The paper can be found
HERE.Escanaba lake is a lake in Wisconsin where DOW controls access and regress from the lake on a permit basis. Anglers check-in and on check-out are required to declare the results of fishing which includes both released and harvested fish. Fish measurements are taken. The DOW samples the lake and incorporates various accepted methods to estimate populations, natural mortality, exploitation mortality, and standing weights. It's a very interesting read.
In the county where the lake resided, a 22 in length limit for NP was enforced starting in 1964. Prior to that, there was no length limit for NP. The study reports the metrics prior to and following the regulation change. Results:
1. There was an increase in the biomass of NP and a corresponding decrease in harvested biomass. These more or less balanced. In other words, the biomass of NP increased by about the amount fisherman were no longer harvesting.
2. Growth, Condition, and average weights of NP declined as a result of the regulation.
3. Natural Mortality increased from 15% annually to 76% annually. Fishing Mortality decreased from 46% to 6% annually.
4. Although there were 47% more NP > 22" after the regulation was enacted, they were of lower RW and average weight. Ultimate weights were also lower. Ultimate weights were lower because growth rates were slower and the average weight attained at age declined.
5. Through much of the study, panfish populations were below detectable levels and crashed as a consequence of the regulation's effect of increased density of NP. It is interesting to also note that these decimated panfish populations still produced enough juvenile offspring to support remarkable weights of predators (NP, WE, SMB, LMB, and Muskies) ... but very few panfish reached adulthood. To give you an idea of how low the populations of panfish were ... the first documented reproduction of NP occurred in 1956 when harvestable adult panfish were very low. The reason for this was not stunting. WE effectively controlled recruitment so that few panfish survived to be adults. From there, additional year classes of NP recruited. The effect of this was to diminish WE populations and this resulted in panfish catch rates increasing during the early 1960's.
During this "hey day" period of the early 1960's the panfish populations provided fishing catch rates of .04 to .06 fish/hour (on average it took 20 hours to catch one panfish of harvestable size). So mature panfish were not plentiful even when they were most abundant.
Aside from what there is to learn about the need to harvest predators in their 2nd or 3rd year of life to grow fish of larger ultimate weight ... the paper is a poignant reminder that the potential for predator biomass (limited by the production of consumable prey) is inversely related to the biomass of panfish brood. We shouldn't need a paper like this to be reminded of this principle. We've known about it for a very long time. We've known that the stocking biomass/density of brooders that produces the greatest number of offspring is small. 20 pairs/acre of BG averaging .5 lbs each (~20 lbs/acre of brooders) produces the greatest number of fingerlings. The more the brooders, the fewer the fingerlings. To be sure, consumable prey biomass and production can be the lion's share of the BG biomass potential or it may only be a minor fraction of it. In the case of Escanaba Lake during the 50s and 60s, the prey consumed annually by predator fish was significantly greater than the biomass of the forage brood fish, many times greater. Although, I would not recommend managing panfish brood at such minimal levels ... it is important to understand that it really isn't possible reduce consumable forage production by removing larger than optimum forage fish by fishing. Just as important, removing larger than optimum forage fish increases the production of consumable forage.
