Estuaries Vol. 19, No.4, p. 769-777 December 1996 Reproductive Conditions of the Atlantic Sturgeon (Acipenser oxyrinchus) in the Hudson River J. J. S. I. DOROSHOV D. G. WATSON S. MOORE G. P. J. LINARES P. VAN EENENNAAM 1 MOBERG University of California Animal Science Department Davis, California 95616 ABSTRACT: Ninety-four prespawning adult Atlantic sturgeon (Acipenser oxyrinchus) were sampled in the Hudson River for age, sex, body size, gonad weight, fecundity, mature oocyte size, and plasma concentrations of gonadotropins, sex steroids, and vitellogenin during the spring spawning migrations in 1992 and 1993. In males, the age and total length ranged from 12 yr to 19 yr and from 133 em to 204 em and in females from 14 yr to 36 yr and from 197 em to 254 em. The majority of males were 13-16 yr old, and females were 16-20 yr old. Some females had residual atretic ovarian bodies, presumably remaining from a previous spawning and indicating iteroparity. Pre-ovulatory condition was recognized by migration of the germinal vesicle or by germinal vesicle breakdown and by significantly elevated plasma gonadotropins, progesterone, and vitellogenin. All pre-ovulatory females were captured upriver from Hudson River kilometer 136. Individual fecundity ranged from 0.4 million to 2.0 million eggs and oocyte diameter from 2.4 mm to 2.9 mm, and both characters exhibited a significant (p < 0.05) positive relationship with female body size. Iteroparous females, tentatively identified by the presence of atretic bodies remaining in the ovary from a previous spawning, had significantly (p < 0.05) higher fecundity and produced larger eggs, compared with females spawning presumably for the first time. Introduction tists since the last century (Ryder 1888; Dean 1893, 1894). More recent reviews were published by Vladykov and Greeley (1963), Magnin (1964), Murawski and Pacheco (1977), Hoff (1980), Van Den Avyle (1984), Smith (1985), and Gilbert (1989). The Hudson River Atlantic sturgeon was studied by Dovel and Berggren (1983) and Young et aI. (1988). These works gave insight into sturgeon biology, population structure, and fishery management. However, there is very little information on reproductive development. Smith (1985) provided fecundity data on the South Carolina Atlantic sturgeon, and Huff (1975) described the ovarian stages of the Gulf sturgeon (Acipenser oxyrinchus desotoi) in Florida. To our knowledge, there is no information available on reproductive hormones and metabolites in Atlantic sturgeon. Our objective was to characterize reproductive conditions of the spawning population in the Hudson River in relation to age and body size. The study was part of a larger project aimed to investigate gametogenesis and reproductive hormones of Atlantic sturgeon during marine and freshwater life. The Atlantic sturgeon (Acipenser oxyrinchus) is a valuable chondrostean species of the eastern coast of North America. It was historically abundant and supported a significant fishery including production of caviar (Murawski and Pacheco 1977). However, in most rivers the harvest collapsed during the early 1900s. In addition to overfishing, industrialization contributed to the population decline by river damming and pollution. In spite of these factors, the Hudson River and its estuary still remain one of the major spawning and nursery grounds for the Atlantic sturgeon. Since 1983 commerciallandings of sturgeon in New York have decreased in the Hudson River and increased in the coastal waters (Brandt 1988). Resource agencies are concerned with these trends and are trying to regulate the fishery to protect the remaining sturgeon population. The biology, fishery, and artificial propagation of Atlantic sturgeon has attracted attention of scien1 Corresponding author; tele 916/752-2058; fax 916/7520175. © 1996 Estuarine Research Federation 769 770 J. P. Van Eenennaam et al. HUDSON RIVER Catskill >- 182 Kilometers MA CT Kingston >- 148 Hyde Park • 136 NY ｾｷ｡ｲｴｳ ･ｶ｡ｈＭｮｯｴ ｲｃ Tappanzee---::::....