Detailed Explanation of Icefish Transplantation and Propagation Techniques in Large Water Bodies

Icefish belongs to the order Salmoniformes, family Salangidae. It is an annual small fish, distributed only in the coastal and inland waters of China, Japan, Korea, Vietnam and the eastern part of the former Soviet Union and other Southeast Asian countries and regions. China has the greatest variety of icefish, about 10-plus species, widely distributed in coastal areas, rivers connected to the sea and their affiliated lakes, reservoirs, etc., feeding on zooplankton and small fish and shrimp. The key points of its transplantation and propagation techniques are briefly described as follows:
I. Transplanted species
1. Large icefish (Protosalanx hyalocranius): body length generally 110-160mm, spawning in winter; fecundity 6000-20000 eggs, mature eggs crystal-clear and plump, egg membrane filaments densely distributed, branched; juveniles mainly feed on zooplankton, and adults shift to feeding mainly on small fish and shrimp.
2. Taihu Lake icefish (Neosalanx taihuensis, small icefish): body length generally 60-70mm; in the middle and lower reaches of the Yangtze River there are two spawning groups, spring and autumn; the peak spring spawning period is early to mid-April, and the autumn one is early to mid-October; fecundity generally 1000-5000 eggs, the egg membrane filaments of mature eggs are branched or in radiating short lines; feeds on zooplankton throughout its life.
The transplantation of these two icefish species is affected by geographical and climatic conditions. Taihu Lake icefish is more suitable for transplantation in areas south of the Huai River, and large icefish is more suitable for transplantation in areas north of the Huai River.
II. Environmental conditions of the transplantation water body
1. Water area: reservoirs and lakes with an area of more than 2000 mu.
2. Water level and water depth: a relatively stable water level is preferable; for irrigation-type reservoirs or through-flow lakes with large water level fluctuations, measures to avoid the loss of icefish resources should be considered. Shallow-water reservoirs are favorable for icefish reproduction. A water depth of about 15 m is preferable, but water depth is not a major limiting factor for transplantation (the transplantation of icefish in Fuxian Lake, Yunnan, with an average water depth of 87 m, has been successful).
3. Water temperature: the suitable spawning temperature for Taihu Lake icefish is 15-20°C, and for large icefish 2-6°C.
4. Water quality and bottom substrate: mineralization can be around 1000mg/L; transparency range 25-100cm; pH should not exceed 9; a relatively hard sandy-mud bottom is preferable.
5. Food organisms: the biomass of zooplankton food should generally not be lower than 1.0mg/L, and its population composition is preferably dominated by cladocerans and copepods. For water bodies where large icefish is transplanted, in addition to a certain amount of zooplankton, there should be relatively abundant small fish and shrimp food resources.
III. Fertilized egg transplantation methods
1. Timing: Taihu Lake icefish in March-April, at a water temperature of about 15°C; large icefish from late December to early to mid-January, at a water temperature of 2-6°C.
2. Transportation: when the transport volume is small and the distance is short, use open containers or plastic buckets filled with an appropriate amount of lake or reservoir water filtered through 120-mesh sieve silk (or absorbent cotton); 1 liter of water can hold 3000-5000 fertilized eggs. When the transport volume is large and the distance is long, use plastic bags filled with oxygen placed in an insulated bucket for transport; 1 liter of water can hold 10,000 fertilized eggs. The temperature of the transport water should preferably be kept consistent with the incubation temperature of the fertilized eggs, to avoid hatching and emergence of fry during transport.
3. Release: put the fertilized eggs directly into the water body so that they hatch naturally on a relatively hard or sandy bottom. The release site should be chosen as a leeward, sunny area with stable water, a water depth of 1.5-2.5m, a gentle bottom, and a hard, sandy substrate (such as a lake bay, an open-water area of a reservoir bay, or a wind-sheltered spot near the shore). When releasing eggs, spread the eggs evenly in the water, and select several points for release. If the bottom substrate at the release point is not ideal, the method of placing hatching boxes can be adopted, that is, making hatching boxes of 4×4×4m3 from window screen, hanging the boxes at 1.5-2m below the water surface, lifting the boxes after 7-10 days, and observing the embryonic development; after all the fry have hatched, remove the net cages.
IV. Transplantation density and effects
1. Transplantation density: the usual rule is that the greater the transplantation density and the larger the number, the shorter the latent period, and the faster fish production is formed.
(1) Fertilized egg transplantation: calculated at a hatching rate of 70-80%, the number of eggs released is 30-70 per mu, and generally another release is made in the second year.
(2) Larval fish transplantation: continue incubation management of the fertilized eggs brought back. For Taihu Lake icefish, the incubation time is 126 hours at an average water temperature of 15.6°C; about 81 hours at an average water temperature of 20°C, with newly hatched larvae 2.5-2.9mm; for large icefish, the incubation time is about 40 days at 2-10°C, with newly hatched larvae 4.2-4.9mm. The best release time for larvae is when the fish body has reached horizontal swimming and the yolk sac has not yet been fully absorbed. For Taihu Lake icefish this is about 2-3 days after hatching, and for large icefish about 3-5 days. The release site should usually be a certain distance from the shore, with a water depth of about 1m; the water temperature of the release water body should be consistent with the water temperature in the container holding the larvae, and the temperature difference should not exceed 2°C. The release number is preferably 20-40 individuals.
2. Transplantation effects:
(1) Biological effect: this is the early-stage effect after icefish transplantation. After being transplanted to a new water body, the icefish can survive, and some individuals reach sexual maturity and naturally spawn and reproduce offspring; this can generally be judged in the same year or the second year after transplantation. Usually after the icefish is introduced, especially during the sexual maturity period or spawning period, gill nets with a diameter of 2m should be used to catch parent fish or larvae to check the transplantation effect.
(2) Production effect: that is, the formation of a population with fishing value after natural reproduction. For Taihu Lake icefish this is generally 3 years, occasionally 2 years. For large icefish it is generally 2 years, occasionally 3 years. After the biological effect is confirmed, production nets should be used in the second year for regular or irregular trial fishing to monitor their reproduction and growth status. Calculate their resource amount according to general fish resource assessment.
V. Resource management after transplantation
1. Based on the monitoring of resource amount by light trapping with gill nets, preliminarily determine the fishing gear, fishing time and fishing intensity.
2. Continuously explore and study the resource status of the formed population and the reasons affecting resource changes, adjust operation time and intensity, and determine the proportion of resources to be retained and the allowable catch.
3. Establish a strict management system, strengthen fishery administration order, and strictly prohibit illegal operations such as blast fishing and poison fishing.
VI. Icefish fishing
1. Principles for determining the start of fishing:
(1) Fish when the population reaches its maximum biomass. When icefish begin gonadal development and the growth rates of body length and body weight slow down, the population biomass is at its maximum, and fishery utilization is more reasonable. Therefore, it is preferable to fish when the icefish has generally reached 7 months of age.
(2) Reasonably arrange the fishing season and make full use of the food base. The biomass of the icefish's food, zooplankton, in the water body varies with the seasons; its peak usually appears in summer, followed by spring and autumn, and is lowest in winter. Therefore, the fishing period should avoid summer and autumn so as to make full use of the food base. Generally speaking, fishing in late autumn and early winter is more reasonable.
(3) Avoid fishing during the reproduction period.
2. Fishing methods:
(1) Trawl fishing: suitable for lakes and reservoirs of more than 10,000 mu.
(2) Hemispherical gill net light trapping.
(3) The fishing time is generally October-November each year.