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AbstractFisheries for American eel Anguilla rostrataoccur mostly in estuaries, yet eel abundance in large estuaries is poorly understood and the methods for estimating eel density underdeveloped. During 1997–1999, mark–recapture experiments were conducted for six consecutive days at six sites spanning the 250-km tidal portion of the Hudson River estuary, New York. Each experiment comprised 36 baited eel traps arrayed at 200-m intervals over 144-ha sampling sites. Estimates of local density were complicated by eel behavior, including trap-shy responses to marking and immigration into the experimental grid in response to bait attraction. We compared two open-population models, both modified Peterson methods: Jolly–Seber and a model created to account for eel behavior termed the mean recapture model (MRM). The biases in model outputs in response to trap-shy behavior and immigration were analyzed through simulations; the MRM showed less bias (−13%) than the Jolly–Seber model (+36%). Density estimates for the sampled regions ranged from 2 to 18 eels/ha for MRM and from 3 to 24 eels/ha for the Jolly–Seber model. The lowest density (1.6 eels/ha) was estimated for Albany (river km 240), but all other sites were estimated to have similar densities (5–18 eels/ha). The mean local density in the Hudson River estuary, 9.5 eels/ha, was much lower than those estimated for other systems. An overall abundance of 118,000 was calculated for Hudson River estuary eels larger than 30 cm (total length) at depths of 2–10 m.

A larval mark-recapture experiment was undertaken to compare dispersal patterns and vital rates of hatchery-produced and wild larvae. The potential of larval stocking to enhance recruitment in an estuarine tributary was also evaluated. Approximately 6.5 million striped bass Morone saxatilis larvae (5–7 mm, SL) were marked by immersion in 25 mg l−1 alizarin complexone, which was deposited in their otoliths. Following release on two dates, the dispersal of marked larvae from release sites was rapid and their distribution 4 days after release resembled that of wild larvae. Growth rates of four release groups ranged from 0.20 to 0.25 mm day− 1 and mortality rates ranged from 0.09 to 0.19 day-−1. The rates were similar to those of wild larvae. More than 70% mortality of released groups occurred from 0 to 3 days after release and larvae stocked at one down-river site apparently suffered complete mortality due to advective down-estuary loss. Hatchery-produced larvae were released during a period when temperatures were favorable for growth and survival. Survivors of released larvae were estimated to account for 10 to 35% of the age 0+ juvenile striped bass in the Patuxent River in 1991. The success of this experiment indicated that marked larvae can serve as “probes” to examine environmental and habitat factors that affect recruitment of striped bass and other anadromous species. In the case of striped bass, large-scale stocking of larvae could contribute significantly to recruitment in Chesapeake Bay tributaries.