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Coral reefs, grass beds, and sand flats are important ecosystems all of which are interrelated. With this in mind, the reefs, grasses, and sand flats supply important food, habitiat, and cover to a variety of fish species both adult and juvenile. Grass beds have been found to be important nursery areas for juvenile fish (Robblee and Zieman 1984). Fish exhibit unique behaviors in their use of coral reefs, grass beds, and sand flats in relation to the changes in daylight. Various studies have been done on the diurnal shift of fish species and their composition change in coral reef systems and grass bed systems. It has been found that a diurnal shift does occur in coral reefs and grass beds of tropical ecosystems (Sogard et. al. 1989). Many coral reef fish species migrate at night to forage in grass beds (Sogard et. al. 1989) and sand flats (Burke 1995). A shift in fish species between day and night is very apparent in the coral reef and grass beds during the periods of dawn and dusk (Colton and Alevizon 1981). Given the information from previous studies regarding diurnal changes and changes during the dawn and dusk periods, we focused our study on comparing the time periods of morning, afternoon, and evening on fish diversity and composition change in three ecosystems, the artificial reef, grass beds, and sand flats in Grahams Harbor, San Salvador, Bahamas. We hoped to answer the question: At what time of day does the greatest fish diversity and composition change occur at three different ecosystems, sand flats, grass beds, and artificial reef, in GrahamÕs Harbor? To answer our question we formulated two hypotheses related to fish diversity:
1) There is a greater fish diversity in the evening in the reef and grass bed.
2) There is a greater fish diversity in the morning in the sand.
We also formulated another set of hypotheses related to fish composition:
1) The rank orders of individual fish species in the sand flat are independent
2) The rank orders of individual fish species in the grass bed are independent
3) The rank orders of individual fish species in the artificial reef are independent
Methodology:
The study was conducted between June 17 and June 20, 2002 in the sand flats, grass beds, and artificial reef ecosystems of Grahams Harbor, located approximaely on the northeastern shoreline of San Salvador, Bahamas. In each of the three ecosystems, two transects were positioned for a total of six transects. We had three sets of transects. One set contained a grass transect and a sand transect, the second set was identical to the first set, and the third set contained two reef transects. Each individual transect was 20 meters long. Within each set, the transects were approximately 30 meters apart. Each set of transects were 30 meters apart except the second set which was approximately 60+ meters apart from the reef. The grass beds and sand flat ecosystems were parallel to the beach in order to maintain a constant ecosystem that did not overlap with the other (Figure 1). Large conk shells wrapped in orange marking tape were put at each end of four of the six transects to indicate transect boundaries. At the reef, orange marking tape was positioned on the reef to indicate boundaries. All six transects were sampled three times, once at 6:30am, 11:00am, and 10:00pm, for a period of ten minutes. In order to locate the transects positioned in the water, orange marking tape was attached to vegetation along the shoreline in the same fashion as conk shells were used in the water to indicate transect boundaries. The shoreline markings were inline with the water boundaries. Data was recorded each time noting the species of fish in the ecosystem and the number of individuals per species. All data was compiled into specific ecosystems and time of day. The Spearman rank test, with an alpha value of 0.1, was used to analyze the data.
Results:
Figure 1. Illustration of transect layout in Grahams Harbor, San Salvador, Bahamas. Orange dots indicate transect boundaries.
Figure 2. Overall fish diversity found in grass bed transects of Grahams Harbor, San Salvador, Bahamas in the morning, afternoon, and night sampling periods. Diversity includes type of species found and number of individual species.
The greatest overall fish diversity in the grass beds occurs in the morning as is evident from Figure 2.
Figure 3. Total number of individual fish species found during the morning, afternoon, and night sampling periods in grass bed transects one and two in Grahams Harbor, San Salvador, Bahamas.
The abundance of specific fish species and the total number of each species found in the grass bed during the specific time periods studied, morning, afternoon, and evening is illustrated in Figure 3. The Slippery Dick is the most abundant fish during the morning. In the afternoon the number of Slippery Dick fish decline while the number of Sandy Tile fish remain the same and a juvenile Yellowtail Damselfish is apparent. No fish were found during the evening.
