How to Do a Pairovet Tow

The standard Pairovet net tow (mesh . 150) is a vertical tow which is deployed to 70 meters at a rate of 70 met/min. The net is rested for 10 seconds at depth, then towed back to the surface at 70 met/min. The wire angle should be no greater than 15 degrees, preferably zero.

Steps for doing a Pairovet Tow :

  1. First, make sure to have at least 70 meters of water depth. Check with the bridge if you are uncertain of the actual depth. If you have a depth which is less than 70 meters, then deploy the net to a safe depth.

  2. Deploy the hydro wire with the 100 lb. weight into the water. Lower the weight a few meters below the surface of the water to allow the wire to steady. Attach the Pairovet frame to the wire and secure tightly. Attach a wire clamp to the wire just above where the cod ends dangle and tighten with a screwdriver. Secure the clip that holds the two nets together under the clamp. This will keep the nets vertical to the wire and from rising up during the tow.

  3. Check the flowmeter numbers in the mouth of the frame to see if they agree with the numbers you have on the tow sheet from the previous tow. If they vary, either change the numbers on the tow sheet to agree to the actual numbers, or spin the flowmeter impeller forward or backward to match the numbers on the tow sheet.

  4. Now you are ready to start the Pairovet tow. Contact the bridge to let them know you are ready to deploy the net. Once you have clearance, have the winch operator zero the net at the surface and start the Pairovet down at 70 met/min. Start the stopwatch as soon as the net starts down. Make sure to record the start time in the appropriate place on the tow sheet. The sink time should be about 1:00 for a standard 70 meter tow. When the net reaches depth, stop the watch and record the sink time on the tow sheet. Count off 10 seconds of rest time in your head, then have the winch operator hall back the net at 70 met/min. Make sure to start the stopwatch again when the net starts up. Record the wire angles with a hand-held inclinometer when the net is at depth, then again halfway back to the surface, then a final one when the net breaks the surface of the water. The angles should be 15 degrees or less during the ascent to the surface. If they are too high, then repeat the Pairovet tow. Stop the watch when the net reaches the surface and record the total ascent time on the tow sheet. It should be about 1:00 for a standard tow.

  5. Read the end flowmeter numbers and record them in the appropriate place on the tow sheet.

  6. Wash down the net to congregate the plankton sample into the cod end. Only rinse from the outside of the net. Never squirt the seawater hose inside the net; it might contaminate the sample or damage the eggs in the sample. Once the Pairovet is washed down, remove it from the hydro wire and hang it up in its' appropriate spot. Remove the wire clamp from the hydro wire.

  7. If you will be doing a Manta tow immediately following the Pairovet tow and are using the same hydro wire, then leave the weight in the water and proceed with the Manta tow. If not, then bring the weight in and secure it in its proper holder.

  8. You must now preserve the Pairovet sample. Remove the cod end with a screwdriver and take it to the pickling area. Preserve the sample in a pint jar using a .150 mesh draining sock if needed. Do not use the .333 mesh draining sock or you will lose the sample. Put an inside label in the jar with all the correct information. Also, make sure there is an outside label on the lid of the jar with the identical information. Store the sample in the appropriately labeled box.

Filling out the Pairovet Tow Sheet :

  1. Make sure to put all the information in the proper place as shown on the sample tow form.

  2. The Sink Time should be about 1:00 for a standard 70 meter tow. The Towing Time is always 10 seconds, also called Resting Time. The Total Ascend Time should equal the Sink Time of about 1:00. To calculate the Total Towing Time, add all three values together to equal a time of about 2:10.

  3. To get the End Tow Time, add the Total Towing Time to the Begin Tow Time. Be sure to calculate the seconds also.

  4. Acceptable flowmeter readings : 250 - 320 revs. (These are normal depending on weather.) If the readings consistently vary too much from this range, then check the flowmeter for any problems. If high or low readings persist, then try using a new meter.

  5. Refer to the Example Tow Form or the Net Tow Data Sheet Instructions for further detailed instructions.
Last modified: 12/24/2014