Variations in the diversity of summer estuarine fish populations in Aransas Bay, Texas, 1966–1973

Estuarine and Coastal Marine Science

(1978)

6,

495-501

Variations in the Diversity of Summer Estuarine Fish Populations in Aransas Bay, Texas, 1966-1973

Richard M. Moore” Department of Biology, Coastal Carolina College, University of South Carolina, Conway, South Carolina 29526, U.S.A. and Belle W. Baruch Institute for Marine Biology and Coastal Research University of South Carolina, U.S.A. Received 28 February I977 and in revised.form 2IJuly

Keywords: composition;

fish; estuarine fisheries; bays; Texas coast

summer;

I977

diversity

index;

species

Trawlcollections of estuarine fishes were made inAransas Bay, Texas during the years 1966-1973. This period encompassed several major environmental changes to which variations in several diversity measurements may be correlated. Over the 8-year period however, an idea of ‘average’ conditions for the bay emerged. These include a species number of about 26, an anticipated catch per hour of about 3000 individuals, and a Shannon-Weiner diversity index of about 14. The results from Aransas Bay are also discussed in reference to these from other studies of estuarine regions on the Gulf and Atlantic Coast of the United States.

Introduction Although the estuarine fish fauna of the bays of Texas is relatively well known (Gunter, 1945; Miller, 1965; Hoese et al., 1968) there have been few published accounts of long term studies, which are invaluable in establishing base-line conditions, including estimates of natural variability and for monitoring long-term effects of natural or culturally induced perturbations. The present study is concerned with the variations observed in species composition and diversity over an 8-year period in the summer estuarine fish community in Aransas Bay, Texas.

Description

of the study area

Aransas Bay is located in South Texas, a sub-humid zone where rainfall averages 30-35 inches per year and is commonly exceeded by evaporation. The bay is roughly shaped like an inverted bottle with the neck pointing south, connecting with the Gulf of Mexico through Lydia Ann Channel. Along its northern border the bay communicates with several other bays, most of which receive direct river drainage. Aransas Bay, however is isolated from these drainages and consequently normally maintains relatively high salinities (Collier & Hedgepeth, 1950). Collections

were made at two stations near channel markers 49 and 69. Both stations are “Contribution #206 Coastal Research.

of the Belle W. Baruch

Institute

for Marine

Biology

and

495 0302-3524/78/0501-0495

$01.00/O

@

1978

Academic Press Inc. (London)

Ltd.

R. H. Moore

496

near the mid-line of the bay in its deepest parts. The former station is about 14 feet deep and has a predominantly mud bottom while the latter station is in 12 feet and has increased amounts of sand with mud on the bottom. Most of the bay is surrounded by relatively shallow (3 feet) flats, which may or may not be vegetated. Methods and materials Fish were collected by means of a 35-foot standard shrimp trawl with a-inch stretch mesh wings and I-inch stretch mesh bag which was towed by the University of Texas research vessel LORENE. Tows were made by day and by night at each of the two stations. In most casesduplicate tows lasting either ro or 15 min, were made. In order to present the results in a comparable manner the catch data were adjusted to that expected from a r-h tow (catch per hour). Most collections were made as a part of the University of Texas Marine Science Institute’s summer course ‘ Ecology of Fishes ‘. Diversity indices and other calculations were performed on the University of South Carolina’s IBM 360 computer. Indices calculated were the Shannon-Wiener Index (H”), the Maximum Information Index (H,,,), Evenness (J), Margalef’s Species Richness Indess (D), and Hurlbert’s Probability of Interspecific Encounter (PIE). These were evaluated using the following formulae: H”

