COLLECTING LIVING PLANKTON FOR EXPERIMENTAL STUDIES

Collecting planktonic animals alive and in excellent condition for experimental work is difficult using conventional nets. Often the delicate animals are damaged when they enter the net and brush against the sides of the netting or when they are crowded and crushed, this usually occurs with small cod-end buckets and during tows longer than 40min. On this cruise, it is very important that we collect the salp, Salpa thompsoni in as pristine condition as possible because individuals will be put into an aquarium on the ship to study how being in different food regimes and temperatures affects the key genes that control physiological and biochemical processes.   “Blue water” diving in the upper 20 to 30 m of the water column from a zodiac away from the ship is one technique used to capture animals without damage, but that option is not available to us. Instead for sampling animals alive, we brought along a simple net system that was devised years ago to capture animals in very good condition. In 1981, Dr Michael Reeve took a conventional ring net and equipped it with a large weighted cod-end. He then lowered the net to a particular depth and then hauled it vertically very slowly back to the surface (5 to 10 m/min).  The animals collected in this manner were usually in very good condition.

While steaming down the Gerlasche Straits during the afternoon of Nov. 7th, we assembled a Reeve Net for use on this cruise. First we undid the lashing holding an old 1-m ring on a 1-m diameter pipe ring.  New nylon line was provided by Kelly Watson, a Marine Technician (MT) on our cruise. This line was used to lash a new Reeve net with 505 um mesh onto the ring. The Dacron collar at top of the net has a series of brass grommets through which the line was drawn and then lock-stitched (see Fig. 1), thus attaching the net to the ring.

Fig. 1. Paola attaching the Reeve net to the ring with nylon line.

The net is 4.1 m (13 ft) long. After getting the net attached to the ring, we assembled the cod-end bucket, which is a large PVC bucket [0.46 m (18") tall; 0.32 m (12.75") in diameter]. The cod end was inserted into a support bag made of mesh netting. The bag was then attached to the back of the net with a heavy-duty zipper.  Then the top of the bucket was taped with black electrical tape and duck tape to prevent chafing. A piece of Tygon tubing with a hose clamp inside was placed just under a lip of PVC to seal the mesh bag and prevent animals from passing down the outside of the bucket and out through the coarse mesh. 

Four ½" nylon lines run from the top of the pipe ring to the cod-end (Fig. 2). These were adjusted in length so that - when attached by shackles to straps on the mesh bag - they were loose and not supporting the bucket. The lines are intended to be used in retrieving the bucket after a tow; during a tow the net supports the bucket.

Fig. 2. Paola standing next to the Reeve Net hanging in the Baltic Room on the LM Gould. She is holding one of the lines used to haul in the cod-end bucket. Photo by P.H. Wiebe

Finally, we made a bridle from which the net will be towed.  Using Stainless steel thimbles also supplied by the MT shop, the remainder of the nylon line was used to fashion a towing bridle. 

Additional sampling gear such as the Multiple Opening/Closing Net and Environmental Sensing System (MOCNESS) and the Isaacs-Kidd Midwater Trawl (IKMT) will be used on this cruise. After looking at samples collected by these gears, we can determine whether we have encountered a patch of higher salp abundance or not. The MOCNESS will also provide information about the depth at which the salps reside. Once we have this information, we can use the Reeve net to target the areas and depths where we can collect salps in good condition for the shipboard experiments.

-- Paola Batta-Lona (University of Connecticut) and Peter Wiebe (Woods Hole Oceanographic Institution)