ATOLL RESEARCH BULLETIN No. 153
FISH IIIVERSITY ON A CORAL REEF IN THE VIRGIN ISLANDS
by Michael
J. Risk
Issued
by
TEE SMITHSONIAN INSTITUTION Washington,,D. C., U. S. A.
3ec enber
31,
:i.3 7
2FISH DIVERSITY ON A CORAL REEF IN THE VIRGIN ISLANDS
by Michael J. Risk'
INTRODUCTION
T e r r e s t r i a l ecologists have accumulated data relating b i r d s p e c i e s diversity to foliage height diversity (MacArthur andMacArthur, 1961; MacArthur, 1964; Recher, 1969); complexity of foliage pattern s e e m s to be a better predictor of b i r d s p e c i e s diversity than i s the t r e e s p e c i e s diversity. T h e r e i s some indication that a s i m i l a r relationship exists on c o r a l r e e f s , between f i s h s p e c i e s diversity and substrate complexity. Talbot (1965, p. 453j, in comparing f i s h collections f r o m a r e a s with either few o r many s p e c i e s of c o r a l s present, s t a t e s "It i s probable that the g r e a t e r variety of coral s p e c i e s . . .allows f o r a m o r e varied f i s h population than the single species," and, "It is probable that the m o r e complex c o r a l population i n the mixed stand provides m o r e ecological niches than a r e available in the single s p e c i e s stand."
Hiatt and Strasburg (1960, p. 118) found m o r e s p e c i e s and individuals on "ramose" than on
"glomerate" c o r a l heads: "Because glomerate c o r a l h e a d s a r e g e n e r a l l y devoid of i n t e r s t i c e s in w!lich s m a l l o r g a n i s n ? -an hide, such heads a r e visited m l y by f i s h s p e c i e s intent on browfjins o r grazing c o r a l polyps." I11 a study on Conus, Kohn (1967, p. 257) found "Type 111 habitats a r e topographically the m o s t complex.. .and these support the m n s t divers? a s s e m - blages of Conus." The author attempted t o m e a s u r e the relationship between f i s h s p e c i e s diversity and the physical makeup of the c o r a l s u b s t r a t e , following the work done on bird species diversity.
PROCEDURE ( a j Study a v e a
The a r e a selected was a portion of a low-lying, shallow patch reef located in the e a s t e r n p a r t of G r e a t e r Lameshur Bay, St. John, Virgin Islands. Substrate types withiti the a r e a w e r e live c o r a l (Montastvea annulavis. Millepova a l c i c o d s , I'oorites jiwcata, and Acavicia agavicitesl, dead coral, sponges (mostly I ~ c i n i a spp.) and carbonate sand.
Two t r a n s e c t s were laid out in the f o r m of a "T", with the upright perpendicular to the shore, and the c r o s s p i e c e f u r t h e r offshore and parallel to the shoreline.
The "upright" t r a n s e c t began about 50 m e t e r s f r o m shore, in 4 m e t e r s 31' water, and extended 16 m e t e r s seaward, to
a
depth of 4.5 m e t e r s . The "crosspiece" t r a n s e c t was a l s o 16 m e t e r s long, and 4.5 m e t e r s deep throughout its length.' ~ e p a r t m a n t of Biological Sciences, University of Southern California, Los Angeles, California 90007
(Manuscript r e c e i v e d Feb. 1971--Eds.)
( C ) D e l e ~ m i n a l i o n of fish s p e c i e s divelmity, " I I Y
Only t e r r i t o r i a l f i s h e s w e r e considered in t h i s study. Several days w e r e spent diving a n d observing in the a r e a , in o r d e r to determ'ne which s p e c i e s were t e r r i t o r i a l . When doubt existed about a p a r t i c u l a r fish, i t was frightened into i t s hom? t e r r i t o r y s e v e r a l t i m e s over the c o u r s e of 2 o r 3 days. Only f i s h e s consistently returning to the s a m e loca- tion w e r e counted; appropriate m e m b e r s w e r e t h r e e s p e c i e s of damselfish, a goby, a squirrelfish, and occasional filefishes, tangs, juvenile grunts and snappers. All caves, c r e v i c e s , and holes viere investigated, often using a light.
Numbers of individuals of the selected s p e c i e s were then determined in e a c h of the sixteen quadrats. E a c h a r e a was counted a t l e a s t twice a day f o r a p e r i o d of t h r e e days.
