The Trinidad Black-Back Snake or Erythrolamprus melanotus nesos is endemic to Trinidad and Tobago. In Tobago, it is known as the Doctor Snake, they are predominately found along streams and waterways (running water) and feed mainly on small frogs or lizards. In this photo, the snake is consuming a Skink, possibly a Greater Windward Skink (Copeoglossum aurae).
Please note that these endemic snakes are harmless and do not grow more than two-feet.