Atlantic sturgeon in the Delaware River: 

contemporary population status and identification 

of spawning areas 

Completion Report: Award NA05NMF4051093

 

 

Philip C. Simpson 

Delaware State University 

Department of Agriculture and Natural Resources 

1200 N. Dupont Hwy. 

Dover, DE 19943 

(302) 284-8587 

philchristiansimpson@yahoo.com 

Dewayne A. Fox* 

Delaware State University 

Department of Agriculture and Natural Resources 

1200 N. Dupont Hwy. 

Dover, DE 19943 

(302) 857-6436 

dfox@desu.edu 
2

Introduction 

The Atlantic sturgeon (Acipenser oxyrinchus oxyrinchus) is one of 27 species 

within the family Acipenseridae and one of nine species/subspecies extant in North 

American waters (Cech and Doroshov 2004). Atlantic sturgeon historically occupied all 

major river systems along the Atlantic coast between Canada (Bachus 1951) and Florida 

(Vladykov & Greely 1963), with the Delaware River historically supporting the largest 

population (Secor and Waldman 1999). Atlantic sturgeon are anadromous with spawning 

in the southern portion of their range typically occurring in the early spring months 

followed by a general trend of later spawning times in northern rivers (Smith and 

Clugston 1997). The adhesive eggs are deposited on rocky or hard-bottom substrate and 

fertilized externally (Smith et al. 1980). The Atlantic sturgeon is a slow maturing fish 

(females reach sexual maturity at 16 years or older, males at least 12 years within the 

mid-Atlantic coast region (Van Eenennaam et al. 1996)). Fecundity of female Atlantic 

sturgeon is estimated to range from 400,000 – 8 million eggs (Smith et al. 1982, Van 

Eenennaam and Doroshov 1998, Dadswell 2006) with a spawning interval of 2-6 years 

(Vladykov and Greeley 1963, Van Eenennaam et al. 1996, Stevenson and Secor 1999). 

Larval Atlantic sturgeon emerge in roughly 4 - 6 days and begin to work their way 

downriver towards brackish waters (Smith et al. 1980). Juveniles may remain in 

brackish waters for months or years before moving out to sea, where they are known to 

migrate great distances (Smith and Clugston 1997). 

Humans took advantage of the migratory behavior of spawning Atlantic sturgeon 

to develop a fishery for Atlantic sturgeon flesh and roe (i.e. caviar). Caviar production in 
3

the United States did not flourish until the late 19th century (Cobb 1900), subsequently 

transforming the Delaware River Atlantic sturgeon fishery from a small, flesh-driven 

business into the caviar capital of North America (Saffron 2002). By the late 1800’s, the 

Delaware River Atlantic sturgeon fishery was the largest in the United States and 

produced 75% of the US sturgeon harvest from 1890-1899 (Townsend 1900). Between 

the collection roe from mature females for caviar production and the canning of the 

smoked flesh, the fishery provided a way of life for many people in search of post-Civil 

War prosperity (Saffron 2002). Delaware River landings reached a peak in 1888 with a 

total catch of nearly 3000 metric tons of Atlantic sturgeon (Smith 1894). The success of 

the fishery was short-lived, and by 1900 the total catch was less than 10% of the peak 

harvest totals (Ryder 1890; Cobb 1900). 

Given the steep decline in landings, management intervention was required. In 

1891 the State of New Jersey passed legislation to protect “mammose” (young sturgeon 

under 1m in length) sturgeon in the waters of Delaware River, bay, or tributaries within 

state jurisdiction (Cobb 1900). In 1990, the Atlantic States Marine Fisheries Commission 

(ASMFC) produced a Fishery Management Plan (FMP) for the Atlantic sturgeon with a 

stated goal of restoring the species throughout its range to allow an annual harvest of 

about 317 metric tons, which is approximately 10% of the peak catch totals (Smith and 

Clugston 1997). Following a petition to list the Atlantic sturgeon under the Endangered 

Species Act, a status review was conducted in 1998 that eventually determined the 

species did not warrant protection, although the ASMFC imposed a coastwide 

moratorium during the same year. In May of 1999, National Marine Fisheries Service 

(NMFS) extended the fishing ban into federal waters (North Carolina Division of Marine 
4

Fisheries 2006). Currently there is no allowable commercial, recreational or tribal 

harvest in the United States as designated by the ASMFC (1998), however the fishing 

moratorium has not been in place long enough to generate measurable recovery (Auer 

2004). In 2005, the NMFS initiated another status review of the Atlantic sturgeon based 

on the conclusions of a 2003 NMFS/USFWS joint committee. The status review team 

released their recommendations in February 2007 which stated three of the five distinct 

population segments of Atlantic sturgeon be listed as threatened under the ESA, 

including the Chesapeake regional population segment. 

Since the height of the fishery in the late 1800s, the Delaware River Atlantic 

sturgeon stock has not recovered. Although most harvest ended by the start of the 20th

century, there is little evidence of an extant spawning population. A number of factors 

have likely contributed to the lack of Atlantic sturgeon recovery in the Delaware River, 

including habitat loss primarily due to dredging, saltwater intrusion, water quality 

degradation, harvest pressure and bycatch mortality (Albert 1988; Stein et al. 2004). 

