Counting Fish the Wrong Way: How Survey Methods Reshape What We Think Coral Reefs Tell Us

On a coral reef, the difference between seeing a thriving ecosystem and a collapsing one can come down to how long a diver holds still. That is the startling implication of a new study published in the journal Coral Reefs, in which researchers from James Cook University, the University of Sydney and partner institutions systematically compared the two most widely used techniques for counting reef fishes: stationary point counts and belt transects. Their conclusion is uncomfortable for a field increasingly reliant on massive, stitched-together datasets. The method a scientist chooses does not merely add a little noise to the numbers. It can produce fundamentally different ecological stories about the very same patch of reef.

The research team, led by Helen Yan, conducted their experiment on two fringing reef systems in the central Great Barrier Reef: Pioneer Bay on Orpheus Island, a sheltered no-take scientific research zone, and Indigo Bay on Fantome Island, a semi-exposed area open to most forms of recreational fishing. Separated by roughly ten kilometres, the two bays host distinct benthic and fish assemblages, making them ideal natural laboratories for testing whether census methods behave consistently across different communities. Between 11 and 17 February 2023, four trained observers surveyed fish larger than ten centimetres in total length, spanning eighteen reef fish families, with individuals identified to species, counted, and assigned to five-centimetre size bins.

The experimental design was deliberately rigorous. Each sampling block, performed by a single diver, comprised five surveys: an instantaneous one-minute point count, a continuous ten-minute point count, and belt transects of twenty, thirty and fifty metres, all using a five-metre-wide survey area. Point counts and transects within a block were conducted on non-overlapping reef areas to prevent the disturbance caused by one method from contaminating the other, and the order of methods was randomised to balance systematic diver effects. In total, the team completed 46 blocks on Orpheus Island and 48 on Fantome Island, with transect tape laid as the diver counted, a technique designed to minimise the fear response that reef fishes show toward approaching divers.

The results were striking. Point counts produced the most extreme estimates of abundance, biomass and species richness of any method tested. Instantaneous point counts recorded the lowest values, with median abundance estimates of just 0.16 individuals per square metre and standing biomass of 32.3 grams per square metre, while continuous ten-minute point counts recorded the highest, at 0.54 individuals and 93.1 grams per square metre. Transects of different lengths, by contrast, produced remarkably similar estimates, with abundance values clustering between 0.26 and 0.30 individuals per square metre regardless of whether the diver swam twenty, thirty or fifty metres. The discrepancy between the two point count variants points to a bias that is rarely recorded, let alone standardised, in reef fish censuses: total survey time. The longer a diver watches a patch of reef, the more fish swim into view, inflating counts that are then divided by area and presented as density.

But the deeper problem lay in what the methods missed, not just what they counted. Using size spectrum analyses, which examine how biomass is distributed across body size classes in a manner analogous to trophic pyramids, the researchers found that both point count methods completely failed to record the largest fishes on the reef. The reason is almost certainly the diver effect. A point count requires the diver to descend into or swim through the survey area and then remain at its centre, meaning every fish counted is within a small radius of a rotating, breathing, bubble-emitting human. Far from being a passive observer, the diver is a perceived predator, and wary large fishes simply vanish. Previous research has shown that some reef fish communities require upwards of three hours to return to their original densities after a diver passes, so it is unsurprising that even a ten-minute wait failed to bring big fish back into view.

The consequences of these size-based biases ripple directly into ecological interpretation. On Fantome Island, the instantaneous point count produced a size spectrum slope statistically indistinguishable from zero, a flat profile typically read as a sign of an inverted trophic structure, the kind of pattern associated with disrupted energy flows or external nutrient subsidies. Yet every other survey method produced clearly negative slopes, the expected bottom-heavy signature of a healthy size-structured community. In other words, one of the most common census techniques, applied for just one minute, would have led ecologists to conclude that the reef’s energetic architecture was fundamentally distorted when, according to all other methods, it was not. The 50-metre transect was the only technique that consistently captured the entire range of fish body sizes detected across both islands, and it also produced the least variable community composition in multivariate analyses.

The study also quantified how much replication any method requires. By repeatedly subsampling their data and measuring the standard error of estimates, the researchers found that every technique needed at least 31 to 36 samples to reach stable precision, defined as estimates within ten percent of the asymptotic value. This threshold matters because many monitoring programs operate with far fewer replicates per site, meaning their estimates may be too imprecise to detect real ecological change. Even the best-performing method, the 50-metre transect, has limits: highly mobile giants such as reef sharks and the bumphead parrotfish Bolbometopon muricatum are so rare and wide-ranging that transects hundreds of metres long would be needed to quantify their densities reliably.

These findings land at a delicate moment for reef science. Coral reefs are transforming under the pressure of climate change, and researchers are racing to assemble macroecological datasets that combine surveys from disparate sources, different organisations and sometimes different decades. Some of the world’s largest monitoring programs, including the United States National Oceanic and Atmospheric Administration’s National Coral Reef Monitoring Program and the Australian Institute of Marine Science’s Long-Term Monitoring Program on the Great Barrier Reef, each rely on their own protocols. The new study shows that pooling such data without accounting for methodological bias risks amplifying artefacts to continental scales, mistaking the fingerprint of a survey technique for a genuine demographic or ecological process. Statistical corrections and detection modelling may help, but the authors caution that when effect sizes are as large as those documented here, even sophisticated analytical tools are unlikely to fully separate methodological bias from real biology.

Fishing pressure adds another layer of complexity. Fishes in heavily fished areas tend to be warier and exhibit greater flight-initiation distances from divers, a well-documented behavioural response. Consistent with this, the maximum fish size recorded by point counts on fished Fantome Island was smaller than on the protected Orpheus Island, suggesting that the size bias of point counts is not fixed but worsens along gradients of human disturbance. A global dataset mixing surveys from pristine reserves and heavily exploited reefs could therefore encode systematically different biases in different places, further muddying macroecological inference.

The authors’ prescription is straightforward, if demanding: adopt uniform survey methodologies, use at least 30 replicates of a minimum 50-metre transect per site, and treat survey time as a standardised variable rather than an afterthought. In an era of big data, they argue, more data is not necessarily better. A smaller dataset collected consistently, with biases that are at least uniform and therefore quantifiable, will yield far more robust conclusions than a sprawling patchwork of incompatible counts. For a field whose findings feed directly into conservation policy, the message is that how we count fish determines not just the numbers we get, but the reefs we think we are saving.

Subject of Research: Methodological biases in underwater visual census techniques for coral reef fish community surveys

Article Title: Common census methods can lead to fundamentally different ecological interpretations of coral reef fish communities

Article References: Yan, H. F., Crisp, S. K., Bellwood, D. R., Siqueira, A. C., & Tebbett, S. B. (2026). Common census methods can lead to fundamentally different ecological interpretations of coral reef fish communities. Coral Reefs. https://doi.org/10.1007/s00338-026-02959-z

Image Credits: AI Generated

DOI: 10.1007/s00338-026-02959-z

Keywords: coral reef fish, underwater visual census, point counts, belt transects, size spectrum, diver effect, Great Barrier Reef, survey methodology, macroecology, biomass estimation, species richness, ecological monitoring