=-- NJ ermen « RKM 64) set their nets (mesh size 3546 em) to the bottom with heavy weights on each end. The nets were checked daily during slack tide. The upriver (RKM 64 and upstream) fishermen set their nets (mesh size 25-35 em) for 15-40 min depending on tide and river traffic, in the shipping channel at a depth of 12-25 m. Fish collection and sampling were affected by selectivity of fishing gear and duration of the commercial fishing season which lasted from May 15 to July 9, 1992 and from May 15 to June 15, 1993. However, the large number of Atlantic sturgeon sampled during two consecutive years provides representative data for characterization of the spawning population. TISSUE SAMPLING Mter capture, sturgeon were either sampled on the boat, or on shore during cleaning and processing. Fork length (FL) and total length (TL) were measured (±0.1 em) in all fish. When possible, total body weight (BW) (±100 g) and gonad weight (GW) (±l.0 g) were also measured. Samples of the gonadal tissue were excised from the mid-portion of the gonad and fixed in buffered formalin. Five subsamples of oocytes from mature females were collected and weighed (±O.OOOI g) for fecundity estimates. Blood samples were collected from the caudal vein with heparinized vacutainers, and plasma was separated by centrifugation and stored frozen. One pectoral finray was removed for ageing. LABORATORY PROCEDURES Fig. 1. The Hudson River. Arrows indicate location of Atlantic sturgeon captures. Materials and Methods FISH COLLECTION Ninety-four Atlantic sturgeon (28 females and 66 males) were sampled from the Hudson River commercial fishery during May-July 1992 and MayJune 1993, in the area between Hudson River kilometer (RKM) 45 and 182. Only two sampled fish were immature: one male (age 9 years, RKM 137) and one female with previtellogenic ovaries (age 15 years, RKM 64). The remainder were broodfish in prespawning,spawning or recently spent gonadal conditions. The locations of capture sites are shown in Fig. l. Eight commercial fishermen assisted in collecting fish. The fishermen used anchor gill nets that were 15-100 m long and 2.5-4.0 m deep, with a stretch bar mesh of 25-46 em. Nets were set perpendicular to the river flow. The downriver fish- Gonadal Development The ovary of sturgeon on the spawning grounds typically contains three clutches of ovarian follicles: small oocytes in the endogenous growth phase with undifferentiated chorion; slightly enlarged previtellogenic oocytes with no yolk platelets in the cytoplasm; and large, darkly pigmented oocytes to be ovulated during spawning. We analyzed the latter clutch. Oocyte diameter (n = 20) was measured using a darkfield dissecting microscope with camera lucida, digital image analyzing tablet (Nikon Microplan II), and a microcomputer interface (accuracy 0.01 mm). Whole oocytes, fixed in buffered formalin, were bisected with a razor blade along the animal-vegetal axis and examined under a dissecting scope with a fiber optic illuminator. Distance of the germinal vesicle from the inner border of the oocyte chorion and oocyte diameter were used to calculate Polarization Index (PI) (distance from chorion divided by oocyte diameter), a morphologic criterion of oocyte ripeness (Dettlaff et al. 1993). The gonads of prespawning Atlantic sturgeon females ate hypertrophied and open to the coelome. Reproductive Conditions of Atlantic Sturgeon Ovarian folds contain a distinct clutch oflarge, uniformly sized, pigmented oocytes, contributing 8090% of the total ovarian mass. Fecundity was measured using a gravimetric method similar to that described by Bishai et al. (1974) and Keenlyne et al. (1992). The oocytes were counted in five-six subsamples (1-6 g wet weight) from different parts of the ovaries of ten females. The subsamples were cut fragments of the whole ovary (somatic tissue and germ cells), weighed (±0.1 mg), and fixed in 10% phosphate-buffered formalin. All pigmented oocytes were separated and counted in each subsample. The measurement unit was the number of oocytes per 1 g of the ovarian tissue (wet weight). Coefficients of variation among subsamples (within female) ranged from 3.1% to 6.5% (mean = 4.6%). Variation between females was 12.9% (mean ± SD: 107 ± 14 oocytes g-l, n = 10). Low variability among subsamples indicates reliable estimation