Estuarine and riverine habitat areas available to Atlantic sturgeon larvae, juveniles, and 

adults have been degraded over the last century (Albert 1988). Because Atlantic sturgeon 

are anadromous and utilize a combination of river, estuary and ocean habitats, damage to 

any of the three can have detrimental effects. The Delaware River became heavily 

industrialized throughout the latter half of the 19th century, and the increased activity 

resulted in rising water pollution to the point where large sections of the river became 

anoxic during summer months when Atlantic sturgeon occupy the system (Albert 1988). 

Siltation caused by the industrial coal industry drastically altered the substrate 

composition of the Delaware River (Albert 1988), which may have reduced available 
5

spawning habitat. The Delaware River has also experienced large-scale dredging to 

accommodate commercial shipping traffic, resulting in changes in substrate composition 

and tidal flows (Di Lorenzo et al. 1993; Walsh 2004). Within the period 1877 – 1987, the 

mean depth of the Delaware River increased by 1.6m and the mean cross-sectional area 

increased by nearly 3,000 m2 (Walsh 2004). By 1973, the US Army Corps of Engineers 

(USACE) estimated that nearly 154,000,000 m3

 of material had been removed from the 

Delaware Estuary (Walsh 2004). The channel deepening process increased the tidal 

range in the upper estuary; simultaneously, extensive water removals and diversions were 

occurring within the non-tidal watershed, resulting in saltwater intrusion in the 

freshwater-tidal reach of the estuary. This displacement of freshwater habitat may have 

negatively affected any potential success for the contemporary spawning population. 

A serious knowledge gap exists regarding temporal and spatial spawning patterns 

of Atlantic sturgeon in most river systems including the Delaware. The most 

comprehensive information regarding Atlantic sturgeon reproduction in the Delaware 

River is nearly 100 years old (Ryder 1890; Cobb 1900; Borodin 1925). Since the 

conclusions drawn concerning the location and timing of Atlantic sturgeon spawning are 

based primarily on information collected in fishery dependent sampling, it is possible that 

information on spawning locations may not be completely valid. The historically noted 

spawning areas were based on the fact that “ripe” fish were collected at these locations, 

where “ripe” pertains to adult Atlantic sturgeon that contained eggs ideal for caviar 

production rather than eggs ready for fertilization. Ripe eggs as defined for caviar 

production are actually premature in a reproductive sense. Eggs suitable for caviar 

typically have a higher polarization index, lighter color and firmer texture than eggs in 
6

the ovulatory stages (Van Eenennaam et al. 1996; Mohler 2003). Places such as Pea 

Patch Island, DE and Penn’s Grove, NJ (rkm 85-110) were noted as likely spawning 

areas, but such locations are currently brackish habitat unsuitable for survival of larvae. 

Additionally, the significance of historical information could increasingly lose 

relevance given the broad-scale environmental context shift caused by global climate 

change. Given the anthropogenic stressors already functioning within the Delaware 

River such as dredging and non-tidal watershed diversion rights combined with 

acknowledged climate change, the reliance of present day fisheries managers on 100-

year-old fisheries dependent data becomes more problematic. 

By identifying where spawning activity is currently taking place, essential fish 

habitat areas can be characterized for Delaware River Atlantic sturgeon. Young-of-theyear fish (342 mm TL) have been reported as recent as 2005 (Hal Brundage, ERC Ltd. 

pers. comm) supporting the existence of a remnant spawning population. Available 

information on the early life history of Atlantic sturgeon indicates a natal 

riverine/brackish water residency period for young-of-the-year (Bain 1997). Genetic 

evidence also supports the existence of a Delaware River specific haplotype (Tim King, 

USGS, Leetown, WV. pers. comm). 

Because the Atlantic sturgeon is currently listed as a species of concern 

(Department of Commerce 2004) and limited data poses challenges for fishery managers, 

we will hopefully provide information that will ultimately help define Atlantic sturgeon 

habitat utilization patterns within the Delaware River. The primary research objectives 

were to 1) determine the contemporary status of Atlantic sturgeon in the Delaware River, 

2) attempt to identify both the spatial and temporal extent of spawning, and 3) identify 
7

critical riverine habitats used during pre- and post-spawning movements and determine 

duration of fresh-water residency. 

Materials and Methods 

 Sampling Locale: The study took place in the Delaware Bay and Delaware River, 

ranging from the entrance of the bay near Cape Henlopen, DE and Cape May, NJ to as 

far upriver as Trenton, NJ (Figure 1). 

 Collection of Specimens: Nine adult Atlantic sturgeon (FL>1.3m FL) were 

captured using monofilament and multifilament gill nets of varying mesh size (primary 

gear consisted of 30.5cm stretch-mesh monofilament nets using 1-4, 122m panels, 6.1m 

height), length, and height, and by fishing a combination of drift and anchored sets. 

