Oakland-based BrightSource Energy has been ordered by federal regulators to halt construction on two sections of its solar thermal farm in the Mojave Desert due to fresh concerns about the vast project's impact on dozens of desert tortoises. On April 15, the Bureau of Land Management issued an immediate suspension of activities, according to a BLM filing. BrightSource has been relocating tortoises to other areas and building fences to prevent the reptiles from returning. BrightSource says it believes the project can still be completed by 2013. On Friday, BrightSource filed for an initial public offering of its stock that could raise up to $250 million.