Originally Posted by Bill Cody
Quote
First, what is an appropriate assumption of natural mortality for planning? For example, 30% a number thought to be average?

Second, should we consider harvest mortality to be additive to natural mortality when the harvest is less in number than what we expect natural mortality to be?

If one is wanting an accurate estimate of mortality I think it is difficult to use a consistent percentage for natural mortality due to numerous natural factors that can contribute to deaths among widely different pond habitats. Predation losses can vary widely among waters depending on type and size of predators. Often in the literature 30% mortality loss is used. I am not sure how accurate that number is in different types of situations. It probably all depends, but when dealing with mortality loss it is probably a good starting percentage. The long term morality study summary presented above by ewest provides a good starting basis from an actual detailed study that measured natural mortality for LMB.

Second - When measuring mortality as a total mortality, I think one has to include harvest mortality. Any form of fish loss from a community should be included in total mortality. Any form of loss is loss from the ecosystem. If one's goal is to subdivide mortality into different categories then harvest mortality can be one form or a portion of the total mortality.

Bill, thank you. I am only beginning to study natural mortality and it does indeed confound me as a variable that may not be very predictable. To be sure ... yes I understand that harvest mortality is part of the total but was just asking whether harvest of individuals which appear at risk of natural mortality would increase the total. IOWs, if a fish would have died from natural causes anyway perhaps taking the fish by fishing would not increase total mortality (because a fish that would have been lost naturally is harvested instead). Also fishing mortality tends to increase growth and condition of survivors and I wondered if improved condition could lead to lower mortality of the survivors than might otherwise take place. These sound like reasonable arguments but if the natural mortality is very low, fishing mortality could exceed natural mortality and certainly in that case would have to add to the total lost. Eric's reference suggest we should expect mortality to be low for middle aged fish.

As I mention above, I am only now beginning to study mortality. So lots of questions. Two interesting articles that got me looking deeper are one by Willis with comments on SMB.
 and this article pertaining to LMB.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers