Abstract.-Populations of American eel Anguilla rostrata along the eastern coast of North America have declined drastically for largely unknown reasons. We examined the population dynamics of American eels in six tributaries of the Hudson River, New York, to quantify their distribution and the impacts of anthropogenic stressors. With up to 155 American eels per 100 m 2 , tributary densities are greater than those within the main stem of the Hudson River and are among the highest reported anywhere. The predominance of small American eels (,200 mm) and wide range of ages (from young-of-year glass eels to 24-year-old yellow eels) suggest that tributaries are an important nursery area for immature American eels. However, upstream of natural and artificial barriers, American eel densities were reduced by at least a factor of 10 and condition, as measured by mass, was significantly lower. Significantly lower American eel condition was also found with increasing riparian urbanization. Density-dependent growth limitations below barriers are suggested by increased growth rates above the first tributary barrier. We suggest that (1) tributaries are important habitat for the conservation of American eels and (2) mitigation of anthropogenic stressors is vital for complete utilization of available habitat and conservation of the species.

We studied the invasion of the exotic nematode parasite Anguillicola crassus in the American eel Anguilla rostrata using tributaries of the Hudson River estuary. Yellow-phase American eels were sampled from 6 tributaries, and their swim bladders were examined for nematode infection. Prevalence averaged 39% with an intensity of 2.4 nematodes per eel. Parasite distribution was not significant along a latitudinal gradient; on the other hand, physical barriers (dams and natural waterfalls) significantly reduced infections upstream. Urbanization may increase the susceptibility of eels to infection; we found significantly elevated infection rates when urbanized lands exceeded 15% of the tributary catchment area. Yellow-phase eel condition was not affected by parasite infection. The invasion of the entire Hudson River watershed is ongoing and therefore will continue to be a management concern. Further analysis of the parasite-host interaction in North America is warranted.KEY WORDS: Anguilla rostrata · Anguillicola crassus · Parasite · Barriers · Urbanization Resale or republication not permitted without written consent of the publisherDis Aquat Org 79: 37-45, 2008 et al. 2000). Intense infections can cause hemorrhagic lesions of the swim bladder, swim bladder fibrosis, skin ulcers and swollen anuses (van Banning & Haenen 1990). There are contrasting findings regarding the potential negative affects of A. crassus on swimming ability (Sprengel & Lüchtenberg 1991, Nimeth et al. 2000, Münderle et al. 2004. Knopf et al. (1998) theorized that infestation of European eels may have been slowed by low water temperatures, and increased parasite intensity was found in thermal effluent when compared to cooler surrounding waters (Höglund et al. 1992b). Examination of American eel in Canadian waters has not yet shown infection with A. crassus (Marcogliese & Cone 1996, G. Verreault, Faune et Parcs Quebec, pers. comm.).Within New York State the tidal Hudson River contains infected eels with prevalence increasing downstream (Morrison & Secor 2003). However, the rate of infection and infected eel condition in Hudson River tributaries is unknown. Densities of up to 1.55 eels m -2 have been found in the tributaries (Machut et al. 2007), suggesting that they are an important habitat for declining American eel populations. As a relatively new definitive host for the parasite, eels in the Hudson River tributaries may be highly susceptible to infection.The purpose of the present study was to examine yellow-phase eels taken from small tributaries of the freshwater tidal Hudson River estuary to determine the burden of Anguillicola crassus on native American eel populations in these habitats, as well as the current effect of this parasite on the health of yellow-eels. We hypothesized that (1) the infection of eel with the A. crassus parasite would be higher in southern tributaries; (2) barriers would inhibit the upstream invasion of A. crassus in the tributaries; (3) disturbances caused by urbanization would increase the infection ...

Adult blueback herring Alosa aestivalis (N ¼ 116) were collected during the 1999, 2000, and 2002-2004 spawning runs from sites on the Hudson and Mohawk rivers, and gut contents were analyzed. Thirty-four fish (33% of those examined) were found to contain food material. Food items were present in 41% of Mohawk River samples and 11% of Hudson River samples; all Hudson River fish containing food were captured in small tributaries above the head of tide. Hudson River fish predominantly consumed zooplankton, while Mohawk River fish consumed benthic aquatic insects in large quantities, including Baetidae, Ephemeridae, and Chironomidae. Using stable isotope analysis and a mixing model, we found that fish collected later in the season had significantly decreased marine-derived C. Condition indices of later-season fish were equal to or greater than those of fish collected earlier in the season. Blueback herring in this system may face increased energy requirements as they migrate farther upstream during spawning runs, and feeding may provide energy subsidies needed to maintain fitness over their expanded migratory range.