866 



Fishery Bulletin 101(4) 



Simulation 1 



60 80 100 



Simulation 3 



Simulation 2 



60 80 100 120 



Simulation 4 



Year 



Figure 3 



Time-trajectories of the "true" and the assessment model-estimated ratio of the spawning output to Sq 

 (depletion) for four simulations. The up arrows indicate the years in which the stock was declared to be in 

 need of rebuilding and the down arrows show the values of T^^^. The horizontal bars indicate the years 

 during which the stock is under a rebuilding plan. Year 41 is the first "projection year," i.e. the first year 

 in which the management procedure is used to determine the catches (the catches for the years prior to 

 year 41 are set equal to the historical catches — see Fig. lA) 



Results and discussion 



Detailed results for a single operating model variant and 

 management procedure 



Figures 4 and 5 and Table 4 summarize aspects of a 

 simulation trial in which the operating model has its 

 baseline parameterization (Tables 1 and 2) and in which 

 the management procedure used to set harvest guidelines 

 is the PFMC management procedure with no constraints 

 on interannual variation in harvest guidelines other than 

 an upper limit of 10,000 t. The lack of any constraints on 

 changes in harvest guidelines has been imposed because 

 the PFMC has not adopted any such constraints. The har- 

 vest guideline is updated every third year. 



Figure 4 shows the time-trajectories of catch, spawning 

 output in relation to the pre-exploitation equilibrium level 

 ("true" and estimated), and the perceived fishing mortality 

 on which the harvest guideline is based for three of the 100 

 simulations that constitute a simulation trial. The horizon- 

 tal bars on the x-axis again reflect the year during which 

 the stock is managed by using the results from the rebuild- 

 ing analysis rather than the 40-10 rule. The most notable 

 feature of Figure 4 is the high variability in annual catches. 



This variability arises for several reasons: 1) the additional 

 information on population biomass obtained each time a 

 survey occurs changes the perceived status of the resource 

 and hence how far the spawning output is from the target 

 level of OABq-, 2) an extension of the assessment period 

 changes the set of recruitments on which generation of 

 future recruitment is based; and 3) a change from being 

 under a rebuilding plan to being managed by means of the 

 40-10 rule can lead to marked changes in catch. The lat- 

 ter is evident by the change in fishing mortality and catch 

 when the spawning output is estimated to reach 0.4Sg (i.e. 

 the end of the horizontal bar). A marked impact due to 

 the addition of data for a single 3-year period may appear 

 surprising. However, effects of this nature have already 

 been observed for West Coast species (see, for example, the 

 2002 update to the sablefish [Anoplopoma fimbria] stock 

 assessment [Schirripa and Methot^l). 



'^ Schirnpa. M. J., and R. Methot. 2002. Status of the sablefish 

 resource off the continental U.S. Pacific Coast in 2001. In Stock 

 assessment and fishery evaluation: appendix to the status of the 

 Pacific Coast groundfish fishery through 2001 and acceptable bio- 

 logical catches for 2002, x + 122 p. Pacific Fishery Management 

 Council, 7700 NE Ambassador Place, Portland, OR 97220. 



