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.

FIG2_SPC

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).

FIG5_FishBiomass

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).

FIG6_PercentCover

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.

FIG7_shark

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.

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.

 

Recruits Finding a Home

By Kevin Lino

As a fish nerd (and biologist), I was excited to hear about unusual events occurring along Hawaii’s reefs this summer. While away on another research mission, reports came in from various sources and agencies about what was being referred to as a “biblical” fish recruitment event. Across many Hawaiian reefs, there were multiple reports from researchers, fishermen, and the public of juvenile reef fishes seen in extraordinary numbers. Not just one or two species either, but a wide variety of species. Species were also appearing in locations where they had previously been very rarely encountered. After seeing the pictures and reading comments about the amazing and unprecedented numbers, our team of scientists was eager to get in the water to conduct dive surveys.

Several juvenile Bluefin Trevally (Caranx melampygus) investigate research divers.

Several juvenile Bluefin Trevally (Caranx melampygus) investigate research divers.

This September, during the fifth year of a research partnership between the Coral Reef Ecosystem Division (CRED), of the NOAA Pacific Islands Fisheries Science Center, and the Division of Aquatic Resources (DAR) of the Hawai‘i Department of Land and Natural Resources (DLNR), we got our chance. We conducted the most recent round of biannual fish and benthic dive surveys focusing on the near-shore (<18m) reef habitats of the Kahekili Herbivore Fishery Management Area (KHFMA) on the coastline of West Maui.

A large school of juvenile parrotfish and surgeonfish cruise over the reefs in the KHFMA looking for suitable grazing spots.

A large school of juvenile parrotfish and surgeonfish cruise over the reefs in the Kahekili Herbivore Fishery Management Area (KHFMA) looking for suitable grazing spots.

Knowing that these surveys were occurring late in the summer, and that the recruitment may be more site-specific, we were surprised by the numbers. Nothing I would refer to as “epic” or “biblical” but definitely impressive. During four days of surveys, there was certainly an increase in recruited (the number of new young fish that enter a population in a given year) butterflyfishes, surgeonfishes, tangs, parrotfishes, and other near-shore fishes as compared to previous years. The variety was remarkable as numbers of protected herbivorous juvenile Yellow Tang (Zebrasoma flavescens), Kole Tang (Ctenochaetus strigosus), and Lavender Tang (Acanthurus nigrofuscus) were rather abundant. Since 2009, herbivorous fishes within the KHFMA have been protected, but other groups of fish can still be harvested. This unusual management approach has proven much more acceptable to the public as compared to full closure measures. Therefore, if proven effective as a means of restoring herbivorous fish populations, and preventing coral-to-algal phase shifts, then this management approach has great potential to be more widely adopted in Hawai‘i and beyond. Observations seem to be trending in this direction with further analysis forthcoming from this data set and potentially continued research.

One of the many Hawaiian Fantail Filefish (Pervagor spilosoma) recruits seen on the reef in the Kahekili Herbivore Fishery Management Area (KHFMA).

One of the many Hawaiian Fantail Filefish (Pervagor spilosoma) recruits seen on the reef in the  KHFMA.

Our dive teams spent many hours underwater each day identifying, sizing, and counting these fish, as well as sea urchins, and collecting benthic imagery for more detailed analysis later. Divers also paid close attention to potential impacts to corals and the benthic community with much warmer waters being brought in by the current El Niño event. At least for now, little impact was noted during the time of our visit. El Niño is typically associated with a band of warm ocean water temperatures that periodically develop off the Pacific coast of South America and affect other areas in the Pacific. Since the mechanisms and impacts of this warmer water are still under study, the data collected this year will be especially valuable.

 A little White Tip Reef Shark (Triaenodon obesus) emerges from its resting spot within the shallows of the KHFMA.

A little White Tip Reef Shark (Triaenodon obesus) emerges from its resting spot within the shallows of the KHFMA.

There were other species sighted, likely enjoying the bounty, including the smallest white tip reef shark (Triaenodon obesus) that I’ve ever seen—at barely two feet long, pretty adorable. A large number of juvenile predators like Bluefin Trevally (Caranx melampygus) and various goatfish species were also fairly common. One colorful omnivore was sighted throughout all reef habitats on nearly every survey. The Hawaiian Fantail Filefish (Pervagor spilosoma) is an endemic species (only found in Hawaiian waters) that is uncommon on most dives. But as recorded in both Hawaiian culture and historical surveys, this species will have a “bloom” or “boom” year that we were fortunate enough to witness. It will be interesting to see how all of this year’s recruits will affect things in years to come.