N55 2015 Southeastern Naturalist Notes Vol. 14, No. 4 J.J. Hoover , A.W. Katzenmeyer, J. Collins, B.R. Lewis, W.T. Slack, and S.G. George Age and Reproductive Condition of an Unusually Large Bighead Carp from the Lower Mississippi River Basin Jan Jeffrey Hoover 1,*, Alan W. Katzenmeyer1, Jay Collins1, Bradley R. Lewis1, W. Todd Slack1, and Steven G. George1 Abstract - On 24 February 2014, commercial fishers caught an unusually large Hypophthalmichthys nobilis (Bighead Carp) from an oxbow lake in northwestern Mississippi. We examined it to determine age, gonadal development, and fecundity. The specimen was 1316 mm total length, 49.7 kg, and 11 years old. It had asymmetric ovaries that collectively represented 15.7% of the body weight, with an estimated 1.9 to 2.7 million eggs, ~40% of which were mature. The data we collected for size, age, and fecundity were all at or near maximum values known for the species. This specimen demonstrates that for life-history studies and demographic models to be representative of introduced populations, rare or hard-to-catch large specimens are required due to their influence on estimates of longevity, mortality, and fecundity. Introduction. Hypophthalmichthys nobilis (Richardson) (Bighead Carp), a highly invasive fish in North America, is the subject of recent models that explore population dynamics and evaluate possible management techniques (e.g., Tsehaye et al. 2013). Such models require information inputs on longevity and fecundity but have, to date, focused almost exclusively on smaller (<20 kg), younger fish (360 km. Historically, Moon Lake supported commercial fisheries for Paddlefish, which were massive (i.e., up to 70 kg) and abundant (i.e., up to 100/purse-seine haul) in the early 20th century (Hussakof 1911). Presently in Moon Lake, Paddlefish are still common and large; in 2014 we routinely collected 2–5 individuals in a single gillnet in less than an hour, including many specimens 25–35 kg (J.J. Hoover, unpubl. data). Numbers of Paddlefish harvested during a single week in 2011 and in 2014 were 609 and 333, respectively, although total numbers caught (including Paddlefish that were released) were substantially greater (Mississippi Department of Wildlife Fisheries and Parks, Jackson, MS, unpubl. data). 1US Army Engineer Research and Development Center, EE-A, 3909 Halls Ferry Road, Vicksburg, MS 39180-6199. •Corresponding author - Jan.J.Hoover@usace.army.mil. Manuscript Editor: Carol Johnston Notes of the Southeastern Naturalist, Issue 14/4, 2015 2015 Southeastern Naturalist Notes Vol. 14, No. 4 N56 J.J. Hoover , A.W. Katzenmeyer, J. Collins, B.R. Lewis, W.T. Slack, and S.G. George We processed the Bighead Carp in the field. We measured the total length (TL) to the nearest mm and the weight to the nearest 0.03 kg, then removed, weighed, and placed the gonads on ice for freezing. We also removed and iced the head and pectoral rays and froze them. In the laboratory, we removed plostcleithrum from the head and sectioned it along the medial portion of the structure (Fig. 2; Johal et al. 2000, Seibert and Phelps 2013). We sectioned the fin rays along the basal portion of the ray to a thickness of approximately 0.70 mm. We used a Buehler Isomet® saw with a diamond wafering-blade (Buehler, Lake Bluff, IL) to make the sections, which were mounted in Flo-Texx (Fisher Scientific, Hampton, NH). Growth rings visible in boney structures of Bighead Carp have not been validated in US populations (e.g., chemical marking, fishes of known age), but their clarity and regularity in pattern are consistent with annular increments (Nuevo et al. 2004, Schrank and Guy 2002); thus we assumed their utility for determining fish age. We viewed annuli with transmitted light using an Olympus SZX16® zoom stereomicroscope equipped with an Olympus DP72® camera system and cellSens® imaging software. Three readers, all experienced in aging river-fishes, e.g., Acipenseridae (sturgeon), Paddlefish, and Hypophthalmichthys spp. (Asian carp), agreed in advance what constituted an annular mark and independently aged the fish by counting annuli. They agreed upon an age by reaching consensus on initially assigned ages. All three readers were aware of the size of the fish. We expressed robustness of the carp as a ponderal index, better known as a condition factor (Carlander1969). Condition factor (KF) was calculated as: KF = [W/L3]*105, where W is weight in grams, L is total length in mm, and 105 is a scaling factor to bring the values close to unity. Lower values indicate fish that are slim, and higher values indicate fish that are comparatively plump; the number is interpreted as a measure of “well-being” of the fish. Figure 1. Bighead Carp collected from Moon Lake, 24 February 2 014. N57 2015 Southeastern Naturalist Notes Vol. 14, No. 4 J.J. Hoover , A.W. Katzenmeyer, J. Collins, B.R. Lewis, W.T. Slack, and S.G. George We used 2 techniques to evaluate the potential effect of a rare old fish on estimates of mortality