Annual Cycles of Species Occurrence, Abundance, and Diversity in Georgia Estuarine Fish Populations

Abstract

A total of 31,637 individuals, comprising 70 fish and 37 families, were trawled in a Georgia estuary. Population structure was influenced by seasonal changes in taxa, numbers of and individuals, and average size of fishes. Species-numbers diversity was studied in regard to seasonal variation and three zones. Four indices were compared. Seasonal variation was found with and evenness but not with the ShannonWiener index or richness. A seasonal change in relative abundance resulted primarily from the influx of juveniles in the late summer and autumn. An apparent increase in diversity from sounds to small based on three indices was not statistically significant. INTRODUCTION Trawl samples of fish collected throughout one annual cycle from three zones in a Georgia salt marsh estuary are analyzed in terms of seasonal cycles and four contrasting diversity indices. Establishing the normal cycle of diversity in this undisturbed estuarine system is part of an inventory designed to establish bases for the future detection, evaluation, and control of pollution that might result from new industries. Species-numbers diversity is generally a more sensitive and reliable index of environmental health than are individual indicator organisms. For example, Wilhm (1967) found that diversity of benthic stream invertebrates decreased with the degree of pollution. However. diversity may also be affected by normal downstream trends related to stream order (size) (Harrel and Dorris, 1968). It is important, therefore, that the pattern of diversity in time and space be determined, if possible, prior to the advent of stress. The behavior of diversity indices needs to be understood, since the various ratios proposed measure different components of diversity. As the indices are variously affected by sample size and seasonal phenomena, it is also necessary to analyze population dynamics, seasonal changes in taxa, numbers of and individuals, and growth. METHODS Fishes were sampled at approximately three-week intervals from January 1967 to February 1968 at 14 stations in the -estuarine system of Sapelo and St. Catherines sounds. The Marine Inistitute's R/V Kit Jones sampled with a 20 ft (6 m) wide otter trawl having 1 ,4 in (32 mm) mesh in the bag. Each station was trawled for 15 minutes. 1 Contribution No. 182 of the Marine Institute, University of Georgia, Sapelo Island 31327. 382 This content downloaded from 157.55.39.83 on Sun, 09 Oct 2016 05:13:42 UTC All use subject to http://about.jstor.org/terms 1970 DAHLBERG AND ODUM: ANNUAL CYCLES IN FISH 383 A representative fish collection is maintained at the Marine Institute. To compare habitats the study area was arbitrarily divided into three ecological areas according to the size of tidal waters: (1) sounds -stations 1, 2, 3, 7, and 8; (2) larger creeks-stations 4, 5, 6, 9, and 14; and (3) smaller creeks-stations 10, 11, 12, and 13 (Fig. 1). The sounds are the widest and deepest of the aquatic habitats compared and have, primarily, sandy bottoms. The lumps of clay occasionally trawled are exposed by erosion of the bottom. The upper creeks are relatively narrow and shallow with sand bottom, and drain the extensive Spartina salt marshes characteristic of Georgia estuaries. A considerable amount of organic detritus, settles here, and sticks and logs are commonly found on the bottom. The large creeks are generally intermediate in location and physical characteristics. Four diversity indices were calculated. The Shannon-Wiener function is: H =-X Pi logPi (1) i=n where Pi is the proportion of individuals in the i-th species. This -J H 9 SAPELO ~~SOUNUND 0 I < X 1~~~~~~~~~~~~~2 #< jW Fig. 1.-Collecting stations in estuarine system of Sapelo and St. Catherines sounds. This content downloaded from 157.55.39.83 on Sun, 09 Oct 2016 05:13:42 UTC All use subject to http://about.jstor.org/terms 384 THE AMERICAN MIDLAND NATURALIST 83(2) formula has been used widely as a diversity index, as suggested by Margalef (1957). The Shannon-Wiener function increases as both the number of and the equitability of abundance increase. It is desirable to consider indices that treat these aspects separately since the two components of diversity may react differently to certain types of stresses. For the species richness component of diversity we selected the following: D = (S-1)/log N (2) where S is the number of and N is the number of individuals. Tbhis index was also used by Margalef (1969) in diversity studies. Relative abundance was measured by two indices. Lloyd and Ghelardi's (1964) equitability index is: