22 Fish as feed inputs for aquaculture – Practices, sustainability and implications
- The use of fishmeal in poultry diets: As with diets for mammalian species,
fishmeal is considered a natural, balanced ingredient for poultry diets with high
protein, mineral and micronutrient contents. The protein in fishmeal is readily
digested by poultry, and it contains all the essential amino acids necessary for
adequate growth and production, especially the growth-limiting amino acid
lysine. However, as with pig diets, the quality of the fishmeal can seriously affect
protein digestion and biological value. Inclusion of fishmeal in poultry diets at
about 4 percent results in improved feed conversion efficiency and growth rates.
Laying performance is also improved by feeding fishmeal.
4. SUSTAINABILITY ISSUES OF REDUCTION FISHERIES AND FEEDFISH AS
INPUTS FOR AQUACULTURE AND ANIMAL FEED
4.1 Impacts of feed fisheries on ecosystems
4.1.1 Direct and indirect effects of feed fisheries
The removal of large numbers of fish from an ecosystem may directly impact their
prey, predators and the viability of target and bycatch populations. The physical effect
of fishing activity will also affect the ecosystem directly through the disturbance of
habitats (Auster et al., 1996; Langton and Auster, 1999) and the death and injury of
non-target species (Kaiser and Spencer, 1995).Feed-fish stocks
Feed-fish species caught for the production of fishmeal and fish oil are largely small
pelagic fish that forage low in the food chain and are preyed upon by fish, marine
mammals and seabirds at higher trophic levels. The population dynamics of many
small feed-fish species are characterized by their high fecundity and early maturity.
The recruitment patterns are highly variable and coupled with extrinsic environmental
drivers (such as sea temperature and associated climatic/hydrological patterns, e.g. the
North Atlantic Oscillation (NAO) and the El Nino in the southeastern Pacific Ocean)
may rapidly influence stock size due to the short lifespan of the species. This will
inevitably lead to uncertainty in the stock forecasts.
Most commercially exploited fish populations are capable of withstanding relatively
large reductions in the biomass of fish of reproductive capacity (Daan et al., 1990;
Jennings, Kaiser and Reynolds, 2001). However, the removal of extremely high
numbers of spawning stock may impair recruitment due to inadequate egg production.
This has been termed “recruitment overfishing” (Jennings, Kaiser and Reynolds, 2001).
Pelagic species are particularly vulnerable to this type of overfishing, as they are short-
lived (Lluch-Belda et al., 1989; Santos, Borges and Groom, 2001).
Beverton (1990) reviewed the collapse of stocks of small, short-lived pelagics by
examining the effect of fishing and natural extrinsic drivers. In four of the stocks
studied (Icelandic spring-spawning herring, Georges Bank herring, California sardine
and Pacific mackerel), the evidence indicated that each stock’s reproductive capability
had fallen, probably due to environmental conditions, but suggested that fishing
accelerated the collapse. Beverton (1990) concluded that although the likelihood of
harvesting small pelagic species to extinction was remote, a major population collapse
may result in subtle changes to the ecosystem that may change the biological structure
of the community.
Others also consider that harvesting an entire industrial fish species to extinction
seems unlikely (Hutchings, 2000; Sadovy, 2001), but the treatment of stocks as single,
panmictic populations means that if there are relatively local and sedentary stocks,
overall catches could conceal community extirpation. This has implications, for
instance, for the management of localized substocks such as in the case of the North
Sea sand eel.