We suggested the above definition because it is a natural extension of the definition used by most other "single pool" models. The main difference is that the new definition is made with respect to EACH REGION. Thus, within each TimeStep, instead of treating the stock as consisting of a coastwide "single pool," we assume the stock is composed of many "single pools," one for each geographic region.

Tom's point is that by defining TimeStep with respect to fishing processes, the definition may cause problems when we want to partition the computations with respect to biological processes.

Time is just time (with all respect to Einstein and Relativity Theory!)... and we can partition it however we want. The critical issue for the model is: What assumptions and restrictions are we going to make about how we model the processes that occur within a TimeStep? For example, we defined a TimeStep by the assumptions we wanted to make about the harvesting process.

We could adopt a more general definition:

"Within a given year, a TimeStep is a period of time during which all independent variables (eg input state variables and parameters) used in process equations are assumed to be fixed."

This definition removes any specific mention of fishing processes, or biological processes. It leaves open any assumptions about what types of equations we can use for the processes (fishing or biological) that occur within a time step. This is not a trivial point, because it may affect how we define other objects and structure the looping process in the main engine.

For example, the assumption that "all fisheries within a given region are ... working on the same abundance of fish" means that within a region and TimeStep, the typical linear catch equations can be computed independently and in any order. Thus, the definition was designed for computational convenience.

The new more general definition would permit more complicated relationships within a TimeStep (eg differential equations) that allow processes to interact. We then would need some type of object to group all the state variables that interact within a TimeStep.

Perhaps we should make a list of all the assumptions we want to make for a TimeStep. Below is a partial list of possible assumptions.

-- input state variables are fixed (eg cohort abundances at start of TimeStep);

-- environmental parameters are fixed throughout the TimeStep (eg average ocean/river temp; average river flow; average predator abundance);

-- fisheries are working on the same abundance of fish;

-- fisheries are allowed to interact with each other during a time step (eg commercial and sport fisheries often occur simultaneously during a week and they could be modeled by a set of donor controlled differential equations);

....

-- others????