A mortality trap (150) for a spar buoy fish pen (100) is configured to receive and trap deceased fish, or morts (M), that sink from the fish pen. The mortality trap attaches to a lower portion of the spar buoy (110) to define a first passage (90). The sinking mort passes into an upper receiver portion (150U) of the mortality trap, and encounters a sloping transverse panel (154). Gravity causes the mort to continue through a second passage (B) into a lower entrapment portion (150L), and further into a region underlying the transverse panel (154) preventing the mort from escaping if it becomes positively buoyant. The entrapment portion optionally includes a converging channel into a valved port, to permit extraction of morts. The mortality trap may be located on the distal end of the spar buoy, or at an intermediate location on the lower portion of the spar buoy.