of sturgeon fecundity. Ovarian weight and oocyte diameter measurements were available for six other females, but no sUbsamples for fecundity were taken. Their fecundity was reconstructed by dividing ovarian weight by "one oocyte weight" (i.e.,l g numbec 1 of ooeytes) calculated from the regression equation: oocyte weight = 0.00869 0.01313 X oocyte diameter (p < 0.05, r 2 = 0.81) that was established from females sampled for fecundity. Iteroparity of females was tentatively evaluated by the presence of atretic ovarian bodies containing dark pigment. Our recent observations with repeatedly spawned domestic white sturgeon, Acipenser transmontanus, indicate that these atretic follicles were derived from the oocytes that failed to ovulate during the previous spawning. Ryder (1888) described this follicle in Atlantic sturgeon as "degenerating ova with the pigment granules not aggregated at the surface, but irregularly distributed throughout the substance of the yolk mass." Faleeva (1965) reported the presence of similar atretic follicles in mature sterlet, Acipenser ruthenus. All gonads in this study were examined histologically (5-j.Lm paraffin sections stained by periodic acid Schiff stain). The description of histological stages and gametogenesis is not included in this paper. Some ovarian stages were previously described by Huff (1975) in the closely related Gulf sturgeon. Plasma Vitellogenin and Reproductive Hormones Total plasma calcium concentrations, reflecting concentrations of plasma vitellogenin (Vg), were measured by atomic absorption spectrophotometry (Perkin-Elmer spectrophotometer, 422.7 A wavelength, with a calcium hollow cathode lamp). 771 For validation, plasma samples from eight fish were analyzed by an enzyme-linked immunosorbent assay (ELISA) with polyclonal rabbit antibody to purified Vg of white sturgeon (Linares-Casenave et al. 1994). The assay format was similar to that developed for siberian sturgeon by Cuisset et al. (1991). The Vg level exhibited a strict linear relationship with total plasma calcium measured in the same samples. We used the linear regression Vg = 42.98 X calcium - 4136.65 (r2 = 0.97) to calculate plasma vitellogenin concentrations in the remaining females with known concentrations of plasma calcium. Plasma testosterone and estrogen (E2) were measured by the radioimmunoassays (RIA) of Gay and Kerlan (1978) and England et al. (1974), respectively (antisera were provided by G. D. Niswender). The RIA for testosterone is cross-reactive with ll-ketotestosterone and reflects total plasma androgens (A). Two sturgeon gonadotropins (GTH-1 and GTH-2) were measured by RIAs developed for white sturgeon by Moberg et al. (1991). The l7a,2013-dihydroxy-4-pregnen-3-one (17a,2013DHP) was measured by the RIA of Scott et al. (1982), and the antibody was a gift from Y Nagac hama (National Institute of Basic Biology, Okazaki, Japan). The following modifications were made in the assay. Endogenous plasma-binding proteins were denatured by addition of a 10-fold excess of 100% ethanol, the sample was centrifuged, and the supernatant decanted, blown to dryness with an air stream, and reconstituted in a phosphate-buffered saline, 0.1 % gel, pH 7.6 to the original sample volume. Four thousand counts per minute (CPM) of tritium label per assay tube was used. Recovery of tritiated 17a,2013-DHP from spiked serum samples was 90%. The assays detection limits were 20.00 ng ml- 1 for A, 0.05 ng ml- 1 for E2, 0.90 ng ml- 1 for GTH-1, 1.90 ng ml- 1 for GTH-2, 0.12 ng ml- 1 for 17a,2013-DHP, and 300 j.Lg ml- 1 for Vg. Age Determination Age was determined from cross sections (0.2-0.6 mm) of base portions of dried pectoral finrays. Sections polished with increasing grades of wet-dry sandpaper were mounted on slides and examined under a dissecting microscope with camera lucida and digital image analyzer (Nikon Microplan II) with a fine-point light cursor that was used to count the annuli and trace their continuity throughout the section plane. In reading and counting annuli we followed guidelines of Cuerrier (1951), Probst and Cooper (1954), Semakula and Larkin (1968), Sokolov and Akimova (1976), and Brennan