Paea Lagoon Aua i'a Fish Pond Monitoring, Group 1: Invertebrate Abundance, Tahiti, FP, 2023
Paea Lagoon Aua i’a Fish Pond Monitoring, Group 1: Invertebrate Abundance, Tahiti, French Polynesia, 2023
Authors: Kylee Dungan, Bianca Berron, Bridgette Castelino, Roo Swain
Metadata:
August 2, 2023
11AM - 2:32PM
Beaufort 0
Cloud 7% cover
Sunny
Invertebrate Survey Methods:
We split our area into three main categories: the fish pond, boulder substrate to the South of the fish pond , and sandy substrate to the north. Within each of these categories we took two 12 meter long transects perpendicular to the shore 4 meters apart from each other. Two surveyors swam along each side of the transect, recording the number of invertebrates within one meter on either side for a total of 2 meters of observation. The invertebrate families we looked for included snails, hermit crabs, sea cucumber, worms, urchins, crabs, clams, shrimp, and sea stars; specifically crown of thorns sea stars due to the occurring outbreak. The data from both surveyors were combined in the end to find the totals of each family per transect.
Additionally, for the fish pond, we separated it into three categories: the interior floor, the interior wall, and the exterior wall. The entire fish pond is 14 by 13 meters not including the walls. For the interior floor, we placed our transects 5 meters from the right side (north) of the interior rock wall facing the ocean for the first, and 9 meters from the same boundary for the second transect. For each side of the wall, we counted the number of invertebrates on and within a meter. This was done the same way as the transects but with only one person swimming and recording along each side of the wall.
Notable Changes From Last Year:
There were two major changes that were made from the surveys last year. Rather than counting the total number of invertebrates as a whole, we tallied the number of individuals found in each family. This gave us an idea of which invertebrates were most abundant, which were the worms and urchins. Another major change we made was sampling both inside and outside of the fish pond. We kept the transect lengths the same and surveyed the walls according to least years methods, but only took two transects rather than six inside of the pond. Per Tomas’s request, we added an extra two transects on either side of the pond, one about 75 meters to the South with rocky substrate and one about 120 meters to the North with sandy substrate. this helped us gauge the effects of current and substrate on invertebrates.
Using the same methods to survey the interior and exterior wall, we found a 155% increase in abundance on the interior and a 34% increase on the exterior of the wall from last year's data.
Data:
https://docs.google.com/document/d/1-Gsuvp-Hr-TmsPparqthh_1WQNe-n0N9h7-dy4j7z4s/edit?usp=sharing
Results:
To determine whether invertebrate abundance and diversity differed between sites, Shannon’s indices were calculated. In order from lowest to highest, Shannon’s index for the sandy substrate was 0, 1.194 for the interior floor, 1.307 for the rocky substrate, and 1.501 for the fish pond wall. Next, a total of 9 t-tests were conducted. Out of these tests, 3 were statistically significant. The amount of invertebrates found in the fish pond interior floor differed significantly from the sandy substrate with a p-value of 0.036. The amount of invertebrates found in the fish pond interior floor also differed significantly from the fish pond wall with a p-value of 0.016. Lastly, the amount of invertebrates found in the rocky substrate differed significantly to the sandy substrate with a p-value of 0.027.
In contrast, the following comparisons were not significantly different: the amount of invertebrates in the fish pond interior floor to the rocky substrate, fish pond wall to the rocky substrate, left fish pond wall to right fish pond wall, exterior left fish pond wall to exterior right fish pond wall, exterior left fish pond wall to interior left fish pond wall, and exterior left fish pond wall to fish pond interior floor. These tests were performed to determine the effect of current on invertebrate abundance since the current was flowing approximately from south to north, or from left to right when looking at the ocean from shore.
Despite the current Crown of Thorns outbreak, we did not observe any individuals in or around the area.
Discussion:
Our client was interested to see the differences in invertebrate population covers across different substrates and various locations along the Paea coastline of Tahiti, especially those differences caused by changes in current flow and the presence of the sheltering fish pond structures.
The several t-tests performed suggest that the effect of substrate is greater than the effect of current on invertebrate presence and diversity. Significant differences were found between boulder and sandy substrates, and no statistical difference was found between sheltered and unsheltered areas.
Shannon’s index also confirms that diversity increases with rocky substrate, something we expected due to the porous nature of the basalt rock that invertebrate species like to take advantage of.
An observation that I would like to point out is the presence of worm tubes in the sandy area. Other than the singular snail found at the northern sample site, there were numerous worm tubes hidden beneath the sand. I thought this was an observation important to notice, as it is an example of a species making use of a relatively barren environment. It may even be an example of a pioneer species beginning the rebuilding of a reef, but more research would need to be done to confirm.
We’d also like to mention how although we could not turn every stone to find every small snail and hermit crab, our methods were fairly thorough given the time and resources. We’d like to thank you for sharing your home and allowing us to learn from the beautiful environment here. Thank you for listening and we hope that our findings can contribute to your work in some way.
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