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Aquaculture

When life gives you dead mussels, make…lobsters?

Paper: Wang G (2016) Potential use of Mussel farms as Multitrophic on-growth sites for American lobster, Homarus americanus (Milne Edwards). Fish Aquac J 07:1–11

Introduction

Aquaculture is an expensive business, in terms of the space needed, food required, and profits that are not guaranteed. There is also considerable risk associated with aquaculture; much like farming – your population could contract disease, or worse, all die off, leaving you with little to no animals for market. The other variable comes from the specific species itself, with different species being “easier” to culture (e.g. oysters, mussels) but worth less than high-value animals that may be more difficult to culture (e.g. lobsters). Recently, aquaculturists have been thinking outside of the box to try to increase profits while cutting down on space, food required, and the risk of disease.

Most of our East Coast American readers will be familiar with the lobster fishery, which is huge in coastal Maine and Atlantic Canada. Maine’s lobstering season is often 5-6 months long, and lobster vessels operate at top capacity for that entire period. Canada’s Newfoundland Island, however, has a more highly regulated fishery, and their season remains open for only two months. Therefore, many lobstering operations have developed holding systems to keep lobsters alive, extending their market supply (and therefore an income source) all year round (Figure 1). These holding facilities have worked well so far; the lobsters are held at cold temperatures (1-3°C) so that their metabolisms slow and they don’t need to be fed. This process causes no long-term damage to the lobster itself, and does not significantly decrease the amount of meat present. Not feeding the lobsters cuts down on the cost of food, but maintaining these facilities in terms of energy and space can be costly.

Figure 1 – Example of a lobster holding facility at Gidney Fisheries, Canada. Lobsters are held at cold temperatures to keep them alive without feeding. Source: http://gidneyfisheries.ca/logistics

Figure 1 – Example of a lobster holding facility at Gidney Fisheries, Canada. Lobsters are held at cold temperatures to keep them alive without feeding. Source: http://gidneyfisheries.ca/logistics

 

 

 

 

 

 

 

 

Lobstering operations are paid more per lobster if the lobster weighs more and can provide more meat. So, what if we can hold the lobsters, but still feed them at the same time, effectively fattening them for market? Can we do that and still keep costs low? The authors of this paper had an idea that could solve this conundrum. They noticed, from reading previous studies, that American lobsters are found often below the lines of mussel farms (Figure 2).

Figure 2 – Example of what a mussel line looks like. When mussels die, they fall off of the lines, bringing nutrients and a potential food source for lobsters. Source: The impact of a mussel farm on water transparency in the Kiel Fjord - Scientific Figure on ResearchGate. https://www.researchgate.net/263201455_fig3_Fig-3-Scheme-of-the-long-line-system-of-the-mussel-farm-and-locations-of-de-fi-ned

Figure 2 – Example of what a mussel line looks like. When mussels die, they fall off of the lines, bringing nutrients and a potential food source for lobsters. Source: The impact of a mussel farm on water transparency in the Kiel Fjord – Scientific Figure on ResearchGate. https://www.researchgate.net/263201455_fig3_Fig-3-Scheme-of-the-long-line-system-of-the-mussel-farm-and-locations-of-de-fi-ned

 

 

 

 

 

 

 

 

 

Previous studies had speculated that maybe the lobsters were hanging out below the mussel lines to eat the dead mussels that fall off. If this was the case, it would solve so many problems – lobsters get another food source, and they’re helping to “clean up” the nutrient waste caused by the mussel farm. If this was a possible option, we’d be turning the waste from one industry (the mussel farm) into the raw material for another (lobster food), much like business eco-industrial parks but happening on an open coastal mussel farm.

To test this theory, researchers held lobsters in the field underneath blue mussel farms in Newfoundland, as well as in the lab, to get the full picture of what lobsters are eating and how well they grow and survive under different circumstances. The goal of using both field and lab experiments was to get a full picture of what’s happening in this symbiotic partnership: it’s difficult to control variables in the field, so scientists can make more robust claims about laboratory experiments.

