Numerous academic studies as well as real life experience prove that stringing monofilament lines above a dock or other area deters seagulls from landing and roosting. The US Department of Agriculture recommends this system as the most effective non-aggressive method to deter seagulls.

• Our system works because seagulls live in wide open spaces and are primarily gliders. Unlike land birds, they are not designed for confined areas like forests. The monofilament lines make them nervous because they need plenty of room to maneuver. Roosting under the lines is uncomfortable for them, a constant reminder that their takeoff is restricted. Another factor is that many birds, including seagulls, have vision in the ultra-violet light range far beyond what we humans can see. Monofilament lines, nearly invisible to us, are bright blue to seagulls. We use a monofilament that is highly UV reflective to ensure that the gulls see and avoid the area it covers.
• See The Internet Center for Wildlife Damage Management (sponsored by several universities) for more confirmation that stringing monofilament is the most effective method for excluding seagulls from open areas.
  Wildlife Service < click for original document
  and the US Department of Agriculture link for more confirmation:
  Wildlife Damage Management < click for original document