The bongo net, referred to as  CB in the CalCOFI database, has been the standard CalCOFI sampler since replacing the CAL1MOBL in 1978. The net frame consists of two anodized aluminum circles, 0.71 m in diameter, connected by a central yoke which is in turn connected to the towing cable.  The central yoke makes this sampler a "bridle-free bongo net" which allows the mouth openings to be vertical during the tow.  The nets are 3 m in length, a 1.5 m cylindrical portion joined to 1.5 m conical portion tapering to a detachable cod end.  At least one ( starboard net), the principal ichthyoplankton sample net, is made from 0.505 mm square mesh nylon. The sample from the port side net is used for other purposes; the mesh size is either 0.505 mm or 0.333 mm depending on requirements. The cod end of each net is constructed of 0.333 mm mesh.  The ratio of mesh aperture area to mouth area is 4.8:1 for the 0.505 mesh net.

Plankton samples are obtained by retrieving the sampler at an oblique angle.  A 22 kg weight is attached to the bottom of yoke frame.  While the survey ship is making  1-2 knots, a fixed amount of towing cable is paid out, 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).  During each plankton tow 300 m of towing cable is paid out; thus the vertical depth sampled is approximately 210 m.  In shallow areas the amount of towing cable paid out is adjusted so that the net fished to approximately 15 m off the bottom.

Last modified: 12/24/2014