help needed with growth estimation

Haha, i need a glass of water with that one! 

I did some small ammendments to your suggestion to accomodate the need for filtering. 

I also did a excel withdrawal of the result. However, as you can see it didn't yield exactly the result i was looking for.. 

So, in essence what i need the [est. size] column to do is to calculate the weight increase of fish at a specific site every week and add it to last week. the starting weight can be found in the column [start size]. This column only holds values when [weeks since sea transfer] = 1. When the [weeks since sea transfer] = 1 a new generation of fish is put to sea and a new cycle is started. Therefore, the column needs to restart everytime [weeks since sea transfer]=1 (which you can see happening in week 25 above).

As there is about 1000 different sites in my dataset i need to be able to filter by [site number].

Hope that clarifies,

Any ideas?

Cheers,