How Degraded Habitats Affect Coral Reef Fishes

When people clear a hillside for development or regrade a dirt road, this exposes soil that can be easily eroded during heavy rainfall that is typical of the tropics. The eroded soil is swept into streams and runs quickly downhill toward the sea. Coastal habitats, such as mangrove forests and seagrass beds, naturally filter out much of this eroded soil, but when these habitats have been damaged or removed, the soil washes out to the coral reef as large brown plumes of sediment that causes stress, damage and death to corals.  These degraded coral reef habitats may in turn create a negative feedback loop to parrotfish populations (sediment impacted coral reef → reduced parrotfish fitness → more harmful algae → further degraded coral reef → reduced parrotfish abundance → more harmful algae…). Chronic sedimentation, due to poor land development and watershed management, can contribute to the persistence of algal–dominated reefs, a common condition in the Caribbean. In order to learn if these negative feedback loop conditions exist, we  are studying the yellowtail parrotfish (Sparisoma rubripinne), which is a common and abundant parrotfish in shallow coral and rocky reef habitats of the US Virgin Islands. The yellowtail parrotfish has a unique reproductive strategy where they migrate to the reef edge and form large spawning aggregations (hundreds of parrotfish gather for reproduction) on a daily basis. This characteristic makes them an ideal model species to test the effects of multiple natural and human factors on feeding and reproductive rates.

Research Goals

Using a long-term coral reef monitoring database, we are examining how past natural disturbances, such as warm water induced coral bleaching and disease events that cause wide spread coral mortality, affect parrotfish populations.  Our field-based research area focuses on the relationship between land-based sedimentation rates (low vs. high), its influence on habitat composition of turf algae (helpful) vs. macro algae (harmful), and how these factors affect the diet, feeding rates and reproduction of the yellowtail parrotfish. Our research is also measuring how oceanographic conditions influence the frequency of parrotfish spawning and their reproductive output.

We expect the research will advance our understanding of:

1. How major coral bleaching and disease events in 2005 and 2010 and chronic sedimentation affected parrotfish abundance and diversity;

2. How different levels of sedimentation and algae composition affect feeding rates and reproduction (frequency of spawning and fertility) of parrotfishes parrotfish;

3. How parrotfish diet changes under different levels of sedimentation and algae composition; and

4. How oceanographic and environmental variables influence the frequency of spawning and reproductive output of parrot fish.

Goals 1 builds upon previous EPSCoR and NOAA funded research by synthesizing and analyzing long-term coral monitoring data for changes in herbivore communities following acute and chronic disturbances such as major coral bleaching and mortality events and variations in sedimentation rates across monitoring sites. 

Goal 2 measured monthly sedimentation and algae composition across a sediment gradient inside and outside the Virgin Islands National Park, from Fish Bay (high sediment), Reef Bay (moderate sediment) and Europa Bay (low sediment), St. John. To determine parrotfish responses to these variables, we established an array of 55 acoustic receivers across our study area and tagged 60 yellowtail parrotfish with acoustic transmitters. Half the fish were tagged in areas of high sedimentation while the other 30 fish were tagged in areas low to moderate sedimentation. Both these groups of tagged fish used different spawning aggregation sites. This acoustic telemetry study tracked yellowtail parrotfish movements, feeding habitats and spawning rates for over a year. 

Goal 3 used environmental DNA metabarcoding (eDNA) to examine the diet of parrotfishes feeding in areas with different sedimentation rates and benthic algae composition. 

Goal 4 is integrating ecological and oceanographic disciplines by using data from oceanographic and temperature sensors to record environmental conditions at each spawning site. Using data on spawning frequency and plankton tows at each site, we will be able to directly measure the reproductive output of yellowtail parrotfish and calculate their reproductive success. We are also exploring novel ways of measuring spawning frequency using audio and video recordings.

The Fish Ecology research team is working closely with other R2R research areas such as watershed management, oceanographic processes and coral reef resilience to provide a more comprehensive picture of factors that affect coral reef resilience.

Photo below:
Post doc Dr. Kayla Blincow recording data at acoustic Doppler current profiler at the western parrotfish spawning aggregation site. Photo R. Nemeth