Counting all the fish in the sea

Image: Even counting larger fish species (like these trevally that we spotted in Mexico) is harder than you might think, the fish never stand still!


The amount and variety of fish species on a coral reef can indicate the current condition and general health of a reef: plenty of herbivorous fish (eg: parrotfish) might suggest that algae cover on a reef will be kept under control (too much algae on a reef can smother corals causing major problems), the presence of large fish (including grey nurse sharks and groupers) might indicate that there are no issues with overfishing in the area, and a large variety of fish in a small area could indicate that the reef provides an important habitat for a range of fish species and is therfore a critical area to manage. 

Ana Herrera from the University of Venezuela, is completing her PhD in the dynamics of coral reef fish populations in the Caribbean area: assessing the movement of different species of fish between reefs during their lifetime. She has joined the Catlin Seaview Survey (on our Caribbean campaign) to conduct her fish surveys. Ana's work relies on her ability to accurately record the abundance and diversity of fish life in a given area: in effect she is performing a fish census.

So how do you count fish?

To conduct a fish survey, a section of coral reef has to be defined. Ana created a 15 x 15 metre rope square which has nine internal squares in a checkerboard configuration. Divers take the rope square and gently stretch it over the reef, weighting it at the corners so that it won't move in the current.

Ana, with her underwater slate and pen then spends exactly six minutes recording the species, number and age of the fish that entered the first internal square quadrant. Over the course of an hour, Ana has covered all quadrants. The process is repeated three times on different parts of the reef and the results are then averaged to indicate the fish present in the area. Over the course of the expedition, covering Belize and Mexico, Ana will repeat this procedure on different reefs in different countries, to get a complete picture of the fish life living on the Mesoamerican Reef.