Conservation and Sustainable Use of the Biosphere - WBGU

Sustainable food production from aquatic ecosystems E 3.4
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tions of 100–10,000 individuals, the genetic diversity
and the survival of some species of baleen whales are
immediately endangered.
E 3.4.1.1
Scientific foundations for safeguarding utilizable
populations
The starting point for combating overfishing and for
the regeneration of fish populations is the determination
of sustainable fish yields.Whereas the technological
options for fishing have continuously
improved in the last few decades, biological and ecological
data on the subjects being caught are still
largely inadequate.
The marine food chain
Although the oceans cover 71 per cent of the Earth’s
surface, only around half of the organic matter
formed on the Earth is produced in them, around
45–60Gt carbon per year (Longhurst et al, 1995).This
means that on average the oceans produce only
around half as much per unit area as the land. The
food chain comprises the microalgae (phytoplankton)
floating in the water as primary producers, phytoplankton-eating
zooplankton, fish, predatory fish,
squid and whales. However, with every transfer stage
within the food chain, 80–90 per cent of the energy
absorbed with the food is lost. Since plankton algae
in nutrient-rich, and thus production-rich, buoyant
areas are usually larger than those in low-production
areas far from the coast, the food chain here is
shorter and therefore the total transfer-efficiency
rate from the microalgae to the fish is up to 4 per cent
higher than in waters far from the coasts, where is it
is usually well below 1 per cent (Lalli and Parsons,
1997).
Determining sustainable fish yields
Because of the large difference in the structure of
marine food chains, only rough statements – based on
assessments of the ocean’s primary productivity –
can be made about potential mean fish yields, and
only for limited marine areas, usually based on correlation
analyses. However, in individual cases these do
not provide a sufficient foundation for the management
of fish populations. Sustainable fish yields can
only be achieved if the natural rate of increment at
least balances out the total mortality of the fished
population. Total mortality is made up of natural
mortality (of which only a rough assessment is possible)
and fisheries-related mortality. In the case of fish
species with large number of young (high-recruiting),
high fisheries mortalities can be tolerated, without
endangering the continued existence of a population.
The recruitment and, thus, the annual size of populations
available for fishing are subject to strong interannual
fluctuations (Box E 3.4-1).These annual sizes
therefore have to be identified and considered for
sustainable fishing quotas to be set. This requires
considerable technical, logistical and administrative
effort. Unfortunately, data availability on the population
development and recruitment of important commercial
fish species has deteriorated in recent years,
due to cuts in research funding.
Box E 3.4-1
Why do the annual sizes of fish populations
fluctuate?
The reasons for the major fluctuations in the annual sizes of
commercial fish stocks have not yet been identified, in spite
of intensive research efforts.This knowledge deficit is one of
the main reasons why the long-term forecasts of population
sizes and, thus, the determination of long-term sustainable
fishing quotas has not been possible to date. Most bony fish
produce very large numbers of eggs or young larva. But usually
only a very small proportion (at most just a few per
cent) of the hatched young reach sexual maturity. Relatively
small fluctuations in the mortality of the larvae therefore
have a great impact on the population size of the young fish
concerned (so-called recruiting). The following attempts at
explaining the variability of recruiting have been made so
far (Lalli and Parsons, 1997):
• Hunger hypothesis: First of all, fish larvae eat the yolks of
their eggs before they start to feed. If no food is available
then, the young larvae die off.
• Predator hypothesis: Below a certain minimum size
young fish larvae are easily eaten, there is then a high
mortality rate in the populations. With unfavourable
feeding conditions the fish grow more slowly than when
the feeding conditions are favourable. For this reason,
the time in which they can be eliminated by being eaten
is longer.
• Advection hypothesis:As a result of sea currents fish larvae
can be drifted into areas abundant in food – or poor
in food – and recruiting reflects this.
• Growth hypothesis: In higher water temperatures the
fish reach sexual maturity sooner with a smaller body
size. Moreover, their growth depends on the nutrient
value of the food (above all the protein content) and
their metabolic intensity (eg the energy required for
acquiring food). A change in the food spectrum therefore
generates different growth rates that have an
impact on the fish yields that can be achieved.
Experimental findings support all the hypotheses that do
not necessarily rule each other out.A great deal of research
is needed to clarify these questions that are essential to fisheries
and to draw up forecasts for the longer term.