LIVINGSTON ET AL.: LONG-TERM FLUCTUATIONS OF POPULATIONS 



1600- 



J 1200 

 < 



D 



Q 800 



Penaeus setiferus 



— r I'l ' i I I' I I I t 'I — 



s s s 



Penaeus duorarum 



I I'l r 



s s s 



Penaeus aztecus 



Palaemonetes 

 pugio 



-1— m— t-TTT 



I I I 



s 



s s 



Callinectes sapidus 



i' " i' I ■»' I r 



s s s 



Lolliguncula brevis 



MJ S DMJ S DMJ S DM 



I I I I I I 



MjSDMJSDMJSDM 



210 



1^8 



84 



42 



234 



E 

 E 



X 



t- 



Z 



LU 



TIME-MONTHS 



Figure 3. — Seasonal changes of numbers of individuals and 

 mean size of six dominant species of invertebrates taken in 

 the Apalachicola Bay system from March 1972 to February 

 1975 (Palaemonetes pugio and Lolliguncula brevis were not 

 measured). 



brevis. Unlike the fishes which usually reached 

 peak levels during different months of the year, 

 the invertebrates tended to increase in numbers 

 during spring and fall periods. 



Annual peaks of abundance often coincided 

 with influxes of juvenile fishes and invertebrates. 

 A more detailed analysis of this is shown for two 

 representative species of fishes (Figure 4) and in- 

 vertebrates (Figure 5). The young stages of Mic- 

 ropogon undulatus entered the bay during the 

 winter at which time there was a continuous re- 

 cruitment for several months. Decreased num- 

 bers coincided with gradual increases in size dur- 

 ing spring and summer months. With Cynoscion 

 arenarius, recruitment of young occurred during 

 spring and summer, with subsequent increases in 

 size during fall and winter months. The blue crab 

 had peaks of young individuals during summer 

 and winter periods although an almost continu- 

 ous succession of young crabs entered the bay 

 during the year. Young stages of Penaeus 



setiferus were found during the summer with 

 growth occurring through fall and winter. The 

 other penaeid shrimps had similar growth pat- 

 terns with recruitment of the young during sum- 

 mer and fall periods. The data indicate that vari- 

 ous patterns of recruitment and growth occur 

 among the different estuarine species, although 

 the inverse relationship of numbers and size ap- 

 pears to hold for most of the dominants. 



Results of the regression analysis are shown in 

 Table 3. Factors such as chlorophyll a , Secchi disk 

 readings, and color repeatedly accounted for some 

 of the variability associated with fluctuations of 

 estuarine populations. Often such associations 

 were made with a 1-mo lag in the independent 

 variable. In most cases, the given independent 

 variables accounted for less than 50% of the var- 

 iability of the population data. There was a dis- 

 tinct correlation with factors related to trophic 

 phenomena such as chlorophyll a and Secchi disk 

 readings; this would indicate that biological func- 

 tions such as feeding behavior and reproduction 

 could play an important role in the determination 

 of population shifts in the Apalachicola Bay sys- 

 tem. These data indicate that no single set of forc- 

 ing functions can account for the population 

 changes of various estuarine species. Species 

 abundance is dependent on complexes of interac- 

 tions and possibly can be accounted for more 

 adequately by relating such processes to djmamic 

 changes in physical variables as well as impor- 

 tant biological parameters. It is obvious that re- 

 gression analysis cannot account for changes in 



Table 3. — Results of the stepwise regression analysis of 

 various independent parameters and species (population) 

 occurrence in the Apalachicola Bay system from March 1972 

 to February 1975. Independent variables are listed by order 

 of importance with R ^ expressed as a cumulative function of 

 the given parameters. 



315 



