evidence: Barely 30 families live on the narrow sliver of sand known as Tuzla Island, fewer when the weather gets bad, or the fishing gets good. The only other landmarks on the half-mile-wide, 4-mile-long sand spit between Russia and Ukraine are two aging, Soviet-era vacation spas, empty except for occasional visits by port workers from the mainland. The issue of who owns Tuzla Island has been a matter of extreme inconsequence for as long as anyone can remember. When Ukraine and Russia were adjoining republics in the Soviet Union, it hardly mattered. When Ukraine declared independence more than a decade ago, it faced no serious argument from Russia when it penciled its national boundary around Tuzla and began administering the narrow shipping strait nearby that links the Black Sea with the Sea of Azov. [...] A phone call between Russian President Vladimir V. Putin and Ukrainian President Leonid D. Kuchma defused the crisis, halting construction of the nearly 2-mile causeway, just 102 yards short of Ukrainian territory. Last week, the two sides agreed to negotiate the real bones of contention: the Kerch Strait, which flows on either side of Tuzla Island, and more importantly, the adjoining Sea of Azov -- a strategic gateway to the Black Sea and a possible new oil frontier.