There is a great deal of science that points to risks/impacts to the marine environment from these exercises, some of it even addressed in the Navy's EIS.  The basic seasonality of migratory marine wildlife (mammals, birds, fish) in the Gulf of Alaska (which I have studied for several decades) is straightforward.  The exercises would clearly present less risk in winter -- period.  Re: why they wish to train now in spring/summer, I think is is an easier schedule for them, or perhaps they are simply fair weather sailors.