Natural Mortality

[e-perl-NOAA]

Discussion board for questions concerning natural mortality (natM). Please subscribe to this discussion so that you get notifications about discussion posts.

[e-perl-NOAA]

Emailed Question:

A user is updating an assessment that was done using an old SS3 setup (v 3.24 beta) and has a question concerning natural mortality. In the old assessment, the M vector was obtained by running SS3 with the internal Lorenzen M scaling option turned on and then extracting the resulting M at age vector and adjusting age 0 M by half under the justification that the fish species recruits July 1st so only suffer 1/2 year of natural mortality and then fixing the adjusted vector as M-at-age input option 3.

Model Setup:

• season: 1
• months/season: 12
• spawning season: 1
• ages input in the model: calendar age.
• growth estimated internally

The fish species is assumed to recruit July 1st (real age 0.0), but settlement in the model is specified as January 1st and all ages input into the assessment have already been adjusted to reflect calendar ages, not real ages. Therefore, the mean length at age 0 used for Lorenzen scaling in SS3 reflects the mean length of fish ages 0-6 months.

Was it correct to cut age 0 M by half?