rate. One technique compares maximum age (tmax) to instantaneous rate of mortality rate (Z) based on a least-squares-regression analysis of 84 population studies of fishes (Hoenig1983). The relationship is: ln (Z) = 1.46 - 1.01(ln[tmax]) Another technique uses age-frequency data to determine annual mortality rate (M) based on numbers observed in each age class susceptible to collection techniques (Robson and Chapman1961). It is calculated as: M = 1 - T / (ΣNi + T - 1), in which T is the sum of coded ages times their frequency and ΣNi is the sum of the age frequencies. Each of these calculations was performed for a population in which the maximum age of a carp was 6 years (Tsehaye et al. 2013) and in which the maximum age of a carp was equivalent to the age determined from our specimen. We removed ten 2.0–2.5-g sub-samples of eggs from each ovary and examined them using a dissecting microscope with 160x magnification. We counted the eggs and sorted them into 2 categories: mature (large, yellow) or immature (small, white). We selected 3 immature and 3 mature eggs from each and determined their diameters by measuring the maximum and minimum dimension with an ocular micrometer and taking the average of the 2 measurements because the eggs were all nearly spherical. We estimated total egg number by multiplying egg density (number per g ovary) times total wei ght of the ovary. Figure 2. Boney structures used to age Bighead Carp: (upper, left) primary pectoral ray; (lower, left) secondary pectoral ray; (right) postcleithrum. Growth rings ar e marked with a solid black circle. 2015 Southeastern Naturalist Notes Vol. 14, No. 4 N58 J.J. Hoover , A.W. Katzenmeyer, J. Collins, B.R. Lewis, W.T. Slack, and S.G. George Size. The carp was 1316 mm TL, weighed 49.7 kg, and had a condition factor of KF = 2.18. Reviews of the scientific literature for Bighead Carp in Asia report sizes of 1300– 1500+ mm TL and weights of 40–50+ kg (Jennings 1988, Kolar et al. 2007, Schofield et al. 2005), but those data are not strongly supported by field studies of populations in the US. Field surveys suggest that large specimens for the Missouri River and Middle Mississippi River are typically 867–1200 mm TL, with weights of 7–19.3 kg (Nuevo et al. 2004, Papoulias et al. 2006, Schrank and Guy 2002, Wanner and Klumb 2009,). Records from Illinois, Oklahoma, Tennessee, and Mississippi lakes include larger fish with weights of 22–48.8 kg (IDNR 2011, IGFA 2014, Long and Nealis 2011, MDWFP 2014,). Condition factors are not reported in most studies, but in the free-flowing Middle Mississippi River and in Pool 26, condition factors averaged 1.03, and ranged from KF = 0.58 to KF = 2.04 (Nuevo et al. 2004). Our specimen from Moon Lake is larger, heavier, and more robust than any of the fish previously reported from the region. Age. The Moon Lake specimen was 11 y old (Fig. 2). Boney structures had identical numbers of annuli, but annuli were best seen in the postcleithrum, and least clearly in the primary pectoral ray, partly due to the lumen in the center of the ray. There was no disagreement among readers on counts and no differences among aging structures. Spacing of annuli indicated that growth was rapid to age 3 or age 4, moderate to age 7, and comparatively slow from age 8 to age 11. Maximum reported age in the US for all populations is typically 7 (Tsehaye et al. 2013), but a 39.8-kg, 9-y-old specimen was reported from Grand Lake, OK (Long and Nealis 2011). Our specimen is older than these US records and, although rare, has measurable effects on estimates of mortality. Mortality rates based exclusively on longevity (sensu Hoenig 1983) decline by 32%: Z = 0.70 for tmax of 6, and Z = 0.38 for tmax of 11. Mortality rates based on age frequency (i.e., catch curves) decline less. In the Middle Mississippi River, the morality rate for eighty-four 4–6-y-old Bighead Carp was M = 64% (Nuevo et al. 2004). This value corresponds to age frequencies of 42–50 for age 4, 22–38 for age 5, and 4–12 for age 6. If a single age-11 fish is added so that n = 85, the mortality rate declines slightly to M = 61%. Addition of a second age-11 fish results in M = 59%, a third in M = 56%, and a fourth in M = 54%. Even when they account for less than 5% of the total number sampled, the presence of a few old fish reduces estimates of mortality by 2–10%. Reproductive condition. The Moon Lake Bighead Carp was gravid. The left ovary weighed 4.2 kg and the right ovary weighed 3.6 kg, collectively representing 15.7% of the weight of the fish. Typically, gonads of mature Bighead Carp in the US range from 7–13% body weight, but prior to our study, one individual had been documented with gonads >15% body weight (Papoulias et al. 2006, Schrank and Guy 2002). The previously mentioned Grand Lake, OK, specimen also had gravid ovaries (J. Long, US Geological Survey, pers. comm.). In winter, Bighead Carp ovaries typically are small and represent less than 7% body weight (Papoulias et al. 2006). The Moon Lake