=

InPi,

&i

H ma* = In S,

Y

= H”IHn,,,

D

= S/In N,

where S is the total number of species in the collection, N is the total number of individuals in the collection, ni is the number of individuals in the i-th species (i = I to S) and pi is the proportion of individuals in the i-th species (pi = q/N). Results Table I presents the various indices calculated for the Aransas Bay summer fish community in 1966 through 1973. These indices were chosen because they have the widest acceptance TABLE I. Aransas Bay diversity Index/year Numbers of species Maximum information (Hmax) Margelef (D) Shannon-Wiener (H”) Evenness (E) Probability of interspecific encounter (PIE) Number of individuals (N) catch per hour

data 1966-1973

1966

I967

1968

1969

1970

1971

1972

1973

25

28

24

28

21

28

36

20

3.22 7’77 2.13 0.66

3’33 8-40 2.04 0.61

3.18. 7’55 1.41 0.44

3’33 8.40 1-38 0.41

3’04

0.60

0.72

0.57

0.53

1082

3256

3998

3439

I’77 o-58

3’33 8.40 2.04 0.61

3.58 10.05 2'10 0'59

3.00 6.68 I .56 0.52

0.65

0.68

0.71

0.64

6.90

3221

3002

5322

2205

Variations

in diversity

of summer fish populations

497

and use, thus permitting comparisons with other published studies on estuarine fishes (Bechtel & Copeland, 1970; Dahlberg & Odum, 1970; McErlean et al., 1971) and are commonly considered to be the most informative (McErlean & Mihursky, 1969; Pielou, 1975). Table 2 presents a composite list of the species collected in order of their relative abundance. TABLE 2. List of species in order of decreasing abundance collected Bay, Texas 1966-1973 I. 2.

3. 4. 5. 6. 7. 8. g. IO. I I. I 2. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

24. 25.

Micropogon undulatus (Linnaeus) Leiostomus xunthurus Lacepede Anchoa mitchilli (Valenciennes) Arius felis (Linnaeus) Lagodon rhomboides (Linnaeus) Cynoscion arenarius Ginsburg Citharichthys spilopterus Gunther Orthopristis chrysoptera (Linnaeus) Etropus crossotus Jordan and Gilbert Polydactylus octonemus (Girard) Brevoortia patronus Goode Bugre marinus (mitchill) Porichthys porosissimus (Valenciennes) Sphoeroides parvus Shipp and Yerger Puralichthys lethostigma Jordan and Gilbert Prionotus tribulus Cuvier Trichurus lepturus Linnaeus Synodus joetens (Linnaeus) Ancylopsetta quadrocellata Gill Larimus fasciutus Holbrook Dasyatis sabina Lesueur Cynoscion nebulosus Cuvier Bairdiella chrysura (Lacepede) Achirus lineatus (Cinnaeus) Trinectes maculatus (Bloch and Schneider)

from Aransas

26. Chloroscombrus chrysurus (linnaeus)

27. Chilomycterus schoepji (Walbaum)

Ophistonema oglinum (Lesueur) 28. 29. Peprillus paru (Linnaeus) 30. Menticirrhus americanus (Linnaeus) 31. Symphurus plagiusa (Linnaeus) 32. Chaetodipterus faber (Broussonet) 33. Goboionellus hastatus Girard 34. Centropristis philadelphica (Linnaeus) 35. Stenotomus caprinus Bean 36. Aleutera schoepfi (Walbaum) 37. Elops sauyus Linnaeus 38. Ophidion welshi (Nichols and Breder) 39. Opsanus beta (Goode and Bean) 40. Mugil cephalus Linnaeus 41. Selene vomer (Linnaeus) 42. Lagocephalus laevigatus (Linnaeus) 43. Paralichthys albigutta Jordan and Gilbert 44. Abudefduf saxatilis (Linnaeus) 45. Eucinostomus gulu (Quoy and Gaimard) 46. Lobotus surinamema’s (Bloch) 47. Peprilus burti Fowler 48. Rhinoptera bonasa (Mitchill) 49. Scomberomorus maculatus (Mitchill) SO* Syngnathus louisianae (Gunther)

