Abstract
AbstractWe used δ18O signatures in otoliths as a natural tag for hatch year to evaluate the scale‐based age determination method used for adult American shad Alosa sapidissima in the York River, Virginia. Juveniles of the 2002 year‐class exhibited high δ18O values in otolith cores that identified adult members of the cohort as they returned to spawn. Recruitment of the 2002 cohort was monitored for three consecutive years, identifying age‐4, age‐5, and age‐6 individuals of the York River stock. The scale‐based age determination method was not suitable for aging age‐4, age‐5, or age‐6 American shad in the York River. On average, 50% of the individuals from the 2002 year‐class were aged incorrectly using the scale‐based method. These results suggest that the standard age determination method used for American shad is not applicable to the York River stock. Scientists and managers should use caution when applying scale‐based age estimates to stock assessments for American shad in the York River and throughout their range, as the applicability of the scale‐based method likely varies for each stock. This study highlights a promising new direction for otolith geochemistry to provide cohort‐specific markers, and it identifies several factors that should be considered when applying the technique in the future.Received March 21, 2011; accepted August 18, 2011
Highlights
Accurate age determination is critical to the assessment and management of fishes
We hypothesized that the δ18O values in the otoliths of juvenile American shad from the 2002 year-class in the York River system were a distinct marker that could be used to identify adult members of the year-class that returned to spawn in the York River
The objectives of this study were to (1) expand upon the work of Walther and Thorrold (2009) by analyzing the δ18O signature in the otoliths of juvenile American shad from the 2003 year-class in the York River system, which would enable us to determine whether the δ18O signature from juveniles of the 2002 year-class were a distinct marker for that cohort; (2) use otolith δ18O signatures to identify adults of the 2002 year-class that returned to spawn in the York River over three consecutive years; and (3) use ages determined from δ18O signatures to evaluate the Cating (1953) method of age determination used for American shad in the York River
Summary
Accurate age determination is critical to the assessment and management of fishes. Age-specific data allow for estimates of mortality, growth, maturity schedules, and production and are used to develop models of population dynamics (Beamish and McFarlane 1983, 1995; Campana 2001). This pattern was hypothesized to be due to interannual variation in total river flows and drought intensity (USGS 2005) Given this finding, we hypothesized that the δ18O values in the otoliths of juvenile American shad from the 2002 year-class in the York River system were a distinct marker that could be used to identify adult members of the year-class that returned to spawn in the York River. We hypothesized that the δ18O values in the otoliths of juvenile American shad from the 2002 year-class in the York River system were a distinct marker that could be used to identify adult members of the year-class that returned to spawn in the York River This unique, cohort-specific marker offered the opportunity to evaluate age determination methods for American shad, as the accuracy of age estimates for this species has been repeatedly called into question by scientists and managers (McBride et al 2005; ASMFC 2007a, 2007b). Natal river fidelity among American shad creates discrete, river-specific spawning stocks along the Atlantic coast (Walther et al 2008), providing assurance that fish analyzed in this study were from the York River stock
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