This wouldn't be that hard too do but I want to avoid as much as I can. It's just duplicating data and is what the multiple spawning dates are for.
I will keep it in mind the checkbox "copy forward" idea as I do the second bit, but most of the data could be different, so I am not sure. It would also encourage [some] folks to just propagate old data which is not good. I would like to promote the provision of new, quality data in all this.
Bear in mind also that the data for a second blog is meant to be different, if you make it too easy to replicate blogs, then folks might get confused between "I want to add another spawn event for that tank/population" and "I spawned another population in another tank".
Jools