The total number of individuals collected was fairly constant (3000-4000) in most years, however relatively few individuals were collected in 1966 and 1973 and considerably more than average were collected in 1972. The number of species (8) collected ranged from 20 in 1973 and 21 in 1970 to a high of 36 in 1972. In most years there were 24-28 speciespresent. The Shannon-Wiener Index ranged between 1.38 in 1969 and 2.13 in 1966, with an overall eight-year average of 1.80. The highest value for H,,,,, (3.58) occurred in 1972 and the lowest (3.04 and 3.00) in 1970 and 1973 respectively. Species richness (0) also exhibited this same pattern of variation. Evenness (J) was greatest in 1966 and least in 1969. The PIE index also varies along this same pattern, however the 1966 value for PIE instead of being the highest was relatively low; the 1967 value for PIE was the highest and that for 1969 the lowest. Two basic patterns for variations were apparent in the indices calculated. The number of species (8, H,,,, and Species Richness (0) all followed the same pattern with the lowest values in 1973 and the highest in 1972. Any one of these three parameters could be graphed by the same line (see Figure I) by altering the values of the y-axis. Similarly the parameters H”, j”, and PIE f o11ow a second pattern of similarity (with the differences in PIE noted above). Because of these similarities only two of the six parameters, H” and S, are illustrated in Figure I along with N, the total number of individuals (catch per hour) and the surface salinities at the collection sites for 1967 through 1973. Concurrent salinity data for 1966 were not available.

498

R. H. Moore

Discussion A comparison of the values of H” obtained in this study with diversity values from other Texas bays and from estuarine habitats elsewhere on the east coast of the United States indicates that the Aransas Bay values are among the highest published values for estuarine fishes. Bechtel & Copeland (1970) reported values for Galveston Bay. Texas which averaged 1.175 during the summer months. The same authors also calculated diversity values for Aransas Bay based on 1964 data published by Miller (1965) and Hoese et al. (1968) and for Cor pus Christi and Redfish Bays based on unpublished data with no year specified. The values for these three bays (a-09, 1.52 and 1.33 respectively) are included in Figure I. Values for the latter two bays are arbitrarily placed between 1967 and 1968. Bechtel & Copeland (1970) attributed the lower values from Galveston Bay to the greater degree of industrial pollution in that bay in contrast to the South Texas bays like Aransas, Corpus Christi and Redfish.

6466

67

68

69

70

71

72

73

Figure I. Total number ( x IOOO), numbers of species, Shannon-Wiener Diversity, and surface salinities for Aransas Bay fish collections 196671973. Diversity index and salinity (x0) for Aransas Bay 1964 (open triangle) based on data from Miller (1965). Indices for Corpus Christi (cc) and Redfish (rf) Bays from Bechtel & Copeland (1970). Studies of estuarine fish communities conducted elsewhere in the eastern United States have generally found that fish species diversity is lower than has been found in Aransas Bay, Texas. A comparison of Shannon-Wiener diversity values from these other published studies is given in Table 3. A number of factors might be responsible for these differences. The fluctuations in temperature and salinity that are characteristic of most estuarine systems, particularly the small tidal creeks or embayments representative of the study sites of Dahlberg & Odum (1970) and McErlean et al. (rg$3), represent environmental stress which may exclude the more stenothermal or stenohaline species from the affected areas. Fish communities with low diversities and which are dominated by only one or two species are commonly associated

Variations in diversity of summer jish populations

TABLE

Diversity

3. Comparison of number of species (S) and Shannon-Wiener Species Values (H”) from various estuarine areas, East Coast of the United States

Location

Source

Aransas Bay, Texas Aransas Bay, Texas Corpus Christie Bay, Texas Redfish Bay, Texas Galveston Bay, Texas Apalachicola Bay, Florida Salt Marsh Estuary North Inlet, S. C. Cape Fear River, N. C. Grass Flat, N. C. Tidal Creek, Virginia Mystic River, Conn. Narragansett Bay, R. I.

499

Miller (1965) Present Study Bechtel & Copeland (1970) Bechtel & Copeland (1970) Bechtel & Copeland (1970) Livingston (I 976) Dahlberg & Odum (1970) Cain & Dean (1976) Copeland & Birkhead (1972) Adams (I 976) McErlean et aI. (1973) Haedrich & Haedrich (1974) oviatt&

Nixon(I973)

s

Diveristy

16

2.09

49 17 9

1.80" I ‘52

7: 70 32 70 39 na 23 99

1’33 I.18 1.12** 1'3-1'7

2.0-2’4 2.02** 1.04

0’9 0'33-1'03 2'5-2'7

* S-year average. ** average for entire year. na, number of species not available.