fish, in contrast, had well-developed ovaries with large, yellow, mature eggs interspersed with smaller, white, immature eggs. Bi-modal development of eggs in Bighead Carp has been confirmed previously (Schrank and Guy 2002) and supports the contention that individual fish can spawn multiple times over a single protracted spawning season (Kolar et al. 2007, Papoulias et al. 2006). Prior to obtaining this specimen, we have observed fish with immature eggs later in the calendar year, suggesting that Bighead Carp in the lower Mississippi River may spawn in the spring and possibly again in late summer. All of our estimates of total number of eggs exceeded 1.8 million, with total-egg numbers comparable but relative number of mature eggs different between the ovaries (Table 1). Mature eggs averaged 1.43 (SD = 0.05) mm in diameter, twice the mean size of N59 2015 Southeastern Naturalist Notes Vol. 14, No. 4 J.J. Hoover , A.W. Katzenmeyer, J. Collins, B.R. Lewis, W.T. Slack, and S.G. George immature eggs, which was 0.68 (SD = 0.04) mm. Because 45% of the eggs from the left ovary were mature (and large), and only 35% of the eggs of the right ovary were mature, egg density was 8% lower in the left ovary than the right: 283.4 (SD = 23.4) vs. 309.6 (17.5) eggs g-1. The larger size of the left ovary, however, more than offset this difference, so that the mean total number of eggs was slightly higher in the left ovary than the right: 1.2 million vs. 1.1 million. Values for total egg number extrapolated from minimum, mean, and maximum egg densities were 1.9, 2.3, and 2.7 million eggs, respectively. These numbers are appreciably higher than egg counts previously documented in the US (less than 0.8 million; Schrank and Guy 2002) and comparable to or higher than maximum egg counts of 1.1–1.8 million reported in reviews of Asian literature (Jennings 1988, Kolar et al. 2007, Verigin et al. 1990). The presence of 2 developmental classes of eggs, 1.4 and 0.7 mm in diameter, is consistent, however, with a previous US study demonstrating modal sizes of 1.3 and 0.5 mm and suggesting protracted, multiple spawns of individual females within a year (Schrank and Guy 2002). The near absence of larger, older, highly fecund fish in US field studies is not a trivial issue. Attempts to develop estimates of harvest rates that are effective for population control necessitate complete characterization of population structure and its reproductive potential. One such effort used an estimate of Bighead Carp mortality that was comparatively high (M = 0.68), in part because it assumed a longevity of only 6–7 y (Tsehaye et al. 2013). That model indicated that harvest rates equivalent to or in excess of 70% of the population would be necessary for population control. The Grand Lake, OK, specimen (Long and Nealis 2011) and the Moon Lake, MS, specimen (this report) suggest that mortality rates are in fact lower than originally believed and, if so, harvest rates for effective control would need to be correspondingly greater. Researchers conducting field surveys should take this into account when sampling and use gear of appropriate strength and size to effectively sample larger fish. Presently, we do not know whether such fish are genuinely rare or simply difficult to catch. Acknowledgments. The Mississippi Department of Wildlife Fisheries and Parks invited us to participate in fieldwork and provided assistance; Garry Lucas (retired) and Larry Pugh (chief) provided data on Paddlefish populations. James P. Parker and Walter Ray collected the Bighead Carp and brought it to us. Glenn Parsons and Bryan Cage assisted in the field. The Aquatic Nuisance Species Research Program provides funds for our Asian Carp research. The Chief of Engineers approved this paper for publication. Our thanks to all of the people and agencies mentioned above. Table 1. Ovary data for a Bighead Carp measuring 1316 mm TL that was collected 24 Feb 2014, from Moon Lake, MS. Ovum density is expressed as number of ova per g of ovarian tissue (ova g -1 ). Left Ovary Right Ovary Weight of ovary (g) 4252 3629 Mean (SD) immature ovum density 156.8 (19.0) 199.7 (15.8) Mean (SD) number immature ova 666,828.2 (80,950.3) 724,767.5 (57,294.2) Mean (SD) mature ovum density 126.6 (10.3) 109.9 (15.1) Mean (SD) number mature ova 538,351.0 (44,006.4) 398,727.7 (54,772.5) Mean (SD) total ovum density 283.4 (23.4) 309.6 (17.5) Mean (SD) total number of ova 1,205,179.2 (99,315.4) 1,123,495.2 (63, 499.0) Total fecundity, minimum ovum density 1,879,121 Total fecundity, mean ovum density 2,328,668 Total fecundity, maximum ovum density 2,742,559 2015 Southeastern Naturalist Notes Vol. 14, No. 4 N60 J.J. Hoover , A.W. Katzenmeyer, J. Collins, B.R. Lewis, W.T. Slack, and S.G. George Literature Cited Carlander, K.D. 1969. Handbook of Freshwater Fishery Biology. Volume one. Life-History Data on Freshwater Fishes of the United States and Canada, exclusive of the Perciformes. 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