May 4, 2010

Technical Question of the Month: How do we count mobile fish?


Each month, Reef Check will answer a technical question regarding the monitoring protocol of our coral reef or rocky reef programs. If you have a question you would like answered, please email

Reef Check California — How do we count fish along a transect without double counting fish that move around?

To count fish in the rocky reef habitat in California, we use visual surveys of 30m transects. To count fish along the transect, a buddy pair of divers swim along the reef, laying out the meter tape while at the same time the lead diver counts all individuals of the 35 fish species targeted by RCCA within a 2 x 2 meter box. Additionally they record the size of each individual in one of five size categories ranging from less than 15 to more than 50 centimeters.

This or similar survey methods are used by many subtidal researchers working on temperate rocky reefs. Since fish move around and can swim in and out of the 2 x 2 meter box along the transect we use an approach that we call: stop, scan and search. To implement this method along the transect the fish-counting diver moves in stages. At the beginning of each stage the diver stops and scans the water ahead for mobile fish within an elongated imaginary box of 2 x 2 meters. During the scan all conspicuous individuals within the imaginary box in front of the diver are quickly counted. How far ahead the diver scans for fish depends on the visibility during the dive. Fish that move into the area after the scan are not considered and fish that leave the area after the scan are still recorded even if they are not present by the time the diver gets to writing down the data. This scan creates a snapshot of a section of the transect and only fish present during this snapshot are counted.

After the scan the diver searches for fish that are hidden in cracks and crevices as he or she moves on through the section of the transect that was scanned. At the end of the scanned area the diver stops and scans the section ahead of him or her creating the next snapshot of data, than moves on and searches for cryptic fish. These steps are repeated until the end of the transect is reached. This method insures that each fish is only counted once. Subdividing the transect into sections makes it more manageable for the diver to count moving fish.