2012 NORTHEASTERN NATURALIST 19(2):217–228 Age, Growth, and Reproduction of the Eastern Mudminnow (Umbra pygmaea) at the Great Swamp National Wildlife Refuge, New Jersey Frank M. Panek1,* and Judith S. Weis2 Abstract - Umbra pygmaea DeKay (Eastern Mudminnow) is one of four species of Umbridae in North America. There is little published life-history information on the species within its native range, particularly on age, growth, and reproduction. This study focuses on these aspects of the life history of this fish at the Great Swamp National Wildlife Refuge in Morris County, NJ. A total of 336 fish of seven species was collected from June 1978 through May 1979, with the Eastern Mudminnow comprising 74% of the total. The average annual growth increment in total length for the Eastern Mudminnow was 15.3 ± 2.06 mm, with age-1 fish averaging 40 mm total length and age-5 fish, the oldest collected, averaging 107 mm total length. The length-weight relationship was log10W = -5.291 + 3.182 log10TL mm for males and log10W = -4.999 + 3.032 log10TL mm for females. We observed no statistically significant sexually dimorphic differences in length-weight relationships in this population. The ratio of females to males increased from a low of 0.6 (predominance of male fish) at age-1 to a high of 4.6 (predominance of females) at age-5. Annual mortality for age 2–5 fish ranged from 40–76% with a mean of 59 ± 13%. Age-specific fecundity estimates ranged from 250 eggs/female at age-1 to 2168 eggs/female at age-5. The relationship of number of mature ova to age was best described by the exponential function y = 149.29e0.5287x, where y = age-specific fecundity and x = age in years. Ova ranged from 0.1–0.2 mm in diameter in June and July and averaged 1.41 ± 0.1 mm (range = 1.29–1.62 mm) in early February prior to spawning. Peak spawning occurred in mid-April at temperatures of 9–12 °C, and all females were spent by late April (13–15 °C). Introduction Umbra pygmaea DeKay (Eastern Mudminnow) is one of four species of Umbridae in North America (Wilson and Veilleux 1982). The Umbridae along with the Esocidae (pikes and pickerels) form the primitive suborder Esocoidei (Nelson 1972). The Eastern Mudminnow is a freshwater species common to Atlantic slope coastal lowlands from southern New York to northern Florida (see Verreycken et al. 2010 for a range map) and can typically be found in slow-moving, mud-bottomed, and highly vegetated streams, swamps, and small ponds (Lee et al. 1980). It has been documented from some oligohaline (salinity < 5%) habitats in Delaware (Wang and Kernehan 1979) and also in some lower Piedmont watersheds along the Atlantic slope (Jenkins and Burkhead 1994). The species is also known to hybridize with Umbra limi Kirtland (Central Mudminnow) in areas of 1US Geological Survey, