Building blocks for comeback by nature

Mangrove forests also have tangled webs of roots that provide little pockets of space for fish to rest or hide in.

However, degraded habitats and human-made coastal defenses typically lack the variety of features found in natural shores, and creating microhabitats is important in improving biodiversity, the researchers said.

To investigate what types of artificial structures are most effective in attracting the return of fish life, the research team tested three different fish house designs of varying complexity, in terms of their shapes and the size of holes within each block.

The most simple fish house design involved 27 concrete blocks stacked up in the shape of a cube, and had 100 holes, each 6.25 centimeters by 6.25 centimeters.

The more complex design was still cube-shaped, but had 100 holes of 25 different dimensions.

The most complex freestyle design was non-cuboid in nature, with the concrete blocks stacked in different shapes. It had holes of different dimensions and other habitat features to mimic little tunnels or crevices in a coral reef or rock.

The study co-author Rachel Mark said the aim was to find out what types of spaces fishes need, so the blocks can be deployed to the sea wall area to attract fish diversity.

The researchers found that the two cube-shaped fish house designs attracted about 27 fish species, while the most complex freestyle fish house design drew in more species of fishes, and in greater numbers.

The study also showed that fish use the houses for different reasons by day and by night.

During the day they typically enter the fish houses to find food and sometimes rest or ambush other fish. The time spent in the houses is also short, about a few seconds to 30 minutes, and they prefer spaces larger than them as they search for food.

At night they primarily use them for resting, which largely explains why they spend a longer time in them and in smaller spaces with less visual exposure where they can snugly fit their body to avoid predators.

Taira and Mark said the findings show that the effectiveness of fish houses depends on their design.

"This study provides more technical information on how fish houses deployed near sea walls can be designed to support higher fish diversity, which can be incorporated into future coastal defense construction," Taira said.