Aquaculture techniques: Purging Fish

Purging is the processes whereby fish are held in clean water and not feed for a period of time. Purging is a useful protocol employed by fish farmers to reduce the stress on fish during handling and transport, and to minimise unpleasant flavours in the fish’s flesh.

Purging is the processes whereby fish are held in clean water and not feed for a period of time. Purging serves two primary purposes: • Reducing the amount of faecal material in the gut which reduces the level of stress encountered by the fish during handling or live transport, and • Reducing or removing off flavour in the flesh prior to slaughter.

Purging for handling or transport

Digestion utilises oxygen. As such, if fish are handled or transported after normal feeding they experience increased stress during handling as well as transport, as the body struggles to obtain adequate oxygen from the water for digestion and dealing with heightened stressors. Furthermore, normal feeding is followed by normal rates of waste excretion from the body, which can quickly pollute the transport water, adding yet additional stress. By withholding feed for a limited period of time prior to size sorting,

harvesting or transporting them, we reduce both the oxygen shortage and metabolite toxicity stressors, allowing the fish to travel and arrive in a far healthier state. The primary factors determining the duration of purging for handling or transport are the size of the fish and the duration of the handling or transport event; small fish and shorter time periods require less purging. Small fish may require 6 hours of purging prior to handling or overnight purging prior to a 12 hour long trip. Larger fish going on a 24 hour trip may require 3 days of purging prior to being packaged and shipped.

Purging for slaughter

The feed that fish consume and quality of water in which they live, can impact its flavour. As such, normally fish is purged prior to slaughter in order to evacuate their gut as this lessens or removes the off-flavour associated with feed. In addition, hold the fish in clean water for the duration of purging to rinse out water borne flavour affecters. By doing these two things simultaneously for 24 to 72 hours you are able to get rid of all unpleasant tastes in the flesh. Clearly you need to know why you are purging your fish in order to apply the more appropriate method of purging them.

Systems differences

Most fish species cannot be purged in earths pond as they will find food to eat in the pond even though all artificial feed is withheld. The exception would be predatory species such as trout or eels that will not be able to find appreciable quantities of feed and can be purged reasonably effectively in ponds. Cages in a green water environment are similarly ineffective for purging fish that feed low on the food web as they can find some feed in the cage and importantly the green water still results in off-flavours persisting.

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Clean water cages are perfect for purging, as are tanks where clean water can be run through the tank and feed withheld.

Weight loss during purging

Fish will lose some body mass and an element of stress will ultimately result from purging but this is unavoidable. The point though is not to purge the fish, especially small fish, for longer than is necessary. Purging is a useful protocol employed by fish farmers to reduce the stress on fish during handling and transport, and to minimise unpleasant flavours in the fish’s flesh. Be sure to purge the fish correctly though to maximise the benefits without incurring undue stress or harm. By: Leslie Ter Morshuizen, AquaAfrica

THE BENEFITS OF AQUAPONICS

The benefits of using an aquaponics system to grow food are becoming more important due to population growth making fresh water more scarce, and pollution more prevalent within the world. Here are some of the reasons why aquaponics is such a hot topic.

FAST PLANT GROWTH

Providing nutrients and an oxygen rich environment for the roots of aquaponic plants makes them grow 3 times as fast compared to soil. The flavour of the plants is better too, as they have more time to focus on healthy growth rather than placing energy into extending their root system to look for water.

HEALTH

The fish you grow are free from pollution unlike many sourced from the sea. This goes for the vegetable and herb produce too; you have control of what goes into the system. All the produce grown within an aquaponics system is important for a healthy lifestyle.

REDUCED CHEMICAL USE

When an aquaponics system is setup within a greenhouse or insect mesh there are very few bugs to manage. Pesticides are extremely dangerous to fish, so they are not recommended for aquaponics. As chemicals can be introduced, you’re guaranteed clean and non-toxic produce, completely organic (depending on where you live!).

FOOD LOCALISATION

An important benefit of an aquaponic system is that it produces fish and plants for the family or grower all year round without leaving their backyard. A family will save time and fuel without having to shop for the same items, and use more healthy herbs and veggies in their cooking.

FOOD SECURITY

Many consider that one of the main benefits of aquaponics is having a replenishing food source at home. Owners can rest assured they will never go hungry, and they have fresh produce to barter with too.

REDUCED WATER USE

Compared to soil gardening, aquaponics uses 90% less water. An aquaponics system continuously recycles water through the plants and fish tank, so the only water lost is through evaporation from the fish tank (when uncovered), and from the plants’ transpiration.

SMARTER FARMING

Aquaponics reduces erosion by eliminating the need to turn or plough the soil. The backbreaking work of turning soil and pulling weeds is gone, reducing the cost compared to a conventional horticultural farm. Imagine gardening with your plants at waist height – now that is smart!

From: Aqua Gardening