Conservation and Sustainable Use of the Biosphere - WBGU

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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.

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