Meanwhile, 26-year-old Zimbabwe international McNab leaves the club having scored 30 tries in 51 appearances since moving to Penzance two years ago.
He had previously spent time at Hartpury and Worcester Warriors and also played in South Africa’s Currie Cup for the Zimbabwe Goshawks.
“Since arriving at the Pirates, Matt immediately showed his strength and ability and a real commitment both on and off the pitch,” Cattle said.
“He is a big personality who will be missed massively, and we would like to thank him for his quite considerable overall contribution.”
McNab added: “It has been the best two years of my rugby career so far, and I have loved every single minute.
“I can’t thank everyone at the Pirates enough for what they have done for me along the journey. It’s been amazing for my development on the field and in my day-to-day life off it.”
