What happens to reef fish after coral bleaching?

by Adel Heenan

For the past month, researchers aboard the NOAA Ship Hi‘ialakai have been navigating across the Pacific Ocean to survey coral reef ecosystems at remote Wake Atoll and the Mariana Archipelago. This expedition includes additional surveys at Jarvis Island, in the Pacific Remote Islands Marine National Monument, to assess the reef condition and degree of recovery from a catastrophic coral bleaching event in 2014-2015.

Jarvis Island is located in the central Pacific Ocean, close to the equator, and is a small island in the direct path of a deep current that flows east (Figure 1). Because of it’s position right on the equator and the strong currents hitting the island, Jarvis sits in the middle of a major upwelling zone—where cold nutrient rich water is drawn up from the deep. This water fertilizes the whole area, elevating nutrient levels and productivity in the reef ecosystem (Gove et al., 2006). As a result, Jarvis supports exceptionally high biomass of planktivorous and piscivorous fishes (Williams et al., 2015).

Because it is unpopulated and extremely remote, Jarvis provides an important reference point and opportunity to understand the natural structure, function, and variation in coral reef ecosystems. The island also offers a natural laboratory in which the effects of ocean warming can be assessed in the absence of stressors that impact coral reefs where humans are present (e.g., fishing or land-based sources of pollution).

El Niño, La Niña and the global coral bleaching event of 2014-2015
The Equatorial Pacific upwelling at Jarvis alternates between warm El Niño years, when upwelling is weak and oceanic productivity low, and cold La Niña years where upwelling is strong and productivity is high (Gove et al., 2006). Unusually warm sea surface temperatures, and a strong El Niño in 2014-2015, triggered the third recorded global coral bleaching event. At Jarvis, these warmer waters led to widespread coral bleaching and mortality. High sea surface temperatures in 2015 also impacted upwelling at Jarvis, as evidenced by a decrease in the primary productivity around the island.

Teams from the Coral Reef Ecosystem Program recently completed ecological monitoring at Jarvis from April 2–5, 2017. They collected data at 28 stationary point count sites (Figure 2) this year, 30 in 2016, 62 in 2015, 42 in 2012, and 30 in 2010.


Figure 2. The stationary point count method is used to monitor the fish assemblage and benthic communities at the Rapid Ecological Assessment (REA) sites.

Main Observations
Fish biomass tended to be highest on the western side of the island where equatorial upwelling occurs (Figure 3). In 2016, we observed somewhat reduced total fish and total planktivore biomass (Figure 4), but this reduction was within the normal range of observed variability.

There were some significant reductions observed for individual species in 2016. These reductions were noticeable across multiple trophic groups, for instance the planktivorous Whitley’s fusilier (Luzonichthys whitleyi), Olive anthias (Pseudanthias olivaceus), Dark-banded fusilier (Pterocaesio tile), the piscivorous Island trevally (Carangoides orthogrammus), and the coral-dwelling Arc-eyed hawkfish (Paracirrhites arcatus) which is strongly associated with Pocillopora coral heads. Some of these species had returned to previous ranges by 2017, but others remain depleted (Figure 5).


Figure 5. Mean species biomass (± standard error) per survey year at Jarvis.

Very high levels of coral mortality were evident in 2016 surveys and coral cover remained low in 2017. Notably, macroalgal cover increased in 2017, approximately by the amount of coral cover lost in 2016 (Figure 6).


Figure 6. Mean percentage cover estimates (± standard error) of benthic habitat per survey year at Jarvis. Data shown for Hard Coral (top, red); macrolagae (middle, green) and CCA: crustose coralline algae (bottom, orange). Note: no benthic data are available for 2008 as we began collected rapid visual estimates of these benthic functional groups in 2010.

Whether this reduction in specific planktivore, piscivore, and live coral-dwelling fish species is a widespread and long-standing shift in the fish assemblages at Jarvis will be the subject of forthcoming research. It seems plausible that they reflect impacts of a prolonged period of reduced food availability and changes to preferred habitat due to the anomalous warm sea conditions in 2014–2015. Our teams will return to Jarvis in 2018 to conduct another assessment in an attempt to answer some of these questions.


An emaciated grey reef shark (Carcharhinus amblyrhynchus) observed during a 2017 fish survey. (Photo: NOAA Fisheries/Adel Heenan)

Additional detail on survey methods and sampling design are available in the full monitoring brief: Jarvis Island time trends 2008-2017.

