The thesis, “No Evidence of Top-Down Control on Sponges by Vertebrate Predators in a Philippine Reef System”, produced by Sara Basso MSc, University of Essex, and supported by CCC, aims to comprehend if small-scaled Marine Protected Areas (MPA) supply an increased spongivore (an animal adapted to feeding on sponges) abundance which would help prevent a shift in the dominant benthic community from corals to sponge.
This paper was developed by Sara for her Msc in Tropical Marine Biology at the University of Essex in 2018.
It is estimated that the total area of coral reefs worldwide has decreased by 19%. Sponges contribute the highest benthic biomass in coral reefs, after scleractinian corals. Although they tend to be in competition with scleractinian corals for substrate, sponges are relevant benthic organisms that perform important roles in coral reefs and in many marine ecosystems. This study assesses and analyses: 1. If small no-take areas maintain increased spongivore abundance; 2. Sponge-coral interactions and growth form assemblages of sponges in an overfished and protected site, in order to evaluate the effect of spongivore in preventing out-competition of corals by sponges; 3. Coral and sponge cover across four reefs, in order to compare their abundances with corals in sites where spongivores have varying abundance levels.
The study was conducted in Sogod Bay in the province of Southern Leyte, Philippines. The reefs surveyed were situated in the barangays of Napantao, Bahay, Cagbungalon, Catig, Tabugon and Gudan. The reefs in Napantao, Catig, Bahay and Tabugon are considered protected as they have established MPAs. As control sites forNapantao and Catig, transects were placed outside of MPAs; whereas Cagbungalon and Gudan were used as control sites for Bahay and Tabugon respectively.
The surveys at all the sites were performed using SCUBA and were conducted at both 6 m and 12 m depths. The data for the spongivore abundance and substrate type were conducted with an augmented Reef Check protocol; whereas the fish data were collected within a 5 m belt transect along 20 m of each replicate of the 100 m transect. Substrate data were recorded along the same replicates of the point-intercept transect at intervals of 0.5 m.
To positively avoid the exclusion of fish that potentially feed on sponges, spongivores were considered at the family level. A deepened review of the literature was performed to identify the spongivore families for the Indo-Pacific region. Eight families (Acanthuridae, Chaetodontidae, Nemipteridae, Pomacanthidae, Siganidae, Scaridae, Tetraodontidae and Zanclidae) were selected for this study. To avoid bias of fish species that were included that do not feed on sponges, the same analyses were repeated considering just Pomacanthidae. From 2017-2018, the mean spongivore abundance was 42.33 inside the MPA and 17.20 outside the MPA; with the lowest value being in Gudan at 16.19 and the highest value being in Napantao at 59.33.
At the survey site in Catig, a total of 2, 125 sponges were counted inside the MPA and 1,791 were counted outside. Encrusting sponges were the most abundant growth form, accounting for 61.67% and 55.90% of the total composition for inside and outside the MPA respectively. nMDS analyses showed the morphological assemblages did not differ between the protected and non-protected area of Catig when all the spongivore families were considered but when only the Pomacanthidae were considered, it showed that they had an influence on sponge morphology between the two sites.
The comparison of sponge abundances with spongivore abundances across the four MPAs and likewise overfished sites showed no relationship between spongivore and sponge abundance. This shows that there is no evidence of top-down control by sponge vertebrate predators along the fringing reefs in Sogod Bay. The absence of sponge abundance control by predators in all the survey sites may explain the lack of diverse morphological assemblages in Catig. In the surveyed reefs, the visibility was often less than 20 m and a moderate current was recorded which could have had an impact on sponge growth as sponges are negatively affected by increasingly turbid environments, particularly in proximity with the coast.
In summary, this thesis demonstrates that there is still no clear definition if sponges are top-down controlled by the presence of their predators or bottom-up regulated by nutrient availability. This paper also shows the possible efficacy of forbidding any fishing practice in incrementing fish abundance, underlining the relevance and importance of no-take areas in maintaining adequate fish abundances.
If you would like more information about the study, you can contact Sara at sara.basso1994@gmail.com or alternatively, our Head of Science at headofscience@coralcay.org
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