Senator John F. Kerry is urging Commerce Secretary Gary Locke to declare a disaster for the Massachusetts fishing industry and raise catch limits that many fishermen say are too low for them to make a living. Kerry wrote a letter to Locke yesterday after meeting with the secretary earlier this week. In January, Locke denied a request from Governor Deval Patrick for emergency increases in New England catch limits, saying there was no new science to justify such increases. Kerry says the decision should be based on the “best available’’ information, even if it’s not new. Kerry also called on Locke to help secure $4 million for a revolving loan fund for small fishermen and to take other steps to rebuild trust following an inspector general’s report that found abusive enforcement of fishing rules.