Prospects are good that the annual striped derby will become a regular feature of the Martha’s Vineyard Rod and Gun Club, and that none save members of the club will participate in the contest. This will mean that, for those who pay the club dues, there will be no further fee for membership in the derby contest or any other tournament which the club may arrange. Action taken at the club on Wednesday night indicates the favorable attitude of the club in making the change, the motion under which it was voted containing the provision that it be subject to legal approval.
The reason for this sensational action results from the ruling by the U.S. Post Office Department that fish derbies are lotteries and that publicizing them by means of the mails is forbidden. This effectually bars every form of advertising and, indeed, amounts to a ban of any fishing contest where a membership fee is charged.
Following prolonged discussion of the plan which was proposed by the derby committee, the club not only adopted it, thereby assuming responsibility for the derby, but also voted to reduce annual club dues from $5 to $4, that amount being the fee charged for derby membership last year.

From the August 25, 1950 edition of the Vineyard Gazette:


President Truman Makes It Official About Derby

President Truman made it official last week - fishing is a matter of skill, not luck. The President signed a bill excluding fishing contests from the postal laws covering lotteries. The measure excludes from the  law “non-profit contests wherein prizes are awarded for the species, size, weight or quality of fish caught by contestants in any bona-fide fishing or recreational event.”
The bill was introduced by Rep. Edward T. Miller, Republican, of Maryland, who said the postoffice department should have known in the first place that fishing is a matter of skill.