Environmental Engineering Reference
In-Depth Information
use the total catch of 93 Mt (average of 1998-2002 rates) augmented by about
17 Mt (18% of the reported total) representing illegal landings, and 8 Mt of dis-
carded by-catch. When conservatively converted (12% C in fresh weight), 118 Mt
of aquatic zoomass are equal to about 14 Mt C, and with the average trophic index
at 3.3 and the mean transfer efi ciency at 10%, the production of 118 Mt of aquatic
zoomass would have needed at least 2.8 Gt C of phytoplankton and aquatic plants.
With a global aquatic NPP of about 50 Gt C, this would have equaled nearly 6%
of the primary marine productivity.
Calculating the global claim made by the rising aquacultural production would
be fairly simple if the output were limited to the traditional pond cultures of
Asian and European cyprinids feeding on natural phytoplankton and detritus or the
maricultures of mollusks capturing their feed from coastal waters by i ltration and
hence feeding near the bottom of their respective trophic webs. The dominant species
of cultured carps (including grass and silver carp) are strict herbivores (trophic
level 2), while the common carp feeds at levels ranging from 2.1 to 2.8 and the
most popular species of mollusks (mussels, oysters, clams, scallops) are herbivores
and detritivores (level 2). But modern aquaculture uses mass-produced commercial
plant-based feeds to increase the efi ciency of herbivorous i sh production and has
been producing increasing amounts of carnivorous species (above all salmon and
shrimp, in the Mediterranean mainly sea breams and sea basses) whose feeding
requires protein derived from captured wild i sh and marine invertebrates processed
into i sh meals and oils.
As a result, the mean trophic level of the Mediterranean marine farmed species
rose from 2 during the 1970s and 1980s to about 3 by 2005 (Stergiou, Tsikliras,
and Pauly 2008). Moreover, some producers used to increase the weight of herbivo-
rous and omnivorous species by supplementary (up to 15% of all energy) rations
of i sh meals and oils (Naylor, Williams, and Strong 2000). But both of these feed
streams have already been accounted for: plant-based aqua-feeds claim just a small
share of cereal, leguminous, and oil crop harvests, while i sh-based feeds claim a
signii cant share of i sh landings (particularly such small pelagic species as anchoveta
and sardines) that are not eaten by humans.
According to the FAO (FAO 2011a), 2006 was the i rst year when worldwide
aquaculture produced more than 50 Mt of i sh and marine invertebrates (it was
about 35 Mt in 2000), and nearly 75% of that output was as species that consume
feeds. As a result, the production of aqua-feeds increased from just 4 Mt in 1994
to about 20 Mt by 2006, and Tacon (2008) estimated that in that year, nearly 70%
of all i sh meal and almost 90% of i sh oil were consumed by farmed species (about
