What will be the impact of dredging on important rocky intertidal communities in the Otago Harbour? School students and marine scientists join forces to find out.
Students from primary and secondary schools across Dunedin have been closely counting the different types of wildlife they find at seven different rocky seashore sites around Otago Harbour – from Acheron Point to Yellow Head.
The students are also measuring the amount of sediment (fine particles of soil and sand) that has moved to these sites, to see if it has come from the area’s recent increase in dredging.
Dredging is where the soil and sand at the bottom of the harbour gets sucked up and moved elsewhere. It prevents the Otago Harbour channel from becoming too shallow or narrow for boats to travel along, but creates plumes of sediment that spread and settle in other areas.
Recently, Port Otago has been given permission to make the channel deeper and wider so that bigger ships can travel down it.
“What will happen to the environment when we widen the channel?” asks Stone, a student at Sawyer Bay School. The students and Otago University’s marine scientists are hoping to find the answer, which Port Otago will use to help protect the channel’s environment.
The investigation began in 2016, when students and experts took a ‘snapshot’ as a starting point to track any changes over time. They did this by measuring the level of sediment and the amount and variety of wildlife at nine different locations from March through to June.
In 2017, the team continued to monitor at seven of the sites, but at a different time of the year – in June and August. This is to know how the shore looks during a different season, since seasonal variations might change the sediment levels and marine community, so the investigators need to make sure seasonal changes aren’t confused with changes caused by dredging.
The students tested out different ways of measuring and comparing the amount of sediment – from using artificial grass to harvest the sediment before drying and weighing it, to visually recording how much sediment covered a 1-metre-square area. The team also developed a way of finding out how mildly or severely different species might be affected by sediment.
Even though these kids have only recently learnt the scientific techniques used to take the measurements, the accuracy of their results is on-par with the independent results from the scientists who also did surveys in the same areas.
With one exception: the students spotted more species than the scientists.
“We were surprised but very pleased to discover that the students are doing such a good job!” says project lead Sally Carson, who heads the New Zealand Marine Studies Centre at Otago University.
“I think this is partly because the kids don’t have a pre-existing idea of what they might find, so they’re more proactive and observant – looking under every rock and in every crevice – than scientists who are familiar with these environments.”
The students have found lots of unusual creatures, some of which are only found in Otago while others are rarely seen in other parts of the country.
Some students found an octopus, while others found a rare nudibranch. Another group found an unusual sea star that copies itself by tearing itself in half and growing back the lost half.
“Last year we didn’t find any fish but this time we saw some,” says 10-year-old Xanthe. “They were all hiding in their nests under the rocks where their eggs were.”
Skylar, 10, adds, “My favourite part was finding the brittle stars - they’re so cool.”
“It’s great to see how much Xanthe knows now,” says Daena, Xanthe’s mum. “We go for lots of coastal walks and she tells us about what lives there. It’s helped me become more aware of the things I’m treading on!”
Teens from Otago Girls’ High School even found two seahorses near Quarantine Island:
“I have never found live seahorses that close to the shore,” Sally says. “That’s very unusual.”
Overall, the team has found that sediment has increased at Dowling Bay, Rocky Point and Quarantine Island for 2017 compared to 2016, while other sites had no change or a decrease in sediment.
There was also a lower number of species and a higher number of turret snails in the areas with more sediment. Turret snails are not normally found on rocky shores but in sandier and muddier areas. Sally points out that the investigators need to explore these findings further as they could just be caused by weather.
Now, the students and researchers are collecting more information to better compare the build-up of sediment over time and between sites, as well as watching for changes in the sea life at these sites.
The students have already suggested some ideas as to how we can start protecting the environment from dredging: carry out long-term monitoring, change how the dredge is used or designed so that it contains as much sediment as possible with fewer ‘spills’, and find out where the safest place is to dump the sediment once it’s been vacuumed from the harbour.
They also posed a great question for us all to think about: “Which is more important: the marine life or the shipping channel?”
About the Project
Sediments and Seashores is run by the Department of Marine Biology at the University of Otago, with support from the Otago Participatory Science Platform.