and Cailliet (1991). Ageing was initially conducted on 85 fish by three independent readers who counted annuli on three replicate finray sections of each 772 J. P. Van Eenennaam et al. 54 TABLE 1. Body size, gonadosomatic index, and age of Atlantic sturgeon. Data are means, standard deviation, and sample size in parentheses. All means between the sexes are significantly different (p :5 0.05). Females Fork length (cm) 194.0 :!: 14.9 (28) Total length (cm) 217.9 :!: 15.8 (28) Body weight (kg) 72.7 :!: 20.3 (22) Condition factor (%) 0.94 :!: 0.11 (22) Gonadosomatic index (%) 13.84 :!: 5.72 (16) Age (year) 20:!: 5 (27) Males 161.7 181.5 37.3 0.83 :!: :!: :!: :!: 14.3 15.4 7.5 0.08 (66) (66) (48) (48) 48 42 ｾ 36 () >z 30 ::> 24 w 0 w cc L1. 18 12 3.97 :!: 1.13 (42) 15 :!: 2 (66) 6 oL-O--------'---------100-115 fish. Agreement between the readers was observed in 33-40% of all cases, and agreement within ± 1 yr was obtained for 84% of all fish. Maximum disagreement of 3 yr was observed in four fish (5%) and it was limited to older fish exhibiting some erosion at the periphery of the finray section, and in rare cases, in the center of the section (first annulus). Following this procedure and discussion of disagreements, ageing was completed by the senior author. Based on our work with semi-anadromous white sturgeon, ageing of Atlantic sturgeon is more accurate, and we subjectively estimate the potential error of 1-2 yr for fish ranging in age from 15 yr to 30 yr. Ageing of younger fish is more accurate, with very rare exceptions of the erosion of the first annulus. Data Analysis Individual condition factor was calculated as 100 X BWg/FLcm3, gonadosomatic index (GSI) as 100 X GW/BW. Data were analyzed by descriptive statistics, analysis of variance, and simple linear regressions. Significant differences between sample means were examined by the Student t-test (two samples) or the Fisher PLSD test (multiple sampies). A contingency table with a Fisher Exact Probability test was used to examine proportions of iteroparous females in different age groups. All comparisons were considered significant at p < 0.05. SAS software (SAS Institute) was used for statistical processing. Results REPRODUCTIVE CONDITIONS OF THE SPAWNING POPULATION The majority (56%) of mature males were caught during the first 10 d of June, and 82% of the females during the first, second, and third weeks of June. The second year sampling period was reduced by new regulations that restricted commercial fishing on the Hudson River to May 15 through June 15, 1993. Capture of spent males in early June and the presence of ovulatory and spent females during the first and third weeks of 115·130 130·145 145-160 160-175 175·190 190-215 215-230 FORK LENGTH (eM) _ B!iIJ MALES (N-66) FEMALES (N-2B) Fig. 2. Fork length distribution in the spawning cohorts of Atlantic sturgeon. June indicate that the breeding season of Atlantic sturgeon in the Hudson River extends from late spring to early summer. In 1992, fishermen reported the first capture of a ripe female on May 21 (RKM 101), and the last capture onJuly 9 (RKM 76). Data on body size, condition factor, gonadosomatic index, and age of broodfish are summarized in Table 1. Females are larger and older fish, compared to males. Body weight ranged from 47 kg to 125 kg in females and from 27 kg to 53 kg in males. The females had a significantly higher condition factor and gonadosomatic index, the latter range was 7-25% in females and 1-6% in males. Length and age histograms show substantial differences between the sexes, with different modal classes in males and females (Figs. 2 and 3). Age of all fish ranged from 12 yr to 36 yr, with approximately a 6 yr overlap between the two sexes. No males older than 19 yr and no females younger than 14 yr were present in our collection. 45 40 36 31 ｾ >z () w ::> 0 27 23 w 18 L1. 14 cc 5 oL-_-----"L.....IL-llOLJ 6 8 __ 10 12 14 16 18 20 22 24 26 28 30 32 34 36 AGE (Years) _ MALES (N-66) B!iIJ FEMALES (N-27) Fig. 3. Age distribution in the spawning cohorts of Atlantic sturgeon. The X-axis labels indicate the upper age class limit. 