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CRAMP Rapid Assessment. Rugosity

Rugosity measurements to determine topographical relief and spatial complexity are conducted along each transect. A 15 m chain marked at 1 m intervals with 1.3 cm links is draped along the length of the transect (10 m) following the contours of the benthos. An index of rugosity is calculated using the ratio of the reef contour distance as measured by chain length, to the linear, horizontal distance (McCormick 1994).

Rugosity measurements taken by Eric Brown at Keanapapa on Lāna‘i.

In concert with this study, 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.

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 Weber 1993).

Rugosity measurements are heavily influenced by coral cover and diversity, which are also found in this study to be highly correlated with fish populations. Thus, measurements of spatial complexity may prove to be a rapid way to assess both coral and fish communities.

References:

Carpenter, K.E., Miclat, R.I., Albaladejo, V.D., and Corpuz, V.T. 1981. The influence of substrate structure on the local abundance and diversity of Philippine reef fishes. Proceedings of the Fourth International Coral Reef Symposium 2:497-502.

Friedlander A.M. and Parrish, J.D. 1998a. Habitat characteristics affecting fish assemblages on a Hawaiian coral reef. Journal of Experimental Marine Biology and Ecology 224:1-30.

Hixon, M.A. and Weber, M.S. 1993. Predation, prey refuges, and the structure of coral reef fish assemblages. Ecological Monographs 63:77-101.

Holbrook, S.J., Schmitt, R.J., and Ambrose, R.F. 1990. Biogenic habitat structure and characteristics of temperate reef fish assemblages. Australian Journal of Ecology 15:489-503.

McCormick, M. 1994. Comparison of field methods for measuring surface

 

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