T o reduce o p e r a t o r bias, p r e v i o u s values r e c o r d e d on underwater s l a t e s were left on s h o r e , a n d the quadrats counted beginning at different ends of the two t r a n s e c t s ( s e e Appendix A).
Census values w e r e in good agreement, a n d ranged f r o m 4 to 7 s p e c i e s and f r o m 7 to 21 individuals p e r s q u a r e m e t e r . F i s h s p e c i e s diversity values w e r e calculated using Shannon's f o r m u l a f o r H ' (Shannon, 1948).
( d ) Detemzinalzon ofsubstvate topogvajhic complexity, "T"
Actual s u r f a c e a r e a of a s u b s t r a t e a s c o m p a r e d to i t s horizontal projection a r e a was a s s u m e d to be a m e a s u r e of spatial heterogeneity. The technique u s e d was a m,xiifi- cation of geological "point-counting," wherein volumes of constituents a r e a s s u m e d proportional to t h e i r representation along line t r a n s e c t s . In the p r e s e n t study, actual s u r f a c e a r e a was a s s u m e d to b e a function of a c t u a l linear dimensions.
T h e author u s e d a fine-link chain, which could be draped and applied o v e r a length of one meter. This was done eight t i m e s f o r e a c h m,?ter-square quadrat; in s o m e c a s e s f o u r m e t e r s of chain were r e q u i r e d to equal one "straight" meter.
Actual s u r f a c e a r e a was a s s u m ? d proportional to the s q u a r e s of t h e s e e s t i m a t e s of a c t u a l l i n e a r dimensions; therefore, the value of T f o r any one quadrat i s the s u m of the eight e s t i m s t e s , squared. The:;,? - ~ a l u e s a r e record.?rl in .4ppend.x B.
( e ) Dete7mination ofsubstvate biological divevsity "B"
T h e s u b s t r a t e was a r b i t r a r i l y divided into s e w n consYtuents: f w ~ r s p e c i e s of living c o r a l , dead coral, spo:izes, and carbonate sand. Different s p e c i e s of living c o r a l w e r e a l m o s t always s e p a r a t e d by a band of dead coral; sponges g r e w only on this dead coral.
Eac!l consl:itu$?n: type was ass1mi.d to have a r e a l c o x r a z e proportional to i t s i n t e r s e c t length along line transects. Again, in e a c h of the sixteen quadrats, i n t e r s e c t length w e r e determined along eight one-mt?ter line t r a n s e c t s . The r e s u l t f o r each quadrat was t h e r e f o r e a total length of eight m e t e r s , divided a m m g up to seven constituents.
B was the11 calculated f r o m i h s s e data, using Shannon's formula f o r H'; values f o r e a c h quadrat a r e r e c o r d e d in Appendix C.
( f ) Data p7,ocessing
Multiple r e g r e s s i o n analysis was p.!rformt?d on the data a t the University of Southern California Computer Sciences Laboratory.
( g ) Undevwatev data g a t l z e v i n ~
The author is familiar with data-gathering by SCUBA, having conducted benthic s u r v e y s and studies in a variety of non-tropical habitats. The amount of time and energy r e q u i r e d to m e a s u r e only sixteen s a m p l e a r e a s was staggering.
F o r this study, the author spent80 hoursunderwater; including the preliminary study, more than 800 fishes were identified, and their territories noted; intersect values f o r substrate types were determined (to the nearest millimeter) along 144 m e t e r s of reef, and 1,028 mt?ters of chain were stretchedover the reef surface. To mt?asure each mt?ter- square quadrat took at least five hours.
RESULTS
Significant relationships between the three main variables (Hf, T and B) were tested for.
In addition, many subdivisions of the main variables, such a s sand cover, total coral cover (live o r dead), and total fishes, were investigated.
Significant correlation exists only between fish species diversity, Hf, and substrate topographic conlplexity, T. Results of analysis of the three m.ain variables a r e given in Table 1.
Table 1. CORRELATION COEFFICIENT MATRIX
*p
<
0.05Regression Equation: Hf = 1.06 + 0.004Ti 0.108B Multiple correlation coefficient 0.633.
Hf = fish species diversity ("nits") T
=
substrate topographic complexity B = substrate biological diversityThe regression coefficient for T i s highly significant (p (0.01); that f o r B is not Significant.
Increase in the mllltiple correlation coefficient on addition of the B effects to those of T i s only 0.009.
