Marine restoration is a lazy business. All you have to do is stop fishing an area and marine ecosystems heal themselves. However this is not the case with green-lipped mussels in New Zealand.
100’s of square kilometres of sub-tidal mussel beds were fished to extinction in each harbour around New Zealand.
The industry collapsed and more than half a century later they have not returned. In the Hauraki Gulf there are a few places you can still find Green-lipped mussels. You would think that these places would be deep under the ocean (Green-lipped mussels have been found at 50m deep), but they are not.
Most are in the intertidal zone on rocky shores. Here there is usually a gradient with mussels thin higher up and getting thicker towards the low tide mark where the abruptly stop. I have asked several local experts and no one has a solid answer why they stop so abruptly.
As we spend 100’s of thousands of dollars restoring sub-tidal beds maybe the key to unlocking a lazier (and cheaper) solution is staring us in the face. Here are some thoughts on why the line exists:
Avian predation. Every exposed mussel bed has at least one pair of Variable oystercatcher eating the smaller mussels every low tide. So living in the subtidal zone would be of some advantage but we are more likely to see juveniles higher up. This suggests there is even more predation from below.
Starfish predation. Eleven-armed star fish were a big problem in the first beds put down by Revive our Gulf. However starfish do okay in the intertidal zone and are not particularly abundant in intertidal beds. We don’t see lots of them waiting below the low tide mark next to intertidal mussel reefs.
Fish predation. This seems like the most obvious cause but surely it can’t be Snapper as they have been fished down to 20% of their natural biomass. Rays are a possibility but I thought I would see more of them in the shallows if this was the case.
Octopus predation. They are nearly invisible and love eating mussels so at first this is a good fit. But octopus leave the shells, I will look for evidence on the next intertidal bed I explore. None of the predation theories show why the line is so strong.
Food. Mussels eat phytoplankton and algae. I am sure there will be more of this close to the surface. I am pretty sure this is why mussel farmers grow their mussels high in the water column. Wild mussels might also benefit from wave action on the rocks as it would increase the oxygen in the water. However I would have thought that these benefits would be offset by the fact they can’t feed while they are exposed to the air.
Sediment. If there is some other benefit to being exposed to the air maybe it’s that the water in the Gulf just has too much stuff in it. Mussels have to work hard sorting out the food from the dirt, maybe mussels are not good at taking a break and being forced to is good for them. Comparing the condition of intertidal and sub-tidal mussels would help dismiss this idea.
Or maybe like a lot of things in biology it’s a mixture of the above factors. As we are slowly losing our intertidal mussel beds it might be wise to set up a long-term monitoring project that might solve this mystery and inspire lazier restoration methods.
I was recently lucky enough to monitor kiwi on Hauturu with the Little Barrier Island Supporters Trust. I have done this many times before at Tāwharanui with TOSSI. The exercise involves a good hike to the destination at night then sitting quietly in the dark listening to the sounds of the forest for a couple of hours. I have met some great people doing this and this trip was no exception. They patiently waited for me as I inched along the tracks on the way back to the hut, inspecting every tree for hidden treasures and some even came out with me again on their free nights. I have posted all my invertebrate observations here on iNaturalist where the community is helping me identify them all. I have also posted some fungi which were a hot topic among the volunteers.
It’s an incredible island where one can get a sense for pre-human New Zealand. Here is a list of some observations I made over the 12 days I was there.
I saw less invertebrates, birds and geckos at elevation.
Piwakawaka following Tieke on two occasions.
By weight there was much more industrial plastic pollution (mussel floats) than domestic plastic pollution on the beaches.
Similar gecko numbers in winter compared to summer. One seen every 20 minutes on night walks.
Cave Weta were the dominant weta by a factor of 10 or more. Tree Weta, Ground weta and Wētāpunga active.
Winter invertebrate abundance similar to mainland sites with predator control (and less birds). At night, in two out of three tress invertebrates were easily found.
Invertebrates not much bigger than raindrops stay hidden during the rain.
NZ Giant Centipede found 3m above ground inside a Kanuka tree that fell in the middle of the day. The species is very arboreal.
Korimoko chasing Ruru during the day.
Flock of 18 Kereru feeding on Muehlenbeckia.
A high of 12 Shore Skinks seen from one observation point. No other skinks observed.
Invertebrate diversity high with many species I had not seen before. Some had not been previously photographed in the wild.
Strange absence of Katydid calls.
Very empty streams with only Shortfinned Eel and Banded Kokopu observed. Huge flushing events the likely cause of low freshwater diversity and abundance.
The population of any given species needs to be strong enough to handle destructive natural and made made events. Without this resilience one event can wipe a species of the face of the planet forever. In New Zealand our rarest endemic breeding bird is the New Zealand fairy tern. With a population of only 40 birds this is the breakdown for the 2017-18 season.
3-4 of last seasons surviving chicks which are too young to breed
1 menopausal female
1-2 ‘engaged’ couples who behave like they are breeding but do not lay eggs
11-14 unpartnered males (the population is short on females)
20 breeding birds (10 pairs)
It’s a fragile population with too many males (1:2 ratio). The other major concern is that during breeding season 80% of the population is on the same 40km stretch of coast. Here is where the breeding happened in 2017-18:
The fragility is particularly exposed when we think about the threat of an oil spill from the RMS Niagara. This wreck is a time bomb just off the coast from this critical breeding habitat. Even a small amount of the submerged oil could easily cover the fairy terns breeding grounds for weeks, starving the birds to death in a matter of days.
Book recommendation: This book by a NZ adventurer and herpetologist is awesome. It’s a refreshingly honest collection of great adventures and scientific discoveries. I throughly enjoyed Tony’s detailed descriptions of New Zealand’s treasures, his passion is inspirational. I really appreciated him sharing the highs & lows of what has been an outstanding career… so far. Get it here.
I have started work helping New Zealanders keep their Parks and Reserves. Building and Construction Minister Dr Nick Smith says that we have to “choose between using land for houses or cows”. This is ridiculous – saying our reserves are for cows is like saying Eden park is for lawn mowers!