LIVINGSTON ET AL.: LONG-TERM FLUCTUATIONS OF POPULATIONS 



the adaptive response of populations to the ex- 

 tremely complex environment of the estuary. The 

 data indicate that, in this case, temperature and 

 salinity might not be as critical in the determina- 

 tion of seasonal fluctuations of estuarine popula- 

 tions as biological functions such as trophic re- 

 sponse and possibly reproduction. 



DISCUSSION 



A review of the literature (Gunter 1945, 1950; 

 Daugherty 1952; Reid 1955; Van Engel 1958; 

 Gunter and Hall 1965; Williams 1965; Tagatz 

 1968; More 1969; Perez Farfante 1969; King 

 1971; Lyons et al. 1971; Swingle 1971; Perret and 

 Caillouet 1974; Stokes 1974; Swingle and Bland 

 1974) confirms that although minor variations 

 were evident (notably among the fishes), there 

 was a generally high level of conformity concern- 

 ing the time of appearance of various dominants 

 in the Apalachicola estuary with previously re- 

 corded data from other northern Gulf areas. Al- 

 though such timing was essentially stable from 

 year to year, there was considerable within- 

 species variability in annual abundance. For 

 example, the bay anchovy was particularly dom- 

 inant during the summer and fall of 1972, while 

 fewer individuals were taken during the succeed- 

 ing 2 yr. The Atlantic bumper, although not con- 

 sidered a common Gulf species (Perret and Cail- 

 louet 1974), was relatively common in the 

 Apalachicola estuary, especially during the first 

 year of collection. Some species reflected particu- 

 lar habitat preferences: Palaemonetes pugio was 

 located primarily in grass-bed areas of East Bay 

 during periods of low salinity while L. brevis was 

 found in outer bay areas during summer and fall 

 periods of increased salinity. Although gen- 

 eralized temperature and salinity preferences 

 have been shown for various estuarine species 

 (Copeland and Bechtel 1974), as a whole these 

 organisms show a wide tolerance for short-term 

 changes in these parameters. This could help to 

 explain the general lack of importance of temper- 

 ature and salinity as critical variables in the 

 multiple regression analysis; quite obviously, 

 other functions such as acclimatization would 

 tend to complicate such a direct approach to de- 

 termination of causative agents. The multiple re- 

 gression technique was limited in its application 

 to causal relationships since various biological 

 functions are probably involved in the determina- 

 tion of a given population curve. 



It is possible that trophic relationships and re- 

 productive cycles are of critical importance in the 

 spatial and temporal distribution of estuarine 

 populations. As in other Gulf estuaries, the 

 Apalachicola Bay system is dominated by 

 juvenile stages of a small number of species. The 

 bay anchovy, abundant in a size range of 35-50 

 mm, is considered to be a generalized zooplankti- 

 vore at this stage, feeding in the water column on 

 copepods, amphipods, mysids, larval and juvenile 

 shrimps and fishes, etc. (Darnell 1958; Odum and 

 Heald 1972; Carr and Adams 1973). Various 

 studies (Roelofs 1954; Darnell 1958; Fontenot and 

 Rogillio 1970) indicate that M. undulatus 

 (juveniles, 10-50 mm) feeds primarily on zoo- 

 plankton (copepods and amphipods) while C. 

 arenarius (juveniles, 40-99 mm) consumes larger 

 zooplanktors such as mysids, shrimp, and larval 

 or juvenile fishes (Darnell 1958; Springer and 

 Woodburn 1960). Juvenile (up to 40 mm) spot also 

 feed on zooplankton; more mature fish of this 

 species (40-200 mm) become benthic omnivores 

 (Roelofs 1954; Darnell 1958; Springer and Wood- 

 burn 1960). Juvenile B. chrysura (16-160 mm) 

 feed on copepods, mysids, shrimp, and small 

 fishes (Darnell 1958; Carr and Adams 1973). 

 Thus, the dominant fishes in the Apalachicola 

 Bay system are primarily planktivorous although 

 possible differences could exist in vertical feeding 

 distribution and the size and species composition 

 of the prey organisms. Previous work has shown 

 that Anchoa mitchilli feeds on small crustaceans 

 and C. arenarius eats the larger, more motile 

 crustaceans. Both Leiostomus xanthurus and B. 

 chrysura feed on small mid-water planktors 

 (mainly copepods) as early juveniles, with later 

 stages becoming benthic omnivores feeding 

 largely on mysids and shrimp. Increased concen- 

 trations of zooplankton occur in Apalachicola Bay 

 during the spring and summer while palae- 

 monetid shrimp are abundant during winter 

 and early spring (H. L. Edmiston pers. commun.). 

 Thus, diversity in feeding behavior would con- 

 tribute to the observed vertical partitioning of 

 prey organisms among various planktivorous 

 species; such data are consistent with the ob- 

 served distribution of fishes in Apalachicola 

 Bay at any given period of time. 



Of the six most prevalent invertebrates in the 

 Apalachicola estuary, five are benthic omnivores 

 and one is a probable planktivore. Juvenile blue 

 crabs consume detritus while larger individuals 

 (20-200 mm) are omnivorous, feeding on detritus 



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