Figure 4. Overall fish diversity found in sand flat transects of Grahams Harbor, San Salvador, Bahamas in the morning, afternoon, and night sampling periods. Diversity includes type of species found and number of individual species.
The greatest overall fish diversity in the sand flats occurs in the afternoon as is evident from Figure 4.
Figure 5. Total number of individual fish species found during the morning, afternoon, and night sampling periods in sand flat transects one and two in Grahams Harbor, San Salvador, Bahamas.
The abundance of specific fish species and the total number of each species found in the sand flats during the specific time periods studied, morning, afternoon, and evening is illustrated in Figure 3. Juvenile fish are the most abundant fish overall but also the most abundant during the afternoon. The juvenile Yellowtail Damselfish is also more abundant in the afternoon as are the Sandy Tile fish. A small blue fish, species unknown, was found during the evening. No fish were found during the morning.
Figure 6. Overall fish diversity found in artificial reef transects of Grahams Harbor, San Salvador, Bahamas in the morning, afternoon, and night sampling periods. Diversity includes the number of types of species found in each ecosystem.
The overall abundance of fish in the reef during the specific time periods studied, morning, afternoon, and evening is illustrated in Figure 4. The greatest abundance of fish is during the afternoon.
Figure 7. Total number of individual fish species found during the morning, afternoon, and night sampling periods in artificial reef transects one and two in Grahams Harbor, San Salvador, Bahamas.
The abundance of specific fish species and the total number of each species found in the reef during the specific time periods studied, morning, afternoon, and evening is illustrated in Figure 7. Although the afternoon period had the most abundance of total fish, there a noticeable changes in composition changes between time periods. Bar Jacks (total 88) are in greatest abundance followed by juvenile Wrasses (total 77) and Blue-Stripped Grunts (total 63). In the afternoon there is a switch with juvenile Wrasses being dominate (total 104) followed by Bar Jacks (total 8), and Yellowtail Snapper (total 53). In the evening those fish in abundance during the morning and afternoon are no longer seen and are replaced by Blackbar Soldierfish (total 55), most abundant, Tomtate (total 35), and Bigeye (total 31).
Table 1. Spearman rank test ( µ= 0.1) results comparing the fish diversity and composition change to the varying times of day in the sand flat, grass bed, and artificial reef ecosystems.
Between Morning/Afternoon Between Afternoon/Evening BetweenMorning/Evening
SAND
P-value 0.386 > µ (0.1) 0.4884 > µ (0.1) 0.2253 > µ (0.1)
GRASSBED
P-value 0.386 > µ (0.1) 0.4884 > µ (0.1) 0.2253 > µ (0.1)
REEF
P-value (<0.0001) < µ (0.1) (0.0153) < µ (0.1) 0.2907 > µ (0.1)
Rho-Value 0.684 -0.346
Results from the Spearman rank test are given in Table 1. Significance is indicated by the p-value being less than the alpha value (0.1). As seen in the table, significance occurs only at the reef during the periods between morning and afternoon and between afternoon and evening. Due the significance, the rho values of the previously stated time periods can be accepted, which suggest negative and positive correlations occurring during those time periods. Negative correlation suggests there to be an inverse relationship where the most abundant species in the afternoon were the least abundant species in the evening. A positive correlation suggests that the most abundant species during the morning were the most abundant during the evening. These correlations are illustrated in Figure 7.
Discussion:
A number of studies have found a relationship between the time of day and the diversity and composition change of fish species in tropical grass bed and reef ecosystems. In determining the greatest diversity in our study locations, we considered the diversity of an ecosystem to include both the type of species found and the number of the individual species (Figures 2, 4). In order to clearly illustrate the diversity for the artificial reef we considered the diversity to refer to the number of types of species found in each transect (Figure 6). As evident from Figure 2, the greatest fish diversity in the grass bed occurred in the morning; however, when looking at Figure 3 there was an additional species in the afternoon. Therefore we could consider there to be an evenness in diversity between both the morning and afternoon, or we could consider there to be a lack of sufficient evidence to make an accurate assumption regarding the diversity of the grass bed. In the sand flat, the greatest diversity occurred in the afternoon (Figures 4,5).
