MULTIPAROUS FEMALES 457 



Another factor influencing the apparent percentage pregnant is the mortality rate. Non-pregnant 

 mature females taken in antarctic waters will on average be about a year older than pregnant females, 

 because most females become pregnant at the first ovulation and do not join the non-pregnant group 

 until a year later. Thus, even if lactating females were fully represented in the samples and the actual 

 average pregnancy rate for individual females was 50% (that is, one pregnancy every 2 years), the 

 apparent ' percentage pregnant ' of the population as a whole, resulting from an annual adult mortality 

 rate of, say, 20% should be 55*5%. For every 100 pregnant females there would be only 80 non-preg- 

 nant, because 20 females die in the intervening year. However, even if the effects of high mortality rates 

 are allowed for, the variation between the values for the apparent ' percentage pregnant ' is considerable, 

 and probably represents a real change in fertility. This might be caused by increased fertility at post- 

 partum or post-lactation ovulations, and to a small extent perhaps by reduced pre-natal mortality. 



It has been shown above that the youngest females have a lower fertility rate, and that fertility is 

 maximal at about 15-25 years, after which it appears to fall slightly. If the age-composition of the 

 population has changed then this would affect the percentage pregnant, but only to a very slight and 

 possibly not appreciable extent. 



100 



z 

 < 



5 SOl 



LU 



a. 



a 



BLUE WHALE- ALL AREAS 



£ IOO 



£ SO 



-n-*-«- 



FIN WHALE-AREA II o 



AREAS m+K« 



1930 



1935 



-1 — i — 1 — r 

 1940 



-1 1 1 1 r- 



1945 



1950 



1955 



I960 



Text-fig. 53. Pregnancy rates of mature females in different seasons and areas. 



Table 28. Variations in the 'pregnancy rate' in the catches of adult females over a number of years 



Blue Fin Blue Fin 



South 

 Georgia 



Pelagic 



South 

 Georgia 



Area 11 



Areas 



III + IV 



Pelagic 



Area 



Area 11 



Areas 

 III + IV 



Season 



1925/26 

 1926/27 

 1927/28 

 1928/29 

 1929/30 

 1930/31 

 I93I/32 

 1932/33 

 1933/34 

 1934/35 

 1935/36 

 1936/37 

 1937/38 

 1938/39 

 I939/40 

 1940/41 



Per- Per- Per- Per- Per- 



No. centage No. centage No. centage No. centage No. centage 



— — — — 72 46 — — — — 

 30 40 — — 18 56 — — — — 



— — — — 12 42 — — — — 

 27 41 — — 74 69 



9 55 — — 188 69 — — — — 



25 76 — — 70 76 — — — — 



— — 184 



57 



— — 25 



44 



Season 



1945/46 

 1946/47 

 1947/48 

 1948/49 

 1949/50 

 1950/51 

 1951/52 

 1952/53 

 1953/54 

 1954/55 

 1955/56 

 1956/57 

 1957/58 

 1958/59 



Per- Per- Per- Per- 



No. centage No. centage No. centage No. centage 



190 



479 

 377 

 347 

 239 

 278 

 142 

 134 

 121 

 68 



26 



54 

 68 

 54 

 68 



75 

 77 

 73 

 73 



77 



284 



32 



130 



163 



108 



69 185 



79 294 



71 333 



— 184 



82 



79 

 72 



118 



174 

 363 



84 

 86 

 86 



222 

 164 



77 

 80 



With these reservations in mind the variations from year to year in the ' percentage pregnant ' may 

 be briefly discussed. In Table 28 the full results are shown, and in Text-fig. 53 some of these values 

 are plotted. 



The pelagic values for blue whales are combined for all areas, because individual areas show a 

 similar pattern. This indicates an apparent increase in fertility in pre-war years levelling-out at about 



17 



