FISHERY BULLETIN: VOL. 70. NO. I 



No/m ahousoKlsl 

 10 20 30 40 SO 

 La_l I I 1 I 



'mMui&ii^^ 



J4N FEB MSH iPR MAY JUN JUL AUG SEP OCT NOV DEC JAN FEB MAR 



Figure 3.-Monthly mean zooplankton abundance 

 (no./m2 in the upper 50 m data from Stations 1 

 and 2 combined). 



7-fold increase which in turn doubled to ca. 

 26,000/m2 by October. The November-December 

 catches of Diaptomus exceeded lO.OOO/m^, which 

 was approximately two orders of magnitude 

 greater than their standing stock 12 months 

 earlier. Substantial numbers of Diaptomus, 

 3,000 to 4,000/m-, were carried through into 

 1971. Bosmina were the most abundant species 

 collected throughout the year. Their numbers 

 ranged from ca. 8,000/m2 in January to ca. 

 60,000/m- in June and again in October. The 

 December concentrations of Bosmina were twice 

 that of the preceding January. However, by 

 January of 1971 Bosmina had virtually disap- 

 peared from the water column, to 50 m. Holo- 

 pedium attained their maximum abundance in 

 July, approximately 3 months after they began 

 appearing in the samples in significant quan- 

 tities, i.e. greater than l.OOO/m-. Following 

 a secondary maximum in October, Holopedium 

 were virtually absent from samples collected 

 from December through March. Daphnia were 

 the least numerous of the zooplankton spe- 

 cies routinely sampled. They occurred in num- 

 bers of 1,000 to 3,000/m2 from June through 

 September. Kellicottia exceeded 10,000/m2 from 

 May through August and again in October 

 and November. Nearly twice as many Kellicot- 

 tia were present in December as were present 

 at the beginning of 1970. The maximum abun- 

 dance of Conochilus colonies (2,000/m") was 

 during July; no colonies were found prior to 



June and by December the number of colonies 

 had declined to approximately 500/m-. 



In toto there were two to three times more 

 zooplankton present in December of 1970 than 

 there were the preceding January. It is of in- 

 terest to note that the greater abundance of zoo- 

 plankton at the end of 1970 was not maintained 

 through the first 3 months of 1971 and further, 

 that Bosmina had been apparently supplanted 

 by Diaptomus in 1971. On a monthly basis 

 there were fewer than 22,000 organisms/m^ 

 , in January while in October, where the max- 

 imum concentration was observed, there were 

 nearly 10 times as many organisms present. 

 Zooplankton counts exceeded 100,000/m- in 

 June, July, September, and October. The de- 

 crease in zooplankton abundance in August was 

 approximately 15 Sr lower than that in either 

 July or September; this decline was attributable 

 mainly to fewer numbers of Bosmina. 



Individual species counts (4-day running mean 

 number/m-) in vertical hauls have been pre- 

 sented in Figure 4 in order to show the seasonal 

 variations in abundance in greater detail than 

 is shown in Figure 3. The general features of 

 both figures are the same but in Figure 4 the 

 rapid increase and decrease in numbers of some 

 species are shown more clearly, e.g. Holopedium 

 and Epischura. From Figure 4 it is possible to 

 infer some relationship between the addition of 

 nutrients and the appearance of Conochilus or 

 the sustained increase in the abundance of Diap- 

 tomus. It is noteworthy that all species, with the 

 exception of Epischura and possibly Daphnia, 

 went through a secondary maximum in October 

 which was nearly as great as or greater than 

 their level of abundance earlier in the summer. 



SEX RATIO 



Adult stages of Cyclops and Epischura showed 

 marked imbalances from an expected 50: 50 ratio 

 of females to males through the year (Table 4). 

 Males of these two species were clearly pre- 

 dominant during the late winter and early spring 

 months. Cyclops females were predominant 

 among the adults taken in June through August 

 whereas Epischura females were never numer- 

 ically dominant for more than two or three sam- 

 pling periods, i.e. July 21 to 31, August 28 to 



30 



