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OceanMOOC | 8.3 | Aquaculture and Mariculture



This lecture is about marine aquaculture,
the fastest growing food sector in the world. You will learn in this lecture that eating
tuna is actually like eating a wolf-eater, if it was an equivalent food chain on land.
You will learn that most of the aquaculture production currently is not sustainable and
that we are partly actually taking protein away from the human population that’s badly
needed. And finally, you will learn that saving or protecting the fish stocks is best done
by managing the fish stocks and not by replacing wild codfish by aquaculture fish.
So let us first look at the global production of food from the sea, be it wild cod here
in light blue or from aquaculture production. What we clearly see is that around the 1990s
the wild capture fisheries leveled off at a level of about 95 million metric tons per
year. Very much in contrast to the dark blue which shows the almost exponential rise of
the aquaculture food production from waters in general. So at a first glance we may think,
okay, so aquaculture alleviates the pressure from the wild fish stocks that cannot yield
more catch to human nutrition. Let us dissect now more what this dark blue sector is, the
aquaculture production. And here we quickly see that aquaculture is actually a lot of
freshwater fish. So 60% of something are cyprinids that are mainly grown in east Asia.
Then we have a large section of crustaceans like shrimp and bivalves such as oysters and
the fraction of fish is actually quite small. So actually here the production of, of seafood
in terms of harvestable fish that could alleviate the pressure on fish populations looks quite
limited already. We also need to have a look at how the aquaculture
is organized. So there are aquaculture settings with a very different carbon footprint. Evidently
recirculating systems based on land need huge infrastructure and have a very high energy
demand. So in terms of their carbon footprint, they’re hardly sustainable. At a medium
level, we have for example net pens for sea bass or for salmon and at the lowest level
we would have in a way passive rope cultures of mussels and algae that need no additional
feeding. This is very important to keep in mind if we look at the general overall sustainability
of an aquaculture setting. What’s now the general problem with having
marine fish in an aquaculture setting? We need to be aware of that the food chains in
the ocean are much longer than on land. So whereas on land we have the cows as the
first consumers of primary productivity, in the oceans these would be tiny crustaceans.
And they are simply not suited for human nutrition. Conversely, if we eat a mackerel, the corresponding
trophic level on land would already be the eater of a wolf eater. And that’s…. So
that’s evidently a fantasy animal which doesn’t exist on land, but it very well
exists in the form of large pelagic predators in the oceans.
So in other words, if we want to keep sea bass or sea bream or raise tuna, we have to
feed them with other fish. And what are these other fish? These other fish are called forage
fish. So those are mainly small pelagics which are caught at a rate of about 20 million tons
per year, so a considerable fraction of the wild fish catch, about a fifth. And they are
mainly then converted into fish meal and go into fish feed for aquaculture production.
So let’s have a look at the budgets. So 18 million metric tons are then converted
into, they are basically dried, they are converted into 6 million metric tons of fish oil and
fish meal. And they then go to livestock feed on land, about half of it. And another fraction
goes to aquaculture. We ignore here now the freshwater portion. And then we see that about
2.1 million of those forage fish derived feed goes into producing fish in marine aquaculture.
And if you remember now, we have about 2.5 million tons production of marine fish in
the aquaculture. You see that we have a one to one relationship which is actually worse
because we have dried fish oil and dried fish meal. So at the moment actually we lose, we
lose protein that could be available for human nutrition in poorer countries when we feed
those small forage fish to larger marketable fishes such as sea bream or sea bass.
So what could be solutions out of that sustainability dilemma? One solution is to replace the fish
derived protein by plant derived protein. For example, from soybean or rapeseed. And
this is happening. So they are, there are really intense research projects trying to
replace that. And there is now considerable progress has been made, but at the moment
this progress is completely eaten up by the increase of aquaculture production. So the
net demand on forage fish still increases and is not decreasing. Another solution is to use different fish
species. So those fish species that are omnivorous like tilapia, pangasius, or carp. With those
species, it’s possible to completely replace fish derived protein by plant derived protein.
So in principle here the feeding problem is completely solved and it’s no surprise that
these fishes now are heavily increasing on the market. But these are all freshwater fish
and we are talking here about health and sustainability of the oceans. What’s now with the argument that marine
aquaculture saves marine fish stocks? And here we can see a clear no. It’s in fact
inverse. So we have to have severely over-fished fish stocks to make aquaculture profitable.
As one example I’m showing you here, the recruitment of the European eel. Recruitment levels of
small eels, so-called glass eels arriving here are down to 1% of the original levels.
And actually eel is one of the few species listed in the Red List and it’s still exploited
by fisheries. And you see in the second diagram, the increase
in the production of fattened eels. So they are not based on a complete cycles, just that
small eels are caught and fattened to then later been sold on the…on the food…global
food market. And this only became profitable when the wild, when the catch of wild eel
decreased dramatically. The same situation happened in cod. Here fortunately the Barents
Sea caught stock recovered due to good management and many of the cod aquaculture facilities
that had been built up during the years when cod was in…was in a very bad shape, they
are now basically not working anymore because it’s not worth to produce cod with the relatively
expensive aquaculture settings. Another sustainability issue with aquaculture
is that it needs space and that it needs very, very pristine and very vulnerable space, in
particular in coastal ecosystems. If you think for example of crustacean and here, shrimp
aquaculture. So this picture shows you a temporal comparison over 20 years how the mangrove
forest in Honduras has been converted, but this, I could show you similar pictures from
other Asian countries, so Honduras is not a particular bad example, but it’s a very
well worked out example from the Millennium Ecosystem Assessment.
So why is this conversion a problem? This conversion is a problem because the very valuable
mangrove forests are destroyed that are…provide coast protection that at the same time collect
sediment so as to help making the land raise in the face of sea level rise. So what could
be a sustainable solution? A sustainable solution could be that we farm down the food web. So
we should not fish down the food web, but we should farm down the food web, meaning
that we should tackle lower trophic levels for aquaculture.
And these lower trophic levels could be filter feeders like oysters or these mussels on rope
culture. Or it could even be algae, so primary producers. And common to both such types of
aquaculture that’s often then called mariculture is that they at the same time tackle an additional
problem which is nutrient pollution. So by not needing to feed them additionally,
we do not have additional input of nutrients which would be the case in fish or shrimp
aquaculture, but we even can extract nutrients that then is incorporated in the biomass of
mussels or algae. Another idea is to have small self-contained,
little enhanced ecosystems in multi-trophic aquaculture. So these systems are very much
still in the experimental phase. So here science still has a lot to contribute. Basically what
we try is to have the primary producers and/or the filter feeders together with the consumers. That’s for sure something that could work, but at the moment it will
not work at a price level that’s competitive on the world market.
So what are our take home messages? I hope I could make clear that at the moment many
aquaculture practices, in particular with marine fish are not sustainable. We are taking
away protein for human consumption and we are not saving the fish stocks. To the contrary,
such aquaculture for very high priced fish is only profitable because the wild populations
have been decimated. And to save fish stocks, we have to better manage fish stocks. Solutions to improve
sustainability is farming down the food chain, go for mariculture, go for bivalves, or maybe
even go for algae and maybe also now still in an experimental phase, go for multi-trophic
aquaculture.

One Comment

  1. ahowl7mx

    I thought the explanation about the tiered food web was great, particularly liked the dragon. Video quality is a little poor but a good if a little long winded summary of mariculture.

    Reply

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