Wild fish and other aquatic organisms as feed in aquaculture in Europe 249
characteristics and the magnitude of other sources relative to aquaculture, and
internal processes such as uptake by phytoplankton, algae, internal recycling,
resuspension of fine material and uptake by biofouling communities that colonize
cage-farming areas. Eutrophication can alter the ratio between essential nutrients
(carbon: nitrogen: phosphorus), as well as absolute concentrations by causing a
shift in phytoplankton species assemblages. The possible interactions between
aquaculture and harmful algal blooms (HABs) are of considerable current
environmental and public interest in Europe. This relationship exists on two
levels: (i) the role of intensive finfish aquaculture in contributing to HAB events
through the ability of fish to input nutrients into the aquatic ecosystem through
uneaten food, faecal material and metabolic by-products; and (ii) the impact of
HABs resulting from wider anthropogenic and natural sources upon aquaculture
systems, especially cultured bivalves. Other studies have looked at the effects
of different shellfish and finfish excretion products on phytoplankton growth- shellfish excreta are generally stimulatory; finfish ammonia compounds are also
stimulatory, but other metabolic products may have an inhibitory effect (Arzul,
Seguel and Clément, 2001). - Sedimentation from faecal solids and uneaten food: Both finfish and shellfish
aquaculture produces particulate wastes that mainly result from the undigested
organic and inorganic elements of the feed materials. While land-based farms
are able to remove these elements from the system through the use of settlement
ponds and filtration, they are more difficult to control in cage farms. Particulate
loss occurs during finfish feeding, and wastes are usually found directly under
the net cages with relatively local impacts. The underlying sediments become
enriched with organic matter that degrades more easily than the natural
particulates in coastal areas. This may have important consequences for sediment
biogeochemistry, especially when microbial activity is engaged. In the marine
environment, sulphate reduction is among the most important mineralization
processes and is stimulated by enrichment with organic matter. This leads to an
increase in the production of sulphides, which may accumulate to levels toxic
for benthic fauna. In moderately enriched sediments, opportunistic species may
survive, but if enrichment is increased further, the fauna may disappear completely.
This leaves the degradation of waste products to microbes only, and such a change
is usually followed by increased burial rates of organic matter. It then becomes
very difficult for a climax benthic community to re-establish itself. The impact of
such sediment deposition may largely be limited to localized effects. However,
the change in such coastal benthic faunal communities may have consequences for
inshore nursery grounds. These are not necessarily negative, as juvenile stages may
benefit from faunal changes, as they are able to consume the copepods or annelids
favoured by organic enrichment.
The use of trash fish in European aquaculture is limited to tuna fattening in the
Mediterranean Sea. The Worldwide Fund for Nature (WWF) has noted that this has
had a number of undesirable impacts, such as increasing the fishing pressure for species
that were not previously fished commercially, such as the round sardinella in the
western Mediterranean Sea, with possible consequences for one of its main predators,
the common dolphin. In addition, they raise the possibility of transmitting viruses
from non-endemic feedfish to local wild fish populations, as has been experienced in
Australian waters (WWF, 2005).
5.2 Examples of environmental “best practice”
Intensive aquaculture in Europe has been driven to improve efficiency by a combination
of lower economic margins and an increasingly strict regulatory environment. This is
reflected by the very low FCRs now experienced in salmonid and seabass/seabream