North Carolina often experiences fish kills during the
summer. One of the main reasons for that is something scientists
call anoxia and fishermen call "dead water." Anoxia
is a layer of water that forms along the bottom of rivers
and sounds that is so depleted of oxygen it cannot support
life. Flounder have been known to back halfway up a bank
to escape the water's clutches. Crabs will scurry on to
dry land, and eels will lay their heads on the bank gasping
for air. Anoxia has always been a problem in the deepest
parts of our sounds during the summer. During hot, dry summers,
when there is little wind and rainfall, the water begins
to stratify, or form horizontal layers. The warmer, fresher
water coming downstream settles atop the cooler, saltier
water, creating what is often called a "salt wedge."
The upper layer of water always contains plenty of dissolved
oxygen because of the exchange that takes place at the surface
and because plants and algae produce oxygen through photosynthesis.
However, in the saltier, lower level, very little photosynthesis
goes on because sunlight cannot filter through. When the
two layers do not mix, what little oxygen exists in the
lower layer is used up and "dead water" is the
Excess nutrients flowing into our coastal waters can compound
problems caused by anoxia. Nutrients such as nitrogen and
phosphorus, are food for algae and other microscopic organisms.
As algae multiply, or bloom, they remove the oxygen from
the water. As they die, the algae sink and decompose. This
processes uses up even more oxygen and results in the death
of fish, crabs, oysters, clams and other creatures. These
nutrients can enter estuaries via any number of sources
upstream including livestock operations, agricultural runoff,
sewage treatment plants, and industrial discharges.
This summer is shaping up to be a bad one for fish kills
in our state. According to a report by the
North Carolina Science Advisory Council on Water Resources
and Coastal Fisheries Management, heavy rainfalls this
past winter and spring have increased the amount of nutrients
in eastern North Carolina's waterways.
Another cause of fish kills in some of North Carolina's
waterways may be linked to Pfiesteria, a toxic dinoflagellate,
sometimes called "killer algae," which paralyzes
fish and then eats their flesh. Scientists suspect Pfiesteria
is stimulated by nutrient pollution.
While a wide variety of fish have been shown to be vulnerable
to Pfiesteria when exposed to it in a laboratory setting,
little is definitively known about the relationship between
fish species and Pfiesteria in the wild. Scientists do know
these organisms graze on other algae and their growth can
be stimulated by nutrient-induced algal blooms. It is quite
possible that stress caused by low oxygen conditions makes
fish more susceptible to attacks from organisms such as
What should you do if you see a fish kill? The
North Carolina Department of Health and Human Services
recommends the following precautions:
1. Do not eat any part of a fish with sores or other indication
2. Do not collect for consumption dead or dying fish (floaters).
3. Do not use fish harvested in a dead, dying or diseased
state to feed domestic animals or as bait.
4. When you are uncertain about the cause and side effects
of a fish kill, you can protect yourself by avoiding consumption
of any fish, shellfish or crabs harvested in the immediate
vicinity of the fish kill.
5. Do not swim in waters near a fish kill. This advice
also applies to other recreational activities that would
involve skin contact with the waters of a fish kill site.
6. If your work requires contact with water, you should
postpone any planned activities in the vicinity of an ongoing
fish kill. If water contact cannot be postponed, protective
gear should be used to reduce water contact.
7. Items that have been immersed in the waters of a fish
kill site should be handled with suitable protective gear
8. A person who falls into the water at a fish kill site
or who has another unprotected water contact should change
any wet clothing and wash the exposed area(s) with soap
and clean water or a solution of 1 part household bleach
to 10 parts water. (Do NOT use undiluted bleach.)
9. Pets should not be allowed to swim in the vicinity of
a fish kill.
10. Persons who experience illness that they think may
be related to exposures at a fish kill are advised to promptly
consult a physician.