painting a bridge

I am making a bridge to go over my pond, a previous bridge rotted away and I am now looking for advice regarding the best paint/stain etc that will preserve the wood while at the same time not be a threat to the fish and wildlife. ive contacted cuprinol and they say they
do not have a product suitable that over time would not leech into the
pond. having
purchased the timber i am looking help.

thanking you ,puzzled pete