Fish Electrocution
Richard D. MocciaOAC Publication 2191
July 1991, Agdex 485/661
Introduction
Fish, like people, are subject to deliberate or accidental
exposure to high voltage electrical current. Electrical
exposure causing death is referred to as electrocution.
Improperly sealed pumps, aerators, or other electrical
devices may result in electrocution, although this is rare
if you are using today's ground fault circuit interrupters
with 110 volt systems. 220 volt circuits can be more
dangerous. Deliberate exposure of fish can occur as a
malicious act, or during the use of electro-shocking devices
which can serve to capture, subdue or kill fish. The only
known natural source of high energy electricity is
lightning, and while recorded incidents of strikes to fish
are infrequent, the results can be devastating especially if
valuable breeding stock are involved.The amount of electrical current necessary to cause death depends upon many factors, but especially the specific conductance of the water. Specific conductance of water is the reciprocal of specific resistance (!) and this varies greatly according to salinity, or hardness, and temperature. Very soft waters have a specific resistance of about 50,000 ohms; average fresh water about 6,000, and hard water about 3,000 ohms. Average sea water has a resistance of only 18 ohms. Lower water temperature increases the resistance in all types of water.
Low hardness inhibits the flow of electricity through the water, and the body of a fish will be a selective path for current flow due to the higher specific conductance of muscle and normal bodily fluids. In very hard waters, the current will often 'flow around' the fish, since the water in this case provides the path of least resistance compared to the fishes body. Hence, lethal voltages must usually be higher in hard waters.
Most outdoor ponds and tanks made out of metal or other conductive materials are prone to lightning strike. Even indoor facilities are susceptible. Lightning can enter a building via utility lines, metal beams, water pipes or drains. Many farmers know well the effects of lightning on the contents of unprotected barns! Most lightning rod experts are able to assess the likelihood of a strike in an enclosed building, and design simple protective devices to divert the electricity harmlessly to the ground.
Clinical Signs of Electrocution
Logically, electrocution occurs suddenly and the onset of
clinical signs are equally acute. Mortality rates in any
given group of fish hit by lightning can vary considerably.
Usually most fish survive - although often with extensive
injuries. Surviving fish often appear to have crooked backs
and are unable to swim properly. Paralysis of the posterior
one-third of the body is evident as a limp, non-responsive
tail. If the animal can swim, the movements of the caudal
fin are often exaggerated and erratic, and the fish are
hyperactive to touch or when netting. Quivering movements to
one side are also common, and like all the above signs
usually results from local damage to spinal and peripheral
nerves which control muscle action. Upon handling, the
backbone may be obviously broken, and a grinding noise or
'crepitus' heard during movement of the tail. The fish can
have an increased respiratory rate, will feed poorly and
take on darker pigmentation. Many other fish in the stricken
tank may appear perfectly normal.
Diagnosis
A recent history of electrical exposure is helpful, and a
check of local weather records is of use to confirm thunder
storm and electrical activity near your farm if you don't
inspect facilities daily.
Dead fish feel abnormally limp. Fractured spinal columns or severe compression of the vertebrae occurs due to violent contraction of the flank musculature. Palpation or X-ray photographs can be used to detect this, with the fractures usually evident below and just posterior to the dorsal fin. This is the centre of flexure of the tail, and the largest muscular forces are exerted here. Consequently, this is the common site for the vertebrae to break. Internally, there can be massive intramuscular hemorrhage close to the fracture, and numerous small haemorrhages may be spread throughout the body. The internal organs appear very reddish or congested. There may be blood pooled around the heart and numerous blood clots or thrombi are present.
Scorched skin or burns are not usually present unless the fish comes into direct contact with the electrical device (such as a shocking probe). Burns are never evident after lightning strikes. Cutting into the flank muscle may reveal torn tissue, and the affected area has a raw, hamburg-like texture. Large amounts of bloody fluid are also present.
Prevention
Unfortunately, once stricken, there is no cure for a
surviving fish. A high level of care will usually keep the
fish alive, but normal healing is slow, and full recovery
unlikely. Prevention of electrocution is the only sensible
course of action for any prudent fish farmer.