TOSSI – Wanted: more crayfish

Officially speaking, the ocean isn’t within the Tāwharanui Open Sanctuary Society’s remit, but we do pay attention to the Tāwharanui coastline because you just can’t separate one ecosystem from another. Hence, Seaweek, organised by the NZ Association for Environmental Education earlier this month, was a good time to reflect on the underwater goings on in the marine reserve that stretches along the sanctuary’s northern coastline. In 1981, fishing was stopped in the area, and it became a marine reserve in 2011. This helped protect the vital relationship between sea and land environments. An obvious connection between the two is the food source the ocean provides for 24 native bird species that have been identified in the Tāwharanui Marine Reserve area. Of these, 18 species are recognised by the Department of Conservation as uncommon, vulnerable, declining, or endangered.

The crayfish population is a good indicator of the health of the marine reserve – these  include packhorse (Sagmariasus verreauxi) and red rock lobster (Jasus edwardsii). Large crayfish and snapper eat kina. When they are absent, the kina population explodes and the spiky urchins devour kelp (seaweed) beds, creating kina barrens. This is bad because kelp beds are essential habitats for fish, sea snails, sea urchins, corals, sponge gardens, sea squirts, orca and dolphins.

In 1977, renowned marine biologist Dr Roger Grace established permanent transects in and outside of Tāwharanui Marine Reserve to accurately monitor crayfish over time. He produced detailed maps that illustrated each boulder and crevice. When he died in 2019, a memorial fund was established to ensure his work continued. In 2020, divers counted every crayfish along each 50×10 metre transect and the results reflect the  decrease of crayfish throughout the Hauraki Gulf. There were substantially more crayfish within the reserve than outside of it, but overall numbers were still down. This could be improved if the reserve boundaries were extended further out to sea. Legal-sized crays within the reserve are vulnerable to fishing when they move out of the reserve to deeper water and shellfish beds.

Among those studying the crayfish is University of Auckland student Benn Hanns. He says it is concerning that populations in the reserve are diminishing and suggests the effectiveness of the Tāwharanui Marine Reserve in terms of conserving lobster populations could be improved.

Jackie Russell, TOSSI
www.tossi.org.nz