How do humans impact shark assemblages on coral reefs in Curaçao?

— Written by Maria Rivera

For my project this summer, I compared the relative abundance of different shark dermal denticle (“skin teeth”) functional morphotypes preserved in reef sediments across two locations on Curaçao, an island located in the southern Caribbean. Denticle morphology can provide insight into the identity and ecology of the sharks that they belong to, allowing me to investigate whether the shark community composition differs between these two reefs. But why should we care? Visual surveys, such as underwater visual censuses performed by divers and baited remote underwater video stations, have recorded few sharks along Curaçao’s reefs. However, sharks are sporadically spotted by divers and anecdotally were quite abundant along Curaçao’s coast in the past. Curaçao also recently declared its waters to be a shark sanctuary. However, it is challenging to protect and manage shark populations when we cannot readily monitor them or determine how they vary across space, particularly given their high mobility.

Using the denticle record, I investigated how shark community composition and size differed between a high and low human impact site on Curaçao. The low impact site (Klein Curaçao) is a small island a few kilometers away from the main island. This island has, for the most part, healthier reefs with higher coral cover and fish biomass than Curaçao proper and has experienced less human interference. Given its low impact and distance from most of Curaçao’s population, Klein Curaçao has been thought of, as least anecdotally, as a window into Curaçao’s historical reefs. In contrast, the high impact site (CARMABI) is located near Willemstad, Curaçao’s capital where most of the island’s population resides. This reef has been exposed to pressures from fishing, pollution, diving and tourism, and boat traffic. These sites were selected to represent a gradient of human impacts on Curaçao. I hypothesized that there would be a higher abundance of sharks and henceforth a higher abundance of denticles on the reef on Klein Curaçao, which has much less human influence, as opposed to CARMABI. I also hypothesized that human activities could alter the composition of shark communities. In particular, I predicted that that there would be more pelagic shark denticles (i.e. drag reduction morphotype) in the sediments on Klein Curaçao given that it is more exposed to the open ocean and historically has experienced less fishing pressure, which tends to selectively remove requiem and hammerhead sharks. In contrast, I predicted to find a higher relative abundance of nurse shark denticles (i.e. abrasion strength morphotype) at CARMABI since this species is not typically targeted by fishermen.

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Figure 1. Shows the differences in shark communities with differences in denticle morphology.

The samples at both sites were full of denticles, which was very exciting. We found a higher number of denticles per amount sediment at the low impact site (Klein Curaçao) than the high impact site (CARMABI). This suggests that there are larger shark assemblages at the low impact site. These sites also had higher time-averaged denticle abundances, unstandardized by reef accretion rates, than other regions in the Caribbean (Panama and the Dominican Republic) that we have surveyed using the denticle record. We also saw a difference in shark communities between sites. Klein Curaçao had a higher relative proportion of abrasion strength denticles, which are found on bottom dwelling sharks such as nurse sharks. In comparison, Carmabi had a higher relative proportion of drag reduction denticles, which are characteristic of open ocean, fast swimming sharks such as a great white shark. This finding was surprising and unexpected. While we have found a pendulum-like shift in community composition from abrasion strength to drag reduction denticles across time in Panama and the Dominican Republic due to human impacts, something different appears to be occurring in this space-for-time comparison across Curaçao. First, there could be spatial differences in habitat quality and environmental features. For example, the site at CARMABI might also be easily accessed by pelagic sharks or be providing habitat for reef-associated requiem and hammerhead sharks. Alternatively, Klein Curaçao has recently been exposed to higher fishing pressures by fishermen who are now venturing farther to access fishing resources given that the inshore reefs have been historically exploited. While our time-averaged samples are likely capturing a signal of the shark assemblage prior to the start of these pressures, we could be seeing a shift in community composition progress – from a community dominated by pelagics to one dominated by demersal sharks.  This could have potentially been due to changes in how humans affect these shark communities, for example, these effects could be caused by how and what humans have fished over the years and how that has changed. These results show us that shark communities do change over time and across regions, and also that humans can have an effect on these changes. This is important knowledge when it comes to better conservation and management of shark communities near the reefs of Curaçao.

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