773 Reproductive Conditions of Atlantic Sturgeon 0,12 , - - - - - - - - - - - - - - - - - - - - - - - - , 0,10 0.08 T ! IT i' ｾ T III ___________ ｾ Ｍ ｪ ｬｾ 0.06 PRE-OVULATORY . « 2.68 w 2.61 e 0 I- w a: 2.46 « ::; 2.39 0 0.00 L-----'_---L_--'--_-'----_.L..-.----'_---L_--'--__-'------l 32 48 64 80 96 112 128 144 0 ::J I- OVULATORY 0 0 2.32 '----'_.....L_--'--_-'----_'-------'_---L_--'--_-'------l 160 170 176 182 HUDSON RIVER KILOMETERS Fig. 4. The relationship between individual oocyte polarization index (mean and standard deviation) and capture site of Atlantic sturgeon females in the Hudson River. The spaces between the horizontal dotted lines approximate different stages of maturity as determined by the position of germinal vesicle in the oocytes. Two females with mature oocytes (germinal vesicle breakdown) are assigned PI = O. The ovaries of 18 females were close to final ovarian maturation, with polarized oocytes and advanced migration of the germinal vesicle. Two females had matured oocytes (germinal vesicle breakdown). Four ripe females near final ovarian maturation were kept alive and induced to ovulate in the hatchery (personal communication, Jerre Mohler, United States Fish and Wildlife Service, Lamar, Pennsylvania). Three other sampled females had spent ovaries and one was immature, in a previtellogenic phase of ovarian development. No overripe females with atretic oocytes have been found in this study. All males were mature, with many males releasing milt during capture and handling on the boat. Seventeen females were examined for PI of the oocyte. Dettlaff et al. (1993) considered a PI below 0.07 as a threshold for oocyte maturation and ovulation in Russian sturgeon. The PI of Atlantic sturgeon females was 0.056 ± 0.030 (mean ± standard deviation, n = 17). Plot of individual PI against the distance of female capture from the river mouth is shown in Fig. 4. The majority of females caught around RKM 136 and upstream were within the ovulatory threshold zone, or in the process of spawning. One fish had freely flowing ovulated eggs and another had matured oocytes still attached to the ovary. In contrast, the oocytes of all four females caught between RKM 45 and RKM 64 were in a less advanced stage, with PI higher than 0.07. At this stage the sturgeon female does not respond with ovulation to hormonal injections and the oocytes do not mature in vitro under stimulation with progesterone (Dettlaff et al. 1993). It appears that females enter the Hudson River with incomplete germinal vesicle migration, which occurs 0 2.54 0 0 -----------------------!Hi--16 0 0 >- {) ! o Rsq .. 0.65 2.75 ::; lJ y - 1.0643 + .0078x 2.83 P<O.OOO5 a: w Iw 1. IlI Tl• 0,04 0.02 2,90 UNDERGOING POLARIZATION 188 194 200 206 212 218 224 230 FORK LENGTH (CM) Fig. 5. The relationship between mean oocyte diameter and female fork length in the spawning cohorts of Atlantic sturgeon in the Hudson River. rapidly as fish move to the spawning grounds. These data also suggest that spawning grounds of Atlantic sturgeon in the Hudson River are located upstream from the tidal saltwater front. The diameter of mature oocytes in sturgeon females ranged from 2.38 mm to 2.88 mm, and the relationship between oocyte diameter and fork length of females was significant (Fig. 5). Individual and relative (number of oocytes per kg of body weight) fecundity ranged from 0.4 million to 2.0 million and 7 thousand to 22 thousand, respectively, and exhibited a significant linear increase with fork length of females (Fig. 6). To examine the effect of iteroparity on fecundity and mature oocyte size, the females were grouped into four age classes, and proportion of presumably iteroparous fish in each group was determined by the presence of atretic ovarian bodies in histological sections (Table 2). The difference in the proportions of iteroparous females between the youngest (15-17 yr) and the oldest (24-29 yr) 2,10 0 Y - -5,888,552 + 35,580x 1.88 >- P<O.OOO5 1.66 l- ez 0" ::J 0 Rsq '" 0.78 1.43 0 w c: u. 0 ..J= 1.21 0 0.99 ｾ ;; e "" 0.77 0.54 0.32 ＧＭ ｟ Ｇ｟ ＭｌＺ ｟Ｎ Ｎ ｌＭ｟ＮｌＭ ｾＧＭ ｟ Ｇ｟ 170 _ 176 182 188 194 200 206 __'__....L-_.L-----' 212 218 224 230 FORK LENGTH (CM) Fig. 6. The relationship between individual fecundity and female fork length in the spawning cohorts of Atlantic sturgeon in the Hudson River. 