As suggested from the Spearman rank test, a significant relationship was found to occur in fish composition during the periods between morning and afternoon and between afternoon and evening in the artificial reef system, a relationship was not found to occur in either the sand flats or the grass beds (Table 1). Despite the lack of a relationship in the sand and grass beds, there was noticeable occurrence in the sand flats. The fish found in this ecosystem were predominantly juveniles (Figure 5), which strengthens the notion of sand flats being used as feeding grounds and nursery areas for juvenile fish (Robblee and Zieman 1984). As seen Figure 5, these fish are the most abundant during the afternoon possibly due to larger amounts of phytoplankton being available due to an increase in the amount of sunlight available as compared to the morning and evening. Also in both sand and grass bed ecosystems (Figures 3 and 5), the fish found both in the morning and afternoon were not at all present during the evening suggesting that these fish feed during the day and shelter at night primarily in the sediment (Robblee and Zieman 1984).
The artificial reef exhibited many of the common diurnal behaviors found to be associated with the fish species dwelling in this habitat. There was an apparent difference in the distribution and abundance of fish species during the daytime and during the evening (Figure 7). This finding was consistent with a study done by Colton and Alevizon (1981) where a difference was also detected in distribution and abundance of fish species during day and night hours. As evident in Figure 7, the species composition changes in the evening where nocturnal fish, with the characteristic larger eyes and larger mouths, are more abundant. Common nocturnal fish species found in this study include snappers, cardinalfish, and squirrelfish. Other studies have found such fish to be common nocturnal species as well (Robblee and Zieman 1984).
Overall the first set of hypotheses related to diversity were all rejected concluding that the greatest diversity occurred during the afternoon for all ecosystems rather than during the evening at the reef and grass bed and during the morning at the sand flats (Figures 2,3,4). We failed to reject the second set of hypotheses for the sand and grass bed time periods and for the period between morning and evening in the reef, which concluded that the rank orders were in fact independent. We rejected the null hypothesis for the time periods between morning and afternoon and afternoon and evening in the reef, concluding that there was a correlation with rank orders. The time period between morning and afternoon had a positive correlation meaning the most abundant species during the morning were also the most abundant during the afternoon. The time period between afternoon and evening had a negative correlation meaning the most abundant species during the afternoon were the least abundant in the evening.
Many of the results obtain from this study were consistent with previous studies related to diurnal behaviors of tropical fish species; however, errors did occur resulting in inaccuracies in our findings. There were inconsistencies in the distance between transects. The grass and sand transects closest to the reef (Figure 1) were approximately 60+ meters away from the reef rather than being 30 meters as it was from the first set of transects farthest from the reef. This distance may have limited the numbers and types of fish species found in the grass bed and sand particularly during the evening and morning periods when coral reef fish use the grass beds and sand for feeding. Also the times during which we sampled were after the periods of greater activity and changer-over behavior, therefore possibly limiting the number and types of fish species found. The sand transects were set approximately 20 meters off shore. This may have been too close to the shoreline, again limiting the abundance of fish. We might have had better results had we started the transects farther out in the harbor. Other errors include only sampling the transects once during the morning, afternoon and evening. Our results possibly would have been improved if multiple samples had been taken each day at different time periods for each transect rather than only having one sample of each time period. Also the time samples are taken is important. During the early morning hours just before sunrise and during sunset possibly would contribute to more accurate results. Other possible errors include too much movement when sampling possibly disturbing fish and keeping them from coming near and too short of a time period sampling.
References:
Burke, N.C. 1995. Nocturanl foraging habitats of French and Bluestriped Grunts, Haemulon
Flavolineatum and H. sciurns, at Tobacco Cayne, Belize. Environmental Biology of
Fishes 42:365-374.
Colton, D.E. and W.S. ALevizon. 1981. Diurnal variability in a fish assemblage of a Bahamian
coral reef. Environmental Biology of Fishes 6:341-345.
Robblee, M.B. and J.C. Zieman. 1984. Diel variation in the fish fauna of a tropical seagrass
feeding ground. Bulletin of Marine Science 34:335-345.
Sogard, S.M, G.V.N. Powell, and J.G. Holmquist. 1989. Utilization by fishes of shallow,
seagrass-covered banks in Florida Bay: 2. Diel and tidal patters. Environmental Biology
of Fishes 24:81-92.
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