Elasmobranchs in Watamu Marine National Park

Elasmobranchs ; Kings of the Sea

Not all that is in the sea is fish. Marine wildlife is broad in the number of different species and they come in many colours and forms. Among these are elasmobranchs which are the apex predators of the marine food chain. What are elasmobranchs you may ask? This is a marine wildlife subclass comprising of sharks, rays, skates and sawfishes. They are cartilaginous fish characterised by having five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins and small placoid scales on the skin.

Elasmobranchs play a critical role in structuring and maintaining healthy marine ecosystems by exerting top-down control on other species. They do this by altering the spatial habitats of their prey. This indirectly helps in the development of coral habitats and seagrass, making them a key indicator of a healthy ocean. However, as important as they are to the ecosystem, elasmobranchs face a challenge when it comes to growing in number.

Sharks and their relatives include some of the latest maturing and slowest reproducing of all vertebrates. They exhibit the longest gestation periods and some of the highest levels of maternal investment in the animal kingdom. The extreme life histories of these elasmobranchs result in very low population growth rates and weak density-dependent compensation in juvenile survival, rendering them highly susceptible to elevated fishing mortality. Overfishing and habitat degradation have had a major impact on the populations of these marine animals.

Surveys are being carried out in the park using Baited Remote Underwater Video Stations (BRUVS). This survey method offers unique insight into understanding the abundance biodiversity and behaviour of sharks and rays species in a multitude of habitats.

Better understanding of the ecology of elasmobranchs is critical in order to promote their conservation and develop better management initiatives.