Total biomass within the Leigh Marine Reserve is five times higher than it was 40 years ago when the Reserve was first established, a study shows.
University of Auckland’s Institute of Marine Science PhD candidate Harry Allard conducted a three-year survey of 22 fish species within the reserve, 10 of them targeted by fishers. Finishing the survey in 2018 meant it was exactly 40 years since a 1978 study, which established a baseline for future research.
Both old and new studies were similar in approach, using biomass as a measuring tool which takes into account both outright numbers and size of individual organisms. Harry counted fish at similar locations to the previous study.
However, he also surveyed 12 fished (unprotected) sites outside the reserve that weren’t done 40 years ago.
Results show snapper are on average six centimetres larger in the reserve than in fished areas outside but overall numbers have remained stable while the number of juvenile fish has decreased.
Red moki and butterfish – both favoured by spear divers – are also larger in the reserve than in fished areas but numbers had also remained stable since 1978. While red moki are similar in abundance compared to 40 years ago, total biomass of the species has increased with a greater number of legal-sized (more than 40cm) fish.
“Overall, the data shows the marine environment within the reserve is now dominated by fewer, larger fish with most of these being of prime breeding age,” says Harry. “The offspring of these fish are now seeding areas outside the reserve.”
Numbers of three target species included in the study – parore, leatherjacket and blue cod – are more abundant inside the reserve than in fished areas but showed a decline in numbers overall since 1978. This could be due to wider environmental factors or the fact sea urchin barrens – areas with a high number of kina or sea urchins – have been widely replaced by kelp forests suiting some species but not others.
“This may reflect larger-scale changes in the Hauraki Gulf rather than local effects, perhaps due to warming winter temperatures,” says Harry. “Blue cod are still larger and more numerous within the reserve than fished areas outside, which shows protection of this species may be increasingly important.”
There are fewer juvenile leatherjackets within the reserve but still more than outside, with a similar finding for parore but large-scale changes due to climate change could also be a factor, says Harry.
One of the most notable changes since 1978 is the replacement of sea urchin barrens with kelp forest and algae species, a sure sign that kina predators – such as larger snapper – are more numerous in the reserve.
Three species, hiwhiwi, spotty (a common wrasse species) and goatfish, all showed a decline in numbers within the reserve compared to fished areas outside. This might be due to a decrease in sea urchin areas or could point to increased predation by the higher number of larger fish within the reserve.
Overall, six out of 10 species targeted by fishers showed positive growth in either weight or abundance in the reserve during the last 40 years – but Harry says further research is needed to investigate longer term changes in the wider marine environment such as those caused by climate change.
However, the research does undermine reports from the public there are fewer fish in the reeserve than there used to be.
“The fish are still there but they don’t come into shore anymore for a feeding frenzy of peas and bread now that feeding is banned,” says Harry. “But it’s amazing how long a snapper’s memory can be, some of the older fish still follow people around expecting food.”
During research dives Harry regularly saw the reserve’s most famous resident – ‘Monkey Face’, the giant snapper – estimated to be at least 40 years old. However, the last time he saw him was 2018 and he’s heard that other divers haven’t seen him for some time. “It may be he’s finally gone to that big marine reserve in the sky.”