| | | 17.7 Controlled cultural eutrophication and aquaculture |
Agriculture has been the backbone of human existence that created the base for the 3 orders of magnitude increase in human population. While intensive agriculture often is considered negative for the productivity in adjacent aquatic environments (e.g. too high nutrient supply, major deviations in nutrient composition, large-scale modifications of water sheds and water supplies) the exploitation of aquatic ecosystems in Europe has until recently been dominated by various forms of fish and shellfish. Compared to agriculture, fishery is still based on the hunter and picker approach that agriculture left several thousand years ago: wild resources are exploited rather than cultivated. While various aquaculture techniques, often in combination with agriculture, have been widely applied in many countries (e.g. China and Japan), aquaculture in sea- or landÐbased enclosures first developed into a major economy in European countries in recent decades. Aquaculture can now be of similar or even greater significance than natural fish and shellfish catches. The majority of marine fish aquaculture depends on feed that derives from natural fish resources. The supply of nutrients that causes eutrophication in coastal regions may support a higher fishery, but this effect has not been quantified. One problem is that the nutrients are not added in close to Redfield ratio proportion and that pollutants such as heavy metals, pesticides and detergents are dumped in concert with the nutrients. For a general discussion of marine production and seafood, see Mariculture Chapter.
Is it possible to go leave the `hunter and picker stage' of fisheries and introduce aquaculture in the meaning of agriculture, i.e. fertilising a region, manipulate the organisms and channel the nutrients into crops that are harvestable items of human food consumption? Can we control the fertilisation of aquatic recipients by using a limiting resource (i.e. nutrients), which at present is dumped into what we wish to be pristine regions or can we generate a controlled fertilisation of certain regions by adding fertilisers? Could we turn the waste of resources that result in eutrophication into a benefit, resources that benefit humans? The lack of knowledge how nutrients are channelled through manipulated food webs prevents extensive sustainable aquaculture in the foreseeable future. However, it is important to investigate the base for a future aquaculture (see Chapt. olesen), in parallel with the development of agriculture that took place several thousand years ago. There is no reason to assume that aquatic environments are in essence so different compared to terrestrial environments that significant aquaculture should be out of question. In particular not in a world that is short in food.
To build up a modern aquaculture know-how that would allow extensive aquaculture if the need arises would be a good investment in future prosperity of coastal populations. This knowledge would simultaneously contribute to a better understanding of eutrophication. Some attempts to obtain such knowledge have been already obtained by the MARICULT programme (see Mariculture Chapter). To endeavor a controlled fertilization of coastal waters (in contradiction to our current uncontrolled experiment) to be subsequently utilized by fisheries or extensive aquaculture is an approach that has not found wide acceptance. On the contrary, the common attitude is rather to prevent eutrophication of coastal water or dump nutrients at depth. This attitude is opposed to use the already and in most poor regions of the world perpetuate eutrophied coastal waters for extensive aquaculture, in analogy to agriculture. There exists a conflict between the wish to (a) experience non-eutrophied coastal waters, (b) to use these waters eventually for aquaculture and fertilize them and (c) todays approach to dump nutrient at large-scale without considering (a) or (b). It can be questioned if option (b) is the best manner to cope favorably with the negative aspects of eutrophication. This in order to support the sustainability of the coastal zone that is rapidly decreasing in many regions (e.g. some estuaries and fjords, the southern North Sea or the `death zone' in the Gulf of Mexico).
In Japan where aquaculture has played a vital role throughout the last thousand years examples exists showing that eutrophication and aquaculture can co-exist without destroying the long-term integrity of aquatic ecosystems. Edo, the capital of Japan during the Tukagawa regime (at present Tokyo), was densely populated and transported manure from the city to fields outside the town. Also, they harvested the undoubtedly eutrophicated bight outside the city through extensive aquaculture establishments (e.g. scallops, fish, seaweed etc.). The discharge and cycling of biomass and nutrients was so balanced that no negative episodes of eutrophication (e.g. HAB, anoxic bottom water) have been reported. The wisdom concealed in the case of Edo reflects a balanced solution, in a setting where resources (here nutrients/food) were strictly limited. The example shows that sustainable development is possible if appropriate techniques are applied. In many respects eutrophication functions like the waste of a limited resource, in this case nutrients. In a phase of our development characterised by excessive use of resources, phenomena such as eutrophication are difficult to omit. In times to come, when the negative consequences of our resource mismanagement will become difficult to cope, and resource and food shortage are difficult to deal with, the waste of nutrients and the accompanied negative effects have to be carefully evaluated. Extensive aquaculture combined with recycling of nutrients back to arable land may be one option to alleviate this conflict.
| | | 17.7 Controlled cultural eutrophication and aquaculture |