All the world's phosphates eventually get washed into the ocean. Sea birds eat phosphate-rich fish, and then dump out phosphate-rich droppings. Historically, people have scooped up bird droppings from cliffs and rocks where they roost. However, not all nesting areas are suitable for collecting droppings (guano), and so much of the nutrient-rich material gets washed back out to sea.
In the 1930s in Namibia, a solution was developed. A flat, watertight platform was built on an islet offshore. Seabirds flocked to the convenient nesting spot, and deposited 5 centimeters of poop each year. The flat surface allowed for the droppings to be easily scraped up, to be then sold as fertilizer.
For cold, windy areas like Atlantic Canada, I think it might be good to build fake lighthouses along the shoreline (just to be aesthetically pleasing). Seagulls and other birds would enter through the top, and nest on wire mesh shelves inside the structure. Nutrient-rich guano would fall down to the base of the tower, for easy collection and sale.
Just as a data point - the Plumb Beach lighthouse in Rhode Island managed to accumulate nearly a tonne of guano a year, despite not having been designed to attract birds or collect their droppings.