222 



Fishery Bulletin 104(2) 



Discussion 



According to surplus production model runs and corre- 

 sponding projections, the horseshoe crab population in 

 the Delaware Bay region has been depleted and current 

 harvest levels may be too high to allow the population to 

 rebuild to S^gy within 15 years. Biomass in this region 

 has decreased steadily since 1995 and is currently well 



below B 



A/.sy 



This decline was evident in models that 



incorporated regional fishery-independent surveys and 

 Delaware fishery-dependent indices. Figure 4 shows 

 current relative biomass and relative fishing mortality 

 for each of the model applications. All model runs indi- 

 cated a depleted population with high fishing mortality, 



45 

 40 

 3.5 

 3.0 

 25 

 2.0 

 15 

 10 

 05 

 0.0 



F/F, 



1993 



1995 



1997 



1999 



Year 



Figure 8 



Production model estimates of relative fishing mortality ^F/F^|^y) of 

 horseshoe crabs iLimulus polyphemus) in the Delaware Bay region, 

 1991-2002. Results from fishery-dependent model runs are shown, 

 and the horizontal line represents FIFmsy-^- ^0% confidence inter- 

 vals for models FD-1 and FD-4 are shown in gray. 



although the estimated extent differed among models 

 (Fig. 4). Projections for some of the model runs predicted 

 a relatively fast population recovery. In the absence of 

 harvest, five of the seven model applications predicted 

 rebuilding to 5^/.,y in five or fewer years. However, two 

 of the model runs estimated such low population biomass 

 that rebuilding the Delaware Bay population to B^,^y 

 would take greater than 15 years, even with no harvest. 

 A precautionary management strategy may therefore be 

 appropriate for this population in the short term as more 

 data are being collected. 



Population model results may have been affected by 

 assumptions that we made during the modeling pro- 

 cess. In order to extend the time period of the model 

 back to 1991, we had to estimate regional 

 harvest based on Delaware harvest data for 

 the early years of the model. Because har- 

 vest data from most states were not avail- 

 able prior to 1995, we assumed that regional 

 harvest was equal to ten times the Dela- 

 ware harvest for the years 1991-94. This 

 was the approximate relationship between 

 Delaware and regional harvest from 1997 

 through 2003 when reporting was manda- 

 tory. Actual regional harvest for 1991-94 

 was unknown because there were no har- 

 vest-reporting requirements. We conducted 

 sensitivity analyses which showed very little 

 difference among runs with 1991-94 harvest 

 equal to Delaware landings, 10 times Dela- 

 ware landings, or 20 times the Delaware 

 landings (the mean difference for Bo(fQ-^IB^,^y 

 from these runs was 0,011). If the actual 

 harvest was substantially greater than 20 

 times Delaware landings, differences in re- 

 sults may occur. 



Results were also influenced by the data 

 sources that were included. We based the 

 inclusion of fishery-independent surveys on 

 positive correlations among surveys, incor- 

 porating the largest number of data sources 

 possible into model runs. However, two fish- 

 ery-independent surveys were not included: 

 the NMFS fall trawl and the Delaware Bay 

 spawning survey. The NMFS fall trawl was 

 negatively correlated with seven of the nine 

 other data sources (Table 1), and we there- 

 fore assumed that it was not a reliable index 

 of horseshoe crab abundance. The spawning 

 survey was negatively correlated with five of 

 the nine other data sources (Table 1), This 

 survey also had a very short time series be- 

 cause it was redesigned in 1999 to improve 

 statistical power (Smith et al,, 2002). With so 

 few data points, the production model would 

 be unable to distinguish population trends 

 from survey variability; therefore spawning 

 survey data were excluded. However, as one 

 of only two horseshoe crab-specific surveys 

 currently in place, the Delaware Bay spawn- 



