mass mortality

Management of reef fish populations requires insight in their resilience to anthropogenic stressors. Studies on temporal variations in reef fish populations and their abundance responses to environmental changes are crucial to the development of ecosystem-based management.

Seventeen years of voluntary fish survey data of reef fish at the west coast of Bonaire Marine Park (Caribbean) were analysed to investigate the effect of environmental changes on local reef fish populations. Various anthropogenic stressors that influence the coral reefs of Bonaire were studied in recent years, this study focusses on population responses to loss of coral cover, the establishment of a fishery protected area, and reef fishery pressure. In addition, reef fish responses to specific events were analysed at species or family level.

In general fluctuations in sighting frequencies were species-specific and difficult to interpret. Three large parrotfish decreased by approximately 50% over time and there were indications of a negative trend in mid-sized parrotfish as well. In fishery targets, the most intensive fished group (large to mid-sized grouper species), decreased strongly. After their abundance decreased to near zero, two species belonging to other fish families showed signs of sequential overfishing. It is unclear if larval import will compensate for the loss in reproductive capacity of the species in decline, recent literature on recruitment distances suggest that the level of self recruitment is high in Bonaire. An exception to short recruitment distances was noted earlier in the balloon fish Diodon holocanthus. The data suggest that the balloon fish population till 2010 originated from the mass recruitment in 1994, from the start of the studied period in 2000 to 2010 their abundance steadily declined to near zero. Another event with direct effect on population dynamics is the mass mortality events in moray eels in 2008. With a lag time of a year, especially two Enchelycore species strongly declined afterwards, which could elucidate the highly variable population densities of these species over the period. A following moray eel mass mortality event in 2022, also occurred at the onset of a period of sea water warming, paralleling various reef fish species in the Red Sea. Increase in warming events carries the risk of more frequent mass mortalities.

The 1983-1984 die-off of the long-spined sea urchin Diadema antillarum stands out as a catastrophic marine event because of its detrimental effectson Caribbean coral reefs. Without the grazing activities of this key herbivore, turf and macroalgae became the dominant benthic group, inhibiting coral recruitment and compromising coral reef recovery from other disturbances. In the decades that followed, recovery of D. antillarum populations was slow to non-existent. In late January 2022, a new mass mortality of D. antillarum was first observed in the U.S. Virgin Islands. We documented the spread and extent of this new die-off using an online survey. Infected individuals were closely monitored in the lab to record signs of illness, while a large population on Saba, Dutch Caribbean, was surveyed weekly before and during mortality to determine the lethality of this event. Within four months the die-off was distributed over 1,300 km from north to south and 2,500 km east to west. Whereas the 1983-1984 die-off advanced mostly with the currents, the 2022 event has appeared far more quickly in geographically distant areas. First die-off observations in each jurisdiction were often close to harbor areas, which, together with their rapid appearance, suggests that anthropogenic factors may have contributed to the spread of the causative agent. The signs of illness in sick D. antillarum were very similar to those recorded during the 1983-1984 die-off: lack of tube feet control, slow spine reaction followed by their loss, and necrosis of the epidermis were observed in both lab and wild urchins. Affected populations succumbed fast; within a month of the first signs of illness, a closely monitored population at Saba, Dutch Caribbean, had decreased from 4.05 individuals per m2 to 0.05 individuals per m2. Lethality can therefore be as high as 99%. The full extent of the 2022 D. antillarum die-off event is not currently known. The slower spread in the summer of 2022 might indicate that the die-off is coming to a (temporary) standstill. If this is the case, some populations will remain unaffected and potentially supply larvae to downstream areas and augment natural recovery processes. In addition, several D. antillarum rehabilitation approaches have been developed in the past decade and some are ready for large scale implementation. However, active conservation and restoration should not distract from the primary goal of identifying a cause and, if possible, implementing actions to decrease the likelihood of future D. antillarum die-off events.

The sea urchin Diadema antillarum commonly occurs on Caribbean reefs in densities sufficiently high to influence characteristics such as community composition and reef growth. We observed an outbreak of mass mortality in this species reducing population densities by 98 to 100 %. Mortality spread from the Curacao harbour mouth along the coast, most rapidly advancing in the down-current direction. Our calculations show a pronounced effect on the carbonate budget of the reef. Recovery of Diadema populations may be facilitated by parthenogenesis. There is continuing recruitment on affected reefs, a possible location of the parent population being up-current unaffected reefs of Bonaire.