World Aquaculture - December 2014

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60 DECEMBER 2014 • WORLD AQUACULTURE •
Potential Effects of Cultural
Eutrophication on Cage Culture
in Lakes and Reservoirs in Nigeria
Moshood Mustapha
Freshwater cage culture is an aquaculture production system
where fish are grown from fry to table size in cages or enclosures that
are fixed, floating or submerged in lakes, reservoirs, rivers or streams.
Cages are usually enclosed on all sides with mesh netting and the
complete system often includes materials such as PVC pipes, bamboo,
wood, used tyres, plastic or steel drums, weights and anchors (mooring)
and ropes. Cage fish culture involves simple technology in operation
and management and can use locally available and cheap materials
for construction. Water exchange occurs between the water body and
the cages.
Cage culture has existed for many centuries in several countries,
especially in Asia (Beveridge 2004), but is now becoming widespread
in other countries, especially in sub-Saharan African countries like
Nigeria, where large bodies of fresh water abound. The expanion of
cage culture can be attributed to the high demand for fish and increased
competition for available resources faced by the existing aquaculture
sector (Foley et al. 2005).
There are numerous advantages of cage culture. Cages use limited
space in existing water bodies and thus eliminate the need to buy land.
Compared to pond culture, the investment or capital needed to con-
struct facilities is relatively low. Production capacity is high arising from
high stocking densities; production in cages can be as much as 20 times
greater than in pond culture (Das et al.2009). Unwanted recruitment,
especially in tilapia culture, can be controlled. Eliminating losses from
predation, simple methods of harvesting, observation and sampling of
fish, and quarantine and disease treatment is rapid and easy. The system
is viable, economical and conserves the fishery of the water body in
which it is sited. Above all, cage culture can contribute to the liveli-
hoods of people through employment, income generation, poverty alle-
viation and provision of low-cost fish protein, ensuring food security.
With these advantages, the success of freshwater cage culture
depends largely on the water quality of the water body in which the
system is sited. Water quality includes all physical, chemical and
biological factors of water that influence the beneficial use of that
water for various purposes. Thus, water quality dynamics must be
taken into account to conformwith the requirements of the species
cultured. Cage culture leaves the fish susceptible to prevailing
physicochemical and biological conditions in the water body.
One of the challenges of freshwater cage culture in Nigeria is
deteriorating water quality stemming from cultural eutrophication of
lakes and reservoirs. Eutrophication from high nutrient loading is one
of the most important causes of water quality deterioration and the
consequent decline and collapse of fish populations and production
in lakes and reservoirs (Allan et al. 2005, Jones-Lee and Lee 2005,
Mustapha 2008, 2011). The focus of this article is to examine the
potential effects of cultural eutrophication on the emergence of cage
culture in lakes and reservoirs of Nigeria and to offer suggestions to
mitigate possible effects.
The Threat of Cultural Eutrophication
Cultural eutrophication is the anthropogenic increase in
loadings of nutrients, especially phosphate and nitrate, into water
bodies. It also occurs through human alteration of the physical
and biogeochemical conditions of the watershed of a lake or
reservoir. Phosphate and nitrate limit the growth of phytoplankton
and aquatic macrophytes and thus have significant impact on the
trophic status and productivity of lakes and reservoirs. Freshwater
lakes are more vulnerable to ecological changes caused by inputs
of phosphorus than nitrogen (Rojas andWadsworth 2007). When
nutrient concentration increases from external loading, excessive
phytoplankton and macrophyte production often results, leading to
water quality problems.
Human activities that lead to cultural eutrophication of Nigerian
lakes and reservoirs include bank erosion, urban runoff, agricultural
runoff of fertilizers, washing and bathing with phosphate-based
detergents and soaps, and runoff from concentrated livestock
operations, all regarded as non-point sources. Point sources include
discharges fromwastewater treatment and industrial facilities. Non-
points source nutrient inputs from the watershed are the leading
causes of cultural eutrophication and water quality problems in
lakes and reservoirs (Carpenter et al. 1998, Mustapha 2009). The
rainy season often exacerbates cultural eutrophication from non-
point sources during the rainy season in many Nigerian lakes and
reservoirs.
Effects of Cultural Eutrophication on Lake and
ReservoirWater Quality that Affect Fish in Cages
There are numerous potential effects of cultural eutrophication
on water quality of lakes and reservoirs where fish cages are sited
and that could negatively impact fish growth and production.
Eutrophication increases the risk of dissolved oxygen depletion,
potentially leading to severe mortality events in cages due to
confinement at high densities. Eutrophication can increase
biofouling of cage nets, restricting water exchange and oxygen
supply. Biofouling can cause the weight of nets to double, reducing
cage buoyancy (Piccolotti and Lovatelli 2003).
Erosion in the watershed can bring excessive suspended
inorganic (mineral) matter, causing gill irritation to fish, causing
stress that can lead to disease outbreaks in caged fish. High turbidity
can lead to stunting of cultured fish populations (Lee and Jones-Lee
1991). High turbidity also reduces the ability of sight feeders, such
as many carnivorous species, to locate feed.
Some algae, especially blue-green algae, produce metabolites
that cause off-flavor in cultured fish. These off-flavors can cause
economic losses because harvested fish are not acceptable by
consumers. Although rare, some blue-green algae excrete toxins
that can kill fish (Jones-Lee and Lee 2005).