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.

 

A Fish That Shapes The Reef

By Andrew E. Gray

Every three years, scientists from NOAA’s Coral Reef Ecosystem Program (CREP) visit Wake Atoll to survey corals, assess the fish populations, and collect oceanographic data for a long-term monitoring effort—the Pacific Reef Assessment and Monitoring Program (Pacific RAMP). Wake Atoll has clear water, healthy coral reefs, and is managed and conserved as part of the expansive U.S. Pacific Remote Islands Marine National Monument. It has a healthy reef fish community with plentiful sharks, jacks, and groupers. As a fish research diver, it’s my kind of paradise. Sitting in the middle of the subtropical North Pacific Ocean, 1,500 miles east of Guam and about 2,300 miles southwest of Honolulu, it may be the most remote place I’ve ever been. But for me, and a few other scientists lucky enough to visit the island, there is one thing that makes Wake a special place: Bolbometopon muricatm, the Bumphead parrotfish.

Bumphead parrotfish

Bumphead parrotfish (Bolbometopon muricatm) at Wake Atoll (Photo: NOAA Fisheries/Andrew E. Gray)

Bumphead parrotfish are an incredible and unique reef fish, differing from other parrotfish by their large size, appearance, diet, and by their ecological impact on coral reef ecosystems. There are a number of other parrotfish that sport a bump on their head, and these may be mistaken for a Bumphead parrotfish—that is until you actually see one. Bumpheads have a presence like no other fish on the reef and when they are around I can’t take my eyes off of them. The first thing I notice is their sheer size: growing to 4.2 feet long and up to a 100 pounds (that’s 130 cm and 46 kg for you scientists). Bumpheads are the world’s largest parrotfish and among the largest of all reef fish. When I get a little closer, I can’t help but focus on their incredible beaks. On coral reefs, all parrotfish species are tasked with the important job of keeping algae from overgrowing reef-building corals.

Corals chomped

Bumphead parrotfish chomp corals and help maintain the health and diversity of the reef ecosystem, Wake Atoll (Photo: NOAA Fisheries/Andrew E. Gray)

Parrotfish bite and scrape algae off of rocks and dead corals with their parrot-like beaks; grind the inedible calcium carbonate (reef material made mostly of coral skeletons) which is excreted as sand back onto the reef. Larger parrotfish species can take small chunks out of the reef, removing algae and the occasional piece of coral. Bumphead parrotfish are unique in that they are continuously crunching large bites out of the reef, about half of it from live coral. In fact, that’s what they do most of the day. Bite the reef. Excrete sand. Repeat. Over the course of a year a single fish can remove over 5 tons of calcium carbonate from the reef! But by selectively eating fast growing coral species over slower growing species, they help maintain a more diverse coral reef ecosystem. Also, by munching down tons of dead corals every year each fish makes room for young corals to settle, grow and build up the reef. This means breaking down “dead reef” into sand rather than it breaking off in a storm and damaging other parts of the reef. And since Bumpheads often travel in groups, sometimes numbering into hundreds and traveling multiple kilometers in a day, this species can have quite an impact on the reef ecosystem. Bumphead parrotfish literally shape the reef.

Bumphead

Large bump on the head of a Bumphead parrotfish (Photo: NOAA Fisheries/Andrew E. Gray)

Then, of course, there is the fish’s namesake, its bump. All Bumphead parrotfish sport a large protrusion on their forehead which is similar in function to a pair of horns on a bighorn sheep. The largest males have the biggest bumps and will occasionally use them as battering rams around spawning time, smashing headfirst into rivals in an attempt to show their dominance and retain territorial and breeding rights. This incredible behavior was observed by CREP scientists in 2009 and first documented and filmed by researchers at Wake in 2011. During these mating events, the parrotfish gather or aggregate around a spawning site and can number into the hundreds, an uncommon site anywhere in the world and one that I hope to see sometime at Wake.

