A modeling study was conducted to determine if the expansion of invasive northern snakehead Channa argus could negatively affect the largemouth bass Micropterus salmoides population size in Potomac River (Chesapeake Bay). Current distributions for both species were generated using catch records. Northern snakehead was not widely distributed during the study period and occurred mainly in upstream areas of tributaries. Many of these areas were moderately or highly suitable habitats for largemouth bass. Of sites where juvenile largemouth bass were collected, 10.6% were associated with northern snakehead. Using population modeling and measured predator–prey interactions, we determined that this level of co‐occurrence would result in a 3.8% reduction in largemouth bass population size. This prediction is consistent with current observations that indicate there has not been a negative trend in the largemouth bass fishery. As co‐occurrence was increased in the model, however, the negative impact of northern snakehead on largemouth bass monotonically increased. The time required for such increases in northern snakehead distribution is not known. If northern snakehead continues to expand its range in the absence of control measures, then our population model, with its assumptions, predicts a 35.5% reduction in the abundance of largemouth bass in the Potomac River. Received February 6, 2012; accepted June 7, 2012

No abstract

Northern snakehead Channa argus, an invasive predatory fish species from Asia, may continue to establish itself throughout temperate areas of the eastern United States, particularly in shallow vegetated habitats of ponds and streams. The species was first collected in the Potomac River in 2004 and has become successfully established in several major rivers within the Chesapeake Bay watershed. The objectives of this work were to develop habitat suitability criteria using a novel methodology that combines geographic information systems technology and fish surveys to estimate population sizes. A combination of catch data and reported or empirically derived habitat relationships were used to analyze seasonal distributions (March–October) in two tidal freshwater tributaries of the Potomac River: Nanjemoy Creek (2013) and Chopawamsic Creek (2010–2013). Adults were collected in relatively deeper sections of the streams (average depth 0.7–1.0 m) with a low cover of submerged aquatic vegetation (0–21% of site). Using additional distributional data, we identified suitability criteria as: 1) edges of submerged aquatic vegetation that included 5 m of vegetation and 5 m of adjacent open water; 2) less than 30% of mid-channel distance from shore, which may or may not include submerged aquatic vegetation; and 3) the upper 15% of the tidal freshwater stream. An adult population estimate derived from a suitable area in Pomonkey Creek (a tributary of the Potomac River) and estimated densities from Nanjemoy Creek and Chopawamsic Creek (i.e., three adults/ha) was not different from that expected using electrofishing surveys. Assuming approximately 7,093 ha of suitable habitat and three adults/ha, the number of adults was predicted to be 21,279 for 44 major tidal freshwater tributaries of the Potomac River. This is our first estimate of population size of northern snakehead for any river of the Chesapeake Bay watershed and its accuracy will undoubtedly improve as additional studies report variation in density for other tributaries. Because of the species’ ability to establish itself in temperate climates, it is important to engage the public to prevent additional releases of northern snakehead, especially to vulnerable habitats.

Northern Snakehead Channa argus, a nonnative species to North America, was discovered in 2004 in tidal freshwater of the Potomac River, the second largest drainage of the Chesapeake Bay watershed. Since then Northern Snakehead has expanded its range throughout much of Maryland's portion of the Chesapeake Bay watershed. We estimated that the species has spread beyond its introduced range at a rate of about 2.7 subwatersheds per year. If that rate is maintained, the species will have spread its range throughout the entire watershed in 52 years. This rate has been consistent over time except for short periods of heightened expansion that followed an introduction of snakeheads into Delaware waters and in years with greater levels of spring precipitation. The expansion by Northern Snakehead is more widespread than that of other invasive fishes of Maryland's portion of the Chesapeake Bay, which include Blue Catfish Ictalurus furcatus and Flathead Catfish Pylodictus olivaris. Attempts to control the spread of the species have included public education, incentives for harvest, agency surveys, and law enforcement. The rapid natural and human‐aided expansion of Northern Snakehead throughout the country's largest estuary highlights the importance of laws that prevent live possession and importation of this species.

