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Fishery Bulletin 104(2) 



ing estimate of this value equal to 0.5. In the absence 

 of other information about starting biomass, B^IK = 

 0.5 (i.e., Bj=BygY' is an appropriate default value 

 (Punt, 1990; Prager, 1994). In some cases B^/K was 

 poorly estimated, and we employed a common solu- 

 tion of fixing the value o{ B^IK (Vaughan and Prager, 

 2002). For model runs based on fishery-dependent 

 indices, we fixed B^IK at 0.1, 0.2, 0.3, and 0.4 (Table 

 2). The CPUE value for the Delaware hand fishery 

 was relatively low in 1991; therefore we assumed that 

 the population biomass was below B^jgy. ie. B^IK was 

 less than 0.5. 



Quantities estimated by the model 



The production model estimated several benchmarks 

 and status indicators useful in understanding horse- 

 shoe crab biology and in improving management. These 

 quantities included relative biomass (B/Sj^jgyl. relative 

 fishing mortality (F/Fy^^y). population biomass (B), and 

 maximum sustainable yield (MSY). Point estimates and 

 80% confidence intervals for each of these quantities 

 were calculated for each model run. 



Population projections 



We used the Delaware Bay population estimates 

 calculated by the surplus production model to 

 project the population forward in time for a period 

 of 15 years to evaluate potential harvest levels. 

 We conducted projections using ASPICP (Prager, 

 1994), with annual landings specified for each 

 year of the projections. We selected a 15-year 

 time period because this is the longest projection 

 period that can be programed in ASPICP, and 

 confidence in projection model results decreases 

 at longer time intervals. We evaluated trends in 

 biomass over time for a range of harvest levels, 

 including harvest at 2003 levels and proportional 

 reductions of that harvest. These harvests were 

 0% of 2003 catch (no harvest), 25% (339 t annu- 

 ally), 50%' (678 t), 75% (1017 t), and 100% (1356 

 t). We identified the number of years under each 

 harvest scenario required for the population (and 

 80% confidence intervals) to rebuild to B^/^y. We 

 also compared relative biomass (with 80%5 confi- 

 dence intervals) in the final year of projections 

 (2018) for each harvest level. 



1991 



1993 



1997 



Year 



Figure 5 



Production model estimates of relative biomass (B/B/^igy'i of horseshoe 

 crabs (Limulus polyphemus) in the Delaware Bay region, 1991-2004. 

 Results from fishery-independent model runs are shown, and the hori- 

 zontal line represents B/B^,^y=l. 80% confidence intervals each model 

 run are the same line pattern in gray. 



Results 



Results differed little between model sim- 

 ulations for surveys weighted inversely 

 to CV and for simulations with equally 

 weighted surveys (Fig. 4). 



Production model runs showed that 

 BIB^i^Y in the Delaware Bay region 

 increased in the early 1990s and has 

 declined steadily since 1995 (Figs. 5 

 and 6). Slight increases since 2001 were 

 evident in some model runs. Relative 

 biomass in 2003 was estimated to be 

 low, with point estimates ranging from 

 0.03 to 0.20 for models with fishery- 

 independent data and 0.20 to 0.71 for 

 models with fishery-dependent data 

 (Table 3). Eighty-percent confidence 

 intervals for B^oos/B^gy ranged from 

 0.005 to 1.16. 



