The very first sentence of the introduction reads:
The complete removal of fish from Third Sister Lake was not carried out for the primary purpose of making a fish population analysis but rather as part of a general investigation on the existing relationship between the fish and fish-food organisms in this lake.The author continues by clinically describing the process:
The destruction of the fish population was accomplished by means of rotenone (Derris root) which was supplemented by netting-and angling during the 3 weeks previous to poisoning. The actual poisoning process and its effect on the fish and other organisms found in the lakes described by the authors elsewhere in this volume.In other words, they poisoned the lake and then collected the fish that floated up to the surface. The first time wasn't good enough to clear the lake, and so they did another application, but didn't include those fish in the survey. Oh, and by the way, no fish were seen until the summer of the following year; likely up-migration from the connecting stream.
The first poisoning operation was carried out on May 6, 1941, before the spawning season and before the aquatic vegetation was extensive enough to hamper the recovery of fish ... Because the first poisoning did not kill all the fish, a second application was made on August 18 and 19 when water temperatures were more favorable. Only a very few adult fish were recovered along with many thousand bluegill and pumpkinseed fry. These were not included in the population total. After that time, however no fish were seen in the lake until June, 1942,when a few long-eared sunfish entered the lake, presumably via the outlet during high water. We believe that the poisoning was 100 per cent effective in the removal of fish in August.
Still, the 1941 survey did a few things, such as provide a good estimate of total fish biomass in Third Sister Lake as well as providing a date certain from when there were absolutely no fish in the lake.