Estimating Stock Composition of Anadromous Fishes from Mark–Recovery Data: Possible Application to American Shad

—Information on the stock composition of mixed-stock fisheries is often needed to develop management regulations for anadromous fishes. Although several methods can be used to infer stock composition, marking studies have long been identified as a promising approach. Hatchery-reared larval American shad Alosa sapidissima are marked with a river-specific mark and released in stock enhancement programs along the U.S. East Coast. We describe and apply a mark–recovery method for inferring the proportion of the catch in a mixed-stock fishery that originates from a particular river. The method is based on comparing the proportion of the mixed-stock catch with marks from the river with the proportion of the fish returning to the river with marks. We explore the utility of using mass marking of hatchery-reared American shad larvae with tetracycline to determine the stock composition of mixed-stock fisheries of American shad in Virginia. Our analysis focuses on the impact of the former Virginia coastal ocean fishery on fish produced in the James and Pamunkey rivers, Virginia, and on the impact of bycatch in Chesapeake Bay pound nets on Susquehanna River American shad. Our results suggest that the coastal ocean fishery harvested relatively small proportions of the James and Pamunkey River stocks and that few American shad captured in pound nets sampled in Chesapeake Bay were from the Susquehanna River system. Management of mixed-stock fisheries of anadromous fishes is often plagued by a lack of information detailing the contributions of the constituent stocks to the total catch (Cadrin et al. 2005). Several methods can be used to assess stock composition, including genetic analyses (Nolan and Grossfield 1991; Epifanio et al. 1995; Nielsen 1998; Brown et al. 1999; Nolan et al. 2003; Beacham et al. 2004), meristic and morphometric characters (Melvin et al. 1992; Waldman et al. 1997; DeVries et al. 2002), life history characteristics and population parameters (Skillman 1989), natural tags (Thorrold et al. 1998; Campana et al. 1999), and tagging or marking studies (McFarlane et al. 1990; Brodziak 1993). Brodziak (1993) noted that tagging studies may be inadequate for determining the contributions of multiple stocks to total harvest unless the tagging programs are undertaken on a large scale. In this paper, we describe a study design that has proved feasible for determining stock composition of American shad Alosa sapidissima in a former offshore mixed-stock fishery and also in the bycatch of pound nets within Chesapeake Bay. American shad is an anadromous clupeid whose native range extends along the East Coast of North America from the St. Lawrence River, Canada, to the St. Johns River, Florida (Walburg and Nichols 1967). Each spring, mature American shad leave their offshore grounds and migrate to their natal rivers to spawn (Leggett 1973). Juveniles usually exit their natal streams by late fall (Hoffman et al. 2008), and immature fish remain in oceanic environments until sexual maturity. The timing of the spawning migration is related to latitude, the southern stocks commencing their spawning runs earlier than the northern stocks (Leggett and Carscadden 1978). During the coastal migration and within the natal rivers, American shad historically have supported sizable commercial and recreational fisheries (Walburg and Nichols 1967). Currently, in-river directed harvest of shad stocks continues except in the Chesapeake Bay and its tributaries. In the Maryland and Virginia waters of the bay (including tributaries), fishing moratoria were enacted in 1980 and 1994, respectively. Allowances are made for pound-net and gill-net bycatch in the Potomac River and gill-net bycatch in Virginia rivers. Since 1998, the spawning runs of Virginia stocks of American shad have been evaluated through in-river monitoring programs (Olney and Hoenig 2001a). Despite the closure of these fisheries and stocking programs in some rivers, monitoring data have shown only marginal increases in shad abundance in Virginia rivers (Olney et al. 2003; Olney and Delano 2006). In 2000, scientists affiliated with the Atlantic States Marine Fisheries Commission (ASMFC) postulated that the coastal ocean mixed-stock fishery was at least potentially responsible for the slow recovery. Annual * Corresponding author: [email protected] Received April 19, 2007; accepted September 11, 2007 Published online March 31, 2008 507 North American Journal of Fisheries Management 28:507–515, 2008 Copyright by the American Fisheries Society 2008 DOI: 10.1577/M07-072.1 [Article]