Field Experiments: Methods

The authors placed ~200 lobsters in cages both beneath a mussel farm, and in a location far enough away (15-20 m) from a mussel farm where the lobsters wouldn’t have access to the resources provided by the farm. Half of the lobsters were placed in small cages, and the other half were placed in larger cages. They monitored growth (length and weight) as well as the health of all of these lobsters (indicated by blood samples) for six months. They also kept careful record of temperature, which was similar at both sites.

Field Results

After six months, the researchers crunched the numbers. They found there was no effect of cage size – lobsters in big cages didn’t grow any larger than the ones in small cages. This is good news for holding operations because they’ll be able to get by on smaller and cheaper cages, therefore reducing costs.

They also found no difference in growth between the lobsters that were held underneath the mussel farm lines and the lobsters held away from the farm. Due to difficulties in monitoring so many lobsters in the field, researchers needed more data to make concrete conclusions. They turned to lab experiments to find why there was no difference.

Lab Experiments

The researchers wanted to figure out how water temperature, diet type, and feeding frequency individually affected lobster growth so they could better understand field results. They tested three temperatures – 5°C, 10°C, and 15°C to see what temperature favored highest growth. They tested two diet types, mixed (squid, shrimp, and mussels) and mussels only, to see if lobsters fed a mussel-only diet would grow as they did in the field (underneath mussel farm lines). Finally, to understand how feeding frequency affects growth, they fed lobsters either twice a week or once a month.

Lab Results

The results from the lab were more clear than field experiments, showing that lobsters were not able to survive on mussels alone. The mortality rates in the lab experiments with lobsters fed only mussels at 5°C and 15°C were much higher than the lobsters fed a mixed died, suggesting that these lobsters do need a variety of food to survive.

They also noticed that lobsters in higher temperatures were more likely to molt (shed their exoskeleton and grow a new, larger one), increasing their size by ~15-20% (Figure 3). However, lobsters can only molt if they have enough food energy. Keeping lobsters at higher temperatures so that they would molt would be great news for lobster holding if they could feed them enough of that mixed diet, but as you can guess, that would get expensive.

Figure 3: Short video of a spiny lobster molting. The video is time lapsed, and the process actually takes about 30 minutes.

The results become clearer after looking at the lobster’s blood. Mussels are deficient in a few essential amino acids, and also don’t have a high concentration of astaxanthin, a protein essential for lobster immune system function. The researchers hypothesized that maybe the lobsters were dying more frequently on a mussel-only diet because they didn’t have these nutrients available to them – it’s the same concept as if you, a human, didn’t get enough calcium or vitamin D in your diet.

Conclusions

Unfortunately, it doesn’t seem like keeping lobsters underneath mussel farm lines will be a silver bullet for lobster holding. The researchers hoped that having lobsters eat dead mussels would reduce all the costs (food, holding space, etc) while increasing the profit margin (larger lobsters = more $$). But, studies like this one are important because they show the value in thinking creatively about how to solve important issues like this. Now that we know lobsters need a more mixed diet, rich in all of the essential nutrients, maybe we can come up with a collaborative solution. Maybe hold them instead under a large farm that grows some mussels, some squid, and some shrimp? The solutions to aquaculture problems are out there, we just need to do some more out-of-the-box thinking like these researchers to try to find a solution that’s financially feasible for everyone involved.

Engage

What do you think are other ways we can use waste products of one industry as raw materials for another? Another creative solution that got a lot of press recently was a plastic six-pack can holder being replaced by an edible, biodegradable six-pack can holder. Comment with some other ideas!

Erin McLean
Hi and welcome to oceanbites! I recently finished my master’s degree at URI, focusing on lobsters and how they respond metabolically to ocean acidification projections. I did my undergrad at Boston University and majored in English and Marine Sciences – a weird combination, but a scientist also has to be a good writer! When I’m not researching, I’m cooking or going for a run or kicking butt at trivia competitions. Check me out on Twitter @glassysquid for more ocean and climate change related conversation!

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