11. Where a person has, in order to comply with the requirements of this Schedule, scoured out and cleansed any watercourse, it shall, in any proceedings taken against him under any statutory provision relating to fisheries in respect of any injury or disturbance thereby caused to any spawn, smolts or fry of any kind of fish, or to any spawning bed, bank or shallow, be a good defence to him to prove—N.I.
(a)if the watercourse was scoured out and cleansed in accordance with a notice under paragraph 4 or 6( c)(ii) and the notice included directions for the purpose of avoiding such injury or disturbance, that those directions were complied with; or
(b)in any other case, that such injury or disturbance was unavoidable.