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CRAMP Rapid Assessment. Other Fish Survey Techniques

Numerous methods have been developed for sampling fishes.  Method selection depends on the focus of the research and the spatial and temporal scales involved.  Accuracy can depend on the number and size of transects and whether transect locations are randomly selected, stratified random (e.g. following depth contours), or fixed.  Fixed transects may not be representative of the entire community of interest but allow for more accurate repeated measurements.

Visual fish censusing methods used in nearshore marine sampling

Method Type Description Limitations

Presence/ Absence

Qualitative

Lists species

No abundance or density data

Timed Swim

Qualitative

Surveyor swims within defined area for specified length of time recording species observed

Records species only

No size estimates

Relative Abundance

Semi-Qualitative

Records abundance of species

No density estimates

Reed Method

(Reed 1980)

Semi-Qualitative

Surveyors record species only once, in the order they are sighted

No size estimates

Rapid Visual Transect (Sanderson and Solonski 1986)

Semi-Qualitative

Species are recorded only once and assigned to a time interval based on when they were observed

Underestimated patchy

Species Abundance

Quantitative

Records number of individuals and size

Time constraints

Line/Strip/Belt transect (Brock 1954)

Quantitative

Line laid prior to observations. Records number of individuals and size

Speed variability: rare and cryptic species overlooked, highly mobile species overestimated

Video Transect

Quantitative

Video recording

Limited resolution

Circular Plot

Quantitative

Visual or video recording of all fishes in a 360o arc in a designated time period

Water visibility estimate necessary High variability

Spatial and temporal variability of fishes can be extremely high due to mobility and large home ranges.  Many fish species are cryptic, rare or transient.  There are also diurnal/nocturnal and seasonal sources of variability.  To quantify absolute values for fish populations an extremely large sample size is required especially for heterogeneous habitats.  Relative values are often used to determine differences between sites.

Our RATs species abundance estimates were selected to maximize data and statistical comparability, allow for length to biomass conversions, and avoid limitations inherent in some other methods.  This method includes two measures of abundance: numerical and biomass.  These are both important population parameters that address different aspects of fish community structure.  Unlike the belt transect method, species abundance estimates do not require additional survey time to allow for fish equilibrium to occur.  The transect line is spooled out as the survey is conducted to avoid fish dispersal.  Although additional dive and training time must be allotted to estimate fish length, post processing of data is relatively rapid.

 

Last Update: 04/21/2008

By: Lea Hollingsworth

Hawai‘i  Coral Reef Assessment & Monitoring Program

Hawai‘i  Institute of Marine Biology

P.O. Box 1346

Kāne‘ohe, HI 96744

808-236-7440 phone

808-236-7443 fax

email: jokiel@hawaii.edu