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.

Finding needles in a hay stack

— Written by Maria Rivera

Collecting hundreds of kilograms of sediment samples is challenging but finding microscopic shark scales in the midst of hundreds of sand grains is another thing entirely. In the process of extracting denticles from large sediment samples, several steps such as lab processing, many gallons of acetic acid, and hundreds of hours of microscope time are required. First of all, we need to collect the samples (bulk of sediments), from a region with high and low shark abundance to be able to make comparisons. For example, with my project on a small island in the Caribbean called Curaçao, there are samples from two sites. One site has high human impacts while the other has less human impacts. A potential hypothesis might be that the more impacted site might have less sharks and therefore less denticles.


Fig.1 shows the microscopes used in the lab when we are searching for denticles in a sample.

Once the samples are collected, we then weigh each sample in the lab. These samples are then separated into different sized fragments using sieves with various mesh sizes (2 mm, 500µm (micrometers), 250µm, 106µm, and 63µm). The denticles, which are the size of several strands of hair, are found in the 106, 250, and 500 µm fractions. The bulkiness of these samples would make it nearly impossible to pick through, like finding a needle in a hay stack, thus, we add 10% acetic acid to reduce the size of the mass significantly. This gets rid of all the carbonate (coral and shell) material, which compromises over 90% of the samples’ mass. We also have to rinse the sample with hydrogen peroxide to get rid of any excess organic material. All that is left is the non-organic and non-carbonate residue that the denticles are found in. Now it is feasible to pick through the different sized grains and identify our denticles under a microscope. Once we identify our denticles in a sample, we measure them and look at the ridges and how they are shaped. This helps us determine the type of shark form which it was shed. When we identify the denticles, we can plot our data and analyze/make inferences about how shark communities vary across different human interference sites. Going back to Curaçao, once we have all the data, we would be able to analyze what types of sharks roam each site and their relative abundance, as well as how that has changed over time. This will shed a light to how humans have impacted the coral reef shark communities in both areas.

It is important, however, to realize that a number of factors can impact the number of denticles we find in a sample. For example, denticle abundance can be affected by denticle shedding rates, transportation, and coral reef growth and sedimentation rates. In regards to shedding rates, we do not know how often sharks shed denticles or whether shedding rates differ between pelagic and demersal species, and this can impact how many denticles we find in a sample. When it comes to transportation, denticles can be transported horizontally by currents as they sink. However, our collection sites are located on very sheltered, low energy reefs, and modeling exercises have shown that sinking denticles do not travel very far. With coral growth, if a coral reef is growing at a fast rate, denticles would be deposited and then buried at a quicker rate. As a result, denticle densities per amount sediment would be lower at faster accumulating reefs. To correct for this, we date corals to determine how much time is encapsulated in each sediment sample. These are just some of the few things that we need to consider when we are analyzing denticle abundance and trying to relate that to shark abundance.

Shark abundance shown with dermal denticles

— Written by Maria Rivera

How can we know how many sharks there are in a specific area? We cannot track or observe them as easily as other animals, especially land animals. This is a major issue because we do not have sufficient data that tells us about shark communities in the past nor present. Without this knowledge, we cannot analyze how humans have impacted shark communities near reefs. Fortunately, there are new techniques being developed that help us measure the relative abundance of sharks near coral reefs. One of these new techniques involves analyzing dermal denticles. Dermal denticles are the microscopic tooth-like scales that cover sharks’ bodies. Sharks shed these denticles, and they are deposited and preserved in coral reef sediments. Hundreds of denticles can be preserved in the sand, recording a timeline of what shark communities where like in the past and how they have changed.


Fig. 1 shows different types of denticles that belong to different kinds of sharks. Denticle A has high ridges for swimming, while denticle B is smooth and thick for protection. Photographs courtesy of Natalie Minouei taken on July 6, 2018.

How do denticles help us estimate/determine the relative abundance of sharks? By comparing the denticles we find in sediments from different regions and time periods, we can qualitatively estimate the abundance of sharks. Not only do the number of denticles help us interpret the abundance of sharks near reefs, but we can also use the wide array of denticle shapes and sizes to learn more about the ecology of sharks. Variation in denticle morphology serves different purposes. For example, thin denticles with high ridges are for drag reduction and allow the shark to swim faster (Fig. 1). These denticle characteristics are common with pelagic sharks that spend a majority of the time in open waters and are fast swimmers, like great whites for example. In contrast, there are denticles that can be smooth and thick for abrasion strength, this provides protection to the shark from the ocean floor (Fig. 1). Denticles like these would be found on sharks that live near reefs, such as nurse sharks. Variations of denticles for different sharks gives us an insight into the structure and composition of the shark communities roaming that reef.

We can also look at denticle distributions in areas across gradients of human impacts to investigate how shark communities are affected by human activities. For example, if there is a lot of fishing in an area, there might be lower numbers of sharks and thus lower number of denticles. If we compare that to a place that is less impacted by humans, we would expect to see more sharks and more denticles. This can help us understand how humans have impacted the marine ecosystem through time and in a context of local baselines. This is a great way to analyze the relative abundance of sharks near coral reefs when there are limited tools and scientists cannot go physically count sharks one by one in the ocean.