SEA-FISHERIES LABORATORY. 231 



We may consider these data more closely. The columns 

 represent the data of the above table. The +'s represent the 

 rough data smoothed by the method described by Dr. Johnstone 

 in last year's Keport.* The mode, or time of maximal spawn- 

 ing, is about 10th April, when about 75 % of all the female 

 shrimps' sample were found to be carrying abdominal eggs. 

 The minimum proportion of berried females was observed 

 in the samples taken during November, and this was the case 

 in each of the years (1912, 1914 and 1915) in which samples 

 were taken in that month. The proportion of berried female 

 shrimps rises from the minimum towards the maximum rather 

 more rapidly than it drops from the maximum towards the 

 minimum. 



The spawning period seems, therefore, to be a prolonged 

 one, and indeed it is common experience everywhere off the 

 Lancashire Coasts to find berried females throughout the 

 year. But it does not follow that it is so prolonged as the figure 

 indicates for, having extruded its eggs, the female carries 

 them for a considerable time attached to her swimmerets. 

 It is not known how long a period the eggs require for incuba- 

 tion, but according to Ehrenbaum* it may be four weeks 

 (or even less) in summer, and four to five months in the winter. 

 This is for the German North Sea Coast, and since the variation 

 in the length of the incubation period depends on the 

 temperature of the sea Ehrenbaum's estimates will probably 

 be approximately true also for the Mersey Estuary shrimps. 



During the spring and summer when the sea temperature 

 is rising rapidly the proportion of female shrimps carrying 

 abdominal eggs will, therefore, tend to be diminished by the 

 hatching out of the larvae. This is, of course, also the case 

 during the latter half of the year when the sea temperature 



* Rept. Lancashire Sea-Fish. Laby. for 1913. p. 83. 



* " Zur Naturgeschichte von Crangoti Vulgaris Fabr." Mitth. /. Scklion 

 /. KHsten-und Hochsee fischerei, Deutschen Fischerei-Vereins, .Jahrgang, 

 1890. Berlin, 1890. 



