Ridge to Reef

We all know that the “ridge to reef” paradigm has become a familiar management theme throughout the Pacific and for centuries has been a central element in the native Hawaiian management scheme known as the ahupua‘a system.

This view of Hawaiian land division and resource management combined watersheds, streams, and coastal regions as integral interacting components of an ecosystem. In this holistic view, the sustainability of watershed and nearshore resources were recognized as being related to human practices on land, which today is a central premise in ecological science. The Hawaiians understood what they did on land would affect their fishing.

The Hawaiians understood that what they did on land would affect their fishing.

However, there was no quantitative data until recently in Hawai’i to verify this linkage on a large scale. So why did we believe so strongly that this relationship was valid, that the watersheds affected the reefs? Well, it’s because there have been links established on a small scale, such as on the south shore of Moloka’i, where a clear connection between poor land practices was correlated with poor reef condition. But we have also worked in regions that we didn’t find that link. There really was no data on a statewide scale to examine this relationship and validate a connection. In order to see if there really is a connection we needed a large scale data on the watersheds and on the reefs (there are 580 watersheds).

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The watershed data came from University of Hawai’i researcher Mike Kido and we used our reef data from our Coral Reef Assessment and Monitoring Program (CRAMP). We developed a watershed and a reef index and compared them to see if there really is a link between the two. We looked at over 60 biological and environmental factors, such as water motion and coral cover, to see which were most important in influencing coral reef communities. The same was done for the watersheds to develop an index of watershed health and reef health. What we found was an overall significantly positive correlation between the watersheds and the reefs. So the healthier the watershed, the healthier the reef.

“What we found was an overall significantly positive correlation between the watersheds and the reefs. So the healthier the watershed, the healthier the reef.“

This provided the first quantitative statewide evidence to support the long standing assumption that there is a significant overall relationship between the condition of watersheds and the condition of adjacent coral reef communities in Hawai‘i. Not surprising.

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What is interesting is that the relationship was strongest for shallow sites on south-facing shores but diminished as you traveled west and east around each of the islands. In addition, as you get to the north facing coasts we found no statistically significant relationship at all between the watersheds and the reefs. High surf conditions along the north shore increase local wave driven currents and flush watershed-derived materials away from nearshore waters. Consequently, reefs in these locales are less vulnerable to the deposition of transported land derived materials. Also, since the south shores of the islands have less wave energy, this is where the most development initially occurred so you have more degraded watersheds on the south shores.

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We conducted field verification of this model. We needed to be able to go to a new site and determine whether it fit this pattern. We also had a very small sample size on north shores, preventing validation of the watershed/reef correlation. So we did this on Kaua‘i. We added north and south facing reefs in 4 categories of watershed health (excellent which is depicted in blue, good, fair, and poor which is colored red). This wasn’t as easy as it sounds. We couldn’t find excellent watersheds on the south shores of any of the islands except in a few rare exceptions that we couldn’t use (such as Waimea on Kaua’i where a huge watershed drains into a small area that has little reef structure and is not surrounded by good watersheds). We added 49 stations at 8 sites, one in each index health category on the south shore and one in each index health category on the north shore.

With this validation data we found the same pattern. Shallow south shores are impacted more heavily by watershed impacts than north facing reefs.

“Shallow south shores are impacted more heavily by watershed impacts than north facing reefs.”
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There are however some factors that can override this watershed/reef relationship, such as heavy fishing pressure or sand scour, but the general pattern is clear. We can now predict reef health from watershed health in certain regions.

“We can now predict reef health from watershed health in certain regions.”

We can evaluate reefs, predict reef health from watershed health, and improve and coordinate conservation and resource management of the entire ahupua‘a. We are now developing an interactive map that shows each of the 580 watersheds and their connection to the reef.

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This can tell managers where restoration efforts in the watershed will be most effective in protecting the reefs. We also found that watersheds need not be pristine for adjacent reefs to contain high coral cover. Thus, watersheds may not need to be fully restored to pristine natural conditions to support thriving coral reef communities in adjacent marine waters.

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Map showing (a) watershed health indices (WHI), (b) reef health indices (RHI), and (c) the difference between RHI and WHI for watersheds on the islands of Ni‘ihau and Kaua‘i. In the lower map (c), negative numbers indicate watershed health is greater than reef health and positive numbers indicate reef health is greater than watershed health.