318 



Fishery Bulletin 101(2) 



Table 4 



Lipofuscin (LF)-based age composition and relative abundance (in parentheses) of each age group in subadult (60 < carapace width 

 <120 mm) and adult (>120 mm) size classes for crabs from 1998-99 and 1999-2000 winter bottom-dredge surveys (WDS). Crabs 

 <60 mm were excluded due to gear selectivity and our exclusion of crabs <40 mm (see "Materials and methods" section). 



1998-99 WDS 



Size class 



1999-2000 WDS 



Size class 



Age group 



(LF based) 



60 < CW < 120 mm 



CW > 120 mm 



60 < CW < 120 mm 



CW > 120 mm 





 1 



2+ 



Total number 



80 (0.63) 

 44 (0.35) 

 3(0.02) 

 127 



31 (0.34) 

 50 (0.56) 

 9(0.10) 

 90 



67(0.57) 

 60(0.51) 

 0(0.00) 

 117 



30(0.33) 

 54(0.59) 

 7(0.08) 

 91 



age structure. Crabs that spawn early may grow rapidly 

 and attain "subadult"size (>60 mm) by the end of the first 

 growth season (Ju et al., 2001). If this is the case, then a 

 significant fraction of subadults (defined as >60 mm CW) 

 may have been misclassified on the basis of past CW crite- 

 ria as being 1-yr-old subadults. We would argue that crabs 

 spawned late in the season (late summer-early fall) may 

 overwinter at small sizes but will emerge the next spring 

 and experience rapid growth in warm temperatures to 

 reach maturity within their first year of life. In support 

 of this view, Ju et al. (2001) have shown in growth studies 

 that pond-reared crabs from late-spawning cohorts over- 

 winter and undergo extremely rapid growth during the 

 following summer months. Most of these crabs attained 

 127 mm CW (size of entry into the hard crab fishery) before 

 their second winter. Occasionally such a protracted period 

 of spawning may result in multimodal patterns in recruit- 

 ment. As reported by van Montfrans et al. (1990, 1995), 

 we observed two subannual cohorts of age-1 blue crabs 

 in 1999-2000; therefore it is likely that these crabs were 

 recruited from a bimodal pattern of juvenile production 

 during 1998. These subannual cohorts were statistically 

 separated into modes (Fig. 3) based on known lipofuscin 

 accumulation rate in blue crabs (Fig. 4). Modal analysis 

 of lipofuscin index for 1998-99 did not show evidence of 

 multimodal recruitment (Fig. 3A). The inconsistent ap- 

 pearance of "subannual" cohorts between sampling years 

 may result from the interannual variation of settlement 

 and spawning patterns, which in turn is related to yearly 

 spawning stock conditions and physicochemical conditions 

 in the Chesapeake Bay region (McConaugha et al., 1983; 

 van Montfrans et al., 1990, 1995). 



According to lipofuscin modal analysis, crabs <2 yr old 

 are a significant fraction of the harvestable (>127 mm CW) 

 stock in Chesapeake Bay. This finding argues that size 

 (CW) criteria, on which past assumptions of greater ages 

 in the Chesapeake Bay blue crab stocks are based, is in- 

 sufficient for assessing blue crab demographics. It appears 

 that size may be more reflective of interannual differences 

 in growth rates than age structure alone. 



Given that lipofuscin-based age distributions show that 

 >2 year old crabs are a minor contributor to the harvestable 

 stock, we speculate that the population dynamics of crabs 



available for harvest is strongly influenced by numbers of 

 juveniles produced in each year and that growth conditions 

 experienced by these juvenile cohorts during their first full 

 year is a dominant determinant of the reproductive poten- 

 tial of the blue crab. It also suggests that seasonal yield 

 patterns in commercial fisheries are strongly influenced 

 by seasonal patterns in time of spawning and subsequent 

 juvenile growth. If relatively older (>2 yr) crabs are minor 

 contributors to the adult stock, then the Chesapeake Bay 

 fishery may essentially depend on an annual crop of crabs, 

 produced over a protracted spawning season. The conse- 

 quence of this pattern is that landings will be more tightly 

 coupled with juvenile production levels (e.g. settlement 

 rates of postlarvae) and environmental conditions (e.g. 

 winter duration) that affect their growth into the adult 

 stock, not unlike the dynamics observed in penaeid shrimp 

 fisheries (e.g. Haas et al., 2001 ). Other attributes including 

 sex ratio and size- and age-specific reproductive rates are 

 other important considerations in assessing the reproduc- 

 tive condition of the Chesapeake Bay blue crab stock ( Jivoff 

 and Mines, 1998; Rugolo et al. 1998). 



Uncertainties remain in applying these results to the 

 assessment of the Chesapeake Bay We note that samples 

 used in our study did not cover the entire Chesapeake 

 system and that the abundance and size distributions of 

 blue crabs are expected to vary locally. Nevertheless, these 

 results strongly suggest that for moderately large sample 

 sizes, crabs can be assigned to annual or subannual cohorts 

 on the basis of lipofuscin measures, thereby significantly 

 improving our knowledge of population dynamics and life 

 history. Natural longevity in Chesapeake Bay blue crabs 

 remains an open question, particularly because crabs (>3 

 yr old) are likely to be very rare because of the combined 

 effects of high exploitation and natural mortality (Miller. 

 2001). 



Acknowledgments 



The authors thank the Maryland Department of Natural 

 Resources (DNR) and the Virginia Institute of Marine Sci- 

 ence winter dredge survey and vessel operations staff for 

 kindly providing subsamples from 1998-99 and 1999-2000 



