The condition of the eggs, indicating the advancement of embryonic development, should be monitored on an ongoing basis by the staff operating the apparatus. This is due to the need to take into account the variable sensitivity of the embryos and adjust the flow force, irradiation and water temperature to the requirements associated with the specific stage of embryonic development. also, in order to properly organize the work during the critical hatching period, you need to know the deadline in advance, in which the first larvae may appear. It is usually adopted to use an indicator for this purpose, which is the sum of average daily temperatures of water flowing through the apparatuses. They are obtained from several measurements repeated at equal intervals during the day and night. Adding daily average temperatures each day gives the number of degree days since the start of incubation. Pike larvae hatch after a period of time 120 degree days, that is, after about twelve days of embryonic development at an average temperature of 10 °. At higher and lower temperatures, the number of degree days it takes to reach the stage, in which the embryos are capable of hatching, undergoes changes as shown in the figure.
Pike roe development period in degree days, at different incubation temperatures (by Linidroth).
However, identifying the stages of development on the basis of a degree-day scale runs the risk of making a mistake, resulting from here, that the speed of egg processes depends not only on the average temperature, but also on the range of its fluctuations, both over relatively short periods of time, and throughout the incubation period, and in addition to the chemical properties of the water fed to the hatchery. More serious mistakes can be avoided by checking the condition of the embryos on an ongoing basis under the microscope.
According to the assumptions of the most common method of rearing young pikes, the larvae hatch outside the Weiss apparatus. Because – usually as soon as the pigment appears in the eyes of the embryos (after the so-called. zaoczkowamiu ikry) the eggs are transferred to Californian apparatuses, where it rests on the net, rinsed with constantly flowing water, but no longer carried away by its current.
Dead eggs must be separated again during transfer, using or the already mentioned Hofer's fluid, or saline solution, in which healthy eggs float to the surface, dead, however, remain at the bottom of the pan. Sakowicz (1939) recommends to prepare a solution with a concentration first 12% (12 g soli na 0,1 1 water) and only after placing the roe in it, add more concentrated roe until the desired concentration is obtained - 15,5 %.
In natural conditions, pike larvae look for a high concentration of oxygen right after hatching and, when stuck to the surrounding objects, they undergo a period of peace. They behave in a similar way in hatching apparatuses, finding conditions there, however, much less favorable due to the flow of water, high density in a small space and the presence of decaying egg membranes, the removal of which bothers the larvae and causes, that they form mass clusters in the corners of the camera. As a result, oxygen deficits arise, leading to the larvae dying by suffocation. That is why hatched larvae should be immediately transferred to specially prepared pools - nurseries. The rubber hose used for this purpose, however, pulls in empty egg membranes with the larvae, and also eggs, from which the larvae have not yet hatched. Kraus (1961) therefore recommends placing eggs transferred to Californian cameras not directly on the camera mesh, but in a wooden frame with the dimensions so selected, that a distance of 5–6 cm remains between its edges and the sides of the apparatus.
Drawing. A wooden frame for hatching pike eggs in Californian cameras (wg Krausa).
The larvae scared by tearing up the net float out into the free space, where they can be easily collected, and rinse off the debris from the eggs left on the frame.
Szczerbowski (1965) describes a successful brood attempt using the Weiss apparatus. After streaking, he put the roe back in the jar (2 litry ikry na 7 liters of water), where the hatching larvae circled for two days, carried by a stream of water, the flow rate of which was kept within the limits 3 l / min., and the draining water took away the empty egg membranes. The advantage of this method - also successfully used in the United States (Hiner, 1961) - there is a significant saving in labor input. It follows from Hiner's description, that the larvae circulate for up to five days, until they begin to actively resist the current and assume a horizontal position.
In the first stage of larval development, young pike should be provided with good oxygen conditions. In order to increase the area, which they could stick to, twigs of coniferous trees are placed in nurseries or linen "aprons" are hung. About five days after hatching, the larvae begin to swim freely and although the yolk sac has not yet been resorbed, are ready to receive food from the outside. If they have not been introduced into natural reservoirs before, further rearing requires intensive feeding. The methods used can be divided into three groups: 1 - the larvae remain in the pools, to which plankton is supplied; 2 - they are placed in the plankton catching devices by themselves; 3 - they are moved to previously flooded and fertilized ponds in order to increase their fertility.