Great Barracuda: A Spotty Guide to Better Fish Counts

Scientists use baited underwater cameras to watch fish. They often report the highest number of a species seen at one time, called MaxN. But MaxN can miss real numbers because it ignores repeated sightings of the same fish. The Great Barracuda has a unique pattern of spots that lets researchers spot individual fish on video. Using over two thousand clips from 43 locations in twelve countries, they matched each fish by its spots. They then compared MaxN with a count that only counts unique individuals, called IND. The two methods agreed well (correlation of 0. 93), yet MaxN was lower on average. When the MaxN number rose above four, the difference grew larger.On average, IND counts were 1. 4 times higher than MaxN. The study also looked at whether the same fish appeared on separate cameras. Only five sites had a fish show up on more than one camera, and less than 4% of all videos. Only once did a fish appear on two cameras that were more than 500 metres apart during the same deployment. This suggests that placing cameras 500 metres apart is usually enough to treat them as independent samples for fish like the barracuda. Identifying individuals takes time and effort, but it makes abundance estimates more accurate. Researchers recommend doing so when precise numbers are needed and when the species has recognizable markings.