312 



Abstract-The blue crab (Callinectes 

 sapidus) plays an important economic 

 and ecological role in estuaries and 

 coastal habitats from the Gulf of Mexico 

 to the east coast of North America, but 

 demographic assessments are limited 

 by length-based methods. We applied 

 an alternative aging method using bio- 

 chemical measures of metabolic byprod- 

 ucts (lipofuscins) sequestered in the 

 neural tissue of eyestalks to examine 

 population age structure. From Chesa- 

 peake Bay, subsamples of animals col- 

 lected from the 1998-99 («=769) and 

 1999-2000 (n=367) winter dredge sur- 

 veys were collected and lipofuscin was 

 measured. Modal analysis of the lipo- 

 fuscin index provided separation into 

 three modes, whereas carapace-width 

 data collected among the same indi- 

 viduals showed two broad modes. Lipo- 

 fuscin modal analysis indicated that 

 most adults (carapace width >120 mm) 

 were <2 years old. The results indicate 

 that use of extractable lipofuscin can 

 provide a more accurate and better 

 resolved estimation of demographic 

 structure of blue crab populations in 

 the field than size alone. 



Demographic assessment of the blue crab 

 {Callinectes sopidus) in Chesapeake Bay 

 using extractable lipofuscins as age markers* 



Se-Jong Ju 



David H. Secor 



H. Rodger Harvey 



Chesapeake Biological Laboratory 



University of Maryland Center lor Environmental Science 



Box 38 



Solomons, Maryland 20688 



E-mail address (for H Rodger Harvey, contact author) tiarveym'cbl umcesedu 



Manuscript accepted 21 October 2002. 



Manuscript received 31 December 2002 

 at NMFS Scientific Publications OfTice. 



Fish. Bull 101:312-320 (2003). 



In fisheries management, age-struc- 

 tured models are the most common 

 method for estimating optimal yields 

 and determining the effect of fishing on 

 population dynamics (Gulland, 1983). 

 For some species of commercial marine 

 fish, age can be determined by using 

 otoliths or other hard parts (Secor et 

 al., 1995). This is not possible for crusta- 

 ceans, including the blue crahiCallinec- 

 tus sapidus), which periodically molts 

 its calcareous exoskeleton to accommo- 

 date future growth and thereby aban- 

 dons any external evidence of age or 

 previous size. As a consequence, modal 

 analysis of length-frequency data has 

 often been used as an alternative to 

 direct aging methods (e.g. Rothschild et 

 al., 1992), Unfortunately, this approach 

 has proven difficult to validate because 

 1) growth is characterized by strong 

 Interannual and seasonal variability 

 and 2) the spawning season is pro- 

 tracted, which leads to a wide, and 

 sometimes multimodal distribution 

 of sizes per year class (Prager et al., 

 1990; Ju et al, 2001), The critical need 

 for understanding the demographic 

 structure of crab populations led us 

 to investigate an alternative method 

 for age determination. Age pigments 

 known as lipofuscins accumulate as 

 stable mixtures in postmitotic tissue as 

 a consequence of peroxidation reactions 

 during normal metabolism (see review 

 by (Jutteridge, 1987). These biochemical 

 measures have been applied to deter- 

 mine age in crustaceans (Ettershank 

 and George, 1984; Belchier et al., 1994; 

 Sheehy et al„ 1995; Sheehy et al., 1996; 



Wahle et al., 1996). Recently, a modified 

 approach that relies on the concentra- 

 tion of these products normalized to 

 tissue protein has been developed and 

 validated for the blue crab (Ju et al., 

 1999), 



We hypothesize that lipofuscin can 

 provide a more robust measure of age 

 than current size-based measures. For 

 example, variance in size among indi- 

 viduals within the same year class, and 

 declining growth rate with increasing 

 age, can result in significant overlap 

 of sizes among adjacent age classes 

 and can reduce the accuracy of age 

 determinations with length-frequency 

 analysis. Additionally, the blue crab's 

 natural longevity is an important fac- 

 tor and one still disputed by scientists 

 because of the lack of reliable long-term 

 tag returns and absence of age infor- 

 mation. If blue crabs reach maturity 

 quickly and live to age three to four 

 as has been suggested (Ju et al,, 1999; 

 Van Engel et al,, 1958, 1999; Reiser 

 and Kahn, 1999), then higher fishing 

 rates may be sustained than those for 

 crabs that mature more slowly and live 

 to age six or older (Rugolo et al,, 1998; 

 Miller, 2001), In the present study, we 

 conducted a large-scale application 

 of the lipofuscin aging method using 

 subsamples from fishery-independent 

 winter dredge surveys (1998-99 and 

 1999-2000), The goal of this study was 



 Contribution 3626 of the University of Mary- 

 land Center for Environmental Science, 

 Solomons. MD 20688, 



