Q Are there any plant sources of the oily wonder-ingredients said to be present in oily fish?
A Yes, there are a number of quality sources. Omega -3 fatty acids is what you're referring to and fish get them directly from algae, or from smaller fish that eat the algae. Some companies do harvest different algae and market it, but there are other great sources that are less expensive and more readily available. Great land sources are the plant purslane, and flax, chia, or hemp seeds.
Here is more information on edible seeds and the essential oils they contain.