MORTALITY IN PIKE-PERCH EGGS IN HATCHERIES.^ 



By Franz Schradkr. forrtterlji FisJi I'atho1o(/isf, T'. t<. Bureau of Fisheries, 



and 

 Sally Hughks Schradkr. Tciiiixiiai-ir AKsistanf. T'. S. liuremt of Fislicries. 



The remarkable losses in the hatching of pike-perch {Sth.ostedion 

 mtreum) 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 i^ermitting. Those that are ripe are stripped immedi- 

 ately, either in the field or at the hatchery, while the rest are retained 

 in pens or live boxes. The}' 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 con- 

 tact 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 af^itation. 

 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 mor- 

 tality is, in general, ascribed to failure of the eggs to be fertilized 

 or else to injuries incurred while the eggs are being handled, espe- 

 ciall}' 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 



1 Appendix V to the Keport of the U. S. Commissioner of Fisheries for 1922. B. F. 

 Doc. No. 920. 



91376° — 22 1 



