What Happens to Reef Fish After Coral Bleaching?

May 08, 2017

By Adel Heenan

A school of dark-banded fusilier (Pterocaesio tile) at Jarvis Island in 2015.

A school of dark-banded fusilier (Pterocaesio tile) at Jarvis Island in 2015 (Photo: NOAA Fisheries/Andrew E. Gray).

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 surveys at Jarvis Island in the Pacific Remote Islands Marine National Monument to assess the reef and degree of recovery from a catastrophic coral bleaching event in 2014–2015.

Diver conducts survey to assess reef condition after coral bleaching at Jarvis Island

Recent surveys at Jarvis Island documented one of the few remaining table Acropora corals in the foreground, the background shows the rest of the reef area covered with a carpet of red turf algae that grows over dead and damaged corals in April 2017. (Photo: NOAA Fisheries/Tate Wester)

Image of wide-band fusilier.

The recent decline of planktivorous fish species has been a focus for fish researchers studying Jarvis before, during, and after the latest global bleaching event. In 2017, several common fusilier species were seen fewer times than in previous years. However, one species never before recorded at Jarvis in prior surveys was observed in 2017: the wide-band fusilier (Pterocaesio lativitta). (Photo: NOAA Fisheries/Adel Heenan)

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. Because of its 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).

Map of sea surface temperatures in the Pacific Remote Island Areas.

Sea surface temperatures around Jarvis Island and Pacific Remote Islands.

Map of seafloor depths around Jarvis Island.

Seafloor depths around Jarvis Island.

Because it is unpopulated and extremely remote, Jarvis allows us to study the natural structure, function, and variation in coral reef ecosystems. The island also offers a natural laboratory where we can assess the effects of ocean warming in the absence of local fishing and land-based sources of pollution that impact coral reefs where humans are present.

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.

Our scientists monitored coral reef ecosystems at Jarvis in April 2017. They collected data at 28 stationary point count sites.

Diagram of stationary point count methodology.

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 Jarvis, where equatorial upwelling occurs. In 2016, we observed somewhat reduced fish and planktivore biomass compared to other years, but this reduction was within the normal range of observed variability.

Fish biomass at survey sites each year from 2008 to 2017.

Fish biomass at sites around Jarvis Island surveyed from 2008 to 2017.

Graphs of fish biomass at Jarvis from 2008 to 2017.

Primary consumers include herbivores (which eat plants) and detritivores (which bottom feed on detritus), and secondary consumers are largely omnivores (which mostly eat a variety of fish and invertebrates) and invertivores (which eat invertebrates).

We observed a significant reduction of some species in 2016. These reductions were noticeable across trophic groups. For instance, the planktivorous Whitley’s fusilier (Luzonichthys whitleyi), olive anthias (Pseudanthias olivaceus), and 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 observations returned to previous ranges by 2017, but others were still depleted.

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

2016 surveys showed very high levels of coral mortality, and coral cover remained low in 2017. Macroalgal cover increased in 2017, similar to the amount of coral cover lost in 2016.

Graphs of percent voer of hard corals, macroalgae, and crustose coralline algae, by year, at Jarvis island.

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). No data are available for 2008 as we began collecting rapid visual estimates of these benthic functional groups in 2010.

Is the reduction in specific planktivore, piscivore, and live coral-dwelling fish species a widespread and long-standing shift in the fish assemblages at Jarvis? It may be due to a prolonged period of reduced food availability and changes to habitat due to the unusual warm sea conditions in 2014 and 2015. Our teams will return to Jarvis in 2018 to investigate.

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

Download the full monitoring brief (PDF) for additional detail on survey methods and sampling design: Jarvis Island time trends 2008-2017.

References

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.