Find F that yields a given SPR spr_x(nages, mort, mat, waa, sel, ret, target_x = 0.35) nagesnumber of ages in age structure mortinstantaneous natural mortality rate matmaturity-at-age vector waaweight-at-age vector seltotal selectivity-at-age vector rettotal retention-at-age vector target_xdesired SPR proportion Use bisection algorithm to identify the level of fishing mortality required to yield an SPR of x