0 In Floating Equipment/ Natives

Experiment – Floating Equipment

So here we are almost four weeks into the COVID-19 lock down with no end in site as of yet, but the show must go on. For this blog we thought we would write about our experiment to grow European Flat oysters (Ostrea edulis) in floating equipment.

Ongoing Maintenance

Firstly a wee update on the farm. As we mentioned previously, the farm is basically mothballed and is on a maintenance footing. This means that one of us continues to work on site but at a much reduced level. The goal is to make sure the oysters are doing fine, as we have a duty of care to these animals in the same way any other farmer does.

Part of carrying out these duties includes grading the animals to make sure they have enough room to grow and they are clean of competitors, in our case blue mussels.

If you do not grade shellfish they start competing for the available space and food and the end result is a poorly shaped product (badly shaped) with an associated poor growth rate. In addition, the animals can become stressed and the mortality rate can rise significantly, particularly if the weather is also warm.

However, grading is usually a team effort that takes place at set points, so at this time we have had to take a different approach. In order to keep things ticking over, I have instead been grading on my own and doing a few thousand every chance I get. Tough work but it makes a difference.

Last week it was time to grade out a few thousand Native oysters (Ostrea edulis) and included in those were a small subset that we have been keeping in floating gear as part of an on going experiment. So let’s talk floating aquaculture and our anecdotal findings thus far from our experiment.

Floating Aquaculture

As you may or may not be aware, we have been involved with the Glenmorangie DEEP project, the goal of which is to help restore a European Native oyster reef in the Dornoch Firth. Native oysters are a fickle creature to try and grow, and experience has shown that they are much harder to grow than the standard aquaculture oyster of choice, the Pacific (Crassostrea gigas).

As part of the working group we have had many interesting discussions around how best to grow Natives and one of the theories that we discussed is using floating equipment as opposed to structures in the intertidal zone.

The Evidence

If you search the internet you shall find that floating equipment for oysters has been all the rage in North America in the last few years. From home made Taylor floats through to commercial products such as the OysterGro.

It is common to see statements along the lines that the floating gear improves growth as the oysters are always immersed and food is constantly available. As opposed to the intertidal zone, where they are exposed for portions of the tidal cycle and food is not constantly available. This sounds plausible but there is an inbuilt assumption here that floating oysters feed around the clock, or at least for a significantly greater time period than those in the intertidal, but is this true?

The answer should be in a study somewhere but the studies I have seen are inconclusive. This study, for example, states that for growth:

“There was no significant statistical difference in growth observed between oysters grown in floating bags and those grown intertidally in bags on racks”

Evaluation of Floating Bags in Off Bottom Oyster Culture

and for mortality:

“Oysters grown using the rack and bag method in their first year of growth experienced a statistically significantly higher mortality rate than those grown using the floating bag method, however, biologically the difference was insignificant. No other differences in mortality were noted for the remaining year classes of oysters”

Evaluation of Floating Bags in Off Bottom Oyster Culture

On the other hand this study concluded that with the scope of its criteria

“this study provides some of the first data on growth performance and shell shape using the OysterGro™ system in Chesapeake Bay and overall, results indicate that floating gear such as the OysterGro™ system may be a valuable method to finish oysters, facilitating good growth, and improving shape over a relatively short period of time”

The effect of aquaculture gear on the growth and shape of the oyster Crassostrea virginica during a “finishing period” in Chesapeake Bay, USA

So mixed messages, however for us there is a key issue with both studies. They concentrate on the Eastern oyster (Crassostrea virginica) not the European Flat oyster (Ostrea edulis) and as a result you can not make a direct comparison. In addition they were carried out in North America and not Europe and the results could be location specific.

Our View

Our working group has taken the view that the floating equipment could well aid in the culture of the Flat oyster (Ostrea edulis) in our environment. This theory is driven by our understanding of what phytoplankton these oysters like to eat and by the fact that in the wild they tend to show a preference for the sub tidal living as opposed to intertidal living.

Biological factors are also only part of the story here. Working an intertidal farm presents major issues related to access and this has a knock impact upon every other farm activity. If it is the case that floating equipment deliveries results ‘at least’ as good as the intertidal setup, then there is an opportunity to make significant productivity gains by using it. In addition, we already operate our farm by boat so incorporating this mode of operation is straightforward for us, and being more ‘boat people’ than ‘tractor people’ it is right up out street.

All this gave us enough encouragement to run an initial proof of concept trial.

Proof of Concept

So in the summer last year we deployed a small floating line on site to try and get a feel for floating native oysters. The key goal of this mini experiment to establish if the native oysters would survive, and as a secondary goal we were interested in the growth performance in comparison to the fixed intertidal gear.

The second goal was going to be difficult to evaluate as we were a bit late in the year in deploying the line and it was unlikely we would have enough data to show any difference either way.

Our floating line was made up using our existing baskets and installed beside the intertidal setup, meaning that the floating line itself was exposed to the tidal cycle. Ideally we would have preferred to install this subtidally but that would have required moorings which require a bit of thought and time to make, time we did not have.

Outcomes

Did it work? Well, yes it did, although we did have a number of issues with the design of the line.

Mortality: This was actually very low, and tracked the intertidal baskets. Our major concern was how these oysters would survive the winter storms. Our site is relatively exposed and all the gear takes a battering during the winter. This is especially the case for the floating equipment as it is on the surface all of the time. This was a great result and the key thing we were looking / hoping for.

Growth Performance: As we thought, there was not enough data to make a proper comparison but the growth seemed very similar to the intertidal. This might actually to be expected because of where the line is; being beside the fixed structures in the intertidal all the oysters shall be exposed to much the same conditions, the only really difference being in the second half of the high tide cycle. Is this enough to make a difference?

Shape / Profile: This is where the biggest difference was observed, and it was very noticeable. The oysters in the floating lines had a much rounder profile, were smooth and just seemed much more chunky that the intertidal oysters.

Fouling: There appeared to be less barnacle and blue mussel settlement on the floating oysters but that is really hard to measure. They were still there but in seemingly reduced quantities. Of course, we were not flipping the gear to expose it to the air as many of the equipment manufacturers recommend.

Next Steps

We have had enough success to continue with the experiment. The next step is going to be move the line into a sub tidal position. This shall hopefully allow us to focus more upon the question of growth performance. We really need this in place shortly to ensure we can collect a decent data set, and plan to have it place in the next few weeks.

Longer term we also want to look at flipping the floating equipment to help with fouling and look at sinking it in winter to reduce the impact of exposure to the elements.

So lots to do and there are some potentially exciting developments happening in the background that tie in directly to these experiments.

More to follow in future.

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