Gove J. et al. (2006) Temporal variability of current-driven upwelling at Jarvis Island. J Geo Res: Oceans 111, 1-10, doi: 10.1029/2005JC003161.
Williams I. et al. (2015) Human, oceanographic and habitat drivers of central and western Pacific coral reef fish assemblages. PLoS 10: e0120516, doi: 10.1371/journal.pone.0120516.


Why do coral reef ecosystems thrive in remote areas of the Pacific Ocean?

by Jamison Gove and Amanda Dillon
Rose Atoll in American Samoa

Rose Atoll in American Samoa

The thousands of coral reef islands and atolls found across the Pacific Ocean are predominately surrounded by a biologically barren oceanic landscape. Paradoxically, the nearshore marine ecosystems around these islands and atolls are teeming with an abundance of reef fish, healthy corals, and other marine species.

Coral reef at Rose Atoll, American Samoa

Four Stripe damselfish (Dascyllus melanurus) and Manybar goatfish (Parupeneus multifasciatus) swim among a variety of corals including Acropora and Monitpora at Rose Atoll, American Samoa.

Led by Jamison Gove of the PIFSC Ecosystems and Oceanography Program, an international team of scientists from NOAA, Joint Institute for Marine and Atmospheric Research, University of Hawaiʻi at Mānoa, National Geographic Society, Scripps Institution of Oceanography, and Bangor University published a study in Nature Communications that provides the first broad-scale investigation of this paradoxical increase in productivity near coral reef islands and atolls—known as the “Island Mass Effect.”

Although observations of the paradoxical increase in biological productivity near coral reef islands and atolls date back to Charles Darwin nearly 200 years ago, the Island Mass Effect was first described by Maxwell S. Doty and colleagues in 1959 as a phenomenon in which the growth of phytoplankton is enhanced close to island-reef ecosystems. Until now, all studies trying to explain the reasons for the Island Mass Effect have been done over small, geographically-confined areas, such as a single island or coral reef group. As such, scientists have known very little about the prevalence, geographic variability, and drivers of this perplexing but ecologically important phenomenon.

In order to investigate the Island Mass Effect on a large scale, we studied 35 coral reef islands and atolls in the Pacific, using satellite imagery and ship-based surveys to assess the distribution of chlorophyll and accumulation of phytoplankton. We discovered that phytoplankton biomass increased at nearly all of the nearshore survey locations, resulting in biological “hotspots” in otherwise barren, tropical waters.

Figure 1. Island Mass Effect

Figure 1. Illustration showing the Island Mass Effect at a human-populated island (left) and unpopulated atoll (right).

We further discovered that the strength of the Island Mass Effect varied between island ecosystems, and that island type, sea-floor slope, reef area, and human habitation are the primary causes or drivers of increased phytoplankton. Overall, our team found that the Island Mass Effect enhances phytoplankton biomass in most (86 percent) offshore ocean conditions, providing increased food resources for higher trophic groups such as tuna and dolphins.

2009Although increased phytoplankton is often beneficial for these coral reef ecosystems, it can also lead to toxic algal blooms, increased fleshy (non-reef building) algal growth and other negative impacts when associated with human activities. The ability to discern human-versus natural-driven changes in phytoplankton biomass is of great scientific interest and has important resource management implications.

That humans can artificially elevate phytoplankton biomass—around entire islands—provides important context for the impact of human activities on nearshore marine ecosystems. These reef ecosystems support human populations through food production and employment opportunities from fisheries and tourism, as well as coastal protection.

Now, our goal is to better understand these impacts and their influence on nearshore phytoplankton production. The Island Mass Effect will continue to be an important phenomenon, especially for island nations, in developing effective strategies to address future changes in climate and the global ocean.

Read the full article online: Near-island biological hotspots in barren ocean basins


Sarigan Island in the Commonwealth of the Northern Mariana Islands

The study was featured in numerous online news articles:

The New York Times: An Island, a Focal Point for a Healthy Marine Ecosystem
The Conversation: Solving ‘Darwin’s Paradox’: why coral island hotspots exist in an oceanic desert
IFLScience: Islands Help Create Phytoplankton Oases In Oceanic Deserts
University of Hawai‘i News: Research explains near-island biological hotspots in barren ocean
UH School of Ocean and Earth Science and Technology: Research explains near-island biological hotspots in barren ocean basins
EurekAlert! Ocean oases: How islands support more sea-life