Podcast: Reducing the effects of fish farms

A fish farm

24 December 2013 by Richard Hollingham

This week in the Planet Earth podcast, Kenny Black and Jo Gosling of the Scottish Association for Marine Science (SAMS) describe the work they're doing to make fish farming sustainable.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Richard Hollingham: This time in the Planet Earth podcast, minimising the environmental impact of fish farms. I'm Richard Hollingham and I'm at the Scottish Association for Marine Science near Oban in North West Scotland, and the chances are if you've eaten Scottish salmon recently it will have come from one of the fish farms near here or dotted around the Scottish coast, and I'm with Kenny Black who has led the development of a computer model used by fish farms all over the world for planning and monitoring. Kenny, just set the scene for us here. We're standing outside the back of the laboratory looking out over the water - this is the sea is it?

Kenny Black: Yes, this is a sea loch, [unintelligible] system, a large one, Loch Linnhe, and what you can see here in the foreground is the low lying limestone outcropped island of Lismore and behind that is Morven and the large island of Mull.

Richard Hollingham: It really is beautiful and there are fish farms out there which we can't quite see, I think, just over the hill.

Kenny Black: They are often quite difficult to see. There's actually some over on the [unintelligible] and there's one just around the corner from SAMS what's called the [unintelligible] site.

Richard Hollingham: What is a fish farm exactly? What is going on in it?

Kenny Black: To look at a fish farm when you approached it you would find that the majority are, maybe, eight to ten or maybe more black plastic circles, called polar circles is the brand but everyone calls them polar circles, and they might be maybe 30 metres in diameter and have a net cage inside which might be up to 20 metres deep, enclosing quite a large volume and each cage might have many tens or even hundreds of tons of fish in there.

Richard Hollingham: So it's really a floating cage of fish?

Kenny Black: Yeah. You've got a floating collar and then below that you've got a net which is tensioned by weights to keep it nice and tight.

Richard Hollingham: And you're looking at or past of the work that you're doing is looking at the impact of that on the seafloor. I mean to put it simply you're interested in fish faeces?

Kenny Black: Fish are fed a mixture of fish meal and plant meals and this diet is closely monitored as it goes into the cages and is eaten by the fish, so relatively little is wasted. So what we're really mostly looking at is their fish faeces. Once fish have eaten and digested this food the residue is then defecated out and then we've been looking at measuring things like settling velocities, and using sediment traps to find out exactly where this stuff goes and then attempting to model this.

Richard Hollingham: When you say model that, you're looking really at computer simulations of the whole process of where this waste goes and the impact it has.

Kenny Black: Yeah that's right. So we start off with a conceptual fish cage filled with fish and we feed conceptually again this diet, according to the way the fish grows, and then we made some assumptions from the literature about digestibility and then we produce the faeces. The faeces then settles according to measurements that we've made about settling velocity and driven by either a hydrodynamical model or by measured currents these particles end up on the seabed.

Richard Hollingham: Now all fish farms in Scotland use this computer model you've developed but you're not just doing stuff on the computer, you have some tanks here. These are covered tanks outside and really look like covered bath tanks. Maybe we could open one up... okay, so that looks just like some water with, I don't know, across the bottom it looks like mould is growing there.

Kenny Black: What you're seeing there is sulfide oxidising bacteria mat which you find where you've got an interface between highly sulfidic sediment, that's lots of sulfide in the sediment, and an oxygen-rich overlying water. The sulfide comes from the reduction of sulfate (SO4) by sulfate reducing bacteria in the sediment reducing as an end product sulfide.

Richard Hollingham: And you could expect something like this under fish farms?

Kenny Black: You could do. In what I call the 'bad old days' when I started this game about 20 years ago that would be a very common sight now fish farms, partly due to the use of the model, are much better fitted to the scale of the farm and the inputs are much better fitted to the capacity of the environment and the environmental conditions, so that seeing large amounts of vegatoa like you are seeing in these tanks is probably fairly unusual nowadays at fish farms.

Richard Hollingham: Nevertheless this is not going to be good news for the bottom of the sea.

Kenny Black: No, if you ended up with a situation where you've got a lot of vegatoa you've by definition got a very highly sulfidic sediment and sulfide is toxic to the majority of metazoan life. You can smell this and you can tell me what it looks like.

Richard Hollingham: Okay, let's put my nose... oh, that is quite a pong isn't it, a sulfurous pong. It's not pleasant.

Kenny Black: What's happening in a rotten egg... poaching an egg that has a fair amount of sulfur in it and amino-acids and when these rot and in anaerobic conditions you will get this classic sulfidic rotten egg smell.

Richard Hollingham: Now, we're also joined by-

Kenny Black: I love it by the way!

Richard Hollingham: You love it?!

Kenny Black: Yes, it's the smell of good interesting sediment.

Richard Hollingham: But a bad seabed?

Kenny Black: Yes, indeed.

Richard Hollingham: We're also with Jo Gosling who is a PhD student. You are doing work on the recovery of the seafloor. What is this if someone moves a fish farm, or the area around the fish farm?

Jo Gosling: Yeah. So what's quite interesting is no one really knows how long it takes for a sediment to recover from these impacts from fish faeces. Some estimate between one to two years but it can be a lot longer and the role of sulfur in this recovery process is really important and it's not fully understood yet, so I will be using these three tanks and once I've put all the fish pellets on there I will be measuring various species of sulfur which have developed in the sediment and tried to quantify that and figure how long it is before you remediate the sediment.

Richard Hollingham: And all this feeds back into minimising the environmental impact of the fish farm, both whilst it is operating but also once it has gone or maybe moved to somewhere else?

Jo Gosling: So the overall aim is to obviously make fish farming a more sustainable way to produce protein for the UK. If you can reintroduce the benthic fauna below the fish farm after its gone then there's a large amount of biodiversity on the sediment and it is seen as a healthier sediment overall and that's what we're really trying to achieve after the fish farm has gone.

Richard Hollingham: And, Kenny, this will all feed back into improving these models that fish farmers use?

Kenny Black: Yes I think so. What the fish farmer are really interested in doing, and the regulators, is getting the answer right at the beginning. So you look for new sites to perhaps put a fish farm and you want to be able to get the scale right at the beginning. You want to be able to also know that it is going to be at a scale you can do it economically so you're looking for a certain amount of size and then ideally what you will be able to do is produce a really robust prediction of what the impact would be and that it wouldn't break any of their quality standards that the regulators are putting out for biological diversity and so forth. So if you can get really good simulations then we've got much more confidence about getting it right first time in terms of scaling the industry.

Richard Hollingham: In the past fish farming has had a bad press when it has come to its environmental impact and yet we all eat farmed salmon. Is that improving or is this going to help that?

Kenny Black: Yeah, I think there's been an enormous change. I mean this is a very young industry. Really the first salmon were farmed in Scotland in the seventies on a very small scale; really we've been learning how to do this for only... we compare this with any domestic agriculture which is typically thousands of years. This is very new stuff and there have been really massive changes in the way that we've done things over this last 20 years which has made a massive difference to the impacts.

Richard Hollingham: Kenny Black and Jo Gosling here at the Scottish Association for Marine Science, thank you both. As ever we will put pictures of our recording today on our Facebook page, you can also find us on Twitter and of course every podcast we've ever recorded for the Natural Environment Research Council will be found at Plant Earth online. I'm Richard Hollingham from Oban, thanks for listening.