Climate driven regime shifts vs rebound potential

Conditions under which coral reefs show resilience and regenerate from bleaching events or dramatically shift in state to less diverse and productive algal states are not well known in ecology yet. This is a severe drawback if one wants to investigate the impacts of climate change and associated bleaching events on coral reef fish. As I have come to understand and explain through previous posts, the potential for coral reef fish to survive bleaching events depends on the frequency and severity of these events. There are also a wide range of biological and behavioral factors that influence this, such as the specificity of diet, range and distribution of these species. We already know that a loss of coral cover significantly impacts juveniles that rely on cover to survive and evade predators.

A letter published to Nature this year by Graham et al., one of the leading researchers on this issue looks to understand the dynamics by which a >90% loss of coral cover can lead to both recovery and  a shifting of regime, two completely different responses. And these two different reef responses translate to two distinctive fish species responses, with a return to predistrubance structure on recovering reefs yet becoming significantly and progressively altered on those that shifted regimes. Using a 17 year dataset the paper assesses the long term ecosystem dynamics of 21 reef sites across Seychelles, the most severely affected area by the 1998 el Nino.

Figure 1. Response or recovery of coral reef assemblages post disturbance (source: Graham et al.,2015)

Results from the study found 12 of 21 sites recovered but this was laboured over the first 7-10 years, speeding up after this period as local recruitment levels increased. The main point to take from the paper is that a trajectory towards recovery occurred when reefs were structurally complex and in deeper water. In these area juvenile corals and herbivorous fish populations were high and nutrient loads low. This is interesting as it suggests a level of symbiotism between coral reef fish and coral reefs; while the fish species rely on the reef for protection and for survival, the coral species depend on the herbivorous fish species to recover. Whilst the paper does give an interesting insight into reef responses to disturbance it is important to remember this was only one rather local study, there could be significant regional differences in factors that influence such response.

Endangered Coral Reef Fish

Below is an introduction to some of the endangered fish species that rely on coral reef cover, it is important to dedicate a post to this so to highlight the beautiful species that are at risk, illuminating the main concerns of this blog. This list has been adapted from a study that assesses the IUCN Red List of Threatened Species focussing specifically on the US Pacific Islands. It is important to mention that the species listed below are IUCN classified as "endangered", "vulnerable" and/or NOAA "species of concern." Notably, there are many more species that are listed as "near threatened" or "decreasing" in population.


Common Name: Sharptooth Lemon Shark
Family: Carcharhinidae
Species Name: Negaprion acutidens
Range: wide ranging, native to Indian Ocean and western Central Pacific
Justification: Narrow habitat range, heavily overfished
IUCN classification: Vulnerable


Common Name: Threadfin Butterflyfish
Family: Chaetodontidae
Species Name: Chaetodon flavocoronatus
Range: endemic to western Pacific
Justification: very limited range
IUCN classification: Vulnerable


Common Name: Tawny Nurse Shark
Family: Ginglymostomatidae
Species Name: Nebrius ferrugineus
Range: Indo-Pacific
Justification: Heavily fished, narrow habitat range
IUCN classification: Vulnerable


Common Name: Humphead Wrasse
Family: Labridae
Species Name: Cheilinus undulatus
Range: Tropical Indo Pacific
Justification: valued economically, heavily overfished, 50% pop decline in past 20 yrs
IUCN classification: Endangered
NOAA specie of concern


Common Name: Manta Ray
Family: Mobidulae
Species Name: Manta alfredi
Range: wide but sparsely distributed and fragmented Pacific, Atalantic, Indian oceans
Justification: used for medicene, low reproductive output, specific resource needs
IUCN classification: Vulnerable


Common Name: Green Humphead Parrotfish
Family: Scaridae
Species Name: Bolbometopon muricatum
Range: wide range
Justification: overfished, extinct in some localities
IUCN classification: Vulnerable
NOAA specie of concern


Common Name: Squaretail Coral Grouper
Family: Serranidae
Species Name: Plectropomus areolatus
Range: Indo-Pacific, Red Sea, Southeast Asia
Justification: wide range but heavily fished, declined 30% over last 20 years
IUCN classification: Vulnerable


Common Name: Blacksaddled Coral Grouper
Family: Serranidae
Species Name: Plectropomus laevis
Range: large from East African Coast to central and Southern Pacific
Justification: targetting of juveniles by fishing
IUCN classification: Vulnerable


Common Name: Queensland Grouper
Family: Serranidae
Species Name: Epinephelus lanceolatus
Range: Indo-Pacific, locally rare but widely distributed
Justification: Fished heavily, cultural significance in China, reproductive difficulties
IUCN classification: Vulnerable


Common Name: Scalloped Hammerhead
Family: Sphyrnidae
Species Name: Sphyrna lewini
Range: circumglobal distribution in warm temperate and tropical seas
Justification: fishing
IUCN classification: Endangered


Common Name: Great Hammerhead
Family: Sphyrnidae
Species Name: Sphyrna mokarran
Range: large widely distributed, restricted to continental shelves
Justification: valuable in market so fished both intentionally and accidentally
IUCN classification: Endangered


Common Name: Zebra Shark
Family: Stegostomatidae
Species Name: Stegostoma fasciatum
Range: Indian, West and Central Pacific Oceans
Justification: overfishing
IUCN classification: Vulnerable


Some important points to take from this are:

• sharks make up the most of the endangered coral reef fish species
• there is very little overlap with the ideas shared in the lecture from my previous post
• fishing is the main cause of species endangering, very little reference to natural (climate driven) or biological reassons for population decreases

Bleaching on Coral Reef biodiversity - A short lecture

This relatively short lecture by a research fellow at the ARC Centre of Excellence for Coral Reef Studies at James Cook University provides very interesting insight into general impacts on coral reef species post bleaching event.