774 J. P. Van Eenennaam et al. TABLE 2. Iteroparity, body size, and reproductive parameters of Atlantic sturgeon females in different age groups. Data are means and standard deviations. Means with different letters are significantly different (p s 0.05). Age (yr) 15-17 (n Iteroparity (proportion) Fork length (em) Body weight (kg) Fecundity (million) Relative fecundity (thousand kg-I) Oocyte diameter (mm) = 4) 18-20 (n = 5) 21-23 (n = 3) 24-29 (n ｾ 3) 0.00 184.2 a ::t: 6.4 55.0 a:!: 7.3 0.49 a ::t: 0.10 0.40 191.3 a :!: 8.8 71.4 ab ::t: 9.1 1.00 b ::t: 0.26 0.67 206.2 b:!: 1.9 82.9 b::t: 14.8 1.58 c::t: 0.35 1.00 213.3 b :!: 9.2 89.7 b::t: 21.2 1.67 c::t: 0.31 8.92 a :!: 1.78 2.53 a ::t: 0.12 14.02 b :!: 2.91 2.54 a ::t: 0.08 19.02 c:!: 0.80 2.67 ab ::t: 0.05 18.89 c :!: 2.83 2.78 b::t: 0.12 fish was statistically significant (Fisher's Test, 2 X 2, p = 0.029), although the multiple-test comparison for all four groups did not show significant difference (4 X 2, p = 0.066). Relative fecundity and oocyte diameter significantly increased with age and assumed iteroparity (Table 2). Data suggest that females reach their peak of egg production when they are older than 20 yr and spawn repeatedly. REPRODUCTIVE HORMONES, VITELLOGENIN, AND PLASMA CALCIUM Plasma concentrations of reproductive hormones, vitellogenin and calcium in the Atlantic sturgeon males and females are shown in Table 3. The females were subdivided into three distinct groups: gravid (with germinal vesicle intact), ovulating (mature oocytes and flowing eggs), and spent. Males exhibited low concentration of estrogen, moderate concentrations of gonadotropins (GTH-1 and GTH-2), and were similar with gravid females in plasma androgens and 17a,2013-DHP. They had no detectable vitellogenin in circulation, and their total plasma calcium was at the basal level of 80-120 J,.Lg ml- I . Females exhibited elevated concentrations of estrogen, gonadotropic hormones, vitellogenin, and calcium. In the ovulating females, concentrations of androgens and estrogen decreased, but concentrations of GTH-1, GTH-2, 17a,2013-DHP, vitellogenin, and calcium increased significantly compared with gravid females. The spent female (only one recently spawned fish was sampled) exhibited a decrease in sex steroids, calcium, and vitellogenin levels. Discussion Capture records support observations of Dovel and Berggren (1983) that males migrate to the spawning grounds earlier than females and probably spend a longer time in the river than females. Classen (1944) indicates that peak migration ofthe males in European Atlantic sturgeon (Acipenser sturio) occurs about 2-3 wk before migration of the females. Our observations on sturgeon captured with gill nets as well as anecdotal information from fishermen, indicate that there are always a few resident males in the spawning areas, but when a ripe female is captured, the number of captured males also increase. Similar patterns of aggregation of several males and one female at spawning were observed in Atlantic sturgeon by Dean (1893) in the Delaware River. The male and female cohorts differ in age and size composition, which was documented by Dovel and Berggren (1983) for the Hudson River and by Smith (1985) for the mid-Atlantic population. The difference in average age between the two sexes in our study was 5 yr, and the mean body weight of females compared to males was almost twice higher. The youngest reproductively active (recently spent) female in our study was 14 yr old, but the majority of ripe females were similar in age (20 yr) TABLE 3. Plasma concentrations of androgens, estradiol, gonadotropins (GTH-l, GTH-2), 17a,20[:l-dihydroxy-4-pregnen-3-one (17a,20[:l-DHP), total calcium, and vitellogenin in Atlantic sturgeon. Data are means, standard deviation, and sample size in parentheses. Means with different letters are significantly different (p s 0.05). Androgens (ng ml- I ) Estradiol (ng ml- I) GTH-l (ng ml- I ) GTH-2 (ng ml- I) 17a,20[:l-DHP (ng ml- I ) Calcium (fLg ml- 1) Calculated vitellogenin (fLg ml- 1 ) a b 126.83 0.33 3.37 1.73 0.50 105 