Most Thames boats will be shellfishing when shrimping finishes. It's seasonal work, and relies mainly on the Dutch market. Low prices then they stay tied up, especially if diesel is expensive.
My experience with Wash fishing boats is that they can make great money for part of the year and nothing for the rest. The hungrier that they are the more they make when the other guys are in the pub.
Commercial fishing has evolved into a strategic game with quota, days, amounts, rules, regulations, allowed sorts, selling prices, cost of operation and then some.