Long-Term Ecological Impacts of Dredging on Coral Reefs in Kaneohe Bay, Oahu

Abstract

The modification of coral reef habitats by construction activities, particularly dredging, is one of the greatest threats to the sustainability of coral reef ecosystems. Dredging physically destroys corals and removes benthic habitat, causes increased turbidity and sedimentation, and alters reef morphology. This study was conducted in Kaneohe Bay, Oahu, and was the first to quantify the long-term consequences of dredging activities on the structure and function of a coral reef ecosystem in Hawaiʻi. Coral reefs in Kaneohe Bay, Oahu, experienced extensive dredging in the 1950s. Benthic community structure, fish assemblage, and physico-chemical parameters were quantified after 60 years of dredging. Results indicated significant degradation of coral and fish community structure at the dredged site. The corals, which experienced direct physical destruction by dredging, did not show any significant recovery, and the corals in surrounding areas showed significantly lower coral cover. The number of coral colonies at the dredged sites were extremely low, and coral size frequency distribution was centralized with a distinctive lack of small colonies. These results may reflect sublethal effects of dredging, including declined growth rate, increased mortality, and lower recruitment. The fish assemblage (fish biomass, fish number, size distribution) was also significantly degraded at the dredged site. At the dredged site, fish biomass was less than one-fifth of that at the control site, and more than 70% of the fishes were in small fish category (≤ 10 cm). Habitat destruction appeared to be one of the greatest threats to reef fish population for the longer time period. This is consistent with numerous short-term studies. Water quality and sediment composition, were similar between the sites, indicating that the impacts such as increased turbidity and sedimentation caused by dredging may be short-lived. Thus, changes in substratum and morphology are the major factors responsible for the lack of recovery. This study has identified the long-term impacts of dredging activities and developed information linking this stress factor with the condition of coral reef habitats. This study documented community change, which was typically not observed during the term of most ecological studies. From a management perspective, this study can assist decision-making when determining the type and degree of dredging operation to be conducted on coral reef environments. This information is also important for improving design and siting of coastal projects, as to avoid or reduce ecological impacts.