Wetzel et at: The effect of reduced data on monitoring overfished fish stocks 
203 
Full data with survey Reduced data with survey Eliminated data with survey 
Assessment year 
Figure 7 
Relative error (RE) of estimated spawning biomass (SB) and relative SB and estimates 
of steepness in each assessment year when survey data are present during stock re¬ 
building for the time-invariant case and all 3 data scenarios (full data, reduced data, 
and eliminated data) for all simulations (top panels). The percentage of stocks that 
had rebuilt to the target biomass during the management period is shown in bottom 
panel within the operating model (OM, solid black line) and the estimation method 
(EM, dashed black line); data collection consequently returned to historical levels for 
the fishery when the EM determined that the stock was rebuilt. The black lines in 
the grey boxes denote the median of the estimates, the gray boxes cover the 25-75% 
simulation interval, and the boxplot whiskers cover the 95% simulation interval for 
each assessment year. 
between assessments (i.e., over time within a simula¬ 
tion). Although the full data scenario had less varia¬ 
tion, the median estimates of spawning biomass (over 
simulations) and relative spawning biomass were con¬ 
sistently below the operating model values for much of 
the management period. This result is contrary to what 
might be expected when additional data are available. 
Simulations in which there was a fishery-independent 
survey that provided an index of abundance and com¬ 
position data (length and age) determined that this un¬ 
derestimation of the true spawning biomass was elimi¬ 
nated if survey composition data were available along 
with fishery composition data. The underestimation 
was driven by 2 key factors: the shape of fishery selec¬ 
tivity curve and data quantity. The specification of a 
fishery selectivity curve as greater than the maturity- 
at-length curve, with the fishery selecting only mature 
fish, resulted in a lag between recruitment to the pop¬ 
ulation and recruitment to the fishery. However, con¬ 
ducting a fishery-independent survey that selects fish 
at smaller sizes yields information about recruitment 
to the population earlier than using data from the fish¬ 
ery that selects larger, mature fish. Additionally, an in¬ 
crease in the number of length- and age-composition 
