334 POPULATIONS 



populations, depending upon the tempera- temperature effect, but believes that this 

 ture at which they are cultured. For ex- factor in some way has a direct influence on 

 ample, at 19.8° C. the asymptote attained the birds because "a relation between re- 

 is 199 of these cladocerans per unit-con- productive vigor and temperature is at least 

 tainer; at 24.8° the asymptote is 429; and indicated by the facts here presented" (p. 

 at 33.6° it is 271 (Fig. 100). They con- 26). 



elude that there is a relationship between Gunter (1941) has reported on the ef- 



the size of the population and the tempera- feet of unusual cold upon the marine fish 



ture optimal for reproduction. In another populations in the Aransas Bay area of 



paper Terao and Tanaka (1928a) measure Texas. The winter of 1939-1940 was most 



the number of births per female day over severe, and local weather reports showed 



a range of ten constant temperatures ex- that, in this region, January of 1940 aver- 



tending from 3.2° to 37.7° C. They report aged colder than any previously recorded 



the highest reproduction at 28.1°, above month. On January 18 a "norther" struck, 



which the rate drops sharply and below and the temperature fell in four hours from 



which it declines more gradually. Cause 65° to 25° F., and then reached 16° F. 



(1931) analyzed their population growth during the night. This drop in temperature 



data mathematically and showed that when resulted in an excessive fish mortality in the 



temperature is plotted on the abscissa general area as detected by experimental 



against asymptotic population size on the seining and fisheries statistics. Many dead 



ordinate, the resulting curve exhibits, es- ^shes were seen along the shores. For ex- 



sentially, a normal distribution with its ample, on January 24 Gunter personally 



central high value at about 28° C. saw 15,000 to 20,000 dead piggy perch 



It is not permissible to conclude that tem- (Lagodon rhomhoides) in the storm basin 



perature is the only factor operating here at Aransas Pass. The air temperature was 



and that the action is entirely density-inde- 1-5° C., and the water temperature, 4.7° C. 

 pendent, since competition must obtain This severe and sudden cold had a de- 



in all the cultures. In fact, Terao and "^ed efiFect upon the Texas commercial 



Tanaka (1928b) demonstrated that in- fisheries catch, as is brought out in the ac- 



creasing density lowers fecundity, and companying table, reporting catch in 



Pratt (1943) clearly shows for Daphnia pounds. 



magna that the considerable effect of tem- u t? K Decline 



perature on mean population size operates ^ ep em er- e ruary- ( er 



r j.f . ^1 ^^. f 1^-1 Year January April Cent) 



by modifying the action of population den- igsy.igsg 2,171,997 1,331,302 39.6 



sity. However, for Moina it seems appar- 1938-1939 1,972,864 911,133 52.7 



ent that the primary cause is density-inde- 1939-1940 1,412,090 335,431 76.2 



pendent and operates through the differen- 

 tial effect of various temperatures on fe- The figures show a general decline in the 

 male reproduction. over-all September to April catch from 



1937 to 1940, but the important point is 



Natural Populations 2^^ ^W ^ecKne of 76.2 per cent from the 



September-January to the February-April 

 Kendeigh (1942) studied losses in the period in the season of the cold spell, when 

 nesting of fifty-one species of birds and compared with the two preceding years, 

 showed that temperature played a role in (The February to April catch is always 

 this matter. The percentage of eggs that smaller since only three months, rather 

 are addled or infertile is high at low and than five, are considered.) Gunter believes, 

 high temperatures, but relatively low at quite correctly we think, that this drop is 

 medial temperatures. For example, if mean largely attributable to cold. When the re- 

 monthly temperature is plotted against per- ports are broken down in various ways, by 

 centage of eggs not hatching, it can be areas, species, and so forth, the same point 

 shown for the house wren that between holds true. Of all the fishes, the flounder 

 about 15° and 21° C. only 5.3 per cent population was most affected. It declined 

 fail to hatch. Above or below this range from 74,306 pounds in the five months be- 

 the percentage increases so that at 12° and fore the freeze to 3840 pounds after the 

 25° it is about 10 to 11 per cent. Kendeigh freeze, a difference of 94.8 per cent. Gun- 

 is not entirely sure of the nature of the ter reported that many of the invertebrate 



