Using bathymetric lidar to define nearshore benthic habitat complexity: Implications for management of reef fish assemblages in Hawaii

Abstract

Habitat complexity plays a major role in determining the distribution and structure of fish assemblages in the aquatic environment. These locations are critical for ecosystem function and have significant implications for conservation and management. In this study, we evaluated the utility of remotely sensed lidar (light detection and ranging) data for deriving substrate rugosity (a measure of habitat complexity) on a coral reef in Hawaii. We also assessed the potential application of lidar data for examining the relationship between habitat complexity and Hawaiian reef fish assemblage characteristics. Lidar-derived rugosity (4 m grid size) was found to be highly correlated with in-situ rugosity and was concluded to be a viable method for measuring rugosity in analogous coral reef environments. We established that lidar-derived rugosity was a good predictor of fish biomass and demonstrated a strong relationship with several fish assemblage metrics in hard bottom habitat at multiple spatial resolutions. This research demonstrates (i) the efficacy of lidar data to provide substrate rugosity measures at scales commensurate with the resources and their environment (ii) the applicability of lidar-derived rugosity for examining fish–habitat relationships on a coral reef in Hawaii and (iii) the potential of lidar to provide information about the seascape structure that can ultimately be used to prioritize areas for conservation and management.

Introduction

Habitat complexity in the coastal environment plays an important role in structuring nearshore fish assemblages. The relationship between habitat complexity and measures of community structure was first observed in the terrestrial realm (August, 1983, MacArthur and MacArthur, 1961, Murdoch et al., 1972, Rosenzweig and Winakur, 1969). A similar relationship between habitat complexity and fish assemblage characteristics has been well documented in both freshwater (Gorman & Karr, 1978) and marine ecosystems (Caley and St John, 1996, Friedlander and Parrish, 1998, Gratwicke and Speight, 2005, Luckhurst and Luckhurst, 1978, Risk, 1972, Roberts and Ormond, 1987).

Structural complexity, a major component of habitat complexity, can be defined as the architecture of the physical environment (McCoy and Bell, 1991, Sebens, 1991). Structurally complex habitats offer more potential niches and increase survivorship by providing fish additional refuge from predation (Almany, 2004, Beukers and Jones, 1998, Hixon and Beets, 1989). Accordingly, areas of high structural complexity harbor high species richness (Gratwicke & Speight, 2005), species diversity (Almany, 2004) and fish biomass (Friedlander & Parrish, 1998).

There are a number of habitat complexity variables that can be measured in-situ (reviewed in McCormick, 1994), and rugosity is the most commonly used in-situ measure. For the purposes of this study, rugosity, or vertical relief, was used to represent a measure of structural complexity. The chain transect method measures in-situ rugosity by obtaining the ratio of the length of a chain laid across the bottom profile along a transect line to the linear distance of the transect line (Friedlander and Parrish, 1998, Luckhurst and Luckhurst, 1978, Risk, 1972). A limitation of the traditional chain transect method is the restriction of the structural complexity measurements to relatively fine spatial scales. Additionally, field measurements are time-consuming, can have high inter-observer variability, and are difficult to obtain over a broad geographic area.

Considering the documented importance of the relationship between rugosity and fish assemblage structure, it is critical to develop faster methods of determining rugosity in the marine environment at broader geographic extents. The current expansion and wide application of remote sensing technology on coral reef ecosystems were recently reviewed (Mumby et al., 2004). Lidar (Light detection and ranging) is an active remote sensor that allows for spatial analysis of structurally complex habitats (Lefsky et al., 2002). Lidar has recently been applied to map coral reef structure (Storlazzi et al., 2003), and to measure reef rugosity (Brock et al., 2004, Brock et al., 2006). Lidar can provide measurements that may be scaled to allow for extraction of information at spatial extents that are more appropriate for coral reef ecosystems and related management actions. Applying remote sensing techniques that can rapidly identify structurally complex habitat may greatly assist resource managers in locating areas that are important to protect and sustain nearshore fish populations.

The goals of this study were (1) to determine whether lidar technology can provide effective rugosity measures on a coral reef in Hawaii and (2) to examine the relationship between reef fish assemblage characteristics and lidar-derived rugosity.