Two goals each from veteran striker Lee Bradbury and James Lawson saw the away side come away victorious in an entertaining game. Fisk had originally levelled for City after Bradbury's opener, turning the ball home after good work by Asmat El Ouargui, but Matthieu Louis-Jean conceded a penalty and Joe Lewis let in a tame effort to see the Shrimpers in 3-1 ahead at half time.
Jarvis reduced the arrears in the second half, but Lawson's second sealed the win. City's first team face a big East Anglian derby with Ipswich on Sunday.