I still plan to try and build a few new ponds at my place when we can get ahead on the family budget. I think I will try a gams experiment instead of FHM as the starting forage on one of the ponds.
FishingRod, interesting thought. It may be that FHM stocked in March before BG and LMB might produce more but I am sure that GAM would perform well, particularly considering that you mentioned this year's first cohort a couple of weeks ago(earlier than I thought for KS). If you have weeds or brush I think the Gams would over-winter and produce limited forage year after year with a classic LMB-BG combination. I base this on the fact that many of the ponds near me have populations that sustain year after year in the presence of LMB and/or BG. The degree of the GAM population depends on cover, at least this has been my experience. FHM will go away and GAM might go away but GAM stand a pretty good chance of hanging around.
In Swingle's treatments he tried GAM as the sole fish prey for both BG and LMB. He reported the GAMs essentially extirpated in both cases. I will be quick to say that his ponds were probably not very weedy (being under fertilization to maintain bloom) and the sides were probably steeper than a typical rec pond. IOWs there was probably not much cover for them. I've always wondered what stocking rates he tried (but didn't note) but I do think that the ones he reported in his work on production were stocking rates that optimized production.
I have tried to more deeply understand the predator-prey interactions of his GAM combos and worked up a simulation to see if I could replicate the results with a model simulation. I have had varying degrees of success but I think the model is pretty close. In a optimal system, all of the food is eaten at just the right time. If consumed too early, the predators begin to decline. It seems to go without saying that fish will eat all they can (if they care too). By this I mean they will eat as much as they able to up to the limit that they can consume. The limit changes with age. What differentiates fish that grow really large is that they have higher rates of consumption. BG could in theory grow as large as LMB if they would consume as much and live as long. So in modeling BG I did modify the consumption (related to maximum growth rates) to be a fraction of LMB consumption. Below is an image of the BG treatment of Swingle in simulation.
Swingle BG-GAMI will mention that the simulation is very sensitive the consumption ratio and the stocking density of the BG. If they were as hungry as LMB, 1530 of them would extirpate the GAM in short order. Also, if the number of BG were doubled, the same would happen impacting the results adversely. The same is true of the LMB. GAM cannot sustain a large population (number of individuals). Below is an LMB simulation. Now Swingle's treatment was for a year instead of only 6 months in my simulation. But his standing crop at 1 year was substantially larger which means that they must have continued growing and probably recruited. In a pond with no cover, this finished off the GAM. But not until after they had gained 172 lbs. The stocking rate is 100 2" LMB and 1 lb of Gam to the acre. With 70% survival, neglecting the weight of recruits, the average weight would have been 2.45 lbs and with 100% survival 1.72 lbs. Whatever the case, it is safe assume that it was probably somewhere between those two numbers. LMB are thought to grow maximally up to 2 lbs in their first year and so this seems to suggest GAMs were capable of growing the LMB at close to the maximum in the first year. The image of the LMB treatment (first 6 months) in simulation is below.
Swingle LMB-GAMSo a few things I learned from the exercise.
1. Stocking rates matter. You can't double the stocking rate without extirpating the GAM food source and destroying a good thing.
2. The size of fish matters. For example. 100 lbs of 1/10 lb predators consume a much greater percentage of their body weight than 100 lbs of 1 lb or 2 lb predators do.
3. What #2 means is that there are standing weights of adult predators where GAM could provide all of the forage needed ... PROVIDED ... the predators were unable to reproduce AND the predators are cropped reasonably. Throw in predator reproduction and the GAMs will not achieve their potential.
The simulations suggest that GAMS can sustain consumption rates of between 400 to 500 lbs annually in fertilized water. When the population of predators WANTS to consume more than this ... they will extirpate the GAMs. To use GAMs as "the" prey species requires predator consumption of GAMs below this number. This means fewer larger fish can live in harmony with GAMs provided they are growing maximally ... eating all they want... and where GAM reproduction can keep pace.
GAM can certainly produce more than 400 lbs of forage annually. One reference noted 800 pounds of production per acre-year in fertile water and no feeding. In this particular case the GAM were harvested periodically through the growing season to prevent fish kills. It is cropping like this that can substantially increase the productivity. The removal of GAMs, whether by netting in this example or by predators allows GAMs to grow maximally and reproduce. The same is true of any prey fish ... so nothing really new there. GAMs, like other prey fish, can produce more than their limiting standing weight every year provided they are cropped so as to prevent them from reaching that limit.
One particular combination that seemed to have potential to me is HSB-GAM where HSB are stocked on a 25/year ladder. Below is the result of that simulation and it suggests that one needs to overwinter a minimum 25 lbs of GAMs to sustain the population. The scenario requires annual harvest of 3rd year fish. Assuming 100% harvest of this age fish, the HSB population would not be able to keep up with GAM production. The scenario allows a harvest of an estimated ~82 pounds annually of fish in the 18"-20" class. GAMs may not be the best forage for the 3rd year fish (they need the biggest GAMs) but HSB do seem a really good candidate because they can be supplemented with a little feed. Here is the image of the HSB simulation.
Possible HSB ?Here is an image of the weights of the ladder stocked fish where 1.2 lbs of growth is assumed each year. This particular growth corresponds with maximum consumption relative to LMB of 0.81. If HSB would consume as much as or more than LMB, they may need to be reduced in number or satiated with more formulated feed.
HSB LadderThough GAM may persist in ponds where LMB and other predators are reproducing, their number will always be far below their potential. So they aren't going to provide >400 lbs of forage in the 2nd year and the years that follow in those cases. It is unclear just how much they provide where they are limited by predation/cover far below their carrying capacity potential. All I can tell you iis that whatever standing weight they can achieve .... the production will be a fair multiple of that.
