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CRAMP Rugosity Measurements

John Rooney of the UH Oceanography Department measuring rugosity at Kaho‘olawe in 1993. Photo by Paul Jokiel.John Rooney of the UH Oceanography Department measuring rugosity at Kahoolawe in 1993. Photo by Paul Jokiel.

Rugosity as defined here is an index of substrate complexity. The term rugose is derived from a Latin term meaning wrinkled. The index of rugosity describes the amount of "wrinkling" of the substrate. Substrate complexity is an important ecological parameter (Friedlander and Parrish 1998). Areas of high complexity are likely to provide more cover for reef fish and more places of attachment for algae, corals and various sessile invertebrates. Prior research has recognized the importance of topographic relief in the structure of fish assemblages throughout the world (Carpenter 1981; Holbrook et al. 1990) and in Hawai‘i (Friedlander and Parrish 1998a). It is evident that fish populations are highly associated with spatial relief for several reasons.

  • Increased substrate provides habitat for benthic invertebrates, which serve as the main diet of many species of fishes, which in turn are utilized at other trophic levels.

  • Increase in coral cover associated with rugosity feed obligate corallivores.

  • Spatial complexity increases habitat heterogeneity, providing increased areas of refuge for fish populations from predation and competition.

  • Topographical relief can expand the availability of resources and their production rate. Increased rugosity results in higher heterogeneity, creating habitat complexity that increases fish diversity. Coral diversity, correlated with fish populations, is also probably a direct result of habitat complexity.

Since habitat heterogeneity is important in structuring fish assemblages, an index of fish abundance may be obtained through rugosity measurements. There are clear advantages to this indirect measure of abundance. A large sample size is necessary due to the high variability among fish populations, many rare, cryptic or mobile species can be under reported, and the power to accurately detect absolute fish abundances can be extremely low. Although the use of a rugosity index cannot substitute for fish abundance data, it can serve as a relative indicator of differences between sites over large spatial scales where abbreviated surveys are necessary (Rodgers 2005). Spatial complexity can be an indicator in determining the distribution of fish size. For optimum protection, fishes select shelter that complements their size, reducing the risk of predation. Size of voids in reef structure are positively correlated to numerical and biomass densities (Hixon and Beets 1989). Rugosity measurements are heavily influenced by coral cover and diversity, which are also found to be highly correlated with fish populations (Rodgers 2005). Thus, measurements of spatial complexity may prove to be a rapid way to assess both coral and fish communities.

CRAMP uses a chain and tape method, which assigns a numerical value to rugosity by measuring the length of chain draped over the reef surface that is needed to cover a given straight-line distance between two points (McCormick, 1994). A longer length of chain will be needed in areas that have an uneven and complex surface. The surveyor tape is used to measure the straight-line distance between the two marker pins on a transect. This value is generally close to 10 m. A light brass chain marked off in 1m intervals is then spooled out over the bottom along the length of the surveyor line. The amount of chain lying on the bottom that is necessary to span the distance between the two marker pins is then divided by the straight line tape measurement to generate an index of rugosity for that transect. The ten randomly selected transects within a grid are all measured in this manner to produce an average rugosity for the reef. These indexes can be used to test correlation with overall coral cover, coral species composition, fish species richness, fish abundance and fish biomass.

Kuulei Rodgers measures rugosity along 3 m transect in Hanauma Bay, Oct 2000 while trying to ignore a curious triggerfish. Video frame by Will Smith.

References:

Carpenter (1981)

Friedlander AM and Parrish JD (1998)  Habitat characteristics affecting fish assemblages on a Hawaiian coral reef. Journal of Experimental Marine Biology and Ecology 224: 1-30.

Friedlander AM and Parrish JD (1998a) Temporal dynamics of fish communities on an exposed shoreline in Hawai‘i. Environmental Biology of Fishes 253: 1-18.

Hixon MA and Beets JP (1989) Shelter characteristics and Caribbean fish assemblages: experiments with artificial reefs. Bulletin of Marine Science 44: 666-680.

Holbrook et al (1990)

McCormick, Mark (1994)  Comparison of field methods for measuring surface topography and their associations with a tropical reef fish assemblage. Marine Ecology Progress Series 112: 87-96.

Rodgers (2005)

 
Photoquadrats -- Video Transects -- Rugosity Measurements -- Sediment Analysis

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