Page 32 - World Aquaculture Magazine - March 2011

30 March 2011
Extensive molluscs production systems
Production of molluscs in inter-
tidal areas is made in parks within
their natural muddy/sandy substrate,
located in areas protected from strong
waves, and always under the influence
of tides, allowing good water renewal.
Most species are reared inmonoculture
systems due to different specificities.
Farmers use the traditional knowledge
together with some scientific support.
In these systems the management is
restricted to a skilful preparation of
the soil, protection from predators,
especially crabs, and algal removal. Technological inputs
are restricted to some areas and include only mechanical
harvest. Harvest is dependent on the tides and in some
protected areas a knife is the only instrument allowed. In
the spring tides, parks are exposed for harvesting during
four to five hours, being possible to catch up to 15 kg of
clams/person. On the other hand, during the neap tides,
sometimes harvest is not even possible. In Ria Formosa
the productivity has been dropping significantly. Before
1980, it was possible to catch between 20 to 50 kg of
clams per person per tide. In 2005 harvesting values had
already droped to 3 to 6 kg of clams per person per tide
(Associação de Produtores em Aquacultura do Algarve
– APAA – personal information). Although this decrease
has been attributed to the presence of Perkinsus marinus a
parasite which might causes high mortalities in the clams,
the area has also suffered of large sewage impacts from
anthropogenic sources which during the summer months
had contribute to anoxia problems in the farming beds. In
some sites mortality can reach 90%.
Oysters extensive rearing is usually performed in bas-
kets or net bags and allows high density production.
These structures can be positioned near the substrate in
trays within the photic zone. The oysters farming in bags
require a more demanding daily management. Routine
operation includes net cleaning to prevent the holes to
collapse by the excess of algal growth, since it is critical
to guarantee the water circulation inside and between the
different bags or baskets. Oysters regular grading also al-
lows a better production control and size selection along
the rearing phase until commercial size.
Some major constrains of these economic activities are of
legal nature, since most of these coastal areas are protected
and classified as nature reserves. In these sites, strict rules are
rigorously enforced, concerning technological improvements
and seed harvesting. Aquaculture practices in intertidal ar-
eas are always extremely dependent on the environmental
conditions and surrounding activities, such as agriculture,
factories, aquatic sports, or even other fish farms.
Aquaculture production in long-lines protects molluscs,
which are not settled in the bottom but suspended in the
water column, from predators incapable of swimming. The
molluscs extensive culture in long lines represents only a
small volume of Portuguese production, but is an activity
Fig. 5. Evolution of Portuguese aquaculture production systems in the period 2000 –
2008. Source INE.
ponds, although some intensive production in cages is now
beginning to be more relevant.
Each of these systems shows some specificity depending
on the geographical region, environment, species produced
and methodology used.
Figure 5 represents the evolution of the prodution meth-
odologies into systems with higher technological compo-
nents since intensive and semi-intensive systems have been
increasing. Despite the technical investments in semi-in-
tensive and intensive systems that have been made in past
years to improve productivity and enhance management
practices, several constrains are still delaying the further
development of aquaculture sector. The farming sites loca-
tion within natural protected areas restricts the expansion
and limits the production of established farms. The request
for new licences is a long and cumbersome bureaucratic
process, with several, local and central, governmental in-
stitutions involved. Coastal wetlands, which are under the
protection of Natura 2000 network for conservation (20%
of Portuguese territory), makes this process even more
complex. The increasing competition for coastal land use
with more profitable activities in the short term, like tour-
ism, is another constrain. However the major problem for
semi-intensive aquaculture in Portugal is that market dif-
ferentiation does not exist between products from intensive
and non intensive aquaculture, which affects farms profit-
ability, when intensive aquaculture products enter the mar-
ket at lower prices. All these problems and constrains also
result to a large extent from the very weak lobbying power
of the Portuguese aquaculture industry, associated with
the small dimension of the industry, but mostly the lack of
articulation between farmers and producers associations.
Actually a cluster of associations is under organization,
whose objective is reverse this situation, and put the farm-
ers speaking at one voice.
The positive effects of extensive and semi-intensive
aquaculture in coastal areas including environmental pro-
tection and restoration in areas of particular ecological in-
terest, employment opportunity and development in rural
and coastal areas, still has to be recognized by consumers,
in order to differentiate and support this production sys-
tems which are more environmentally and socially sustain-
able.