with such stressed habitats (Nixon & Oviatt, 1973 ; Livingston, 1976). The relatively high diversity values reported by Cain & Dean (1976) f rom a tidal creek in North Inlet, South Carolina are probably due as much to their collecting methods (block netting and poisoning the creek) as to environmental differences inherent in the estuarine system they studied. Although their results may, in fact give a better estimate of estuarine fish populations in these shallow creeks, they are not strictly comparable with the other studies because of the differences in collecting methods. Larger bodies of water, which are physically less susceptible to change, provide a more stable habitat which might be expected to yield higher diversity values, all other factors being the same. Aransas Bay, as well as other large bays such as Narragansett Bay, Rhode Island (Oviatt & Nixon, 1973) and the estuary of the Cape Fear River, North Carolina (Copeland & Birkhead, 1972) represent such large bays and possess the highest values for H”. The higher diversities associated with these inherently more stable systems may be modified by other factors, such as pollution, to produce lower species diversity values (Bechtel & Copeland, 1970; Haedrich & Haedrich, 1974). One additional factor, which as yet has not been satisfactorily investigated for fish communities, is the possibility of latitudinal gradients in species such as those discussed by Pianka (1966). The 8-year period covered by this study included a number of major environmental changes that might be expected to exert considerable effect on the diversity and composition of the Aransas Bay fish Community. Major changes in salinity regimes may have rapid influence as demonstrated by Hoese (1960). During the study period two major hurricanes, a tropical storm, a very ‘wet’ spring, and the beginning of a drought caused considerable variation in the Aransas Bay salinities (Figure I). Hurricane BEULAH in late summer 1967 contributed about 20 inches of rain in a little more than 24 h lowering salinities to levels that were still noticeable one year later. Accompanying this drop in salinity were the relatively low values for H” observed in 1968 and 1969. The number of species was also reduced somewhat in 1968, but had recovered by the following year. Following these rains salinities increased for the next three years. Diversity (H”) increased during this period; however the number of species (5’) decreased to its lowest

500

R. H. Moore

value in 1970 just prior to hurricane CELIA which again caused a reduction in salinity. At the peak of salinity (33x0) in 1970 several stenohaline species, normally not encountered in Aransas Bay, such as Larimus fasciatus and Stenotomus caprinus, were collected. Besides altering salinity conditions hurricanes also exert a considerable ‘scouring’ effect on estuaries. While this was particularly noticeable among the invertebrates following both BEULAH and CELIA (unpublished data) the latter hurricane appears to have had little effect on the fish community except for lowering the salinity and the exclusion once again of stenohaline, Gulf species. Temperature also appears to have exerted some influence over the fish community in Aransas Bay. 1 have shown elsewhere (Moore, 1975, Figure I) that the period 1967-1972 was marked by an increase in summertime water temperatures which could be correlated withthe occurrence of several species of tropical fish in the inshore Gulf of Mexico. Not only were temperatures higher in 1972 but they also remained above 25 “C for a longer period of time than in the previous years. The composition of the Aransas Bay fish community did not become conspicuously tropical in 1972 and only one species which was normally absent from the bay (Centropristis philadelphica) occurred there in that year. The greater number of species, however, and the greater diversity found in 1972 was apparently due to increased numbers of species such as Syngnanthus louisianae, Eucinostomus gula, Mugil cephalus, and Lagocephalus laevigatus normally found on the grassflats and shallows surrounding the bay. Perhaps these and other species such as pipefish, spiny boxfish, and filefishes which were notably abundant in 1972 had moved into the open baywaters to escapethewarmer temperatures on the flats. In conclusion, the 8 year period 1966-1973 encompassed several major climatological changes which presumably should affect the estuarine fish fauna. Evidence is presented for such effects due to salinity and temperature changes. Over the entire period, however, these extreme conditions should tend to average out to something that might be called ‘normal’. Based on these data then normalcy for Aransas Bay during this period would include 24-28 species of fish, a catch per hour of about 3500 individuals, and a Shannon-Wiener species diversity index of about 1.80. Future variations from these values may be attributable to climatic conditions or to human-caused changes such as pollution, In the present study, because most of the commonest diversity measures fell into one of two different patterns of variation, the calculation of more than two indices seems rather superfluous. The number of species and the Shannon-Wiener Index are representative of these two patterns (which reflect changes in dominance and information content of the fish community respectively) and are widely used mother studies making them the most logical choices for inclusion. Other indices such as Evenness or PIE might be more applicable if seasonal variations within the community were to be studied or if geographically distant communities were to be compared more rigorously than here. In any case, the species composition of the community and especially the appearance of unexpected .or indicator species can often tell the investigator more about the ecological conditions than can any numerical index. Acknowledgements Most of the data presented in this study were derived from collections made as a part of the ‘Ecology of Fishes’ course taught by Dr D. E. Wohlschlag at the University of Texas Marine Science Institute, Port Aransas Laboratory. I would like to thank Dr Wohlschlag, the numerous students who participated in these collections and Mr John Thompson, the Laboratory Manager.