Fishing occurred primarily in the region around Woodland and Primehook Beaches 

Delaware during April, near the Artificial Island anchorage area during May, then into 

the Cherry Island Flats region and the Marcus Hook anchorage area during May and 

June. Although the drift method had been used successfully to capture adult Gulf 

sturgeon by Fox et al. (2000), the local Delaware River fishing contingent as well as 

Delaware Natural Resources and Environmental Control (DNREC) fishery managers 

suggested using anchored net techniques as well as drift. Anchored fishing modeled that 

of the anchored striped bass commercial harvesters, where 30.5cm stretch-mesh gillnets 

were moored using commercial gillnet anchors and allowed to fish continuously, being 

tended at least once per day. Fishing locations were based on three variables: 1) advice 

from the local fishers and DNREC fishery managers, 2) having the capacity from a 

physical standpoint to allow fishing of nets exceeding 2000m2

 in size, and 3) striped bass 

fishing reports provided by DNREC that documented adult Atlantic sturgeon as bycatch. 
8

Upon capture, the sturgeon were placed in a live well, approximately 680L in capacity, 

where water was pumped in directly from the river system, maintaining ambient 

temperature, salinity and dissolved oxygen levels. Each fish was then implanted with an 

ultrasonic transmitter (Vemco Ltd. (V-16, 6-H, 69.0 kHz). 

 A reward system was also established to allow us to work with local commercial 

striped bass harvesters, where they were paid $300.00 for a live Atlantic sturgeon of adult 

size (>1.3m FL). Each harvester involved in the program was first required to obtain 

permission from the DNREC to handle the species beyond a normal release. Authorized 

local striped bass commercial harvesters were allowed to retain an adult Atlantic sturgeon 

whenever they encountered one as bycatch. Once the fish was secured to an anchorage 

point, the catch was reported to Delaware State University researchers. An immediate 

response was made to each reported catch, whereupon Dewayne Fox or Phil Simpson 

took possession of the fish and implanted it with an ultrasonic transmitter. 

Tags not utilized for adult Atlantic sturgeon were implanted into juvenile Atlantic 

sturgeon after water temperatures exceeded 25o

 C, the likely threshold for spawning 

activity (Dovel and Beggren 1983). There is presently only limited research available 

regarding juvenile Atlantic sturgeon seasonal migrations, nursery areas, overwintering 

habits, and movement patterns in the Delaware River. The most recent findings (Shirey 

1997), identified two important holding areas (near river kilometers 88 and 125, 

respectively) utilized by juvenile Atlantic sturgeon during the spring and summer months 

in the Delaware Bay and Delaware River. Additionally, sampling reports from Shirey 

during the period 1991-1997 showed a decline in catch rates from a reported 32.2 fish/hr 

to 1.4 fish/hr. This additional juvenile information may aide future researchers by 
9

determining optimal sampling times and locations, as well as policy makers who may 

better identify habitat zones important to the growth and survival of Atlantic sturgeon. 

Surgical Procedures: The ultrasonic transmitters used in this study were 90mm in 

length, 16mm diameter, and weighed 14g (in water). The transmitters were coated with 

Dow Corning Silastic® biologically inert silicone elastomer in an attempt to minimize 

rejection rates (Fox et al. 2000). 

Upon capture, each fish was placed in the live well. The TL and FL (cm) and, 

when available, weight (kg) was measured and recorded. Any captured Atlantic sturgeon 

were scanned thoroughly for the presence of an existing PIT tag. If no tag was present, a 

16mm Biomark® PIT tag was inserted on left side of fish at the base of the dorsal fin. A 

barbel sample was taken and placed in 95% ethanol for future genetic analysis. A plastictipped dart tag was inserted on the left side of the fish below the fourth dorsal scute. 

Surgical implantation of the Vemco ultrasonic transmitter followed the protocol 

established by Fox et al. (2000). 

 

Telemetry: Telemetered Atlantic sturgeon were monitored once a week using the 

telemetry techniques described below. Riverine waters were surveyed weekly, weather 

and personnel permitting, from June until October, or until most or all of the fish had 

moved out of the survey range. Fixed survey stations were set at one kilometer intervals 

and listening occurred for at least two minutes using an omni-directional hydrophone. 

Once a tagged fish was detected, a directional hydrophone was used to determine the 
10

exact location of the fish. The location and depth were recorded along with water quality 

parameters (temperature, salinity, conductivity, and dissolved oxygen). 

A passive array of VR2 receivers was also be established and periodically 

checked to determine broad scale fish movements (Figures 1-2). These receivers were 

attached to the United States Coast Guard navigational aides (Figure 3) and were 

distributed throughout the river and bay in order to maximize the potential for monitoring 

upstream and downstream movements of telemetered Atlantic sturgeon. An additional 

array for a separate DSU research project which utilized anchored receivers in two linear 

arrays was positioned between Slaughter Beach and Cape Henlopen, DE. Data sharing 

with the privately owned research firm Environmental Research and Consulting, Inc. also 

utilizing Vemco VR2 receivers within the Delaware River, allowed for an overall 

increase in the amount of data collected by all parties. 

The average detection range of the VR2 receivers was determined by a series of 

tests. Each test documented a percent of possible tag detections at varying distances and 

directions from the VR2 receiver. Starting at the receiver, a test transmitter similar to the 

ones used for this study was attached to a weighted line and deployed at a depth 

approximately two meters off the river bottom to mimic the presumed benthic nature of 

Atlantic sturgeon. Simultaneously using a VR100 manual hydrophone and a handheld 

GPS unit, the time of each transmittance was recorded along with the location. 

