Parrotfish, Algae, and the St. Croix Transect
On a reef transect near St. Croix, students count organisms along the same 50 m strip in 2019 and again in 2024. In 2019 they record 45 parrotfish, algae covering about 20% of the reef surface, and live coral covering about 55%. In 2024, after several years of heavy fishing, the same strip has 9 parrotfish, algae covering about 61%, and live coral covering about 22%. Water temperature, depth, and clarity readings were nearly the same in both years, and no new pollution source was found nearby. A student writes, "The algae grew because the water got dirtier. Fish don't affect algae." The class must judge which explanation - more pollution or fewer algae-eating fish - better fits the survey data.
1. Which explanation fits the survey data better? Cite the specific numbers that support your choice and describe what evidence would be needed to make the pollution explanation believable.
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The fish explanation fits better.
Supporting evidence
- The one measured change tied to the reef was the parrotfish count: 45 → 9, a drop of about 80%.
- Algae rose from 20% to 61% while coral fell from 55% to 22% - the two are moving in opposite directions at the same time, which fits algae taking over space that coral lost.
- Water clarity, temperature, and depth were nearly unchanged, and no new pollution source was found - the conditions pollution would have to change stayed the same.
Why pollution is not supported yet The student's claim predicts dirtier water, but clarity readings were about the same. A believable pollution case would need something like measured nutrient or sediment levels rising between 2019 and 2024, or a new outfall or runoff source, plus evidence that algae grew at sites with the pollution and not at sites without it.
The reasoning chain Parrotfish scrape algae off coral → fewer parrotfish means less algae removed → algae grows faster than coral can recover → algae shades and crowds coral. The data match this chain; they do not match the pollution claim.
2. Predict what the same transect will show in 2029 if fishing pressure stays high. Describe one consequence for another reef organism and explain the links in your reasoning.
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Prediction
- Parrotfish stay low or fall further (roughly 9 or fewer).
- Algae cover climbs above 61%.
- Live coral cover drops further below 22%, because algae block the sunlight coral polyps need and take up space coral would need to settle and grow.
Consequence for another organism Coral provides the physical structure of the reef - nooks, branches, and holes. As coral dies and algae mats over it, fish and invertebrates that shelter or feed in living coral lose habitat, and the animals that eat coral or live inside it decline too.
Reasoning links, step by step
- Fewer parrotfish → less grazing on algae.
- Algae grows faster than coral and shades it → coral cover drops.
- Less coral → less shelter and less reef structure.
- Fewer sheltered small fish → less food for larger predators, so the whole food web narrows.
What would weaken this prediction Evidence that algae stop spreading on their own (for example, a disease or a storm that clears algae) or that parrotfish numbers recover.
3. The class proposes a no-fishing area on this reef. Describe the data they should collect over two years and give one result that would count against their idea.
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Data to collect, using the same 50 m transect each year
- Parrotfish counts (same method, same time of day, several counts per year averaged).
- Percent algae cover and percent live coral cover, from photos of the same quadrats.
- One control transect outside the protected area that is still fished, so the class can compare change where fishing continues with change where it stopped.
- Water clarity and temperature, so any change in algae can be checked against pollution or heat instead of being credited to fish.
What would count against the idea
- Parrotfish counts rise inside the protected area but algae cover does not fall, and coral does not recover. That would suggest grazing alone does not control algae here, and something else (nutrients, heat, storm damage) is driving it.
- Or: algae fall just as much at the fished control transect, which would mean the change happened whether or not fishing stopped - so the protection was not the cause.
Why a control matters Without a comparison site, any improvement could be due to a good year, so the class could not tell whether their rule caused it.