Tiny floating phytoplankton (microscopic algae) are the basis
of most marine productivity. These convert sunlight and water-borne
nutrients into plant matter. These are fed upon by the tiny animals
called zooplankton. Small fish feed on both and larger fish feed
upon them in turn. This is a simple food chain with four trophic
(feeding) levels. The plants are the primary producers at the base
level. The herbivores that eat the plants are at level two. The
carnivores that eat the herbivores are at level three .The top
carnivores are at level four.
However, there are only a few such simple straight food chains.
Most connections are much more complex, forming a food web. The
links are intricate and vary according to life stage. For example,
a larval cod may feed upon eggs and larvae in the zooplankton while
becoming prey for both juvenile cod and adult herring.
If man removes, or seriously depletes by fishing, a major part of
the web, it is likely that several other components in the rest of
the web will be altered. This might be either through depletion of
food of a predator, or removal of predators, or through removal of
a competitor. Fishing usually affects a number of components in the
marine food web, so the food web effects of fishing are therefore
complex and, usually, it is difficult to prove cause and effect.
Herring © Chris
Martin/SNH
The North Sea provides a good example. The quantities of mackerel
and cod, two of the main fish predators in the North Sea, have
decreased steeply in the past 50 years, as has the quantity of one
of their main planktivorous (plankton eating) prey – herring. These
changes have primarily been caused by fishing. On the other hand,
there is reasonable evidence that quantities of sandeel (another
plankton-eater) have increased (but this is not certain due to the
difficulties of surveying this species). Some sandeel predators,
such as common guillemots and grey seals, have increased greatly
over this same period. It might thus be reasonably suggested that
the increases in sandeel, guillemots and seals are a consequence of
the reduction, caused by fishing, in their fish predators and
competitors.
Such effects are likely to prove difficult to reverse, if such
reversal is possible at all, without a great decrease in fishing
pressure on the depleted stocks. Even then, it is likely that both
competitive and predatory relationships in the food web have
changed and these may not revert to their earlier condition. Cod
stocks provide a good example. Full-sized cod prey on smaller
species such as herring that in turn compete with small cod.
However, fishing has reduced the number of older, larger individual
cod present.
Herring © Chris
Martin/SNH
Cod can live for approximately 40 years, however, due to high
fishing pressure, 90% of the cod in the North Sea are only one or
two years old and less than 0.5% are five years old or more.
Without many adult cod to reduce competition between small cod and
other species, it may be that stocks of cod can never recover to
their former sizes. In addition, this skewed age structure also
leads to the selection of smaller, faster maturing individuals. The
long term affects of such genetic drift are unknown, but may reduce
resistance to further environmental changes.