Deploying the tag for no less than three minutes, the number of detections recorded by 

the VR2 receiver was compared to the number of possible detections to produce a percent 

transmittance. This process was repeated at distances of 500m, 1km, and 1.5km for each 

north, south, east and west headings. Although numerous tests have been conducted in 
11

prior research (Domeier 2005; Szeldlmayer and Schroepfer 2005) and claims made about 

the detection capabilities of the VR2 receivers are well documented (Miller and Sadro 

2003; Simpfendorfer 2006), the detection range can vary significantly because of both 

physical and biological factors (Heupel et al. 2006). The probable detection range, 

according to the available literature, may reach 90% in a static water setting (Clements et 

al. 2005) but may decline and potentially reach a maximum between 400m – 800m 

(Heupel and Hueter 2001; Hildebrand and Hildebrand 2004), but can also be as low as 

300m (Semmens et al. 2005), or in extreme cases as low as 45m (Finstad et al. 2005). 

Telemetry methods and habitat characterization techniques were identical for adult and 

juvenile Atlantic sturgeon. 

Habitat Characterization: Habitat parameters such as water depth (m), 

temperature (o

C), dissolved oxygen (mg/L), salinity (ppt) and conductivity (mS) were 

measured and recorded at each capture location and relocation point. The manual 

telemetry relocations of Atlantic sturgeon were plotted using ArcMap and overlaid with 

sediment-type data collected by Sommerfield and Madsen (2003) to determine habitat 

selection. Sommerfield and Madsen utilized sidescan sonar methods to produce a benthic 

map of the Delaware River from the region just north of Pea Patch Island (approximately 

river kilometer 110) upriver to approximately Bridgeport, NJ. Each relocation point was 

classified by sediment type based on the location of the point within the benthic mosaic 

developed by Sommerfield and Madsen. The total area comprised of each habitat type 

within the region of relocations was calculated, and an individual Chi-square analysis 

was performed to determine if Atlantic sturgeon are selecting habitat based on sediment 
12

type. Similar methods were utilized to assess Atlantic sturgeon depth preferences using 

data obtained from the USACE. 

Additionally, overall movement of telemetered Atlantic sturgeon was crossplotted with water temperature (o

C) as well as river flow (ft3/s) data obtained from the 

United States Geological Survey gaging station in Philadelphia, PA to determine if a 

correlation exists between overall migration patterns of juvenile Atlantic sturgeon and 

either of the two environmental variables. 

Reproductive status: Gonadal biopsies were prepped using standardized 

histological techniques and were staged using the classification criteria established by 

Van Eenennaam and Doroshov (1998). 

Egg Collection: Abrasive surface artificial substrate pads suitable for adherence 

of eggs have been successfully used for white and Gulf sturgeon egg collection (McCabe 

and Beckman 1990; Marchant and Shutters 1996; Fox et al. 2000). Egg mats (diameter 

56cm) were deployed in areas of the river where spawning was believed to be occurring 

based on active telemetry data and substrate sampling. When rocky or gravel substrate 

was collected using a petite ® Ponar grab, egg mats were deployed approximately 10-

15m from each other and checked for deposited eggs at a maximum of every third day 

(based on the hatch times established by the Northeast Fishery Center (Mohler 2003)) 

until spawning fish were no longer in the area. 

Results 
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 The 2005 sampling season began on May 10, and concluded on September 22. 

The 2005 fishing efforts produced only juvenile Atlantic sturgeon (Table 1). Beginning 

May 10, fishing efforts were directed towards adult Atlantic sturgeon. On June 30, 

efforts shifted to juvenile Atlantic sturgeon as water temperatures approached 26o

C. Out 

of the 23 total juvenile Atlantic sturgeon captured, 12 were implanted with transmitters 

(using a minimum total length 80.0cm as size criteria) . The sizes of fish captured ranged 

from 65.3cm – 131.0cm (Table 2). The fishing efforts for the season concluded on 

September 22, 2005. During May and early June, fishing occurred primarily in the upper 

bay off Woodland Beach and Bombay Hook. Efforts shifted towards Artificial Island 

from mid-June to the end date of September 22, with some nets deployed near Cherry 

Island Flats (rkm 110) and Marcus Hook (rkm 125) as well (Figure 4). 

 During 2006, netting began April 12, however the first three Atlantic sturgeon 

implanted with transmitters were obtained as a result of the reward system. The first of 

these fish was below the adult size threshold (FL >1.3m) and therefore was considered 

juvenile. The reward program ceased on May 1 as the commercial striped bass season 

came to a close. From May 10, under the guidance of commercial striped bass fisherman 

Larry Voss, six 30.5cm stretch-mesh gillnets, each 91 meters in length, were anchored in 

upper Delaware Bay. These nets were checked at least once per day for captured adult 

Atlantic sturgeon. Using these methods, a third Atlantic sturgeon of adult size was 

captured and tagged May 18. We continued to fish using this style of anchored sets until 

May 26. At this point, netting efforts were moved upriver (river kilometer (rkm) 85-90) 

as potentially reproductive Atlantic sturgeon were believed to move upriver. 
14

 Starting May 30, we moved our fishing efforts towards Artificial Island (river 

kilometer 88). Gill nets were anchored at slack tide and allowed to fish until the 

subsequent slack water event. A fourth adult Atlantic sturgeon was captured June 1 and 

fitted with an ultrasonic transmitter. Sampling continued until June 18, and gradually 

moved upriver into the Cherry Island Flats/Marcus Hook Bar region (rkm 110-125) as 

well as riverine sections approaching Trenton, NJ (Figure 4), using relocations of the four 

tagged adult Atlantic sturgeon as a guide to direct fishing efforts (Table 3). The gonadal 

biopsies collected from each fish identified all four fish as males, each of them 

potentially of reproductive size. Histological examination of the 2006 gonad samples 

indicated potentially two out of four individuals had reached a near-term spawning 

maturation phase. 

