Limits to marine food production

Published on
December 11, 2020

A new study finds that making better use of the sea as a food source is difficult because the trophic efficiency of marine ecosystems is already high. The increase in food yield from the sea is possible by fishing at lower trophic levels, but this course of action requires a thorough risk assessment. Ecologist Jaap van der Meer published an article in the scientific journal Nature Food about the possibilities and limits of the marine ecosystem for food production.

The world population is growing steadily, and will reach 10 billion in a few decades. Researchers are looking hard for more sustainable food production strategies, such as nature-inclusive or circular agriculture, in order to continue to feed the growing world population. Attention is also increasingly focused on the seas and oceans, which currently produce only a paltry 1-2% of human food, but cover more than 70% of the Earth's surface. There are high expectations worldwide about the possibilities of increasing marine yields. Until now, however, these predictions have been based solely on estimates of the available area for mariculture, without taking into account the nutrients or food required for this. Could mariculture really provide so much more food than fishing, just as agriculture produces so much more food than hunting? To this end, Van der Meer studied the fundamental trophic ecology of land and sea.

Differences between land and sea

What is striking, when comparing the ecology of land and sea, is that the food chain at sea is much longer than that on land. While productivity at the base of the marine food chain is much lower. The so-called primary productivity of microscopic algae in the open ocean can be compared to that of plants in deserts. Nevertheless, predatory fish such as tuna live in the ocean, which also eat predatory fish themselves. In terms of its place in the food chain, tuna can be compared to eaters of lion eaters. It is inconceivable that they could occur on land. Despite the lower primary productivity, much more fish can be caught at sea than can be hunted on land.

The North Sea supplies about 5,000 kilograms of predatory fish per square kilometer every year, while sustainable hunting in the much more productive tropical rainforest amounts to no more than 130 kilograms of herbivores. This remarkable difference has everything to do with ecological efficiency. In nature on land, much more is lost at every step of the food chain. In a forest, almost all plant production disappears as dead material on the forest floor and is digested by fungi and bacteria. Very little plant material is eaten by herbivores. This is not the case at sea. The algae production is largely grazed by the so-called zooplankton, which in turn largely serves as food for fish.

On land, humans could make food production much more efficient through agriculture. By replacing the natural crops - which mainly benefited fungi and bacteria - with crops that are edible for humans or pets, ecological efficiency could be greatly increased. But at sea, that efficiency is naturally very high, which is why the potential of fish farming to get more food from the sea is very limited. Feeding farmed salmon from wild caught fish really does not contribute to an increase in overall food security. On the contrary.

Lower fish in the food web

At each step of the marine food chain, an average of 94% of energy and biomass is lost. So it could be rewarding to harvest at the lowest possible food level. Unfortunately, the microscopic algae or the less than a millimeter large zooplankton are practically impossible to harvest. Primary producers that grow on the coast, such as the multicellular seaweed, can be harvested. The same is true of coastal herbivores, such as the many bivalve species (mussels, cockles, oysters) that are suitable for human consumption. However, these coastal organisms naturally only occur in a fairly narrow strip. The only way to significantly increase their contribution to world food production is therefore through artificial constructions in the open sea.

Using the North Sea for fish farming does not make sense, according to researcher Jaap van der Meer. “Do not start or make sure that you can feed them lower down the food chain - for example based on seaweed. Producing more shellfish would be a much better alternative, because they are herbivores by nature and are therefore already low in the food chain. ”

More insight into the nutrient balance at sea is essential

Yet there are also risks associated with large-scale seaweed or shellfish farming on the open sea. The amount of nutrients is limited. Gaining more knowledge about marine nutrient balance and ecological efficiency, with or without "agriculture" management practices, and about harvesting at different trophic levels, is key to pursuing a form of marine circular agriculture, says Van der Meer. “Large-scale seaweed cultivation can have enormous consequences for the ecosystem, including fishing and biodiversity, apart from current issues concerning value added and production systems. Because there is limited phosphorus and nitrogen present in the sea, which is also consumed by micro-algae. Modeling will certainly help us to better understand this mutual competition between algae and micro-algae and to discover boundaries in the total food web at sea. ”