In the U.S., 525,000 horseshoe crabs (Limulus polyphemus) per year have been captured during 2013–2017, brought to biomedical facilities, and bled to produce Limulus amebocyte lysate (LAL), then mostly released to the area of capture. The Atlantic States Marine Fisheries Commission estimates short-term bleeding-induced mortality to be 15% (4% to 30%), resulting in mortality of approximately 78,750 horseshoe crabs annually in recent years comprising a minor portion (<13%) of the up to one million annual coastwide landings dominated by harvest for bait. However, the long-term effect of bleeding for LAL on annual survival and spawning behavior is unknown; thus, results from short-term studies alone might underestimate bleeding effects at the population level. To address this knowledge gap, we analyzed data from the U.S. Fish and Wildlife horseshoe crab tagging database to estimate the differences in survival and recapture rates of bled and not bled horseshoe crabs tagged in the same years and geographic area. Contrary to expectation, survival was not lower for bled crabs compared to unbled crabs. Differences varied, but survival estimates tended to be higher for bled crabs than for unbled crabs. However, biomedical culling and selection for younger or healthier animals could have resulted in biomedically tagged individuals representing a healthier subset of the overall population with subsequent higher survival. Furthermore, the tagging analysis revealed a post-bleeding reduction in capture probability, which could indicate decreased spawning activity, evident in males more than females. Continued tagging of bled and unbled crabs in the same geographic area while recording age class and sex will contribute to the further resolution of LAL production’s effect on horseshoe crab populations.

No abstract

The Northern Snakehead Channa argus, a species originating from Asia, was illegally introduced into ponds of Maryland in 2002. Later discovered in tidal freshwater of the Potomac River, the species has become successfully established in many Maryland rivers. The magnitude of impact on preferred prey of Northern Snakehead in North American ecosystems has not been widely studied, leaving unresolved whether there are deleterious effects owed to introduction. To add insight into potential impacts of Northern Snakeheads, we calibrated a published, temperaturedependent maximum consumption model using field data and informed its results with diet and prey preference data to compare consumption levels among prey species. The total annual maximum consumption estimated from the model for a population of 600 Northern Snakeheads, which is an abundance approximated from published population sizes in streams of the Potomac basin, amounted to consumption of 2,189 kg of fish for the population per year. We found no evidence of strong prey preferences and partitioned this consumption to relative proportions of some fishes preyed on by Northern Snakehead as 661 kg/year of sunfishes Lepomis spp., 522 kg/year of Yellow Perch Perca flavescens, 332 kg/year of Goldfish Carassius auratus, 250 kg/year of White Perch Morone americana, and 174 kg/year of Banded Killifish Fundulus diaphanus. Some of these coastal fishes comprise important recreational and commercial fisheries. The maximum consumption estimates modeled here are greater than would be observed naturally, and whether predation affects persistence of the prey species in the ecosystem will depend upon the resiliency of the prey population and its current population size, which are unknown. We conclude that, as indicated by earlier risk assessments, the Northern Snakehead is an important predator in Maryland ecosystems because it is opportunistic and is capable of consuming significant biomasses of several recreationally and commercially important fish species.

American Eel Anguilla rostrata populations along the Atlantic coast of the United States have been in decline over the past several decades. One suggested cause of the decline is construction of barriers that block access to upstream tributaries where they can spend a significant portion of their lives. Success of reintroduction efforts above barriers has rarely been evaluated. Within the Susquehanna River (Chesapeake Bay watershed), over 1 million eels were released above four major downstream barriers in the past decade. We used backpack electrofishing and tagging to monitor growth, sexual differentiation, and population density of reintroduced eels in Buffalo Creek, a tributary to the Susquehanna River (Pennsylvania). From 2012 to 2019, we caught over 2,000 individuals, tagged more than 1,800, and recaptured 229. Recaptured eels provided insight into growth, sexual differentiation, and movement. Nearly 99% of recaptures remained near stocking locations. The average growth rate was 47.8 mm/y and ranged between −5.8 and 116.0 mm/y. Females generally grew significantly faster than males, and growth rates of several females exceeded 100 mm/y, a rate typically associated with estuarine residents. The population density within stocking sites was over 2,300 eels/km, roughly four times higher than Susquehanna River tributaries below the most downstream dam, and exceeded the target stocking goal of 529 eels/km. While we caught most eels in areas sampled near stocking locations, we captured some eels in smaller upstream tributaries away from stocking locations. Our study is the first to examine how reintroduced eels grow following stocking above four major dams on the Susquehanna River. We suggest that managers considering moving eels above blockages account for release location and density to achieve desired benefits to the overall population.