Passing by sharks leave their DNA in the water and this can be used to identify the species from an ecosystem
DNA traces in the water give away the identity of sharks - short science news - biodiversity and environment

The biodiversity of sharks is impressive and often scientists are not able to count all species in a give area. A team of scientists compared three different methods for sampling sharks in the New Caledonian archipelago: sending divers to identify and count species, video camera recordings, and analyzing the DNA left in the seawater. The winner was the DNA analysis. It identified 13 species, six of which were missed by the two other approaches. Analyzing environmental DNA could be the next step for species identification; however, it can not provide information about the number of individuals in a habitat.

Read the full story: ScienceBriefss
Scientific publication: Science Advances