(GERMANY OUT) Blacktip Shark, Carcharhinus limbatus, Aliwal Shoal, Indian Ocean, South Africa (Photo by Reinhard Dirscherl/ullstein bild via Getty Images)

ullstein bild via Getty Images

Researchers working in Palm Beach County set out to understand how blacktip sharks use nearshore habitats and whether beach nourishment projects were changing the physical environment these animals depend on. On paper, it looked straightforward: measure the shoreline, track shark aggregations, connect habitat features to behavior. But the ocean rarely stays on script and nature often does not like to listen to human whims.

Instead of clear water and visible fins cutting through the shallows, the researchers were met with something far less cooperative. Over a two-year study period, they documented 24 turbidity plumes stretching from 0.37 miles to 9.26 miles (0.60 to 14.91 kilometers) alongshore and up to more than 820 feet (250 meters) offshore. In other words, entire sections of coastline turned into shifting underwater fog banks.

The ocean essentially became unreadable, with visibility collapsing not for hours but for months at a time.

These plumes were a minor inconvenience for the scientists trying to get… well, science, done. Obviously, the poor water clarity made it impossible to carry out planned surveys of blacktip shark aggregations within the direct project area. It also blocked behavioral observations that were meant to link shark presence to bathymetry and seafloor conditions collected during the study. The irony is a bit hard to ignore — the very environment being modified to protect or maintain beaches was simultaneously obscuring the ecological processes happening just offshore. Oops! And yet, science rarely stops just because one window closes.

Just south of the study area where water clarity was better, blacktip sharks were observed gathering in the hundreds, often within 164 feet (50 meters) of the shoreline during peak aggregation periods in February 2020 and March 2021. A member of the family Carcharhinidae, blacktip sharks (Carcharhinus limbatus) are common to coastal tropical and subtropical waters around the world, including the western Atlantic, Indian Ocean and Indo-Pacific regions. They are best known for their seasonal gatherings in shallow coastal areas, especially when prey moves in; they typically feed on small schooling fish such as sardines, mullet and anchovies, using quick bursts of speed and tight turning ability to hunt in turbid, nearshore environments where visibility can be limited. Despite their abundance in some regions, blacktip sharks are generally not considered dangerous to humans, though their preference for shallow waters means encounters can occur, especially in surf zones where people are also present.

Turbidity changes more than just visibility for scientists. For a predator that relies on sight and fast reaction distances in shallow water, murky conditions can alter the entire hunting landscape and prey may become harder to detect. Movement cues may be distorted and the edge between successful strike and missed opportunity can shrink to being razor-thin, or shift in ways we are only beginning to quantify. At the same time, there when sharks and people share shallow coastal zones, clarity of the water matters for how both predators and humans perceive each other. The study suggests that persistent turbidity plumes created by coastal modification projects may interfere with shark prey capture and could increase the likelihood of unintended human interactions. But it also raises a deeper question: how much of what we assume we know about coastal shark behavior is shaped by when and where we can actually see them?

In science, absence of evidence is not evidence of absence, yet in turbid water, absence of observation becomes its own kind of uncertainty. If we cannot see the sharks clearly, can we fully understand how they are responding to environmental change? And if we rely only on visual surveys, what important behaviors are we missing entirely? It is for this reason the team recommend expanding methods beyond direct observation: “Further study is recommended within turbidity-producing project areas using alternative methods, such as tagging, to investigate the effects of changing water clarity on the blacktip shark locations, prey acquisition, and migration patterns.”