Scientists have identified an ‘ecological bottleneck’ in the ability of juvenile snapper to survive into adulthood. Research has shown that young snapper prefer three-dimensional habitats on the seafloor, particularly seagrass meadows but also horse mussel and sponge beds. It is not known why these areas are preferred, but it is thought that they provide a good source of food, protection from predators and shelter from seawater currents (reducing the need to expend energy to swim against them).
If there is not sufficient area of these habitats available, particularly in places close to important spawning areas, then this creates an ‘ecological bottleneck’ and the survival rate of juvenile snapper is likely to be lower. This has a flow on effect for fish stocks as it reduces the number of snapper that are recruited into the fishable adult population.
In the Hauraki Gulf, the majority of these habitats have been lost over the past 170 years, through a combination of increased levels of sediment and other pollutants entering the marine area from land, and direct damage from reclamation, harbour works, trawling and dredging. Habitat loss is still occurring in important fish nursery areas such as the Mahurangi Harbour.