5 Non-Native Species in Aquaculture
167
Guangxi and Yunan from the Yangtze River system, has severely affected and has
contributed to the extinction of some local finfish species (Li and Xie 2002). The
Nile perch Lates niloticus was introduced into Lake Victoria in the 1960s apparently causing the extinction of many cichlids species – viewed as the biggest
vertebrate extinction of the 20th century (Witte et al. 2000). However, Gurevitch
and Padilla (2004) suggest that development of the railroad in the 1920s caused
erosion and shoreline destruction (Verschuren et al. 2002) and urbanization during
the 1970s increased eutrophication and decreased lake transparency from 8 to
1.5 m (Verschuren et al. 2002; Aloo 2003). Increased nutrient loading and anoxic
events resulting in fish kills are now common. The increase in nutrient loads, however, has favoured the non-native water hyacinth Eichhornia crassipes, which
alters nursery areas for juvenile fish (Witte et al. 2000).
Species in the marine environment are typically considered to have a lower risk
of extinction because of the large continuous habitats they occupy and the life history characteristics of many species that results in extensive dispersal potential
enabling the recolonisation and repopulation of impoverished areas (Gurevitch and
Padilla 2004). Caution should be taken, however, as this perception was derived
from experiences when marine populations were much larger than they are today
(Dulvy et al. 2003) and when the current rate of exploitation of marine species and
the level of associated by-catch of non-target species was significantly lower
(Worm et al. 2006). Unintentional introductions of non-native aquaculture species
are likely to increase with the rapid expansion of the aquaculture industry on a
global scale and there is an urgent need for more research into the role of non-native
species in pushing native species towards extinction and to evaluate their impact
relative to that of other factors (Gurevitch and Padilla 2004).
5.3.4 Disease Transfer
To be economically viable, cultivation of a species must normally take place at a
high density either within contained units (on account of the capital costs of the
equipment) or as bottom culture on shores (where space may be limited). Under
these conditions introduced pests, parasites and diseases are provided with increased
opportunities to thrive (Minchin and Rosenthal 2002).
There are many cases of stock movements introducing unwanted pests, parasites and diseases and some have had serious economic impacts on aquaculture
production, for example, oysters (Heral 1990), shrimp (Kinne 1984; Sindermann
1993) and fishes (Kinne 1984). For example, the trematode Gyrodactylus salaris
was carried with Atlantic salmon Salmo salar from Swedish hatcheries to Norway
(Johnsen and Jensen 1991) and resulted in serious salmon mortalities in the
recipient region.
Movements of harmful biota over larger distances, however, are more common.
For example, consignments of half-grown Pacific oysters have resulted in a large
suite of invertebrates being spread throughout the world (Gruet et al. 1976), with
167
Guangxi and Yunan from the Yangtze River system, has severely affected and has
contributed to the extinction of some local finfish species (Li and Xie 2002). The
Nile perch Lates niloticus was introduced into Lake Victoria in the 1960s apparently causing the extinction of many cichlids species – viewed as the biggest
vertebrate extinction of the 20th century (Witte et al. 2000). However, Gurevitch
and Padilla (2004) suggest that development of the railroad in the 1920s caused
erosion and shoreline destruction (Verschuren et al. 2002) and urbanization during
the 1970s increased eutrophication and decreased lake transparency from 8 to
1.5 m (Verschuren et al. 2002; Aloo 2003). Increased nutrient loading and anoxic
events resulting in fish kills are now common. The increase in nutrient loads, however, has favoured the non-native water hyacinth Eichhornia crassipes, which
alters nursery areas for juvenile fish (Witte et al. 2000).
Species in the marine environment are typically considered to have a lower risk
of extinction because of the large continuous habitats they occupy and the life history characteristics of many species that results in extensive dispersal potential
enabling the recolonisation and repopulation of impoverished areas (Gurevitch and
Padilla 2004). Caution should be taken, however, as this perception was derived
from experiences when marine populations were much larger than they are today
(Dulvy et al. 2003) and when the current rate of exploitation of marine species and
the level of associated by-catch of non-target species was significantly lower
(Worm et al. 2006). Unintentional introductions of non-native aquaculture species
are likely to increase with the rapid expansion of the aquaculture industry on a
global scale and there is an urgent need for more research into the role of non-native
species in pushing native species towards extinction and to evaluate their impact
relative to that of other factors (Gurevitch and Padilla 2004).
5.3.4 Disease Transfer
To be economically viable, cultivation of a species must normally take place at a
high density either within contained units (on account of the capital costs of the
equipment) or as bottom culture on shores (where space may be limited). Under
these conditions introduced pests, parasites and diseases are provided with increased
opportunities to thrive (Minchin and Rosenthal 2002).
There are many cases of stock movements introducing unwanted pests, parasites and diseases and some have had serious economic impacts on aquaculture
production, for example, oysters (Heral 1990), shrimp (Kinne 1984; Sindermann
1993) and fishes (Kinne 1984). For example, the trematode Gyrodactylus salaris
was carried with Atlantic salmon Salmo salar from Swedish hatcheries to Norway
(Johnsen and Jensen 1991) and resulted in serious salmon mortalities in the
recipient region.
Movements of harmful biota over larger distances, however, are more common.
For example, consignments of half-grown Pacific oysters have resulted in a large
suite of invertebrates being spread throughout the world (Gruet et al. 1976), with