Mexico is monitoring possible damage from the Gulf of Mexico oil spill on marine species, especially shrimp and tuna, the environment minister said on Wednesday. 'The impact on the environment is not apparent for now,' Juan Rafael Elvira told a news conference, where he announced the study on species including birds and dolphins. Elvira said Mexican authorities were still on alert as US efforts to contain the giant oil leak grow more desperate, following the April 22 sinking of the Deepwater Horizon rig two days after an explosion that killed 11 workers. Planes, boats and Mexican oil platforms were being used to monitor the situation, Elvira said. Investigators would shortly monitor the migration of Lora turtles, which cross the Gulf to lay eggs in Tamaulipas, northeastern Mexico, he added. The minister underlined that in case of damage to its eco-systems Mexico would take legal steps to ensure that energy giant BP assured clean-up costs, as it has promised. Experts say the main Gulf current flows northwards, which reduces the risk of the slick reaching Mexican waters. 