Monitoring our marine reserves: A snapshot into a southern marine eco-system

Marine Rangers Tom MacTavish and Tom Brough take us through the fourth installment in our blog series from the marine reserve monitoring project at Banks Peninsula using baited underwater video (BUV). Using BUV, deployments were made across five different areas on the peninsula; two marine reserves (Pohatu and Akaroa) and three ‘control’ sites that were outside of the reserves. 

An abundance of Moki caught on baited underwater video camera.

So what were our results? Well, firstly, using the BUV method for our survey was a bit of a gamble, as it was a first for the east coast of the South Island, and the fact that it worked was pleasing in itself. On top of that we think that the fish data worked up from the video provides us with a thought-provoking first ‘snapshot’ of fish assemblages in Pohatu and Akaroa marine reserves and in the three controls sites.

There are a couple of things to consider when interpreting our results. Firstly, we don’t have pre-reserve fish data for Akaroa Marine Reserve, so patterns in fish abundance and/or size cannot automatically be attributed to protection status – there is a possibility that those patterns already existed. Secondly, many variables affect fish trends, which is why multiple rounds of monitoring are needed before any conclusions can be drawn.

A blue cod takes a bite at the underwater video camera.

So…what do you catch when you lower baited hook onto a South Island reef? Blue cod, sea perch, maybe a parrotfish, and occasionally a trumpeter? Well, with our baited underwater video we happened to catch all those species plus 25 others.

30 marine species that were encountered during the survey. Take a look at the total number of sites at which each species was present (out of 140).

One or two of these species aren’t too familiar, and certainly had us scratching our heads. It’s a great reminder of the diversity of fish species inhabiting our southern coasts.

But when considering the effect of the marine reserves, we’re not interested in all these species. Marine reserves protect areas of our ocean from the effects of fishing, so when testing their success we focus on popular harvest species – like blue cod.  We call these ‘indicator species.’

When Tom Brough crunched the numbers on blue cod counts (MaxN), we learnt that abundance did vary between our marine reserve and control sites – have a glance at the bar graph below. You’ll see that both marine reserves had higher blue cod counts than two of the controls. But interestingly, their counts were similar to that of a third control site.

And size? In total, Tom Brough was able to measure just shy of 650 blue cod that ranged from 50mm to 550mm. Check out Figure 2 to see how blue cod size structure varies depending on whether they were measured from marine reserves or control sites. Small blue cod in the 150-250mm range were obviously most common regardless of the site. However, if we focus on legally harvestable fish (>300mm) we see some differences. Namely, blue cod of >300mm were, on average, 3.6 times more common in Pohatu Marine Reserve and 2.1 times more common in Akaroa Marine Reserve relative to the three control sites.

An Octopus rests below the BUV unit.

Before the survey we’d also wondered whether blue moki could serve as an indicator species too. Although they’re a popular harvest fish, they are also tricky to catch on a baited hook, so we weren’t convinced that they’d actually visit our baited camera. But they did! Well, they were present at more than 50 of the sites. Furthermore, Tom Brough’s counts showed that they were present at a higher proportion of the video deployments and in greater numbers in the two marine reserves than at the control sites. Blue moki abundance is shown in the graph below.

Histogram comparing the relative abundance of blue cod and blue moki (as MaxN) in Akaroa Marine Reserve, Pohatu Marine Reserve, and at three fished control sites. Error bars denote the standard error about the mean.

Our results show some interesting trends which we’ll now aim to scrutinise and monitor with ongoing surveys. However, we think some of these results also raise questions about the importance of factors other than protection status in shaping fish communities. Can we account for some of these in the future? They’ve also forced us to start challenging some routine assumptions. How well do these marine reserves represent non-fished sites, and, equally, are our control sites actually fished sites?

In our fifth and final blog in this series we’ll wrap it up and perhaps look ahead a bit. We’re also hoping to then make the final 46 page peer-reviewed report available for folk who’re short of a bit of light bed-time reading. Stay tuned!