Electrofishing and Acoustics

• Lecture Outline

– Electrofishing

– Acoustics

• Assignments

– pp. 353 - 379 in Fisheries Techniques

Circuit Theory

1) Circuit – closed path along which a charge moves

2) Current (amps) – rate at which charge is carried per unit time

3) Voltage (Volts) – amount of energy per charge

4) Resistance – ratio of voltage to current

a) Resistance = V/A (Ohm’s Law)

DC versus AC

1) Frequency – number of cycles per second

2) Pulse width – duration of each pulse (milliseconds)

3) Duty cycle – ratio of pulse to total time between pulses

Conductivity

1) Inverse of resistance

2) Measure of an aqueous solution to carry an electrical current

3) Most important environmental factor that can influence electrofishing

Power transfer

1) Power density may be best indicator of electrofishing intensity

2) Power transfer depends on the mismatch between water conductivity and fish conductivity

Effects of electrofishing on fish

1) Electrotaxis – fish movement caused by electric current

a) AC – Thrashing movement (oscillotaxis)

b) DC – Swim towards the electrode

2) Tetany – when a fish is immobilized by the current

Effects of electrofishing on fish

1) Injuries

a) Hemorrhages

i) Capillaries in skin “branding’

ii) Muscle tissue

iii) Gills

2) Fractures or dislocation

a) vertebrae column

3) Egg mortality

Minimizing damage to fish

1) Continuous DC is the best

2) AC is the worst

3) Pulsed DC is variable

a) Low frequencies, low duty cycles, low voltage

Efficiency/biases

1) Selective for certain species

a) Morphology, behavior

2) Fish size

a) Voltage gradient increases with length

b) Large fish more visible to netters

3) Environmental conditions

a) Conductivity, transparency, substrate, cover, stream width

4) Catchability

a) Effected by environmental conditions, netters, driver

Hydroacoustics

1) Use of transmitted sound to detect fish

2) Can be used to estimate fish biomass, numerical abundance, and size distribution

3) Vertical vs horizontal beaming

a) Horizontal useful in shallow waters

b) Use combination when > 10 m

c) Fish must not be tightly shoaled

4) Season or time of day may be important

5) Air bubbles reduce efficiency

Hydroacoustics: Advantages

1) Quick assessment of stock

2) Can cover large area or frequent time intervals

3) Can have unattended monitoring of fish movement

Hydroacoustics: Limitations

1) Cannot identify fish species

a) Concurrent sampling of fishes

2) Cannot easily sample all parts of the aquatic environment

3) Trained personnel required