If you have a measuring tape this will be easier ,

General fish weight estimationincluding Trout

Length x Girth² ÷ 800
(length x girth x girth ÷ 800)

Pike estimate

Length³ ÷ 3,500
(length x length x length ÷ 3,500)

Bass

Length² x Girth ÷ 1,200
(length x length x girth ÷ 1,200)

If in doubt LIE