By mid-July, tracking data indicated the telemetered adult Atlantic sturgeon had 

moved out of probable spawning areas and returned to brackish waters. Additionally, 

water temperatures began to exceed the reported maximum (25o

C) for spawning activity 

(Dovel and Berggren 1983). At this point, the focus again turned to juvenile Atlantic 

sturgeon. Beginning July 24, smaller mesh nets (15-20cm) were deployed near Artificial 

Island (rkm 88) in addition to the large mesh nets used previously. Ten more juvenile 

Atlantic sturgeon were tagged in the summer of 2006 (Table 4). 

During 2005 and 2006 a total of 27 Atlantic sturgeon were tagged, producing 

181,999 passive relocations coupled with 183 manual relocations (Table 5). Telemetry 

results point to a late spring/early summer inward migration, followed by a period of 

dampened movement during the warm summer months of August and September, then an 

outmigration during October and November. During the summer months juvenile 
15

Atlantic sturgeon displayed little movement compared to spring and fall months (Figure 

6a – 6c). Juvenile Atlantic sturgeon concentrated in three specific areas, two of which 

(Artificial Island, Cherry Island Flats) had been identified by previous research conducted 

on the Delaware River (Figure 5). The previously unidentified area, however, is situated 

in the Marcus Hook anchorage zone (rkm 125). According to manual relocation data, 

juvenile Atlantic sturgeon are selecting for depth (Figure 7a) but are not selecting for 

substrate type (Figure 7b). 

Of the four adult size Atlantic sturgeon tagged during 2006, one individual, 

Vemco ID #1744, migrated into freshwater-tidal reaches of the Delaware River (Figure 

8), although the histological examination of the gonads of this fish indicated it was 

sexually immature. Atlantic sturgeon #1744 moved upriver to Tinicum Island (rkm 136) 

immediately after it was tagged, and remained there from June 4 – June 10. Initial 

sediment sampling was performed near the relocation points at Tinicum Island during this 

time period, however by June 12; this individual had migrated towards Trenton, NJ (rkm 

211) where it remained until June 27. By July 7, Atlantic sturgeon #1744 had returned to 

the location where it was originally captured. 

 Once Atlantic sturgeon #1744 entered the upper river near Trenton, NJ, habitat 

characterization began through the use of a Ponar grab in order to classify the substrate as 

hard-bottom, and egg mats were deployed where hard-bottom, rocky substrate was 

prevalent (Figure 9). A total of 75 egg mats were deployed on June 16 between river 

kilometers 197 – 210 in clusters of 12-16 pads and fished until June 28, as river flow 

measured at Trenton, N.J. rose from 8,000 CFS to over 200,000 CFS within a 24-hour 
16

period. Only one-third of this gear was recovered following the flood event. No eggs 

were collected during the study. 

 Preliminary range tests following our described methodology indicate that 

transmitter detection success is less than 50% at distances of 500m or more. These 

findings stem from tests performed on nearshore receivers in depths not exceeding 8m. 

However, based on other analysis methods, there is evidence that the VR2 receivers can 

detect transmitters at distances of greater than 2km. There were multiple instances where 

two receivers deployed in close proximity to one another detected a transmitter 

simultaneously. The most extreme example of this scenario occurred at Tinicum Island 

(rkm 136 and rkm 141), where the inter-receiver distance is 5.7 km. If a transmitter is 

detected by both receivers, then each receiver must have a minimum detection range of 

2.8 km. While percent transmittance values are not obtainable using this analysis 

method, we can get some idea as to the maximum detection range of the receivers. 

 

Discussion 

The Delaware River once supported the largest population of Atlantic sturgeon 

known to exist (Secor and Waldman 1999) and although our telemetry results are not 

conclusive, coupled with the collection of ripe adults, and preliminary genetic analyses, 

they strongly suggest that a remnant spawning population exists today. The telemetry 

results from Atlantic sturgeon obtained during 2006 provide some insights about the 

probable spatial and temporal extent of spawning behavior. Given the fact that sexually 

mature male Atlantic sturgeon were captured in Delaware Bay between mid-April and 

June 1, our findings for the Delaware River agree with those of Dovel and Beggren 
17

(1983) regarding Atlantic sturgeon spawning times reported for the Hudson River. 

Additionally, the rapid upriver migration made by Atlantic sturgeon #1744 to freshwater 

habitat where hard-bottom substrate is prevalent would indicate that spawning for 

Atlantic sturgeon may occur much further upriver than the historical reports of “ripe fish” 

observations made by Cobb (1899) as well as Borodin (1925). Spawning habitat of 

Atlantic sturgeon may also be shared with the closely related shortnose sturgeon (A. 

brevirostrum) in the Delaware River. These findings differ from Bain’s (1997) 

conclusions for Hudson River shortnose and Atlantic sturgeon where such a spatial 

overlap is not thought to occur regularly. 