Variations

in diversity

of summer fish populations

501

References Adams, S. M. 1976 The ecology of eelgrass, Zostera marina (L), fish communities. I. Structural Analysis. Journal of Experimental Marine Biology and Ecology 229, 269-291. Bechtel, T. J. & Copeland, B. J. 1970 Fish species diversity indices as indicators of pollution in Galveston Bay, Texas. Contributions in Marine Science, University of Texas 15, 103-132. Cain, R. L. & J. M. Dean 1976 Annual occurrences, abundance, and diversity of fish in a South Carolina intertidal creek. Marine Biology 36,369-379. Collier, A. and Hedgpeth, J. W. 1950 An introduction to the Hydrography of tidal waters of Texas. Publications of the Institute of Marine Science, University of Texas I, 125-194. Copeland, B. J. & W. S. Birkhead 1972 Some ecological studies of the Lower Cape Fear River Estuary, ocean outfall, and Dutchman Creek, 1971. First Annual Report, Carolina Power and Light Co. Raleigh, N. C. Contr. No. 71-q. 105 pp. Dahlbere. M. D. & Odum. E. P. 1970 Annual cvcles of sDecies occurrence. abundance. and diversitv in Gkorgia estuarine f&h pop&ions. Amekan Mid&d Naturalist S;, 382-392. ’ Gunter, C. 1945 Studies on marine fishes of Texas. Publications of the Institute of Marine Science, University of Texas I, 1-190. Haedrich, R. L. & Haedrich, S. 0. 1974 A seasonal survey of the fishes in the Mystic River, a polluted estuary in downtown Boston. Estuarine and Coastal Marine Science 2, 5973. Hoese, H. D. 1960 Biotic changes in a bay associated with the end of a drought. Limnology and Oceanography 5326336. Hoese, H. D., Copeland, B. L., Moseley, F. N. & Lane, E. D. 1968 Fauna of the Aransas Pass Inlet, Texas III. Die1 and seasonal variations in trawlable organisms of the adjacent area. TexasJournal of Science 20,33-60. Hurlbert, S. H. 1971 The nonconcept of species diversity: a critique and alternative parameters. Ecology 52, 577-586. Livingstone, R. J. 1976 Diurnal and seasonal fluctuations or organisms in a North Forida estuary. Estuarine and Coastal Marine Science 4, 373-400. McErlean, A. J. & Mihursky, J. A. rg6g Species diversity-species abundance of fish populations: an examination of various methods. Proceedings of the zznd Annual Conference, Southeastern Game and Fish Committee, pp. 367-372. McErlean, A. J., O’Conner, S. G., Mihursky, J. A. & Gibson, C. I. 1973 Abundance, diversity, and seasonal patterns of estuarine fish populations. Estuarine and Coastal Marine Science I, 19-36. Miller, J. M. 1965 A trawl survey of the shallow gulf fishes near Port Aransas, Texas. Publication of the Institute of Marine Science, University of Texas IO, 80-107. Moore, R. H. 1975 O&urrence of tropical marine fishes at Port Aransas, Texas 1967-1973, related to sea temperatures. Copeia 1975(1), 170-172. Nixon, S. W. & Oviatt, C. A. 1973 Ecology of a New England salt marsh. Ecology Monographs 43, 463-498. Oviatt, C. A. & Nixon, S. W. 1973 The demersal fish of Narragansett Bay: an analysis of community structure, distributions, and abundance. Estuarine and Coastal Marine Science I, 361-378. New York, 163 pp. Pielou, E. C. 1975 Ecological Diversity. Wiley-Interscience,