94
T. Dempster and P. Sanchez-Jerez
While such nutrient inputs may have impacts at the local scale, the overall impact
of increased nutrient loading from aquaculture may be unnoticeable at a macroscale. Mediterranean aquaculture produces little detectable increase in nutrients in the
entire Mediterranean, compared to the input of nutrients from other anthropogenic
activities and from atmospheric and terrestrial sources (Karakassis et al. 2005).
Similarly, the input of nutrients from fish-based aquaculture along the Norwegian
coast represents only a small proportion of the overall nutrient budget of the coastal
ecosystem (Ervik and Aure 2006). At the local scale, some studies have indicated that
seasonal variation of nutrients can be more important that differences between impact
and control locations, indicating that farm activities do not always produce a detectable
increase of nutrients. Maldonado et al. (2005) found that neither surface nor bottom
waters at the fish farms showed abnormal concentrations of nitrite and nitrate relative
to controls. This result was unlikely to be the result of uptake by phytoplankton, as
chlorophyll a values under the fish cages were low relative to control sites. Ruiz et al.
(2001) found that the major differences in nitrate and nitrite concentrations along the
SE coast of the Iberian Peninsula were due to seasonal changes in the environment
rather than caused by fish-farming activities. Therefore, at a local scale, the probability
of individual fish farms at their current sizes affecting themselves and their immediate
environment is low (Pitta et al. 1998; Karakassis et al. 2005).
Urban development and human pressure in coastal areas can likewise affect aquaculture. Anthropogenic nutrients from waste water and agriculture run off are responsible
for a large component of the nutrients that cause marine eutrophication (Costanzo et al.
2001). If sewage treatment for coastal cities is inadequate, the introduction to coastal
areas of insufficiently treated waters can have large adverse effects, especially during
summer when water temperatures are high. For example, eutrophication can affect fish
production directly by reducing dissolved oxygen (Page et al. 2005). Alternately,
decreases in primary production in coastal areas are also possible if freshwater runoff is
dramatically reduced, which may affect coastal oyster and mussel production. The
Marennes-Oléron Bay region in France is a major oyster and mussel production area.
A decrease in the amount of freshwater entering the bay, due to a fourfold increase in
the amount of irrigated land in the catchment area combined with greater water usage
on a per hectare basis, led to reduced nutrient supply to coastal culture sites of oysters
and mussels. This resulted in decreased survival rates of oyster and mussel spat which
require reduced salinity (see review by Goulletquer and Le Moine 2002).
Input of nutrients and pathogens from sewage to the marine environment can
reduce the health of cultured fish. Mortalities of cultured fish have resulted from
Vibrio harvey (Saeed 1995) infections. Streptococcus sp. infections have also
been responsible for mortality of wild mullet associated with aquaculture.
Streptococcus sp. could originate from terrestrial or aquatic sources, and the
associated increase in nutrient loading around aquaculture sites may allow indigenous Streptococcus sp. to flourish. However, streptococcal species typically
introduced by sewage are not pathogenic to fish (Gilbert et al. 2002). Important
human health concerns exist with regard to culture of shellfish in coastal waters,
since sewage contains pathogens which can contaminate shellfish and may be
passed on to the consumer; several diseases can be transmitted through human
ingestion of contaminated shellfish (Hill 2005).
T. Dempster and P. Sanchez-Jerez
While such nutrient inputs may have impacts at the local scale, the overall impact
of increased nutrient loading from aquaculture may be unnoticeable at a macroscale. Mediterranean aquaculture produces little detectable increase in nutrients in the
entire Mediterranean, compared to the input of nutrients from other anthropogenic
activities and from atmospheric and terrestrial sources (Karakassis et al. 2005).
Similarly, the input of nutrients from fish-based aquaculture along the Norwegian
coast represents only a small proportion of the overall nutrient budget of the coastal
ecosystem (Ervik and Aure 2006). At the local scale, some studies have indicated that
seasonal variation of nutrients can be more important that differences between impact
and control locations, indicating that farm activities do not always produce a detectable
increase of nutrients. Maldonado et al. (2005) found that neither surface nor bottom
waters at the fish farms showed abnormal concentrations of nitrite and nitrate relative
to controls. This result was unlikely to be the result of uptake by phytoplankton, as
chlorophyll a values under the fish cages were low relative to control sites. Ruiz et al.
(2001) found that the major differences in nitrate and nitrite concentrations along the
SE coast of the Iberian Peninsula were due to seasonal changes in the environment
rather than caused by fish-farming activities. Therefore, at a local scale, the probability
of individual fish farms at their current sizes affecting themselves and their immediate
environment is low (Pitta et al. 1998; Karakassis et al. 2005).
Urban development and human pressure in coastal areas can likewise affect aquaculture. Anthropogenic nutrients from waste water and agriculture run off are responsible
for a large component of the nutrients that cause marine eutrophication (Costanzo et al.
2001). If sewage treatment for coastal cities is inadequate, the introduction to coastal
areas of insufficiently treated waters can have large adverse effects, especially during
summer when water temperatures are high. For example, eutrophication can affect fish
production directly by reducing dissolved oxygen (Page et al. 2005). Alternately,
decreases in primary production in coastal areas are also possible if freshwater runoff is
dramatically reduced, which may affect coastal oyster and mussel production. The
Marennes-Oléron Bay region in France is a major oyster and mussel production area.
A decrease in the amount of freshwater entering the bay, due to a fourfold increase in
the amount of irrigated land in the catchment area combined with greater water usage
on a per hectare basis, led to reduced nutrient supply to coastal culture sites of oysters
and mussels. This resulted in decreased survival rates of oyster and mussel spat which
require reduced salinity (see review by Goulletquer and Le Moine 2002).
Input of nutrients and pathogens from sewage to the marine environment can
reduce the health of cultured fish. Mortalities of cultured fish have resulted from
Vibrio harvey (Saeed 1995) infections. Streptococcus sp. infections have also
been responsible for mortality of wild mullet associated with aquaculture.
Streptococcus sp. could originate from terrestrial or aquatic sources, and the
associated increase in nutrient loading around aquaculture sites may allow indigenous Streptococcus sp. to flourish. However, streptococcal species typically
introduced by sewage are not pathogenic to fish (Gilbert et al. 2002). Important
human health concerns exist with regard to culture of shellfish in coastal waters,
since sewage contains pathogens which can contaminate shellfish and may be
passed on to the consumer; several diseases can be transmitted through human
ingestion of contaminated shellfish (Hill 2005).