- 24 traps (12 + 12 of similar mesh size)
- 1 well-charged marine battery (in lab)
- 1 electric outboard motor
- 1 large garbage can, to mix the rotenone
- 1 kg of 5% rotenone per pond
- several buckets
- 2 boats
- 1 length of rope
- many jars
- MS222
- gloves and mask
- 1 big tweezers for moving fish between jars etc while avoiding
formalin and MS222
- lots of 10% formalin
Don't pack the fish too densely into jars with formalin, as this
will squash them and also dilute the formalin too much.
- 3 dipnets attached to the ends of long poles (8 ft or so).
The white dipnets with very fine mesh are inferior to the green
ones with more open mesh, as they get more gummed up with
sediment. Get dipnets as long as possible for better retrieving.
- paddles for the boats. The large heavy white dipnet on the end of a
fat pole doubles as a paddle and as a dipnet.
Weather is important, as you need good visibility on the bottom of the pond. Sunny is best, but that may be a hard one to achieve. Rain is awful, as it makes it hard to see below the surface.
Do only one pond per day. And do both sides of a pond on the same day. Try to expend equal effort on the two sides, so that the capture rates are equivalent. Time your effort on the first half and apply a similar effort on the second half; as well, use the same number of people on the two sides. We never manage to retrieve every fish, so equal effort expended is important (although we get the vast majority).
The night before you nuke a pond, throw in a dozen traps on both sides. This may catch a sizeable fraction of all the fish in the ponds, which helps a lot. Pull the traps up from both sides of the pond at the start of the day so that the catch is comparable. These fish are useless for later stomach analysis. Therefore they need to be preserved in separate jars (clearly labelled) from those fish obtained by rotenone. You can retrieve the traps while the rotenone is mixing, as this takes 15 minutes or so.
The basic procedure is listed below for a half-pond. The most important thing to watch out for is that the motor is not ever turned too high within each half-pond. A student did this once and the force generated by the motor caused the divider to lift up off the botton, thus mixing the fish from the two sides and destroying the experiment. Also, make sure the battery is charged up the day before you start this.
Mix a half-kilo of rotenone in water: Mount the electric motor on the back of the truck. Sit the propellor in the large garbage can 2/3 full of water. Turn on the motor at low speed. Add the 1/2 kilo. Continue stirring until well-mixed. Rotenone is not very water-soluble, so it will tend to float dry on the water surface when you add it. You can increase the motor speed to improve the mixing.
Fling the mixture as evenly as possible over the surface of one half-pond. Carefully drag the garbage can onto one of the boats. Let one person paddle around the pond while the other stands and dips into the garbage can with a pail and flings the rotenone mixture onto the surface. Distribute more of it over the deeper water.
Stir the water using the electric motor. The best way to do this is to attach one end of the rope to the front end of the boat, and attach the other end to the shore close to one end of the pond divider. Leave about 12 m of rope; when the line is taut the boat should be in about the middle of the half-pond, or maybe a bit further. I then turn the motor around 180 degrees and start it. In this way the motor pulls the line taut, stirring the water. I usually tilt the motor a bit so that the water from the surface is pushed downwards into the pond depths. Youu can increase the speed of the propellor but not too much. Keep an eye on the divider---the point receiving the greatest pressure is in front of the boat at about mid-depth, where water rises from the bottom. Continue stirring for about 15 minutes. If all goes well you should see the water moving around the pond. The other person should now be in the second boat, paddling around looking for fish rising.
Stop the motor and start dip-netting fish that come to the surface. The fish may take a 1/2 hour or longer to start rising, but sooner if the water temperature is still warm. Try to concentrate on the fish rising above the deeper water. The fish moving into the shallows will be easy to get from the bottom later. It will be frantic at first but settle down later as they stop rising.
Continue dip-netting fish off the bottom after they are all expired. This is easy in the shallows, more challenging over the deeper water. The deepest fish will most likely be overlooked, but the fish also tend to concentrate in the shallows. Look carefully at the margins of the pond---the rotenone is suffocating the fish, interfering with their oxygen uptake through the gills. This is why they rise. They will also jump out of the water along the shore. Once all the visible fish are removed, poke under leaves, etc. Also look carefully at the edges of the pond divider, where many fish will collect. Toss collected fish into a plain bucket, but once the action slows, transfer to formalin so the fish don't get to soggy.
At the end of the day, after both sides are done, put one boat on each side of the divider and scan once more for stragglers. Ensure that the effort expended on each pond side is the same.