The Atlantic tomcod is a unique species that adapted resistance to the toxic effects of the PCBs polluting the river. Scientists identified the genetic mutation that conferred the resistance, and found that the mutated form was present in 99 percent of the tomcods in the river, compared to fewer than 10 percent of the tomcods from other waters. The hogchoker flatfish have been historically abundant in the river, where farmers would use them for inexpensive livestock feed, giving the fish its name. Other unusual fish found in the river include the northern pipefish, the lined seahorse, and the northern puffer.