Toward Integrated Coral Reef Monitoring

COMPARING

BRUVs and Long Swim UVC

A Complementary Approach to More Comprehensive Coral Reef Monitoring

You cannot protect what you cannot see, yet some of the most ecologically important predators on coral reefs are also among the hardest to detect. Large predatory fishes can be highly mobile, occur at low densities, avoid divers, or inhabit deeper reefs beyond safe diving limits. This raises a simple but important question: are we using the right tools to monitor the species we most need to understand and protect?

In our latest study, led by Kennedy Osuka-Edeye and co-authored by CORDIO Director Melita Samoilys and colleagues from partner institutions, we compared two widely used survey methods—Baited Remote Underwater Videos (BRUVs) and Long Swim Underwater Visual Census (LS-UVC)—on the coral reefs of Pemba Island, Tanzania. The study found that each method has distinct strengths, detecting different groups of predatory reef fishes. Used together, BRUVs and Long Swim surveys provide a more complete and ecologically meaningful assessment of reef fish communities than either method alone. The article, ”Cameras or Divers? How Baited Remote Underwater Videos and ‘Long Swims’ Underwater Visual Census Complement Each Other on Coral Reefs,” was published in April 2026 in Ecology and Evolution. 

Overview

Traditional underwater visual census methods have been used for decades to monitor reef fish communities, but they can underestimate large-bodied, cryptic or highly mobile predators. Divers can only cover limited areas, are constrained by depth and bottom time, and some species actively avoid them.

BRUVs overcome several of these limitations by using baited cameras that can remain underwater for extended periods and sample habitats beyond normal diving depths. However, because bait attracts some species more than others, BRUVs also have their own biases.

The study set out to determine how these two methods compare when surveying predatory reef fishes and whether combining them provides a more complete assessment than using either method alone.

It is the first direct comparison of two complementary monitoring methods on the coral reefs surrounding Pemba Island, Tanzania:

  • Baited Remote Underwater Videos (BRUVs) – stationary underwater cameras baited to attract fish.
  • Long Swim Underwater Visual Census (LS-UVC) – a rapid, timed survey where a diver swims over a large reef area recording predatory fishes.

The objective was to determine whether one method performs better than the other, or whether combining both provides a more complete picture of predator communities.

Read or download the full paper, right. 

https://doi.org/10.1002/ece3.73533

Study Area and Methodology

The research was conducted on the outer reefs of Pemba Island, Tanzania, an oceanic island in the Western Indian Ocean known for its steep reef slopes, strong currents and relatively intact predator communities. These reefs also face increasing fishing pressure, highlighting the need for reliable monitoring tools.

Researchers surveyed seven reef sites across northern, central and southern Pemba, using two complementary methods to monitor large predatory reef fishes from nine key families across shallow and deeper reef habitats. The study included 61 BRUV deployments and 23 Long Swim surveys. BRUVs used baited GoPro cameras to attract and record predators without divers being present, while Long Swim surveys – as stated above – involved divers conducting timed swims across large areas of reef. Unlike traditional fixed transects, Long Swims allow divers to follow mobile fishes through complex habitats, increasing the likelihood of detecting roaming predators. Together, these methods provide different perspectives on reef predator communities. For full details on the survey methods, see the published study.

A BRUV set to be deployed
A long swim UVC in progress

Findings

The two methods recorded different fish communities

A total of 42 predatory fish species were recorded.

  • 28 species were detected only by BRUVs.
  • 2 species were detected only by Long Swim surveys.
  • 12 species were recorded by both methods.

This relatively small overlap demonstrates that the two techniques are sampling different ecological components of the reef fish community rather than simply duplicating each other’s observations. 

BRUVs performed best for mobile and deep-water predators

After accounting for differences in sampling effort and depth:

  • BRUVs detected higher species richness.
  • They recorded more unique species.
  • They were particularly effective at detecting:
    • trevallies (Carangidae),
    • sharks,
    • rays,
    • other highly mobile or bait-attracted predators,
    • fishes occurring on deeper reefs.

Long deployment times and bait increased the likelihood of recording elusive species that may not approach divers.

Long Swim surveys were better for reef-associated species

LS-UVC was particularly effective at detecting:

  • reef-resident predators,
  • demersal fishes closely associated with coral habitat,
  • species such as Monotaxis grandoculis, which occurred more frequently and in higher abundance during diver surveys.

Because divers actively move through reef habitat, they can better observe species that remain close to reef structure and are less likely to respond to bait.

Differences were not simply caused by bait

One concern with BRUVs is that bait may artificially inflate observations of species that readily respond to food, particularly trevallies.

To test this, the researchers repeated their analyses after removing the family Carangidae from the dataset. The results remained essentially unchanged. Community composition still differed significantly between BRUVs and Long Swim surveys, indicating that the differences reflected the inherent strengths of each method rather than simply the attraction of bait.

The black tailed trevally, from the family Carangidae, were particularly detected through BRUVs

Significance and Recommendations

The study shows there is no single “best” survey method for monitoring large coral reef predators. Instead, the choice of survey technique should depend on the management question being addressed.

Where monitoring targets highly mobile predators, deeper reef habitats or areas beyond safe diving depths, BRUVs provide clear advantages. Conversely, where managers wish to understand the distribution of reef-associated predators in shallow, structurally complex habitats, Long Swim surveys remain extremely valuable.

Most importantly, the study demonstrates that combining both methods provides a more comprehensive assessment than either technique alone. The low similarity between their species lists shows that each contributes unique ecological information that would otherwise be missed.

For coral reef monitoring programmes in the Western Indian Ocean and elsewhere:

  • BRUVs should be incorporated where monitoring includes deeper reefs, highly mobile species or fisheries-important predators.
  • Long Swim surveys remain valuable for shallow reef habitats where divers can document habitat associations and resident predators.
  • Integrated monitoring provides more reliable information for fisheries management, marine protected area assessment and biodiversity conservation than relying on a single survey method.

 

 

What this means for managers

There is no single best survey method for large reef predators. Match the technique to the management question – or run both. 

Managing an MPA?

Use both methods. Their species lists barely overlap, so each one records predators the other misses.

  • Cameras (BRUVs) for sharks, trevallies and deep-reef species

  • Divers (Long Swims) for reef-resident you can see in the structure

Short on budget?

Pick the method that matches the predator group that matters the most for your question.

  • Cameras (BRUVs) for highly mobile and fisheries-important predators; on reefs beyond safe diving depths

  • Divers (Long Swims) to investigate habitat associations in structurally complex reefs

CORDIO will continue to apply integrated BRUV and Long Swim monitoring across the WIO as part of its reef monitoring programme, ensuring that management decisions for species most at risk from fishing pressure are based on the most complete evidence available.

About this Publication

Title: Cameras or Divers? How Baited Remote Underwater Videos and ‘Long Swims’ Underwater Visual Census Complement Each Other on Coral Reefs

Authors: Kennedy Osuka-Edeye, Melita Samoilys, Bryce D. Stewart, Colin J. McClean, Peter Musembi and Saleh Yahya

Journal: Ecology and Evolution
Year: 2026
Volume: 16, Issue 4
Article: e73533
Published: 22 April 2026

DOI: 10.1002/ece3.73533

Study location: Pemba Island, Tanzania
Research type: Open access research article

Full paper: Read the full article