It is believed that the fish may be invading new territories by slipping aboard fishing boats. The fish has been established in some islands east of the Wallace Line, in eastern Indonesia and Papua New Guinea, and is also believed to be advancing toward Northern Australia.
This passage describes how the fish is spreading into new territories, likely through a process of being transported on fishing boats. The fish has already been found in certain areas, including islands east of the Wallace Line, in eastern Indonesia and Papua New Guinea, and it's believed to be moving towards Northern Australia.