Can farmed fish ease the enormous pressure we've placed on the world's wild fisheries without creating a new agricultural headache? Yes – if it's done in truly sustainable fashion.
Aquaculture, or fish farming, seems at first glance to offer a solution to overfishing. It takes up much less space than terrestrial agriculture, and doesn't involve resource-heavy ocean-going vessels. It's also very specific: the desired fish can be grown in select, contained areas, precluding the problem of bycatch (unintended capture of non-target species, which often includes endangered animals like turtles).
But environmentalists have warned of serious negative impacts to aquaculture too, which range from needing large amounts of caught wild fish to feed farmed fish, to local environmental degradation from waste, to passing diseases to adjacent fish populations.
If scrupulous attention is paid to each potentially damaging impact however, the majority can be largely avoided. Katse Dam Fish Farm in the Lesotho Highlands, where rainbow trout are raised to supply Woolworths, is an example of a sustainable aquaculture venture that has addressed many of these issues, some of which are discussed below.
Feeding fish to fish?
Carnivorous species like trout need to be fed fish, which is often sourced from wild fish and would thus seem to defeat the purpose of easing pressure on the oceans. The amount of “fish in” to the system needs to be not much higher than the amount of “fish out”, and the “fish in” needs to come from a sustainable source. Optimally farmed fish species are nonetheless one of the most efficient converters of feed, considerably lower than many terrestrially farmed animals: Katse has acheived a ratio of 1:1.
Do fish leave footprints?
Fish farming can leave a substantial carbon footprint if there is heavy reliance on water pumping and recirculation. In Katse's case, fish pens are situated within the waters of the dam and require no pumps. Where Woolworths Farmed Trout doesn't score well on carbon output – and this is likely it's main environmental negative – is transportation. Woolworths Trout begins life as hatchlings at Three Streams Hatchery in Franschoek, and the fry are flown up to Lesotho to continue growing. Once they reach harvestable size, they are then transported back to Franschoek for processing.
Fish farm “escapees”
If farmed fish escape, they can compete with and put pressure on wild species, transfer disease and in some cases interbreed and alter genetic diversity negatively. This can be avoided with continual monitoring of high quality container nets. Monitoring of species in surrounding waters is also important; the dominant species around Katse Dam is yellowfish, which supports local fisherman. The nets are also very effective in protecting the farmed fish from predators.
Fish health and welfare
If farmed fish are kept in overcrowded, stressful conditions, they are more vulnerable to disease. As Katse managers point out, this is not to the advantage of the industry as stressed fish tend to be smaller with poorer flesh. Disease can be avoided by stress-reduction tactics, primarily low stocking densities (at Katse this is 10kg of fish per m3), and by continual monitoring and vaccination.
- Olivia Rose-Innes, EnviroHealth Editor, Health24, August 2012