animals.
- Shrimp nets, until recently, caught many turtles. The requirement
that all nets have Turtle Excluding Devices TED has reduced the number
of turtles caught and accidentally killed.
populations are influenced by offshore fishing as well as by bay fishing.
And, they are influenced by coastal pollution, destruction of marshes
and mangroves, and coastal development. In Texas, shrimp fishing has
the greatest influence.
To reduce over fishing shrimp and the environmental destruction related to
over fishing, the Texas Parks and Wildlife Department has spent $12.8 million
since 1995 to buy back shrimp-fishing licenses using funds produced by a $3
surcharge on recreational fishing licenses. Through January 2009, the department
has bought back 1,978 licenses, helping reduce the number of licenses from
3,231 in 1995 to 1,253. By 2008, peak bay shrimping effort has
decreased by 91 percent since 1994 according to Texas
Department of Parks and Wildlife.
The over fishing of shrimp in many regions, the rising demand for
shrimp, and the high prices paid for shrimp have led to major expansion
of shrimp farming. Thailand is one of the major sources of farmed shrimp.
The U.S. currently imports around 80% of the
shrimp consumed. In Texas, about half
the shrimp sold today are raised on farms. The remainder come from
such areas as the Texas gulf coast, which supports a major shrimp industry
($194,000,000 in 1999 compared with $9,000,000 for all fish from the
Texas coastal waters). For the latest statistics on US shrimp imports see
the Fish Information Network's Report.
Click on the latest date on the right column.
Texas Shrimp Fishery: A report to the Governor and the 77th Legislature
of Texas. The link will download
a 5 MByte pdf file published by the Texas Parks and Wildlife Department
US seafood price indicators.
What factors influence shrimp farming?
Americans now eat more shrimp per person than any other marine food product.
During 2001, shrimp for the first time surpassed
canned tuna as the most “popular”
seafood item, with the per capita consumption of shrimp being 3.4 lbs
compared to 2.9 lbs for
canned tuna. Apparent domestic consumption of shrimp during this same
period was 1.3 billion
pounds (heads-off weight), up from 750,000 lbs in 1991. To meet this
growing demand, imports of
shrimp products have become the dominant source of product for the
domestic market. Of the total
domestic supply of shrimp during 2001, imported product represented
85%, the remainder being
provided by domestic harvesters.
Charles Adams and Sal Versaggi (2004). U.S. Shrimp Import Controversy.
on International Agricultural Trade Disputes: Case Studies in North America.
For a good overview of shrimp farming read the article by
News (updated October 2003).
How does shrimp farming influence the environment?
Dr. Louis Landesman has written
in 1994 on the negative
influence of aquaculture on the environment.
The environmental problems associated with shrimp farming include:
- Habitat loss. Shrimp farms are located in coastal areas, replacing
mangroves, wetlands, and other essential habitats of marine organisms.
- Waste water from the farms pollute coastal waters.
- Over fishing of brood stock in offshore waters.
- Spread of viral diseases from farmed shrimp to native shrimp.
Disease outbreaks on US shrimp farmers-like
Taura Syndrome Virus (TSV) in Texas in 1995 and 1996 and in South Carolina
in 1996- has raised concerns among aquacuturists and wild harvest fishermen
alike, as well as among regulators charged with overseeing these industries.
Although TSV and other disease recently found in cultured shrimp pose
no threat to humans, they can be devastating to the shrimp themselves
and to the people whose livelihoods depend on them. The 1995 TSV outbreak
in Texas resulted in the loss of more than 95% of the P. vannamei crop.
Disease events in South Carolina in 1996 resulted in estimated losses
of 30%~ 50% on affected farms. Although there is little evidence that it has
been or ever will be a problem, especially considering the significant economic
value of the industry. Currently, the US harvests approximately 200 million
pounds (tails) of shrimp each year and imports another 600 million pounds (tails),
collectively valued at more than $3 billion (Fish Farming News, 1998). However,
exposing native shrimp populations to these diseases could decimate current
populations and set off a chain reaction throughout the coastal ecosystem.
Shrimp Farming in Texas.
- Introduction of exotic species for culture which later escape into
the surrounding ecosystem.
Texas marine shrimp culture has traditionally relied on non native shellfish
species (exotic species) because consumer demand for large species of shrimp
has been increasing. Exotic shrimp grows rapidly, and their ability to survive
in confined conditions is much higher than native species in an open ocean.
Shrimp Farming in Texas.
Shrimp farming needs a constant supply of shrimp larvae or brood stock
(See Kona Hawaii
shrimp ponds replace mangroves, the growing shrimp need lots of high
protein food, some of which comes from the sea, and they
produce waste water. Shrimp farmers can reduce environmental damage,
and by raising shrimp reduce the need to fish wild shrimp. Find out
more at the Texas
Shrimp Farming site.
I have searched for more up-to-date information,
but everything I found was from the early to mid 1990s. Because environmental
problems also lead to failure of the shrimp farms, many of the worst
practices are being replaced by better practices.
Shrimp News publishes a guide to the Farmed
Species Shrimp. The North Carolina Division
of Marine Fisheries has a web page giving a simple
description of shrimp ecology.
Farley, B. (2002). Fishing Yesterday's Gulf
Coast. College Station,
Texas A&M University Press.
3 August, 2009