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The importance of benthic monitoring for aquaculture sustainability

An interview with Kate McEwen, head of environmental services at PHARMAQ Analytiq


Q: Just how crucial is monitoring the seabed at aquaculture production sites?

KM: There’s quite a lot of public concern about the impact of fish farming, and one aspect is what happens to the waste products from farms.

In order to obtain a fish farm license, the operator needs to have carried out an environmental assessment as part of the application and, if a license is obtained and once production is under way, at the end of every production cycle the operators need to monitor the seabed.

People are less likely to buy farmed fish if they think the rearing process is harming the environment. Monitoring results, which are available for public scrutiny, show the level of seabed impact that every farm is having.

That’s why it’s so important; it is a method of ensuring that the carrying capacity of any water body isn’t being exceeded, as a water body needs to be available for use by everybody, and everyone needs to work together to ensure sustainability.


Q: How does the monitoring work?

KM: Sites have a licenced area within which to operate. At the end of every production cycle when the depositional impact is expected to be at its peak, monitoring is carried out to determine how far the enrichment effect of the waste extends. Monitoring over successive production cycles will show whether there is a change in impact area.

What you want to avoid is incremental deterioration or expanding impact on the seabed with every production cycle. It needs to be contained within the licensed area. And this monitoring shows whether it’s being contained, whether the footprint is remaining stable or whether it’s changing.


Q: You mention containment. What exactly would that look like?

KM: Level of enrichment affects the composition of the seabed faunal community. If enrichment spreads too far you would need to reduce the amount of waste and allow the seabed to recover, and this could be achieved in a variety of ways — most simply (but not necessarily economically) by a reduction in biomass for the subsequent cycle.

During a production cycle it is possible to manage biomass in different ways — for example, by changing distribution of biomass across a pen group, reducing biomass by harvesting or stock transfers to other sites with more carrying capacity. This is a simplification; however, all sites are different and impact-mitigation techniques will therefore also vary.


Q: To what extent do feeding practices contribute to matter ending up on the seabed?

KM: It used to be that people thought overfeeding was the main reason for sediment enrichment, but because food is so expensive and feeding rates and stock behavior are carefully monitored, you’re highly unlikely to find waste feed on the seabed; it’s all about fecal matter.


Q: What are the key indicator species you’re looking for in monitoring?

KM: We look at marine benthic invertebrates over a millimeter in size and, specifically for sediment reworking, two species of enrichment-tolerant polychaetes, which are marine worms that process the waste.  What the regulatory authority looks for is that you have sufficient density of those at the cage edge where the deposition is expected to be greatest.

Then we sample different locations, moving out from the pens in the four main directions, looking for tailing off in density of those species as the waste material reduces and a corresponding increase in faunal diversity.


Q: Has anything changed in the methodologies used in sampling?

KM: The requirements for monitoring are actually in transition at the moment. It used to be that sampling would be carried out at a few predetermined points — generally just on the main direction of deposition — but modeling to determine these points (and the footprint from the site) didn’t always accurately reflect what was found during ground truthing.

So now, from a group of pens, sampling is carried out along both long axes (the main direction of flow and expected deposition) and the lateral axes from the pens, with the aim of determining the point on each transect where you are back to what you would consider unimpacted or background conditions.


Q: And what technologies are used in acquiring and analyzing the samples?

KM: You basically take scoops of the seabed using a “grab” from a boat, bring them up to the surface, sieve them to remove fine particles, fix the fauna with preservative and bring the samples back to the lab.

From there, it’s a very time-consuming process where the scientists in the lab pick out all the fauna that was alive at the time of sampling. Then each specimen is identified and counted by taxonomists. Subsequent data analysis then categorizes the faunal community/sediment status at each sampled point.

Environmental DNA (eDNA) is now being adopted to speed up this process. You still have to take grab samples, but it doesn’t require removal of the fauna. We are developing an eDNA service, but at the moment the conventional benthics take priority, as that’s what our clients require.


Q: Have eDNA and bioinformatics currently got the buy-in from industry that conventional approaches have?

KM: Here in Scotland, various companies have been working with the Scottish Environment Protection Agency (SEPA) to accept eDNA results instead of the taxonomic results to determine impact area. We are working with our clients to run eDNA and conventional benthics in parallel, to ensure that they are getting the same impact assessment. Once all this is validated and approved by SEPA, I can see that there will be a bigger move to eDNA.


Q: How engaged are those working on fish farms on the impact of their work on the seabed?

KM: There’s quite a common perception that fish farmers don’t care, that they are just producing fish. I’ve never seen that on sites where I’ve visited. They often ask questions about what I’m seeing in terms of sediment health, what we’re actually looking for and how the process works. They are interested, and they do care.


Q: It’s clear that standards in this area are high, and data is transparent,– but is there enough public awareness of this? 

KM: From my perspective — as a marine biologist, ecologist and ex fish farmer — I still don’t think the industry as a whole does enough to say “look what we’re doing, look how hard we’re looking at the seabed. Look at the level of investigation work that’s done and what is required to achieve compliance.”

But with the increase in certification schemes, such as the Aquaculture Stewardship Council, that take environmental monitoring into account among a breadth of other criteria, public awareness and reassurance about safeguards are improving.