Index | Overview | Coral standards | Fish standards | Calibration | Data processing | Data sheets Coral reef fishes
For fish sampling, biases are often due to difficulties arising from seeing fishes, accurately counting or estimating length, and differentiating among certain similar species (Sale 1997). An effective way to census such mobile organisms as fish are through a method that most imitates an instantaneous survey of a pre-defined area (Sale 1997). The AGRRA fish method includes two different methods to characterize reef fish populations – a belt transect and roving diver method.
Estimating 2 m wide belt transect The first challenge is to visualize the boundaries of a 2-m wide X 30 m long belt transect for fish censuses. The sample unit has been specifically delineated to a small 2-m wide width because it is a small enough width for the observer to be able to estimate the distance easily while allowing for a large enough sample area. A 1 m t-bar (with 5 cm markings) is used to assist observers in estimating the distance (below left). To estimate the 30 m length, try to visualize the transect as a 2-m wide square tunnel (from the bottom to the surface of the water). Start the first box at least 1-2 m ahead of t-bar. When sampling the transect belt, the observer should give uniform attention to each successive 2-m segment (below right). This requires swimming at a more or less constant rate, and looking consistently about 2 m ahead, except when actually recording data
Uniform attention should be given to each successive 2-m segment of the transect when surveying. It is important to swim in a consistent manner while actually sampling the fish, although it is permissible to pause while recording data, and then to start swimming again. A speed that counts each 30-m transect in 6-8 minutes should be attempted. High densities of counted species will slow this rate in some cases. By remembering to keep effort equivalent on all segments of the transect, you can focus on counting just those members which happen to be within the boundaries of the transect and limit the tendency to count all members of a large school of fish on either side of the transect. In other words, keep your eyes focused straight ahead of the transect and do no get distracted side to side as a large school swims by. Large groups of individuals of a species that occur within a 2-m segment will be classified by attempting to put them into one or more size categories as necessary. Estimating
length Fish observers should be trained to estimate fish lengths by using consistency training methods both on land and underwater (e.g., GBRMPA 1979, Bell et al. 1985, English et al. 1994). The AGRRA method assigns fish lengths to the following size categories (0-5 cm, 6-10, 11-20, 21-30, 31-40, >40cm). A 1 m T-bar with 5 cm increments is used to assist in estimating sizes. We use a variety of fish models cut out of closed-cell foam into the different size classes (plywood or plastic can also be used). The shape of the fish models used represent some of the species included in the AGRRA survey.
Roving diver Fish surveyors should be proficient in identifying Caribbean reef fishes. For minimizing and recognizing observer bias and differences with the REEF roving diving technique, we suggest reading the guidance provided by Schmitt et al. (1998). The “Reef fish identification (Florida Caribbean, Bahamas)” book and cd-rom by Paul Humann are very useful for improving and testing your fish identification.
Index | Overview | Coral standards | Fish standards | Calibration | Data processing | Data sheets
© 2000-2006 - AGRRA |
