A fish pen comprising: a spar buoy having an upper end portion and a lower end portion; a mesh enclosure having a top end attached to the upper end portion of the spar buoy, and a bottom end attached to the lower end portion of the spar buoy, wherein the mesh enclosure defines an enclosed volume; a mortality trap (150; 250) attached to the lower end portion of the spar buoy, the mortality trap comprising: a receiver portion that engages the bottom end of the mesh enclosure to define a first passage from the enclosed volume into the mortality trap (150; 250), the receiver portion having a tubular wall and a transverse panel that slopes downwardly from a first side of the tubular wall toward an opposite side of the tubular wall, wherein a second passage is defined between the transverse panel and the tubular wall; and an entrapment portion disposed below the receiver portion, the entrapment portion comprising a floor having a first section that slopes downwardly from the opposite side of the tubular wall to a position below the transverse panel, and a second section that extends from the first section; wherein a negatively buoyant mort sinking through the fish pen will pass through the first passage into the receiver portion of the mortality trap (150; 250) and continue through the second passage to a location under the transverse panel, wherein the entrapment portion defines a converging channel and further comprising a coupling that engages the converging channel and that is configured to couple to an extraction hose, and characterized in that the mortality trap (150; 250) further comprises a flow port positioned opposite the coupling, wherein the flow port is configured to permit a user to initiate a flow from the flow port to the coupling. a spar buoy having an upper end portion and a lower end portion; a mesh enclosure having a top end attached to the upper end portion of the spar buoy, and a bottom end attached to the lower end portion of the spar buoy, wherein the mesh enclosure defines an enclosed volume; a mortality trap (150; 250) attached to the lower end portion of the spar buoy, the mortality trap comprising: a receiver portion that engages the bottom end of the mesh enclosure to define a first passage from the enclosed volume into the mortality trap (150; 250), the receiver portion having a tubular wall and a transverse panel that slopes downwardly from a first side of the tubular wall toward an opposite side of the tubular wall, wherein a second passage is defined between the transverse panel and the tubular wall; and an entrapment portion disposed below the receiver portion, the entrapment portion comprising a floor having a first section that slopes downwardly from the opposite side of the tubular wall to a position below the transverse panel, and a second section that extends from the first section; wherein a negatively buoyant mort sinking through the fish pen will pass through the first passage into the receiver portion of the mortality trap (150; 250) and continue through the second passage to a location under the transverse panel, wherein the entrapment portion defines a converging channel and further comprising a coupling that engages the converging channel and that is configured to couple to an extraction hose, and characterized in that the mortality trap (150; 250) further comprises a flow port positioned opposite the coupling, wherein the flow port is configured to permit a user to initiate a flow from the flow port to the coupling. a receiver portion that engages the bottom end of the mesh enclosure to define a first passage from the enclosed volume into the mortality trap (150; 250), the receiver portion having a tubular wall and a transverse panel that slopes downwardly from a first side of the tubular wall toward an opposite side of the tubular wall, wherein a second passage is defined between the transverse panel and the tubular wall; and an entrapment portion disposed below the receiver portion, the entrapment portion comprising a floor having a first section that slopes downwardly from the opposite side of the tubular wall to a position below the transverse panel, and a second section that extends from the first section; wherein a negatively buoyant mort sinking through the fish pen will pass through the first passage into the receiver portion of the mortality trap (150; 250) and continue through the second passage to a location under the transverse panel, wherein the entrapment portion defines a converging channel and further comprising a coupling that engages the converging channel and that is configured to couple to an extraction hose, and characterized in that the mortality trap (150; 250) further comprises a flow port positioned opposite the coupling, wherein the flow port is configured to permit a user to initiate a flow from the flow port to the coupling.