How do fish feel about how you store your boat?

How exciting! The first publication from my PhD research has been released in Marine Pollution Bulletin!!, and here’s a little summary of the work… 

Even if you don’t own a boat (like most of us), you’ve likely seen a boat mooring from the surface. Moorings are a very common form of boat storage. They pepper embayments around the world, allowing owners to easily monitor and access their boats for a reasonable price (in comparison to some other on water storage options).

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A traditional, or ‘swing’, mooring, consisting of a heavy anchor block and large, heavy chain that runs to a surface buoy. Photo: Kingsley Griffin

But why am I bringing this up?

Well there’s a dark side to boat moorings and some colleagues and I recently published work further exploring how they interact with the marine environment.

Why do we need to bother researching these structures? The answer to that is simple, they can devastate the seabed underneath the boat and we need to understand how that threat can be removed or mitigated. The concrete block has an immediate impact after installation by smothering any vegetation that may have been there previously, and the chain you can see in the picture above moves as the connected boat on the surface gets moved by wind or currents. The chain then “scours” the seabed, removing any vegetation and preventing it from regenerating.

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Those light rings are chain scour from boat moorings. The dark areas are marine vegetation (or seagrass) and the moorings have removed the seagrass, leaving barren “halos” around the mooring block. Photo: SIX Maps (NSW Government).

While we’ve known this since the 1980’s, what we didn’t know until recently is how this effects other ecosystem components. For example, what does chain scour and the loss of vegetation around boat moorings mean for the fish that utilise these habitats?

I’m 100% confident everyone reading this really wants to know that answer to that question, so it’s your lucky day. It has been answered!

In 2015, we jumped on a boat with 20 GoPro’s and a highly accurate GPS and set off to six different locations in Sydney Harbour. Once inside the mooring fields within these embayments, we dropped the GoPro’s off the boat at predetermined positions at varying distances from boat moorings. This allowed us to see where the fish were spending their time and quantify their feeding behaviour over fine scales (0 – 30 m).

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A bit of camera work in the winter of 2015 with Kingsley and Pete.

After watching over 180 hours of video and crunching some numbers we found a number of interesting responses:

  • We discovered that fish were less likely to be found close to boat moorings. Meaning that fish are being pushed away from boat moorings into the surrounding areas (the gaps between boats).
  • This means that fish aren’t feeding evenly over the area. The implication of this is that the tiny invertebrates that live in the sand, and are the prey of many of the fish, are more likely to be eaten further away from moorings. This is amplified because moorings create unsuitable habitat for many of these species, putting them in the same spot as the hungry fish.

An other important finding was that at Manly Cove, this effect wasn’t as prominent. This is due to a number of reasons:

  • Manly Cove, unlike the other locations in this study, still has seagrass beds within the mooring field, including an endangered species (although, it is rapidly declining). This hints that the presence of seagrass is more important than the presence of boat moorings (and therefore restoring seagrass to it’s former glory will help mitigate the effects of boat moorings).
  • Seagrass friendly moorings (or SFMs) also exist at Manly. These structures reduce the damage to the seabed by removing the heavy anchor and chain and therefore, removing the effects of chain scour. These structures are also stronger than swing moorings.
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A seagrass friendly moorings at Manly Cove, surrounded by seagrass and a Banjo Ray. Photo: Kingsley Griffin

So why are these findings important?

First of all, there’s a common misconception that boat moorings attract fish. This may be due to that fact that fish generally aggregate around man made structures, but also due to anecdotal evidence. For example, when you’re own a boat you’re seeing fish nearby because that’s where you physically are, creating the impression that there’s more fish there (when you’re on a boat you aren’t seeing the seabed 30 m away, so your perception of the where the fish are is skewed).

Another, more important, reason is that the habitats in which boat moorings exist are highly important for a whole suite of marine organisms, including fish that we catch recreationally and commercially. Seagrass is extremely important for a range of reasons (from acting as fish nurseries to carbon storage). Therefore, its decline has flow on effects, as seen here with how the distribution of fish is shifted.

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A porcupine fish enjoying the safety of seagrass at Manly Cove. Photo: Kingsley Griffin

What can we do?

Luckily, there are solutions! Progress is being made but we’re not there yet. The main way we can restore the habitats that have been damaged by swing moorings is the introduction of engineering solutions (e.g. SFMs); instead of removing moorings completely, replace them with designs that remove the chain and therefore chain scour.

To maximise the positive effects of the improved designs, their implementation can be done concurrently with seagrass restoration efforts. Recent work in Sydney has seen the restoration of seaweed that had gone locally extinct, giving hope to the endangered seagrass that exists in Sydney Harbour and within the mooring fields.

Endangered seagrass, Posidonia australis

 

The full article is available free here for 50 days after publication and here thereafter.

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