Mariculture in Australia
Aquaculture Impacts on the Environment
Using Natural Fish Stocks to Feed Farmed Fish
Fish meal and fish oils from fish caught in the wild are the main components of artificial feed. Usually the fish used to make artificial feed are not eaten by humans. Additional fish feed comes from bycatch which would otherwise be discarded as waste. About ¾ of the fishmeal and oil are produced from the harvest of small, open-ocean (pelagic) fish such as anchovies, herring, menhaden, capelin, anchovy, pilchard, sardines, and mackerel. These fish have short life cycles and are capable of rapid reproduction and stock replenishment. The other ¼ is generated from the scraps produced when fish are processed for human consumption.
Farmed fish and shrimp eat feed that is specially formulated to contain all the essential nutrients they need to keep them healthy and growing and maintain the human health benefits of seafood consumption. The ingredients are formed into pellets, similar in many ways to dry dog food. There are about 40 essential nutrients needed by all animals. Categories of essential nutrient include vitamins, dietary minerals, essential fatty acids and essential amino acids. These are provided by a number of feed ingredients including fish, plant, and processing waste meals and oils.
Through research, we are learning that other combinations of ingredients can achieve the balance of the 40 essential nutrients. Affordable replacement ingredients for fishmeal and fish oil are becoming increasingly common, which is leading to declining percentage on those ingredients in farmed fish diets. Including ingredients like oils from phytoplankton maintain the nutrient requirements of the final product without depending on fish oil. The economics of using blended oils is improving as fish oil prices rise and the technology to produce phytoplankton (and other replacement ingredients) improves.
Almost 31,000,000 megatonnes (a megatonne is a million tonnes) of the world's total wild fisheries production is used for animal feed each year, 15% of which is used in fish feed. Growing a kilogram of farmed salmon may require 3 to 5 kilograms of wild fish. However, when aquaculture is considered as an aggregate industry, the answer is no. Globally, aquaculture uses about half a metric ton of wild whole fish to produce one metric ton of farmed seafood, meaning that aquaculture is a net producer of protein.
Feed conversion ratios (the amount of feed eaten by a fish related to the amount that fish provides for human consumption) vary among species, but farmed fish are far more efficient at converting feed than wild fish or other farmed animals such as cows and pigs.
To reduce the amount of fish need to produce farmed fish more herbivorous species such as tilapia, catfish, carp, oysters and clams could be farmed. However most of the world’s aquaculture production is based around these species, and it is much more profitable to grow species like salmon and prawns that rely heavily on artificial fish feed.
Impacts on Natural Stocks
Feeding fish to fish leads to a loss of protein in a world running short of protein as food and catching these fish has a direct impact on natural fish numbers, but this is only one impact of aquaculture. Almost all marine or brackish water farming depends on natural fisheries for some part of its operations. Although more and more hatcheries are being constructed to provide breeding stock for shellfish and finfish culture, most farms still capture wild animals for breeding or for a source of larvae. In some cases, collection of wild-caught shrimp larvae to stock ponds has destroyed thousands of other larval species in the process.
The full consequences of removing natural fish stocks from food webs are difficult to predict. When fish are removed to make fish meal, less food may be available for commercially valuable predatory fish and for other marine predators, such as seabirds and seals.
The world supply of fish from pelagic fisheries has remained relatively constant over the past twenty years at around 6 million metric tons. These types of fish generally are capable of rapid reproduction and stock replenishment. Many pelagic fisheries are recognized as successfully regulated and many stocks are fished at levels below the biomass that achieves maximum sustainable yield. Careful fisheries management, including quota and catch limit systems, maintains the sustainability of these fisheries over time.
Next: Genetic Conservation & Aquatic Biodiversity