Commentary for Beitzah 46:6

1- "We don't trap fish" even though slaughtering, baking and cooking are major categories of labor [which are prohibited on the Sabbath], and those labors are permitted for the needs of the holiday, the reason there [for permitting those labors] is that it would be impossible [to do these labors] before the holiday, because with slaughtering we are concerned that the meat will spoil. However, with trapping you can trap the fish while its still the day [before the Holiday], and put it aside in water and it wont die, and on the next day [i.e. the holiday] you can take it from the water.