Based on our findings, it seems likely that the present day lower limit of Atlantic 

sturgeon spawning may be in the area of the upper limit of salt water intrusion near 

Tinicum Island (river kilometer 136) while the upper limit is likely at the fall line near 

Trenton, NJ (river kilometer 211). Dovel and Beggren (1983) reported that early in the 

season (late May), Atlantic sturgeon spawning in the Hudson River occurred near the salt 

wedge, and then continued upstream into June and early July. Atlantic sturgeon #1744 

appeared to fit the profile for a spawning adult sturgeon (e.g. Fox et al. 2000), occupying 

areas around Tinicum Island (negligible salinity) during early June then making a rapid 

directly movement upstream towards Trenton, where it remained until late June before 

making a downriver migration in early July. However, the histological analysis of the 

gonad sample obtained from Atlantic sturgeon #1744 proved inclusive and indicated this 

fish had undifferentiated gonads. However, we hypothesize that the gonadal biopsy of 

this individual missed the reproductively active portion of the gonad since the gonads of 
18

Atlantic sturgeon may exhibit different stages of development throughout the gonadal 

tissue (Joel VanEenennaam, pers comm). 

 Atlantic sturgeon #1225 migrated as far upriver as the mouth of the Schuylkill 

River (rkm 148) on May 31st, 2006; however, the duration of the stay was less than 24 

hours and did not appear indicative of spawning behavior. After quickly returning to the 

upper bay waters (rkm 80), Atlantic sturgeon #1225 returned to the area of Marcus Hook 

Bar (river kilometer 134) on June 28. Again, the visit was short and, within one day, the 

fish returned to the upper bay. Nevertheless, this migration may indicate this fish was 

searching for a mate and could indicate the presence of other spawning adult Atlantic 

sturgeon near the regions of Marcus Hook Bar and Tinicum Island during the month of 

June. Additionally, the histological results from this individual indicate that it was 

sexually mature and ready to spawn, suggesting that spawning may occur between 

Tinicum Island (rkm 136) and the mouth of the Schuylkill River (rkm 148). 

 The discussion pertaining to the section of river between Marcus Hook Bar and 

the mouth of the Schuylkill River becomes more significant when the idea of habitat 

selection is introduced. Atlantic sturgeon not only require freshwater for spawning but 

also hard-bottom substrate (Gilbert 1989, Smith and Clugston 1997). Based on 

Sommerfield and Madsen (2003), there are multiple zones of hard-bottom substrate 

within the Marcus Hook-Schuylkill River area that are considered freshwater. This area 

has been classified as freshwater (0.0 to 0.5 ppt upstream of Marcus Hook) by the Army 

Corps of Engineers (1997). Sommerfield and Madsen classify much of this area as 

“Mixed-grained reworking” where the dominant sediment types are “mixed gravel, sand, 

and mud,” or “Coarse-grained bedload” with “moderately well-sorted sand and gravel” as 
19

the dominant sediment types and “Non-deposition where the dominant sediment types are 

“cobble and bedrock.” 

 Based on the findings of Sommerfield and Madsen (2003), the substrate 

composition between Marcus Hook and Tinicum Island may represent suitable spawning 

habitat for Atlantic sturgeon. The majority of the hard-bottom substrate zones, 

particularly the coarse-grained bedload areas, either neighbor or are within the shipping 

channel, indicative of suitable spawning depths (Smith and Clugston 1997). However, 

the presence of hard-bottom substrate within the shipping channel may also be a limiting 

factor in terms of spawning success, potentially exposing adult Atlantic sturgeon to 

mortality due to boatstrike. 

 The identification of spawning areas in the Delaware River is difficult because of 

the limited data received from telemetered adult Atlantic sturgeon and the lack of 

fertilized egg collections. The Delaware River, between Marcus Hook (rkm 125) and 

Trenton (rkm 211), could be considered potential spawning habitat based on availability 

of freshwater and hard-bottom substrate. Because males tend to be more active (Fox et 

al. 2000) than females during spawning events, it is difficult to apply the telemetry data 

to spawning habitat quantification. For example, Bain described males as occupying a 

stretch of the Hudson River greater than 100km in length during the spawning season; 

however females were relegated to two concentration zones totaling about 20km in length 

(1997). Although telemetry of a mature female and egg collections is nearly essential to 

begin the quantification of potential spawning habitat, some assumptions can be drawn 

based on the telemetry of adult male Atlantic sturgeon. 
20

The upriver migrations made by Atlantic sturgeons #1744 and #1225 are also 

significant when compared to the movements of juveniles documented during 2005-2006. 

Only in a few isolated incidences were relocations of telemetered juvenile Atlantic 

sturgeon made above the Commodore Barry Bridge (rkm 135); the majority of 

relocations occurred in the area of Marcus Hook (rkm 125) or below. This may indicate 

some spatial separation between adult and juvenile Atlantic sturgeon, and would be 

inconsistent with the findings of prior research (Bain 1997; Fox et al. 2000; Hightower et 

al. 2002), where no spatial separation existed between spawning adults and juveniles. 

 Although a population assessment is not one of the specific objectives of this 

research, the low numbers of Atlantic sturgeon collected given the effort expended is 

worth some discussion. Our low CPUE is troubling considering in 1888, nearly 3.0 

million kilograms of Atlantic sturgeon were harvested from the Delaware River (Secor 

and Waldman 1999). Ultimately, during 2005 and 2006, fishing efforts were almost 

completely fruitless, especially those targeted at adult Atlantic sturgeon. Although 

sampling restrictions (not being able to sample within the shipping channel) may have 

limited our catch rates, restrictions alone cannot account for such a low CPUE. 

