An Investigation Into How Two Factors, Average Depth and Water
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AN INVESTIGATION INTO HOW TWO FACTORS, AVERAGE DEPTH AND WATER EXCHANGE, AFFECT JUVENILE FISH POPULATIONS IN THE WAI OPAE TIDEPOOLS AT KAPOHO VACATIONLAND ON THE ISLAND OF HAWAI’I. Kosta Stamoulis Abstract: Depth and water exchange are the two main physical factors that differ servation District (MLCD). This makes the area off limits to Þshermen between tidepools at Kapoho on the Island of Hawaii. Eight tidepools and aquarium collectors. Baseline data is needed to compare to future that have different combinations of these two factors were examined to surveys in order to monitor the effects of the MLCD designation on the determine the effects that depth and water exchange have on juvenile juvenile Þsh populations. My Þrst objective was to obtain comprehen- reef Þsh distribution. Each tidepool was surveyed a total of Þve times sive data on the density of the juvenile Þsh populations at Wai Opae. during the month of July 2003. Average depth, water exchange and an There is increasing literature on how the distribution of habitat, interaction between the two factors were all found to have a signiÞcant both in size and spatial arrangement, can affect population abundance effect on the abundance of juvenile Þsh in the tidepools. Further analy- and occupancy patterns (PÞster, 1998). The Wai Opae tidepools vary in sis was done to determine the tidepool characteristics preferred by each terms of their size, shape, and spatial arrangement in regard to the ocean. juvenile Þsh species observed. The principle physical characteristics that vary among the tidepools are average depth and amount of water exchange with the open ocean. These Introduction: two factors are likely to have signiÞcant effects on the populations of ju- The Wai Opae tidepools are located just south of Kapoho Bay, which venile Þsh living in the pools. The increase in species diversity with an is located 20 nautical miles southeast of Hilo, on the island of HawaiÕi. increase in island or insular habitat area is well documented empirically The area is dotted with clear, shallow tidepools and small coral reefs, (PÞster, 1998). Therefore I expected to see an increase of diversity in which support an abundance of marine life (Ford, 1973). Data collect- the deeper pools. I hypothesized that though the open pools might have ed by the West HawaiÕi Aquarium Project (WHAP) in this area over a greater overall diversity because they are more accessible, the juvenile three-year period, suggests that these tidepools may provide a nursery Þsh populations would be smaller because they prefer more sheltered function for local reef Þsh populations, due to the high percentage of areas. In the system described in PÞster (1995), habitat size (tidepool juvenile Þsh in this area. volume) correlates positively with population size. My second objec- The ecological processes operating in nursery habitats, as com- tive was to examine the effects of tidepool depth and amount of water pared with other habitats, must support greater contributions to adult re- exchange on the juvenile Þsh populations at Wai Opae. cruitment from any combination of four factors: (1) density, (2) growth, The houses in the Vacationland HawaiÕi subdivision rely exclusive- (3) survival of juveniles, and (4) movement to adult habitats (Beck et al., ly on cesspools or septic tanks for domestic sewage disposal, potentially 2001). I investigated the Þrst of these factors within my study. The fact impacting Wai Opae through seepage of wastewater. Suspicions of sew- Wai Opae is a juvenile Þsh habitat as well as a popular Þshing location age leakage into the tidepools were conÞrmed (Flanders, 2002). Sewage makes this area a prime candidate for protection, and roughly half of can affect the abundance, mortality, fecundity and size of Þsh, lead to the Wai Opae area has recently been designated as a Marine Life Con- toxic effects, increase susceptibilities to infections and alter behavioral 41 responses (Smith et al., 1999). How does cesspool leakage affect juve- ation, I considered them juvenile if they were less than 10 cm, assuming nile Þsh populations at this site? With the new understanding of the af- of course that their adult size was more than 20cm. For smaller Þsh with fects of physical characteristics of the tidepools, this question can be ad- no distinctive juvenile coloration I considered them juveniles if they dressed. In this way, variations in the juvenile Þsh populations unrelated were less than 5 cm. After separating out the juvenile data I performed a to sewage can be accounted for, and speciÞc tidepools where sewage has Shannon-Wiener diversity index for each pool individually. I then calcu- a negative effect on juvenile Þsh populations can be identiÞed. My