38
The Last Resource
powers arc required—«up to 10,000 kw to give a shing range of only
30 feet radins. However, scientists have found that pulses of
electricity not unlike those of electric shes can be effective, offering
prospects of a saving in the power needed for the process. Although
the anodic attraction is not as strong, another effect helps counteract
this deciency; low—frequency pulses at a rate close to the natural
swimming rhythm cause more swimming activity. The overall result
is generally to bring the sh towards the anode where they can be
netted or pumped on board the shing boat.
Some experts argue that using the anodic effect is an unnecessary
complication. They suggest 'ring electrodes into the middle of
shoals, delivering a short, sharp shock and collecting the stunned
sh. This seems a drastic and not very selective technique. Far more
ingenious is a system being developed by the US. Bureau of Com—
mercial Fisheries to collect brown and pink shrimps. The shrimp
eets have to spend half their time lying idle over the shing grounds
because the shrimps stay buried in the muddy seabed until the
evening, when they come out to feed. By patient study, the American
scientists have found out how much electrical stimulation is needed
to force the shrimps out of their mud burrow. They have even
worked out what strength and frequency is needed to make the
crustaceans move to a suitable height for a particular speed of
trawling. The prototype electric trawl—a converted Gulf of Mexico
trawl——has ve electrodes which stretch in front of its mouth
_dragging over the sea oor. The electrodes receive pulses of
electricity from a generator attached to the trawl. At the time of
writing the best results seem to be given by discharges of three volts
at four pulses per second, but the settings vary according to the
nature of the bottom. Apart from this promising technique, the only
commercial use of electricity in marine sh catching is on some tuna
lines. Once the sh is hooked, the sherman gives it a short sharp
shock in order to cut short its struggles.
Pump shing combined with a light lure provides more evidence
than electric shing of its potential value to commercial sheries.
After all, shermen in the Caspian Sea have been using this tech—
nique for a decade or so. The lights bring the sh within the range
of a suction pump which draws them out of the water and dumps
them in the holds of the boat. No one knows how lights attract
sh; the area illuminated by even the most powerful lamp is fairly
limited. It seems that they attract and hold the attention of sh which
The Last Resource
powers arc required—«up to 10,000 kw to give a shing range of only
30 feet radins. However, scientists have found that pulses of
electricity not unlike those of electric shes can be effective, offering
prospects of a saving in the power needed for the process. Although
the anodic attraction is not as strong, another effect helps counteract
this deciency; low—frequency pulses at a rate close to the natural
swimming rhythm cause more swimming activity. The overall result
is generally to bring the sh towards the anode where they can be
netted or pumped on board the shing boat.
Some experts argue that using the anodic effect is an unnecessary
complication. They suggest 'ring electrodes into the middle of
shoals, delivering a short, sharp shock and collecting the stunned
sh. This seems a drastic and not very selective technique. Far more
ingenious is a system being developed by the US. Bureau of Com—
mercial Fisheries to collect brown and pink shrimps. The shrimp
eets have to spend half their time lying idle over the shing grounds
because the shrimps stay buried in the muddy seabed until the
evening, when they come out to feed. By patient study, the American
scientists have found out how much electrical stimulation is needed
to force the shrimps out of their mud burrow. They have even
worked out what strength and frequency is needed to make the
crustaceans move to a suitable height for a particular speed of
trawling. The prototype electric trawl—a converted Gulf of Mexico
trawl——has ve electrodes which stretch in front of its mouth
_dragging over the sea oor. The electrodes receive pulses of
electricity from a generator attached to the trawl. At the time of
writing the best results seem to be given by discharges of three volts
at four pulses per second, but the settings vary according to the
nature of the bottom. Apart from this promising technique, the only
commercial use of electricity in marine sh catching is on some tuna
lines. Once the sh is hooked, the sherman gives it a short sharp
shock in order to cut short its struggles.
Pump shing combined with a light lure provides more evidence
than electric shing of its potential value to commercial sheries.
After all, shermen in the Caspian Sea have been using this tech—
nique for a decade or so. The lights bring the sh within the range
of a suction pump which draws them out of the water and dumps
them in the holds of the boat. No one knows how lights attract
sh; the area illuminated by even the most powerful lamp is fairly
limited. It seems that they attract and hold the attention of sh which