However low the CPUE, the fact remains that sexually mature Atlantic sturgeon 

were captured in Delaware Bay. A problem that is of growing concern to the continued 

existence of Atlantic sturgeon in the Delaware River is mortalities of Atlantic sturgeon by 

boat strikes. This is not a new problem as it has been observed anecdotally over the last 

few years. During each of the past two years approximately 5-10 adult (>1.3m) Atlantic 

sturgeon have been observed with blunt trauma that is indicative of being struck by a 

boat. Results provided through this study indicate that Atlantic sturgeon (juveniles and 
21

adults) often inhabit main channel habitats (Figure 7a) The navigation depth of this 

channel is currently maintained at 12.2m by the USACE. Aside from being the largest 

free flowing river on the East Coast, the Delaware River is also the busiest freshwater 

commercial port. Large commercial ships often have limited bottom clearance (<2m) 

when transiting the Delaware River. This fact coupled with Atlantic sturgeon occupying 

main channel habitat increases the probability that a boat strike incident will occur. 

 Our results indicate that juvenile Atlantic sturgeon occupy specific sections of the 

Delaware River for prolonged periods during the summer months. Based on our 

telemetry results we have identified three riverine concentration areas of telemetered 

juvenile Atlantic sturgeon: Artificial Island (rkm 89), Cherry Island Flats (rkm 110), and 

Marcus Hook Anchorage (rkm 125). The lower two sites, Artificial Island and Cherry 

Island Flats, were previously identified and it was noted that substrate composition may 

play a role as a unifying characteristic for the two sites (Shirey 1997). A recent study 

looking at the substrate composition of the upper two areas note that they are 

predominated by mixed gravel and sand sediments with areas of non-depositional 

substrate, however not necessarily due to the presence of slag deposits (Sommerfield and 

Madsen 2003). 

 Based on our recapture data, many juvenile Atlantic sturgeon within Delaware 

Bay and River likely do not stem from Delaware River genetic stock. While a specific 

genetic haplotype has been identified for the Delaware River Atlantic sturgeon 

population (Tim King, pers comm.), our netting efforts and telemetry results, as well as 

mark-recapture data collected by DNREC (Greg Murphy, pers comm.) indicate juvenile 

Atlantic sturgeon collected and or spawned in other rivers are using the Delaware River 
22

and Bay as a nursery habitat. In terms of management implications, the lack of a 

distinctive stock could significantly alter recovery efforts. If stocking programs were 

ever to be implemented, such genetic considerations would come into play. 

 

Conclusions 

Our telemetry results coupled with the collection of ripe male Atlantic sturgeon 

although not conclusive provide strong evidence to suggest that a remnant population of 

spawning Atlantic sturgeon exists in the Delaware River. Although the location of 

spawning habitat has not been confirmed, our telemetry results together with information 

on substrate and water quality data indicate that spawning may be occurring between 

mid-June - late June in freshwater-tidal reaches between north Philadelphia, PA (rkm 

176) and Trenton, NJ (rkm 211). These spawning areas are located much further upriver 

than historically reported spawning grounds (rkm 75 – 130; Ryder 1890, Cobb 1899) 

which may, in part, be due to increased saltwater intrusion. The effects of increased 

saltwater intrusion on Atlantic sturgeon in the Delaware River although not well 

understood, may continue to play a role in the slow recovery of this population especially 

with the political and socio-economic pressure to expand dredging efforts in the river 

potentially furthering the saltwater intrusion and reducing available habitat. 

Atlantic sturgeon bycatch mortality is known to occur in both the Delaware 

Estuary and nearby continental shelf regions (Stein et al. 2004), and undoubtedly has 

played a role in slowing the recovery of this population. We became aware of an 

additional source of mortality during the course of this study as we documented 

numerous reports of adult and juvenile Atlantic sturgeon mortalities, many of which were 
23

likely caused by propeller strikes. The Delaware River serves as one of the most 

important shipping ports in the US and is the number one east coast port for petroleum 

shipments. This strategic importance must be weighed against the costs to the ecological 

integrity of the Delaware River Estuary. At present there are numerous large-scale 

shipping and refining projects proposed for the Delaware Estuary. We propose that 

managers should apply a precautionary approach to the approval of any such projects and 

take into account how their potential impacts can alter the recovery trajectory for what 

was once the largest population of Atlantic sturgeon. 

Acknowledgements 

We would like to thank the following individuals for their assistance in 

completion of this research: Lori Brown, Collette Cairns, Naeem Willett, and Don 

Wujtewicz, Delaware State University; Hal Brundage, Environmental Research and 

Consulting, Inc; Greg Murphy and Craig Shirey, Delaware Natural Resources and 

Environmental Control; Clyde Roberts, Larry and Lawrence “Limbo” Voss; Captain 

Thomas Flynn and Lieutenant Brian Huff, United States Coast Guard; the crew of the 

USCGC William Tate; Russ Allen, New Jersey Department of Environmental Protection; 

and John Jacobs, NOAA Oxford Laboratory. 

