CalBOBL (Bongo) Mesozooplankton Net Deployment


SUMMARY: The CalBOBL net is towed obliquely through the water from 210 meters to the surface and samples organisms > 505 µm. Samples are collected within the California Current Ecosystem, preserved and analyzed for ichthyoplankton and mesozooplankton.


1. Principle

Southwest Fisheries Science Center collects CalCOFI Bongo Oblique (CalBOBL or Bongo) net tow samples for all CalCOFI and SCCOOS stations. The bongo frame consists of two anodized aluminum circles that are 71 cm in diameter and has a 505 µm mesh net, 3 meters in length, attached to each side. The bongo is towed obliquely through the water column from 210 meters to the surface, while the ship is underway at 1-2 knots. While underway, a fixed amount of towing cable is paid out (300 m); the net is held at depth for 30 seconds and then retrieved at a constant rate (20 m/min). Ship speed is adjusted to maintain the towing cable at a constant angle to the vertical (45 degrees +/- 3 degrees). In shallow areas the amount of towing cable paid out is adjusted so the net samples approximately 15 m off the bottom. The starboard side bongo sample is preserved in formalin and the port side sample is preserved in ethanol.

2. Deployment Methods

A detailed method description for the bongo deployment can be found on the Southwest Fisheries Science Center at

3. Sample Preservation

Starboard side of the bongo tow is preserved in a 5% solution of Formalin in seawater, buffered with sodium tetraborate and the port side of the tow is preserved in 95% ethanol.


Sample preservation on the starboard side of the bongo tow (Kramer, 1972):

  1. Concentrate sample using a 333 µm mesh draining sock if needed, rinse sample into a pint jar (quart jar if sample is large), and fill jar with seawater until it about 90% full.
  2. Add saturated sodium tetraborate (10 ml to pint jars and 20 ml to quart jars) to buffer the sample. Formalin is slightly acidic and the sodium borate will raise the pH of the sample to neutral.
  3. Add 37% formaldehyde to each sample jar (25 ml to pint and 50 ml to quart jars). Put label in jar, tighten lid and carefully rotate jar to mix the preservative within the sample.


Sample preservation on the port side of the bongo tow:

  1. In 24 hours, refresh the sample with 95% ethanol by removing the sample into a 333 µm mesh draining sock, place sample back into jar and add fresh 95% ethanol until the sample contains 80% ethanol and 20% sample and mix well.
  2. May need to repeat mixing and step 2 several times to ensure the sample will not deteriorate.

4. Flowmeter Calibrations


General Oceanic Flowmeter 2030R is calibrated before and after each cruise at 12 different speeds (14 – 36 seconds) over a 16 meter measured distance.


Calculate the average revs, average time in seconds, rev/sec and met/rev (total distance/rev) for each speed.


Obtain common slope and intercept for the flowmeter calibrations from pre- and post-cruise data.

x axis = 1/revs/sec (reciprocal of rev/sec)

y axis = met/rev

5. Sample Calculations

Once slope and intercept are defined, you can solve for MRev for each tow in which the specified flowmeter was used. By defining MRev for each tow, you can solve for VWS.

To calculate Revs per second:

RPS = TotalRevs / TowingAndAscentTime (s)

For calculating meters per one revolution, or MRev, (intercept and slope of regression line) for each plankton tow:

MRev = Intercept + (Slope / RPS)

To obtain the Volume Water Strained (VWS m3) through the tow:

VWS = (Revs) * (MRev) * (Area)
Revs = total flowmeter revs during the tow
MRev = meters per one revolution
Area = mouth area (m2) of frame opening (Πr2)

The Bongo net tow is an oblique tow from 210 m to the surface. Since multiple angles are recorded, the cosine of the average angle of stray is taken and this is derived from a tangent of all angles and not just the terminal angle.

The actual depth is calculated by:

ADT = (Cos(Radian of the AveAngleofstray)) * (Wire out)

To calculate the Standard Haul Factor (10m2 / unit area):

SHF = (10) * (ADT) / VWS

For Standardizing Plankton Volumes:

(Raw Volume * (1000)) / VWS = Volume in ml/1000m3 of Water Strained

6. Equipment/Supplies

  • Clipboard, stopwatch, ratchet wrench
  • Waterproof inside and outside labels
  • Bongo frame, hose clamps and hardware to secure flowmeter
  • 333 µm Nitex mesh plankton concentrating sock
  • Wire clamp and Inclinometer
  • Pint and quart jars
  • Flowmeter, Digital, Mechanical, w/standard rotor (General Oceanic Cat. No. 2030R)
  • For bongo frame and tow net specifications refer to
  • -20°C Freezer for sample storage
  • Turner Designs 10-AU Fluorometer, fitted with a red-sensitive photomultiplier tube (PMT)

7. Reagents

  • Sodium tetraborate with filtered seawater for a saturated solution
  • 37% Formaldehyde
  • 95% Ethanol

8. References

  • General Oceanics Digital Flowmeter Mechanical and Electronic Operators Manual 
  • Kramer, D., M.J. Kalin, E.G. Stevens, J.R. Thrailkill, and J.K. Zweifel. 1972. Collecting and processing data on fish eggs and larvae in the California Current region. U.S. Dept. of Commerce, NOAA Technical Report, NMFS Circ-370, pp. iii-iv, 1-38.