Not available. Below detection level (300 fLg ml- I ). a ::t: 62.52 a ::t: 0.15 a ::t: 1.95 a :!: 1.67 a ::t: 0.27 a:!: 9 N.D.b (51) (51) (46) (46) (17) (47) (4) 11 1.56 ab ::t: 35.02 5.10 b ::t: 4.05 11.15 b ::t: 10.06 10.98 b :!: 13.77 0.24 a ::t: 0.24 156 b :!: 38 2,516 a ::t: 1,204 Spent female Ovulating females Gravid females Males (10) (10) (9) (9) (8) (10) (10) 56.63 b::t: 38.59 1.99 c ::t: 1.32 25.88 c ::t: 8.48 38.28 c::t: 12.26 3.70 b::t: 3.09 244 c:!: 131 6,533 b ::t: 5,824 (4) (4) (4) (4) (4) (4) (4) 18.40 (1) 0.60 (1) NN NN 1.32 (1) 127 (1) 1,322 (1) Reproductive Conditions of Atlantic Sturgeon and body size with previous observations of Dovel and Berggren (1983). In the South Carolina fishery, mature females ranged from 10 yr to 30 yr in age and from 145 yr to 233 cm in FL (Smith et al. 1984). Differences in age and size between sexes may be explained by younger maturity age and possibly, annual reproductive cycles of males, whereas the females may have greater individual variation in age at maturity and longer than 1-yrovarian cycles. In addition, the females may have an extended life span, determined by a longer duration of reproductive processes and the importance of larger body size for efficient egg production. Holcik et al. (1989) indicated that males of European Atlantic sturgeon are sexually active for a period of 8 yr whereas the reproductive life of females is extended to 30 yr. Both European and North American Atlantic sturgeon are fast growing and relatively short-lived species compared with other anadromous chondrosteans. However, because of the long history of the caviar fishery, the proportion of large and old females may be reduced at present time, compared to historic records indicating the average TL of gravid females to be 264 cm (Ryder 1888) . Previous descriptions of Atlantic sturgeon spawning grounds in the rivers were based on captures of ovulating and spent females. Borodin (1925) reported that sturgeon in the Delaware River gathered for spawning in an area with swift current and a hard clay bottom. The ovulating females in our study were captured in the area of RKM 136. The majority of ripe and spent females and males were captured between RKM 136 and 182, and the PI of the oocytes in ripe females as well as their plasma levels of reproductive hormones and metabolites implicate this part of the river as a major spawning area. Dovel and Berggren (1983) suggested that Atlantic sturgeon spawn between Croton Point (RKM 55) and Hyde Park (RKM 136). Our observations suggest that spawning grounds of Atlantic sturgeon in Hudson River are located further upstream, between Hyde Park and Catskill. It should be noted that the most upstream tidal salt front in the Hudson River reaches RKM 98 (Dovel and Berggren 1983), and chondrostean embryos and larvae are more sensitive to low salinity than other bony fish, due to water intake by the early embryo (Zotin 1953) and lower osmolality of maternal body fluids (McEnroe and Cech 1985). In addition, the egg and larval dispersal strategy of sturgeon (Dettlaff et al. 1993) may require that a sufficient portion of the tidal river downstream from the spawning site remain fresh water throughout the spawning season. Our estimates of fecundity in the Hudson River 775 females are in general agreement with other reports for North American Atlantic sturgeon (Ryder 1888; Vladykov and Greeley 1963; Smith 1985) and are very similar to data for European Atlantic sturgeon (Holcik et al. 1989). The preliminary data suggest a significant increase in fecundity and mature oocyte size with age, body size, and iteroparity of females. Higher fecundity of older iteroparous females may lend support to a fishery regulation based on a lower and upper size limit, as it is practiced on the Columbia River (Hanson 1992), with the maximum size to protect large fecund females from harvest. However, such regulations should be approached with caution since it may result in selective pressure on reproduction. Published information on reproductive endocrinology of chondrosteans is fragmentary, with most observations limited to cultured white and siberian sturgeon (Moberg et