There are several reasons why a pier structure like the Exmouth Navy Pier is so rich in marine life.
Firstly, the pier structure itself provides plenty of hard substrate onto which corals and soft corals, sponges and many other animals and plants can attach and grow. The Navy Pier is located in the Exmouth gulf, which consists mostly of soft substrate: sands and muds, with very little hard substrate for sessile life to attach, so therefore this structure does provide one of the few areas where it can – and it does in abundance!
Secondly, and one whose understanding is a bit more complicated, is that it provides an oasis effect. This is a self-perpetuating feedback, in which more prey attracts more predators, where more hard bottom life attracts more grazers (and therefore more predators on them), and so on. Like an oasis in the desert, life accumulates here – an ecosystem that can usually only be seen across large areas is concentrated into one small space.
Thirdly we have the effect of the structure providing a ‘roof’. In fishing terms, this is known as the FAD effect. FADs are Fish Aggregation Devices, a widely used and destructive fishing capture method, where a structure simply provides a covering under which fish will always gather. Some structures are fixed while others are floated across the oceans for days, during which time fish species that are usually wide ranging and highly mobile simply remain underneath them. They have the effect of providing a shelter. The mother ship then comes up to it, spreads a net around it, and catches the whole lot, a fishing method that is as destructive as it is profitable to the fishing ships. But, in the case of the Navy Pier, there is no fishing – just years of fish gathering beneath them. These include a wide range and high congregations of fish that have made their home here, prey and predators alike, in huge abundance.
Fourthly, and specific to this area: There are huge tidal movements. This supplies copious amounts of larvae of marine species from a wide area, such as surrounding sponge gardens, and of course the world-renowned Ningaloo Reef, so therefore there is a constant source of new life.
These factors all combine to create an oasis which has a high quantity of life and a high diversity of different kinds of life. The pelagics (the free swimmers) the demersals (those that feed on the sea bed) and the benthics (those that are attached to the pier structure) all congregate in this relatively small area. So do come and see it for yourself!