Black walnut, especially their nut hulls are extremly toxic to fish. They were used for centuries by Native Americans for that exact purpose. Some other nut trees have lesser, but still toxic effects.
However, in a pond with about 2 million gallons of water, a few sparsely placed logs and slash of many other species will not have much of an effect on the fish. If anything, it provides good cover and places where fish can hide and grow.
The biggest problem of using pine (and to a certain extent, fir), is that they don't sink easily. It takes them a while to become water logged. In general, they also break down faster than most deciduous trees in warmer waters, like you may have in California. In cold lakes like Lake Superior, they stay pretty well preserved for centuries. Yes, there are some reported issues of using softwoods in aquariums because they can leach turpentine. From the little bit that I know about that issue, removing the bark eliminates much of that problem.
For anyone who has fished in Northern Wisconsin, Northern Minnesota, Upper Michigan, and the Canadian provinces of Ontario and Manitoba, many of the lakes and rivers are tea colored. That tea color is because of hardwood tannin from trees like red oaks, white oaks, and a number of other kinds of oaks. If you have fished these tea colored tannin filled waters, think about the monster small mouth bass, pike, walleye and so many other great trophy fish you may have caught.
A little searching will lead you to many great publications similar to this one:
Pond and Lake Fish Cover Regards,
Ken