third lated the juvenile/adult ratio for each pool. Using Minitab, a statistical objective was to provide tide pool-speciÞc population data accounting analysis program, I performed a General Linear Model (GLM) ANOVA for depth and water exchange so that the effects of water quality, i.e. test using the total juvenile counts by survey for each tidepool. This sewage leakage, on juvenile Þsh populations can be identiÞed. gave me a sample size of 40. In order to satisfy the assumptions of the I hypothesized that there are correlations between tidepool depth ANOVA I had to transform the data using a square root transformation. and level of exposure on the distribution of juvenile Þsh populations. My I used exposure and depth as my Þxed factors and survey number as goal was to Þnd out if such relationships existed and if so, how these two my random factor. This test told me which factors were signiÞcantly factors affected juvenile Þsh allocation. affecting the assemblage as a whole. I followed up the ANOVA with a TukeyÕs pairwise comparison test, which showed me what kind of effect Methods: each factor was having. After this I used the ANOVA and a main effects The study had a two-factor design. Eight tidepools were used in this plot to test each of the juvenile species to see which ones were affected study. Four of the tidepools were ÒclosedÓ meaning they have minimal by which factors, and what the effect was. surface connection with the ocean. Four of the tidepools were ÒopenÓ, meaning they have signiÞcant surface connection to the ocean. These Results: designations are largely relative and based on observation. The closed General Results: tidepools appear so in comparison to the open tidepools, however no I counted a total of 2,496 Þsh comprising 54 species. 1,551 of those actual measurements were taken to this effect. In fact at a high tide all the Þsh were juveniles of which 33 species were observed. 62% of the total tidepools are connected to each other and to the ocean. Two of the open number of Þsh counted were juveniles. The basic statistics for each tide- tidepools were ÒshallowÓ and two of the closed tidepools were ÒdeepÓ. pool are shown in Table 2. The most abundant juvenile species observed Shallow is deÞned in this study as an average depth less than 0.8 m. Deep were: Thalassoma duperrey, Scarus psittacus, Chlorurus sordidus, is deÞned as an average depth greater than 1.3 m. The average depth Stethojulis balteata, Acanthurus triostegus, Scarus dubius, Gomphosus of each tidepool was determined by running a transect line across the varius, and Stegastes fasciolatus. See Fig. 5. For a complete juvenile long axis of the pool and taking depth measurements every three meters, species list see Table 3. then running a second transect line perpendicular to the Þrst and halfway across it and again taking depth measurements every three meters, then averaging all the depth values. The depth measurements were made with Statistical analysis: a third transect line with a weighted end. I noted the time of the mea- The Shannon-Wiener diversity indices for each tidepool came out large- surements and corrected the average depths in regards to the tide. The ly similar and exhibited no signiÞcant pattern with regard to exposure or length and width measurements were noted for volume estimations. The depth. These values are listed in table 2. dimensions of each tidepool are listed in Table 1. The total juvenile counts by survey for each tidepool were com- The survey technique I used was the belt transect method. I found piled into one data set. The sample size for this data set was 40. Before and noted a natural marker at one end of each of the tidepools. I then ran this data could be put into an ANOVA to test the signiÞcance of each a transect line from that point on a given compass heading for 25 meters. factor, it had to be transformed using the square-root transformation in I secured the transect line and I would slowly swim in a wide circle back order to satisfy the assumptions of the ANOVA. The normality and vari- to the beginning of the transect allowing the Þsh to return to equilibrium. ance test results follow. I then would slowly swim along the left side of the transect line, count- ing all the Þsh that were within two meters of the line, noting them on Normality test: my data sheet and being careful not to count any Þsh twice. I noted not H0: Distribution is normal. only the number of Þsh but also their life stage and estimated their size. HA: Distribution is non-normal. After completing my count I would do a Þve-minute free swim around R: 0.9726 the tidepool, noting any additional Þsh species that were not present P: 0.0608 in the transect. This method is the one utilized by WHAP, as taught to P>0.05, therefore accept the null hypothesis and conclude that the data me by Anneke Scout.