80
M. Holmer et al.
exposure, together with the size of the fish-farming operation and the farm management
programme at a specific locality, appear to be crucial in determining the magnitude
of the adverse impact. The results of the water quality monitoring programmes for
sea bream and sea bass farms did not indicate any large adverse changes in water
quality attributes resulting from the fish-farming activities.
There is currently only a low level of production of sea bream and sea bass in
Malta, as the attention of aquaculture operators is presently on tuna penning.
However, some operators still retain a permit to culture these species in addition to
penning tuna (Schembri et al. 2002), while it is likely that production of sea bream
and sea bass, as well as of additional species that are being introduced into aquaculture in the Mediterranean, will increase in the future, depending on the vagaries
of the market for tuna and other species, and as new operators enter the field and
wild tuna stocks dwindle. The environmental impact of any new (non-tuna) farms
is not likely to be as severe as that of the early sea bream and sea bass farms since
it is unlikely that such farms will be allowed to locate inshore or close to sensitive
habitats, particularly since the technology for siting farms in deep water now exists,
and because the environmental impact monitoring requirements of aquaculture
projects are nowadays much more rigidly enforced.
Overall, the results of environmental monitoring of tuna penning operations during the last 6 years (2000–2006) revealed a consistent pattern of a localised adverse
impact that mainly resulted from the uneaten feed-fish which accumulate on the
seabed during the tuna farming season (July to December). The amount of feed-fish
present decreases only when all the tuna have been harvested, following which, any
feed-fish remaining on the seabed continue to decompose slowly. These results are
characteristic of a “pulse disturbance” where the physical and biological characteristics of the seabed are temporarily altered during the tuna penning season but
return back to more or less the pre-disturbance condition before the start of the next
tuna penning season. Nonetheless, repeated accumulation of feed-fish on the seabed in the vicinity of the tuna pens may prevent complete recovery of the benthic
assemblages following each tuna penning season, potentially leading to a “press
disturbance” where environmental conditions become permanently altered.
The observed differences in the amount of feed-fish present on the seabed below
the cages indicate potential differences between different tuna farms and/or cages
within the same farm in: (1) feed management, or (2) the rate of food intake by the
tuna, or a combination of (1) and (2). It appears that the key to preventing this from
happening is to implement a rigorous feed-management strategy that includes:
●
careful monitoring of the feeding behaviour of the tuna and stopping the supply
of food as soon as the tuna are satiated in order to avoid as much as possible
uneaten food ending up on the bottom; and
●
removal of dead uneaten feed-fish from the bottom should inordinate amounts
accumulate below the cages either due to overfeeding or to accident.
On the other hand, the results of recent (2004–2005) monitoring surveys indicated
an overall large improvement in feed-management at local tuna farms. For example,
M. Holmer et al.
exposure, together with the size of the fish-farming operation and the farm management
programme at a specific locality, appear to be crucial in determining the magnitude
of the adverse impact. The results of the water quality monitoring programmes for
sea bream and sea bass farms did not indicate any large adverse changes in water
quality attributes resulting from the fish-farming activities.
There is currently only a low level of production of sea bream and sea bass in
Malta, as the attention of aquaculture operators is presently on tuna penning.
However, some operators still retain a permit to culture these species in addition to
penning tuna (Schembri et al. 2002), while it is likely that production of sea bream
and sea bass, as well as of additional species that are being introduced into aquaculture in the Mediterranean, will increase in the future, depending on the vagaries
of the market for tuna and other species, and as new operators enter the field and
wild tuna stocks dwindle. The environmental impact of any new (non-tuna) farms
is not likely to be as severe as that of the early sea bream and sea bass farms since
it is unlikely that such farms will be allowed to locate inshore or close to sensitive
habitats, particularly since the technology for siting farms in deep water now exists,
and because the environmental impact monitoring requirements of aquaculture
projects are nowadays much more rigidly enforced.
Overall, the results of environmental monitoring of tuna penning operations during the last 6 years (2000–2006) revealed a consistent pattern of a localised adverse
impact that mainly resulted from the uneaten feed-fish which accumulate on the
seabed during the tuna farming season (July to December). The amount of feed-fish
present decreases only when all the tuna have been harvested, following which, any
feed-fish remaining on the seabed continue to decompose slowly. These results are
characteristic of a “pulse disturbance” where the physical and biological characteristics of the seabed are temporarily altered during the tuna penning season but
return back to more or less the pre-disturbance condition before the start of the next
tuna penning season. Nonetheless, repeated accumulation of feed-fish on the seabed in the vicinity of the tuna pens may prevent complete recovery of the benthic
assemblages following each tuna penning season, potentially leading to a “press
disturbance” where environmental conditions become permanently altered.
The observed differences in the amount of feed-fish present on the seabed below
the cages indicate potential differences between different tuna farms and/or cages
within the same farm in: (1) feed management, or (2) the rate of food intake by the
tuna, or a combination of (1) and (2). It appears that the key to preventing this from
happening is to implement a rigorous feed-management strategy that includes:
●
careful monitoring of the feeding behaviour of the tuna and stopping the supply
of food as soon as the tuna are satiated in order to avoid as much as possible
uneaten food ending up on the bottom; and
●
removal of dead uneaten feed-fish from the bottom should inordinate amounts
accumulate below the cages either due to overfeeding or to accident.
On the other hand, the results of recent (2004–2005) monitoring surveys indicated
an overall large improvement in feed-management at local tuna farms. For example,