On a stunning late August afternoon, researchers from the Anderson Cabot Center for Ocean Life at the New England Aquarium set out on a small boat to write the next chapter in that story. Their mission is to implant tracking devices in juvenile sand tiger sharks. Their mood is buoyant, perhaps even giddy. That atmosphere is largely the product of Ryan Knotek, the shark researcher leading the tagging efforts. Most days, his work involves sitting at his computer writing proposals, he explains.