Just a thought that occurred to me. Fish emulsions, oyster shells, and various forms of seaweed are all popular organic gardening additives. As much as many people avoid bringing in outside sources of nutrients to their properties, I think these being carried onshore by animals (which we count as) is just the natural cycle that recovers soil nutrients that have been washed to sea. This doesn't address the issue of contamination, I know. But I think by looking at it as part of a larger system, there are times when it is a permaculturally sound action to import materials.