Low recapture rate in jellyfish aquaculture: causes and countermeasures

    I. Causes of low recapture rate in pond-cultured jellyfish

    1. Non-standard selection of aquaculture ponds. First, poor pond water quality, individual ponds that are too small, or water that is too shallow. Second, old ponds used for many years, with thick silt, a small slope ratio, incomplete pond clearing, and many harmful organisms. Some ponds already contain Manila clam, hard clam or jackknife clam, etc., and were not thoroughly sun-dried, cleared and disinfected before aquaculture. Some are shrimp ponds in contiguous reclamation areas, with varying species and sizes. In addition, severe pond seepage results in incomplete sun-drying of the bottom. The intake and drainage in the outer embankment ditches is seriously polluted. Third, poor water intake and drainage conditions; in some reclamation-area shrimp ponds, water cannot be exchanged during small tides. Fourth, lack of freshwater sources, and the specific gravity of the seawater is on the high side.

  2. Poor feed cultivation and insufficient feed. Jellyfish feed continuously day and night, and after one full feeding the food is digested after 4 to 5 hours. Jellyfish have a very large appetite; taking Artemia nauplii as feed, the daily intake of an individual is 6,000 at an umbrella diameter of 20mm; 100,000 at 70mm; 260,000 at 110mm; and more than 2.9 million at 230mm. They grow extremely rapidly: an ephyra larva weighing 3mg can reach more than 10kg after 2 months of growth, a weight gain of more than 3 million times. The growth rate of jellyfish is closely related to the feed organisms in the water body.

  3. Timing of seedling release. Improper specifications and density, or poor seedling transport and release techniques, result in a low seedling survival rate and affect the recapture rate.

  4. Aquaculture management techniques lag behind. First, inadequate netting measures: the soft top of the jellyfish umbrella easily rubs against and is damaged by the pond wall, causing jellyfish to strand on the bank and die. Second, water quality management techniques. To raise jellyfish, first raise the water; the fertility and levels of physical and chemical factors of the water body must suit the growth needs of jellyfish, yet many aquaculturists only know that water fertility is related to feed and neglect the three elements of water quality: "fertile, lively and refreshing".

    II. Countermeasures to improve the recapture rate of pond jellyfish aquaculture

 

1. Choose standardized ponds, thoroughly clear the pond, sun-dry the bottom and disinfect, and at the same time implement reasonable netting measures; a surrounding net or plastic sheeting should be added at a water depth of 0.5m along the bank, supported by bamboo poles, to prevent jellyfish from stranding on the bank and dying.

  2. Before releasing seedlings, do a good job of fertilizing the water to increase the plankton biomass in the water, so as to ensure the feed organism biomass in the early stage after release. The fertilizer concentration must be appropriate, to prevent water quality from deteriorating and affecting the seedling survival rate. When using chicken manure to fertilize the water, be sure to thoroughly ferment the chicken manure for 7 to 8 days before use. In the high-temperature season, it is best not to use chicken manure as a top dressing, to prevent water pollution. Before releasing seedlings, test the water quality, mainly water temperature, salinity, plankton biomass, pH value, etc. At the same time, attention should be paid to the water temperature, salinity and other indicators of the nursery pond and the grow-out pond, and the gap between the two should be adjusted to the minimum through the nursery room as far as possible, so as not to affect the survival rate of the jellyfish seedlings due to too large a gap.

    3. Selecting strong, disease-free, larger-sized superior seedlings is the prerequisite for successful aquaculture.

  4. Do a good job of seedling transport and release to improve the seedling survival rate. The release time should be chosen on a morning or evening with good weather, preferably with no wind and no direct sunlight. During seedling transport, oxygen should be added, light should be blocked, and ice should be added to prevent a temperature rise from affecting the transport survival rate. After the seedlings arrive at the pond edge, do not rush to put them into the pond; first untie the bag opening, observe their vitality, then pour the seedlings into a relatively large container, add some water from the pond, and let the seedlings adapt for a period (10 minutes to 20 minutes) before putting them into the pond. When releasing, a small boat should be used to carry the seedlings to the middle of the pond and sprinkle them evenly into the pond; the operation must not be rushed and should be carried out slowly to prevent injuring the seedlings.

    5. Do a good job of daily aquaculture management, especially water exchange and artificial water fertilization.