24 Fish as feed inputs for aquaculture – Practices, sustainability and implications
With the exception of the industrial shrimp trawl fishery, most Asian fisheries have
low discard rates, as most are small-scale, short-trip ventures with any bycatch being
landed for trash/low-cost fish use in aquaculture and livestock feeds. An arbitrary
discard rate of 1 percent has been assigned to the fisheries of Thailand, Malaysia and
Cambodia, which are considered to generate combined discards of less than 50 000
tonnes (Kelleher, 2005). Similarly the fisheries of Viet Nam and China are considered
to have insignificant discards.Seabirds
The methods used for catching fish species depend on the behaviour of the fish. Many
fish species shoal, and small-mesh trawls and gillnets are used to capture them. Many
of the feed-fish fisheries use trawls, and birds are less likely to be caught by this type
of gear (Tasker et al., 2000). A study in the Baltic Sea assessing the bycatch of common
guillemot (Uria alga) indicated that a small unquantified degree of bird mortality could
be attributed to trawls, but the researchers did not identify the trawls as specifically
targeting an industrial fish species (Österblom, Fransson and Olsson, 2002). Bycatch of
birds is potentially an issue in the purse-seining for anchovy, but the level of interaction
is little researched (Majluf et al., 2002).
Seabirds are long-lived, producing few fledglings that breed only if they survive for
several years, and normally have various mechanisms to overcome periods of low food
supply. Specialist seabirds, such as small, surface-feeding species with energetically
expensive foraging methods are the most vulnerable to local depletion and (natural)
variability in prey availability. The relationship between the reproductive success of
black-legged kittiwakes on Shetland and sand-eel abundance has been proposed as
an indicator of local sand-eel availability in the North Sea (ICES, 2003a). Potential
conflicts between fisheries and seabirds are likely to arise only on a local or regional
scale (Tasker et al., 2000). Industrial fisheries can affect seabirds by reducing prey stock
biomass, leading to declining recruitment or alterations in the food-web structure.
Although seabirds consume only an insignificant proportion of North Sea sand-eel
stocks compared with fish predators (Bax, 1991; Gislason, 1994; ICES, 1997), this
relationship is sensitive to the population levels of key predators such as mackerel and
gadoids, therir levels are currently low in the North Sea.
A classic example of how the removal of large quantities of feedfish by industrial
fisheries might reduce food supply to seabirds has been reported in Peru. Extrinsically
driven dramatic decreases in numbers of guano seabirds occur regularly during El
Niño events, but historically, species were shown to recover between events, showing
cyclic fluctuations in populations. However, as the Peruvian anchovy fishery activity
increased, seabird numbers began to fail to recover after El Niño-driven crashes, and
the seabird population fell to only a small fraction of its earlier numbers (Duffy, 1983).
Jahncke, Checkley and Hunt (2003) modeled the guano-producing seabirds (cormorant,
Phalacrocorax bougainvillii; booby, Sula variegata; and pelican, Pelecanus thagus) that
feed almost exclusively on Engraulis ringens to determine if there is a response in the
annual population size of the birds to changes in primary and secondary production
of the Peruvian upwelling system. The seabirds were shown to respond positivily to
the increased productivity of the Peruvian upwelling system, and declines in seabird
abundance after El Niño events were likely due to competition with the fishery for
food.(^5) Weighted discard rate (%) = [Summed discards (tonnes) x 100) / (Summed discards + summed
landings (tonnes)]