Age and Growth of Atlantic Sturgeon in the James River, Virginia, 1997–2011

Historically the Chesapeake Bay supported a large population of Atlantic sturgeon Acipenser oxyrinchus, but loss of suitable spawning habitat and overfishing coincided with dramatic in-system declines throughout the 20th century. Atlantic sturgeon harvest moratoriums were implemented in 1974 for Virginia waters and were expanded coastwide in 1998. In 1997, researchers became aware that commercial fishers in the James River, a tributary of the Chesapeake Bay, were catching juvenile and subadult Atlantic sturgeon as bycatch in various fisheries. Genetic studies showed that the Chesapeake Bay population has maintained genetic integrity and qualifies as a distinct population segment. Between 2007 and 2011, almost 150 adults have been caught in the tidal–freshwater portion of the James River during putative spawning runs. Pectoral fin spines from juveniles and subadults collected in the Burwell Bay (rkm 40) and Cobham Bay (rkm 60) areas and mature adult samples from vessel strikes in freshwater around or above rkm 120 were analyzed to create a length-at-age curve for Atlantic sturgeon in the James River. Five models were used to analyze the data, and the double von Bertalanffy (k1 = 0.054, k2 = 0.097, t1 = −2.85, t2 = 1.09, tp = 6.03 years, L∞ = 2241 mm) provided the best fit to the observed data. We estimated an increase in growth coefficient at tp, which could be an artifact of low sample size or due to ontogenetic changes in habitat use as older fish spend more time in oceanic waters than younger fish. Atlantic sturgeon in the 6–9 year age range are rarely encountered in the James River compared with younger and older age-classes, so a more in-depth analysis of the increased growth coefficient would require ocean sampling. Prior to extensive commercial harvesting in the late 19th century, many tributaries of the Chesapeake Bay supported abundant stocks of Atlantic sturgeon Acipenser oxyrinchus *Corresponding author: [email protected] Received July 28, 2011; accepted March 6, 2012 (Murawski and Pacheco 1977; Smith 1985; ASSRT 2007). After the Virginia fishery collapsed in the early 1900s, Atlantic sturgeon continued to be harvested typically as bycatch until 1974 when the Virginia Marine Resources Commission placed a moratorium on harvest. In 1998, the Atlantic States Marine Fisheries Commission (ASMFC) implemented a moratorium on Atlantic sturgeon harvesting for all U.S. waters (ASMFC 1998). Many researchers considered the Chesapeake Bay population functionally extirpated (Secor 1996; Speir and O’Connell 1996; ASSRT 1998), resulting in it being a candidate for federal protection under the Endangered Species Act (Federal Register Notice RIN 0648-XZ76). The James River is the southernmost major tributary to the Chesapeake Bay. Historically, the James River supported a large population of Atlantic sturgeon, but loss of spawning habitat and overfishing coincided with dramatic in-system declines throughout the 19th and 20th centuries (Smith 1985; Balazik et al. 2010). Observations in the 1990s of subadults caught by commercial fishers, as well as rare discoveries of dead adult Atlantic sturgeon in the James River (A. Spells, U.S. Fish and Wildlife Service, personal communication), suggested that Atlantic sturgeon inhabited the river. Genetic studies using samples from the James and York rivers showed that the Chesapeake Bay metapopulation has maintained genetic integrity and is distinct from other coastal populations (King et al. 2001). Successful spawning in the James River was verified in 2004 when a 135 mm fork length (FL) young-of-year Atlantic sturgeon was collected at river kilometer (rkm) 110 (Figure 1). It is currently unknown if the James River is the only river in the Chesapeake Bay that sustains Atlantic sturgeon reproduction. However, the James 1074 D ow nl oa de d by [ V ir gi ni a C om m on w ea lth U ni ve rs ity L ib ra ri es ] at 0 9: 49 2 6 Ju ne 2 01 2