4
DISCUSSION
Although the number of s a m p l e s is quite sm.111, t h e r e exists a striking positive correlation between f i s h s p e c i e s diversity and degree of s u b s t r a t e topographic complexity. T h i s r e s u l t i s in accordance with predictions f r o m t e r r e s t r i a l bird studies.
It is interesting that there is no significant correlation between fish s p e c i e s diversity and the biological nature of the substrate; the choice of a t e r r i t o r y would s e e m to be controlled by predation p r e s s u r e , r a t h e r than by feeding preferences.
T h e r e is a l s o no significant positive correlation between total numbers of fishes and e i t h e r s u b s t r a t e topographic com:?lexity o r s u b s t r a t e biological diversity; the relationship does not appear to be a simple one of "more holes, m o r e fish.'' T h e r e apparently is occurring s o m e selective partitioning of the habitat by individual f i s h species.
ACKNOWLEDGMENTS
Field work was c a r r i e d out during the s u m i n e r of 1969, when the author was a Teaching Assistant f o r the Organization f o r T r o p i c a l Studies course in Tropical Marine Biology. This was a n invaluable opportunity to study and work on c o r a l r e e f s .
Drs. Gerald J. Bakus and P e t e r W. F r a n k have been generous of t h e i r time in reading and criticizing the m:~nuscript. Any remaining e r r o r s o r omissions a r e the author's.
LITERATURE CITED
Hlatt, Robert W. and Donald W. Strasburg, 1960. Ecologrcal relatronshrps of the fish fauna on c o r a l r e e f s of the Marshall Islands. Ecol. Monographs 30: 65-127.
Kohn, Alan J. 1967. Environmental complexity and s p e c i e s diversity in the gastropod genus Conus on Indo-West Pacific reef platforms. A m s r . Nat. 101: 251-260.
MacArthur, R. and J.W. MacArthur. 1961. On b i r d s p e c i e s diversity. Ecology 42: 594-598.
MacArthur, R. 1964. Environmental patterns affecting bird s p e c i e s diversity. Amer. Nat.
98: 387-397.
Planks, E.R. 1966. Latltudmal gradients in s p e c l e s diversity. a revlew of concepts. Amer.
Nat. 100: 33-46.
Randall, John E. 1963. An analysis of the f i s h populations of artificial and natural r e e f s in the Virgin Islands. Carib. J. Sci. 3: 31-47.
Recher, H a r r y F. 1969. Bird s p e c i e s diversity and habitat diversity in Australia and North Africa. Amer. Nat. 103: 75-80.
Shannon, F.W. 1948. The commonness, and r a r l t y , of species. Ecology 29: 254-283.
~ a l b o t , F.H. 1965. A description of the c o r a l s t r u c t u r e of Tutia Reef (Tanganyika T e r r i t o r y , E a s t A f r i c a ) and i t s f i s h fauna. Proc. Zool. Soc. London 145: 431-470.
APPENDIX A
POPULATIONS O F TERRITORIAL FISHES IN SAMPLE AREA
T r a n s e c t No. 1 { ~ e r p e n d i c u l a r to s h o r e )
-- ----
Quadrat No.
Total No. of f i s h e s
T r a n s e c t No. 2 ( P a r a l l e l to shore)-
--
-Quadrat No.
Total No. of f i s h e s
APPENDIX B
VALUES O F T , SUBSTRATE 'TOPOGRAPHIC COMPLEXITY (rn2)
T r a n s e c t No. 1
Q u a d r a t No.
-
T1 (shallow) 75.5
2 95.7
3 28.2
4 97.2
7 103.1
8 (deep) 37.2
T r a n s e c t No. 2 Q u a d r a t No.
-
T1 (north) 36.5
2 73.6
3 21.3
4 44.2
5 35.4
6 84.1
7 53.7
8 (south) 60.0
APPENDIX C
VALIJE O F B, SUBSTRATE BIOLOGICAL DIVERSITY ("nits")
T r a n s e c t No. 1 T r a n s e c t No. 2
Q u a d r a t No.
-
B Q u a d r a t No.-
B1 (shallow) 1.34 1 (north) 1.21
2 1.23 2 1.40
3 1.17 3 1.06
4 1.12 4 1.34
5 1.34 5 1.43
6 0.98 6 1.23
7 1.11 7 1.68
8 (deep) 1.17 8 (south) 1.08
