Natural mortalities do effect the populations of fish and is something that I experienced at my pond. It is hard to determine how much they effect the pond but they do effect it. Over the past 6 yrs I have had at least 7 or 8 otters that showed up at different times and it might take days before you see they are there. I did find fish bones and fish scales along with scatt where the otters dined. And the Osprey that shows up every year, i have seen it carry off some pretty good sized cnbg. Then there is the Bald Eagle that shows up and stays around for a while. They can easily carry off a 5 lb lmb. And those Great Blue Herron that's there most every day of the year. Not to mention the lmb that are large enough to eat an 8" or better lmb. I am pretty sure that happened alot at my pond prior to the fish kill. Takes me back to how many lbs per acre of fish in the pond?
Tracy, one thing is for sure. It was the combined weight of all fish that tipped the balance. So we can't blame the weight of predators in a vacuum and just target them in our attempts to manage standing weight. Most ponds have three times the predators standing weight in forage fish. But it most cases, the majority of these fish are only contributing to forage the predators can eat by reproducing. At any given moment in time, the standing weight of forage the predators are actually eating is small in relation to the standing weight of forage fish and the standing weight of the pond as whole.
Consequently, harvest should be targeted both for predators having attained target weights (or ages) and for forage fish that can longer contribute to the food chain except by reproduction. Most of the harvested weight, therefore should be concentrated in forage fish too large to contribute significantly as food for predators. This will also stimulate production of prey the predators can eat. It isn't a static system, it quickly reaches stalling limits and only intervention by managers or nature (through mortality) can bring the system below the limit again.
There is an exception to this. When the predators are so numerous and of small lengths, they are able to prevent the prey fish from overpopulating. Under these conditions the system remains below capacity and growth rates and conditions of the prey fish are very good but the predators tend to be lean. This is natures way of achieving sustainable balance.
