MORTALITY IN PIKE-PERCH EGGS IN HATCHERIES
By
FRANZ SCHRADER
The remarkable losses in the hatching of pike-perch {Stizostedion.^fedion ”vitrev ) eggs have frequently made this phase of fish culture a ground for investigation. It is, however, not unjust to say that very little detailed study has been made and that no definite conclusion as to the causes of the high death rate has ever been reached.
The methods used in the handling of parent fish and eggs are. in the main, very much alike at the various stations that hatch pike perch. The fishes are caught in nets that are usually pulled once a day, weather permitting. Those that are ripe are stripped immediately, either in the field or at the hatchery, while the rest are retained in pens or live boxes.
They are examined in turn daily until found to be ripe. In some years the sexes are found to be disproportionate in number. If males are scarce, the same individual may be used to obtain milt on several -successive days. Fertilization is by the dry method, no water, or very little water, being used in the process. Milt and eggs are stripped into a bowl in more or less regular alternation, and the whole is gently stirred at frequent intervals to insure the contact of the eggs with the sperm.
When the bowl is sufficiently full — generally after 10 to 15 minutes—the contents are diluted with water which after a varying period is poured off and renewed until the eggs are contained in clear water. Cohesion of the eggs, which at this time are extremely sticky, is prevented by active stirring or by adding silt or starch to the water in addition to such mechanical agitation. Finally, after several hours, the eggs are put into the hatching jars through which a gentle current of water is kept flowing.
As already indicated, the losses are very great. Nevin (1887) considers a hatch of 50 per cent a very fair success, and this would be agreed to by most fish- culturists. The cause of this great mortality is, in general, ascribed to failure of the eggs to be fertilized or else to injuries incurred while the eggs are being handled, especially the active stirring and the addition of foreign materials to prevent cohesion. It seems almost certain that these last-named crude procedures—which so far are unavoidable—are very apt to be harmful, and a certain amount of loss is probably due to them.
Reighard (1890) found that by very careful handling of the eggs at and immediately after fertilization the percentage of eggs that started segmentation soon after fertilization could be considerably increased. Describing the loss met with in the ordinary course of the routine methods, he states that in 252 samples examined 11 per cent had died due to lack of impregnation and 33 per cent due to injury.
Unfortunately, although he designates the day and hour at which these observations were made, the age of the eggs is not specifically given. From the context it would appear as 29 hours. It seems that an egg was designated as dead when it showed an opaque white color, a criterion which was adopted also in the present investigation.
The figures of the losses in ordinary handling of pike-perch eggs given by Reighard are somewhat at variance with those given by L. H. Almy in some unpublished notes on the pike perch. His findings and those of the authors follow:
It will be seen that in contrast to the 33 per cent of white eggs given by Reighard, Almy observed only 5.4 per cent at 27 hours, while our own observations are lower still. Almy’s and our figures agree fairly well, the latter being lower up to two days and a trifle higher at four days.
It is not quite clear to what such a discrepancy could be due, although Reighard’s hypothesis of injury as a cause of mortality would, of course, itself allow for large differences on account of the varying skill and care bestowed on the eggs. (Temperature conditions were in all cases apparently the same, the water being in the neighborhood of 45° F.) Reighard describes the in- jury as taking place most easily over the oil globule, and there is no reason to dispute the observation.
However, the following explanation which he advances to account for this phenomenon does not seem to rest on a very firm physical basis (Reighard, 1890, pp. 33, 34):]
In the natural position the yolk sphere lies with its lower half against the egg membranes. These membranes, therefore, support this half of the yolk, surrounding it as if it were resting at the bottom of a cup. The upper half of the yolk is, on the contrary, not of the same form as the investing membrane; its spherical surface is interrupted by the protruding oil globule.
The result of this arrangement is that when any pressure is brought to bear on the egg membranes, so that the space within which the yolk lies is reduced, the yolk is able to resist this pressure by fitting itself against the egg membrane at every part of its surface except over the oil globule.
The strain, therefore, comes on that part of the protoplasmic investment of the yoke which covers the oil globule and here it hursts. In almost every case the white spot which indicates the rupture of the yoke investment makes its appearance at the oil globule. usually at the equator.
READ MUCH MORE INSIDE… “MORTALITY IN PIKE-PERCH EGGS IN HATCHERIES” Collector’s Item
DOWNLOAD YOUR COPY NOW $12.99
Click on the order button below for Instant Access!
Just use your name and valid email address – I will never sell or share your email address with anyone. Never. You may unsubscribe anytime. I hate spam just as much as you do.
Regards, Coyalita
Copyright © 2021-2024 1-Stop-Sporting-Goods.com All Rights Reserved Privacy Policy – Earnings Disclaimer – Terms of Use – Contact Us
Please note. The ebook is in PDF. file format. You need an Adobe Acrobat Reader to be able to read the ebook. If you do not have Adobe Acrobat Reader installed in your computer, you can download it at http://www.adobe.com. It’s completely FREE.
About Author