Historically, Bumphead parrotfish were plentiful throughout much of the Western Pacific, Indian Ocean, and Red Sea. In recent decades, fishing led to sharp declines in abundance and they are now only common in protected or very remote areas. Bumpheads have a few traits that make them particularly vulnerable to overfishing, which has led to local disappearances in many parts of their range. Bumphead parrotfish can live to be 40 years old; they do not reach sexual maturity until 5-8 years old and likely have low natural mortality as adults so there is not high natural turnover in the population. However, most detrimental to their survival in a human-dominated world is their aggregating behavior and preference for shallow water. Groups of Bumpheads could be easily netted, as they feed during the day, and at night sleeping parrotfish are easy targets for spear fishermen. With the introduction of scuba gear in the 1960’s and 1970’s there was a steep decline in Bumphead abundances as entire schools could be removed in a single night while they slept. Juvenile Bumpheads are also hard to find or study throughout much of their range and raises concerns that some adult populations are too far from juvenile habitats. This distance prevents new youngsters from entering the population to replace adults that have been caught. In areas where juveniles can be commonly found, such as Papua New Guinea and the Solomon Islands, they are associated with mangrove, rubble, and sheltered lagoon habitats. And this is why Wake Atoll may be such a hotbed of Bumpheads.

Reef at Wake Atoll

Coral reef at Wake Atoll in the Pacific Remote Islands Marine National Monument (Photo: NOAA Fisheries/James Morioka)

In addition to having a sizable healthy coral reef around the island, Wake Atoll has an expansive, sheltered lagoon. This may be the perfect habitat for the juvenile parrotfish and allows Wake to have a healthy, self-supplying population of Bumpheads. And since Wake is protected from fishing, it may be as close to a pristine home as the Bumphead parrotfish are going to encounter in today’s world. Wake actually has the highest concentration of Bumphead parrotfish in U.S. waters and possibly the world (although certain areas of the Great Barrier Reef in Australia also have very healthy adult populations). During my time at Wake Atoll, I had a number of chances to see them, from loose groups of just a few individuals, to a school of thirteen.

School too

School of Bumphead parrotfish at Wake Atoll (Photo: NOAA Fisheries/Andrew E. Gray)

As I write this, the NOAA Ship Hi‘ialakai heads west to Guam, our next survey site where I’ll be spending 8 days surveying reef fish. Bumpheads were once thought to be extinct around Guam due to overfishing, but there have been a few sightings by CREP and partners in the past few years, of both adults and juveniles. So while my expectations of encountering these giant bulbous-headed, coral-chomping fish are low, I sure hope I do, given how important they are to the natural function of coral reef ecosystems.

References
  1. Bellwood, D., & Choat, J. (2011). Dangerous demographics: the lack of juvenile humphead parrotfishes Bolbometopon muricatum on the Great Barrier Reef. Coral Reefs, 30(2), 549-554.
  2. Bellwood, D. R., Hoey, A. S., & Choat, J. H. (2003). Limited functional redundancy in high diversity systems: resilience and ecosystem function on coral reefs. Ecology Letters, 6(4), 281-285.
  3. Bellwood, D. R., Hoey, A. S., & Hughes, T. P. (2011). Human activity selectively impacts the ecosystem roles of parrotfishes on coral reefs. Proceedings of the Royal Society B: Biological Sciences. doi: 10.1098/rspb.2011.1906
  4. Donaldson, T. J., & Dulvy, N. K. (2004). Threatened fishes of the world: Bolbometopon muricatum (Valenciennes 1840)(Scaridae). Environmental Biology of Fishes, 70(4), 373-373.
  5. Green, A. L., & Bellwood, D. R. (2009). Monitoring functional groups of herbivorous reef fishes as indicators of coral reef resilience: a practical guide for coral reef managers in the Asia Pacific Region: IUCN.
  6. Kobayashi, D., Friedlander, A., Grimes, C., Nichols, R., & Zgliczynski, B. (2011). Bumphead parrotfish (Bolbometopon muricatum) status review. NOAA Technical Memorandum NMFS-PIFSC-26. NOAA.
  7. Muñoz, R. C., Zgliczynski, B. J., Laughlin, J. L., & Teer, B. Z. (2012). Extraordinary Aggressive Behavior from the Giant Coral Reef Fish, Bolbometopon muricatum, in a Remote Marine Reserve. PLoS One, 7(6), e38120. doi: 10.1371/journal.pone.0038120
  8. Munoz, R. C., Zgliczynski, B. J., Teer, B. Z., & Laughlin, J. L. (2014). Spawning aggregation behavior and reproductive ecology of the giant bumphead parrotfish, Bolbometopon muricatum, in a remote marine reserve. PeerJ, 2, e681.
  9. Sundberg, M., Kobayashi, D., Kahng, S., Karl, S., & Zamzow, J. (2015). The Search for Juvenile Bumphead Parrotfish (Bolbometopon muricatum) in the Lagoon at Wake Island.