al. 1991; Pelissero and LeMenn 1991; Moberg and Doroshov 1992; Pavlick et al. 1993). Plasma concentrations of sex ｳｴ･ｲｯｩ､ｾ in the Atlantic sturgeon were similar with domestic broodstocks of white and siberian sturgeon during the reproductive season. Concentration of gonadotropins increased 3-4 times at ovulation (Table 3), particularly GTH-2 which we believe is involved in final ovarian maturation. The identity of the maturation-inducing hormone is not known in chondrosteans (Dettlaff et al. 1993), but 17a 2013DHP was a potent inducer of in vitro oocyte maturation in white sturgeon (Lutes 1985). The female Atlantic sturgeon with ovulated or matured oocytes had significantly elevated plasma concentration of this hormone, and ripe males also exhibited detectable concentrations (Table 3). The potential role of this hormone as a natural egg and sperm maturation steroid in chondrostean species deserves further investigation. Ripe and ovulating females of Atlantic sturgeon had relatively high plasma concentrations ofestrogen, vitellogenin, and calcium (Table 3) compared to wild white sturgeon in which vitellogenesis ceases sometime before spawning (Chapman et al. 1987; Doroshov et al. 1991). This difference may relate to the season of spawning migration in these two species. The white sturgeon migrate up the river during the late fall and winter and complete ovarian vitellogenesis 1-3 mo before spawning. North American Atlantic sturgeon and the closely related European species (Holcik et al. 1989) migrate in rivers during spring and summer seasons, and may complete vitellogenesis shortly before spawning. The cessation of vitellogenin uptake by the oocyte before ovulation and a long half-life of circulating vitellogenin in chondrosteans result in elevation of plasma vitellogenin and total calcium in the ovulated females (Table 3). 776 J. P. Van Eenennaam et at. In conclusion, our data describe normal reproductive conditions of the spawning population of Atlantic sturgeon in the Hudson River as indicated by ovarian development, gonadal hormonal levels, fecundity, and the absence of females with overripe eggs or atresia in the advanced clutch of pigmented ovarian follicles. Normal reproductive development in sturgeon is important but not the only factor determining the success of natural recruitment. The Hudson River and estuary provided excellent conditions for reproduction of the Atlantic sturgeon as indicated by its historically large population, topography of the river bed, availability of spawning substrate, high river flow, and significant production of zooplankton essential for larval survival (Moran and Limburg 1985; Dettlaff et al. 1993). Regulation of river flow, combined with dredging, diking, pollution, and climatic fluctuations could change the qualities and carrying capacities of Atlantic sturgeon spawning and larval habitats in the Hudson River. Maintenance of sturgeon spawning and nursery habitat in the Hudson Estuary deserves special consideration in management of the Atlantic sturgeon. ACKNOWLEDGMENTS This research was supported and funded by the Hudson River Foundation (grant number 007/91A). We would like to thank all the Hudson River fishermen who assisted in fish collection and sampling: Ian Burliuk, Jim Carey, Tucker Crawford, Cal Greenburg, Everett Nack, Steve Nack, Jon Powell, Ian Powell, and Ian Reed. Without their support and enthusiasm, this research would not have been possible. We appreciate the assistance of Thomas Lake who provided valuable advice and help during organization of the sampling program, and assistance from staff at the Hudson River Foundation and the Riverkeeper field station. Dr. Y Nagahama, National Institute of Basic Biology, Okazaki, Japan, provided the antibody for the 17a,2013-dihydroxy-4-pregen-3-one assay. Special thanks to Robert Boyle who inspired this study. LITERATURE CITED BISHAI, H. M., M. M. ISHAK, AND W. LABIB. 1974. Fecundity of the mirror carp, Cyprinus carpio L., at the Serow fish farm (Egypt). Aquaculture 4:257-265. BORODIN, N. 1925. Biological observations on the Atlantic sturgeon, Acipenser sturio. 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