 
24

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28

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29

Figure 1. Passive receivers (Vemco VR2) used for Atlantic sturgeon telemetry, 2005. 
30

Figure 2. Passive receivers (Vemco VR2) used for Atlantic sturgeon telemetry, 2006. 
31

Figure 3. Schematic of VR2 deployment using USCG navigational aide. 
32

Figure 4. Atlantic sturgeon netting and capture locations, 2005-2006. 
33

Figure 5. Passive relocations for juvenile Atlantic sturgeon, 2005-06. 
34

Figure 6a. 2005 Atlantic sturgeon movement. 

0

20

40

60

80

100

120

140

160

06/30 07/20 08/09 08/29 09/18 10/08 10/28 11/17 12/07 12/27

Date

river kilometer

Figure 6b. 2006 Juvenile Atlantic sturgeon movement. 

0

20

40

60

80

100

120

140

160

03/14 05/03 06/22 08/11 09/30 11/19

Date

river kilometer
35

Figure 6c. 2006 Adult Atlantic sturgeon movement. 

0

50

100

150

200

250

04/18 05/18 06/17 07/17 08/16 09/15 10/15 

Date

river kilometer

Figure 7a. Atlantic sturgeon utilized vs. available depth. Based on manual relocation data, 2005-2006. 

 

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0 - 2.00 2.01 - 4.00 4.01 - 6.00 6.01 - 8.00 8.01 - 10.00 10.01 - 12.00 12.00 - 14.00 > 14.01

Depth (m)

Relative Frequency

Expected

Observed
36

Figure 7b. Atlantic sturgeon utilized vs. available substrate type. 

Based on manual relocation data, 2005-2006. 

0.00

0.10

0.20

0.30

0.40

0.50

0.60

cobble/bedrock mixed

gravel/sand/mud

mud mud/fluid mud well sorted

sand/gravel

Sediment type

Relative Frequency

Expected

Observed
37

Figure 8: Summary movement patterns for ripe male adult Atlantic sturgeon which was 

implanted with transmitter # 1744 on June 1, 2006. 
38

Figure 9. Atlantic sturgeon egg pad deployment locations, 2006. 
39

Table 1. Catch per unit effort, 2005. 

Total Net Fished (m2

) 115998.7

Total Hours Fished 253.3

Total Catch 23

CPUE (catch/m2/hr) 7.83E-07

Table 2. Length summary for captured and telemeterd Atlantic sturgeon, 

2005. 

Minimum Maximum 

Fork Length (cm), non-telemetered captures 56.0 94.0 

Weight (kg), non-telemetered captures 1.6 2.9 

Fork Length (cm), telemetered captures 78.4 122.5 

Weight (kg), telemetered captures 4.2 10.0 

Table 3. Catch per unit effort, 2006. 

Total Net Fished (m2) 73046.9

Total Hours Fished 2585.0

Total Catch 18

CPUE (catch/m2/hr) 7.41E-08

Table 4. Length summary for captured and telemeterd Atlantic sturgeon, 

2006. 

Minimum Maximum 

Fork Length (cm), non-telemetered captures 57.4 71.2 

Weight (kg), non-telemetered captures 1.5 2.1 

Fork Length (cm), telemetered captures 67.8 158.0 

Weight (kg), telemetered captures 2.5 28.4 
40

Table 5. Atlantic sturgeon relocation summary 2005-2006. 

Fish ID Date 

Captured 

Fork 

Length 

(mm) 

Passive 

Detections

Manual 

Detections

Total 

Detections 

Date Last 

Relocated 

1232 6/30/2005 820 761 10 771 8/18/2006 

1234 6/30/2005 913 4132 5 4137 5/5/2006 

1233 7/7/2005 880 1002 15 1017 11/3/2005 

1229 7/7/2005 851 50754 20 50774 10/31/2006

1224 7/11/2005 784 2121 13 2134 4/25/2006 

1223 7/19/2005 950 7606 8 7614 4/26/2006 

1222 7/27/2005 1225 499 11 510 10/28/2005

1221 8/1/2005 940 2641 7 2648 11/25/2005

1230 8/1/2005 970 534 6 540 5/28/2006 

1231 8/2/2005 1140 881 8 889 9/30/2005 

1227 9/13/2005 785 5133 3 5136 11/1/2005 

1235 9/22/2005 857 4900 5 4905 1/6/2006 

1226 4/18/2006 1210 1770 0 1770 10/5/2006 

1228 4/23/2006 1550 1436 0 1436 6/3/2006 

1225 4/27/2006 1580 3169 8 3177 9/26/2006 

1747 5/18/2006 1370 714 1 715 9/9/2006 

1744 6/1/2006 1370 13877 15 13892 10/27/2006

1731 7/24/2006 796 15722 7 15729 10/3/2006 

1732 7/24/2006 969 5604 5 5609 10/9/2006 

1733 7/24/2006 782 14788 6 14794 10/8/2006 

1740 7/24/2006 800 4596 7 4603 10/8/2006 

1742 7/24/2006 780 14292 7 14299 10/21/2006

1746 7/24/2006 818 9459 6 9465 11/1/2006 

1739 8/2/2006 722 1 1 2 8/2/2006 

1741 8/2/2006 678 8720 7 8727 10/27/2006

1750 8/25/2006 808 800 0 800 8/29/2006 

1734 9/6/2006 754 6087 2 6089 10/9/2006 

Total Passive Detections 181999 

Total Manual Detections 183 

Total Detections 182182 
41