Using seafloor mapping to help assess marine protected areas

One part of my PhD research was working with NSW DPI and UOW to assess the performance of marine protected areas (MPAs). We recently had an article published in Biological Conservation where we used seafloor mapping to improve the assessment of the Lord Howe Island Marine Park (free download here till July 20th). Here’s a quick rundown on what we found…

The study:

We assessed the performance of the Lord Howe Island Marine Park in protecting a heavily targeted species of fish; yellowtail kingfish. The novelty of our study was that we compared two assessments. The first, a conventional assessment where the number of kingfish was compared inside and outside areas open to fishing. The second assessment accounted for reef topography (whether the reef was flat or steep), measured using high resolution SONAR. Reef topography was included in our assessment as previous research has indicted that yellowtail kingfish prefer reefs that are steep and complex, presumably because these habitats have greater resources such as food. Therefore, we predicted that the response of kingfish to MPA protection may be influenced by underlying reef topography.

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The World Heritage listed Lord Howe Island must be one of the most amazing places on Earth. This pic was taken from the lagoon looking up to the 800 m tall Mt Gower.

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Lord Howe Island and Balls Pyramid is located 600 km east of Port Macquarie on mainland NSW, Australia. The region is positioned in a mixing zone of subtropical and temperate waters which results in the word’s most southern coral reefs and a suite of endemic marine life.

Rationale:

Marine Protected Areas are becoming established around the world at an unprecedented rate and as a result it is important that we accurately assess whether they are working. One of the best ways to assess MPA effectiveness is using a Before-After-Control-Impact (BACI) design, where surveys of marine life are completed multiple times before the park is implemented. Subsequent surveys are then repeated over time inside and outside park zones following the marine park’s enforcement. An advantage of BACI assessments is their ability to separate the effects of MPA protection on marine life from underlying natural variation (the inherent differences in species abundance among locations and seasons, for example).

Unfortunately, most MPAs do not have biological data collected prior to their establishment. Hence, most assessments on MPA effectiveness are Control-Impact studies that compare the diversity and abundance of marine life between protected and unprotected zones. This approach may be adequate if the study is well replicated, i.e. there are many protected and unprotected areas surveyed. However, in the case of small MPAs, or MPAs with limited number of zones, the results of Control-Impact assessments may be clouded if habitat characteristics, such as reef topography, varies greatly across the marine park. In our study we were interested in whether accounting for reef topography would improve the accuracy and precision of the assessment of the Lord Howe Island Marine Park.

Methods:

We used baited remote underwater video to survey yellowtail kingfish in 2009 and 2013 on the Lord Howe Island and Balls Pyramid shelf. We chose yellowtail kingfish as they are heavily targeted by charter and recreational fishers in the open fishing zones within the marine park. Approximately 20,000 kg of yellowtail kingfish are captured by charter fishers on the island per year for the local restaurant trade. Consequently, we predicted that kingfish would show a positive response to the removal of fishing inside closed zones. Also, previous research has shown that kingfish prefer topographically complex reefs (steep with large drop offs) compared to simple, flat reefs. Therefore, we predicted that accounting for differences in reef topography among our survey sites was likely to improve our assessment of marine park zoning for this species.

To calculate reef topography of each survey site we used pre-existing high resolution depth data collected using multibeam SONAR.

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Top: screen grab from a baited remote underwater video system of a yellowtail kingfish chopping on our bait bag. Below: screen shot of high resolution bathymetric (depth) data we used to calculate reef topography.

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A yellowtail kingfish caught during a Lord Howe Island fishing charter. Approximately 20,000 kingfish are captured per year in areas open to fishing to supply the local restaurants.

Results:

We found considerable variability in reef topography on the Lord Howe Island and Balls Pyramid shelf. For example, within 200 metres of one site there was a difference in depth of 17 metres. In contrast, within 200 metres of another site there was a difference in depth of only 2 metres.

On average there were almost 3 times more yellowtail kingfish in zones closed to fishing compared to openly fished zones. However, a number of sites in closed zones recorded no kingfish raising uncertainty over this finding.

Adding reef topography into the assessment explained much of the variability in kingfish numbers between sites in closed zones. Substantially higher numbers of kingfish were recorded in closed zones but only on steep, complex reef habitat (see graphic below). In contrast, there was no difference in the number of kingfish between closed and openly fished zones on reefs which were simple and flat. This result was not surprising given kingfish are heavily targeted in openly fished areas and prefer topographically complex reef habitat.

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Graphical representation of our findings. The numbers of yellowtail kingfish were greater in zones closed to fishing but only on topographically complex (steep) reefs. In contrast there was no difference in the number of kingfish between open and closed zones on flat reef habitat.

What does this mean?

By accounting for reef topography, we demonstrate with greater precision and accuracy that no-take zones in the Lord Howe Island Marine Park are benefiting kingfish. Our results suggest that the current zoning arrangement of the Lord Howe Island Marine Park is adequately protecting this species of fish. Our findings also provide compelling evidence for the use of habitat mapping in assessments of MPA effectiveness, as it is likely to lead to a better understanding of ecological change due to this conservation strategy.

Article details:

Rees, M.J., Knott, N.A., Neilson, J., Linklater, M., Osterloh, I., Jordan, A. and Davis, A.R., 2018. Accounting for habitat structural complexity improves the assessment of performance in no-take marine reserves. Biological Conservation, 224, pp.100-110.

*If you would like to read the paper in full and don’t have institutional access to pass the paywall, send us a message and we will happily send through a copy.

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Acknowledgements:

This work would not have been possible without support from NSW DPI. We thank Ian Kerr and Jimmy Maher for assistance with fieldwork and Sallyann Gudge and Hamish Malcolm for their comments on earlier drafts of the article.

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