Improve rationale for dome-shaped selectivity

[seananderson]

In particular, include in methods around line 690 in main.tex.

US rationale:

Updated selectivity assumptions. In this assessment, we estimated dome-shaped selectivity curves for several fleets that were fixed asymptotic in the 2011 assessment. We also estimated additional selectivity parameters to estimate a sex-specific offset to selectivity in each fleet. Rationale: To achieve better fit to length composition data and account for sex-specific habitat preferences observed for this species.

Models that had dome-shaped selectivity for all fleets did not have reasonable estimates for female L∞. However, setting the selectivity for non-trawl landings and the IPHC Survey as asymptotic in the base model (as discussed under Selectivity Parameters below) allowed that parameter to be well estimated.

Non-trawl landings and IPHC Survey have the largest individuals. Assuming selectivity for these fleets asymptotic also allowed estimating asymptotic length of females (L∞) within the model.

Furthermore, results of model runs assuming either dome-shaped or asymptotic patterns with L∞ fixed were virtually identical. Therefore, selectivity of these two fleets was fixed asymptotic.

Selectivity was fixed asymptotic for non-trawl landings and IPHC Survey. For the rest of the fleets, selectivity was estimated to have a various degree of dome-shape, except for the recreational fleet, which was estimated to be asymptotic;

The AFSC triennial, AFSC slope and NWFSC Slope Survey selectivity curves were estimated as dome-shaped, which is consistent with the fact that those survey had only a limited spatial coverage of the assessment area and species range within the assessment area (Table 3).

[seananderson]

I added a paragraph. Leaving open because perhaps the rationale could be improved still.