MARCO ISLAND, Fla. (AP) - Researchers are trying to determine what killed a cluster of catfish near Marco Island. The dead catfish surfaced last week. Rookery Bay National Estuarine Research Reserve spokeswoman Renee Wilson says the fish were ghostly white and had reddened lips and fins, a sign that the fish were under some kind of stress. Researchers haven't said what may have caused the fish kill, but theories range from red tide poisoning to a disease or virus. Tissue samples have been sent to a research institute for further testing, which would take weeks. Naples Daily News reports in 1996 an especially deadly red tide coincided with another catfish kill in Collier County. That fish kill was blamed on a virus.