How to Do a Manta Tow

Prior to Each Deployment:

  1. Check the net rigging: all shackles and connections should be tight and secured with wire where necessary.

  2. Check the net for rips or holes: small holes can be repaired easily with silicone sealant (provided in tool box).

  3. Check the cod end: be sure it is clean and secured properly to the coupler.

  4. Check the flowmeter: it should turn freely and be approximately 3/4 full of flowmeter oil. If the meter requires additional oil, the oil and a syringe can be found in the tool box (please refer to the section at the end of these instructions for a detailed description). If the flowmeter is not turning freely, replace it with the spare provided.

  5. Read the flowmeter: only the first 5 digits of the flowmeter need to be read (this number should coincide with the FINAL reading of the previous tow). Record this reading in the INITIAL space of the NET TOW DATA SHEET in the column labeled PORT 505. NOTE: Before deploying the net, make sure that the impeller of the flowmeter is pointing towards the net and that the nose cone of the flowmeter is pointing forward.

  6. Lower the 100 lb. hydro weight: place the weight 3-5 meters below the surface of the water.

  7. Secure the hydro clamp: make sure the hydro wire is entirely within the channel of the hydro clamp prior to tightening. The clamp should be tightened by placing a screwdriver in the hole of the eye bolt for additional leverage.

  8. Attach inclinometer to hydro cable.

  9. Notify the winch operator when you are ready to lower the net: at this point, the ship's speed should be approximately 1.5 knots. Ask the winch operator to lower the hydro wire so that the clamp is just below the surface and zero the stopwatch.

Deploying the Manta:

  1. Lower the net: after the winch operator indicates that the bridge is ready, lower the Manta frame to the water with the green polypropylene line attached to the wings of the frame . Do not let this line enter the mouth of the frame after the frame is in the water since it will impede or prohibit the flowmeter from spinning. DO NOT throw the Manta into the water since this may damage the flowmeter. Also, if the frame flips upside down it will be towed under the vessel.

  2. Start timing: when the flowmeter begins turning, start the stopwatch (this may be done by the winch operator). The wire angle should be 20-25 degrees 1 through the entire tow.

  3. After 14 min. and 30 sec., retrieve the hydro wire slowly. Using a boat hook, grab the bridle and pull the net frame out of the water. When the flowmeter stops turning, stop the watch (this may be done by the winch operator). The net should break the surface at approximately 15 minutes. Continue pulling the net aboard, read the flowmeter, and record the final reading on the NET TOW DATA SHEET in the space for the FINAL reading. Calculate the difference between the final and initial reading. The difference should be between 1700 and 2500 revolutions depending upon the calibration factor of the meter.

  4. Wash the net down thoroughly: this can be done by using the high pressure sea water hose and spraying the net from the outside. Do not direct the spray into the inside of the net since this will damage the organisms. Begin spraying at the outside of the mouth opening working your way down to the cod end. Remove the cod end and quickly preserve the sample. If the net becomes clogged with phytoplankton or debris and is visibly discolored, soak it in a Biz solution for 24 hours and thoroughly rinse it before the next use.

    1 Towing speed should not exceed 87 cm/sec, otherwise fish eggs and larvae will be extruded through the mesh. If average velocity exceeds 87 cm/sec, discard sample, rinse the cod end, and repeat tow.

Filling out the Manta Tow Sheet :

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

  2. The Total Time should b e about 15:00. Place the Total Time in the Total Box and cross out the Sinking and Towing Boxes.

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

  4. Acceptable flowmeter readings : 1500 - 2500 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