He draws data from 17 studies to look at specific species that have been impacted by previous bleaching events such as the galapagos coral-damsel (Azurite eupalama) which disappeared after the 1998 El Nino event.

As you will observe, he displays optimism with regards to coral reef fish extinction as most species have a large geographical range. Also, when we consider most impacts of coral bleaching are regionally limited it means that there is persistence in unaffected areas which facilitates future recovery. However he does stress that there are significant species whom have limited abundance and geographical range but with a high speciality of diet, ultimately rendering them significantly at risk.

In my next post I will introduce some of the coral reef fish species that are at risk of extinction.

Conservation gone wrong: Marine Protected Areas and Coral Reefs

Marine Protected Areas (MPAs) are widely considered from a conservationist perspective to be the most appropriate and effective way of preserving the structural complexity of marine habitats and in preventing overfishing and consequent decline in fish stocks. If one was to transfer this understanding to coral reef habitats we’d assume that MPAs would be largely beneficial in protecting the diversity within these rich havens. However, in the face of climate driven impacts a study by Graham et al. in 2007 found that the lagged impacts on coral reef fish species are enhanced by assignation of MPAs. The study assessed the long term impacts of the bleaching events on species, their size structure and the effectiveness of MPAs by investigating sites that were fished and those under conservation. The 1998 event reduced coral cover over these sites by 90% and upon revisiting the sites in 2005, recovery had only reached 7.5% overall. The results showed that juvenile species were most affected by long term impacts and the slope of species size decreased significantly, as juvenile species rely heavier on coral cover for shelter and protection from predation. The implications for a loss in juvenile species are severe considering that these species ensure future stable population size. While all sites (both fished and protected) showed a marked decrease in juvenile species the study found that greater predator biomass in MPAs (as these are the targeted species in sites with active fishing) actually results in higher predation rates on the vulnerable juvenile species, ultimately increasing the severity of the lag effect. Looking at species specifically, surgeonfishes (Acanthuridae) and parrotfishes (Scaridae) which generally span a large number of size classes showed a significant loss of size variation as juvenile habitat was destroyed.

The interesting findings from this study raises questions about the efficacy of conservation techniques in coral reefs. As is well known, in the face of climate change bleaching events are only to become more frequent, longer lasting and severe as enhanced teleconnective processes between the atmosphere and oceans (such as el Niño events) also become more frequent. Despite this however, one must consider whether the lag impacts on species size composition after bleaching events pose more threats than overfishing does in general? Though the paper doesn’t weigh up these considerations it will be interesting to see if there has been further thought on the effectiveness of MPAs in light of long term bleaching impacts.

Cold water bleaching - Florida 2010

When we think of coral reef bleaching it is understandable for most to only think of warm water bleaching events as these are the ones we typically learn about in the media. A good example of this is the widely reported warm water bleaching that occurred as a result of the 1997/1998 ENSO event. However, this post aims to look at some cold water events that can be equally as disruptive to coral reef ecosystems, looking at studies that consider exposure under the opposite threshold. That threshold is 16 degrees, temperatures below this can cause fast widespread coral death unprecedented in extent and degree of severity. 

in January 2010 the biggest cold-water event occurred in the Florida Reef Tract (FRT), an extreme anomaly that caused rapid mortality of Acropora spp. An interesting paper on this by Lirman et al. aimed to investigate these impacts through conducting reef surveys. The coral mortality patterns they observed were directly related to satellite derived cold water temperatures, and their results showed severity of impacts similar in extent and intensity to global warm water bleaching events. The reason for the event was surface wind anomalies and southward advection of cold arctic air caused by unprecedented negative values of the Northern Atlantic Oscillation (NAO) index. It is important to mention that this isn't an exclusive cold water event for this area, Shinn reported mortality in 1962 where temperatures fell to 13.3 degrees and further reported events occur in 1981, 1969-1970 and 1977-1978, though all of these were more constrained to local levels than the 2010 event.

Figure 1. Oceanic El Niño Index (ONI) from 1986 to 2011 (source: Paz-Garcia et al, 2011)

The study reports that tissue mortality was high (30-40% in studied areas) and 7.8% of colonies sampled in 2010 were severely lacking of live tissue. They further noted that the Upper and Middle Keys were less severe, with the richest most diverse inshore assemblages being the most impacted (despite being well known as resilient). Another interesting finding was that the response of corals to warm water anomalies was a poor predictor of the response of corals to this cold water event, with those most resistant to warm water temperatures the most vulnerable to cold water temperatures. The paper in general provides a decent analysis of the spatial response of coral reef species to this event however it is limited to analysing stony corals and not soft corals and benthic organisms such as sponges.

The take-home message from this study is that cold water bleaching events are equally as destructive as warm water events but affect different coral species that may be less resilient to the cooler conditions.

Figure 2. Bleaching event of Pocillopora colonies in Punta Galeras (source: Paz-Garcia et al. 2011)

Interestingly as I read further articles on cold water bleaching, it appeared a much more frequent occurrence than first expected (see Figure 1). There also occurred a cold water bleaching event that in the Gulf of California in 2011, despite its time and extent being less severe to that of Florida 2010, a study by Paz-Garcia et al. notes that the same significant visual signs could be seen in 85% of the surveyed corals. Linked in to the Lirman study these academics wanted to explore the different responses of Pocillopora morphospecies (figure 2) to cold water events and analyse the differences between cold and warm water events. They concluded that P. damicornis responded most negatively and that  cold water bleaching events events needs more study in general.

I will conclude this post with a question I asked myself as I read around this topic, why, if cold water events are both heavy impacting and not as temporally spread out as once imagined, is there very little coverage both in the media and in academic study? are these ecosystems being ignored for protection despite being seemingly as vulnerable as those in warmer waters?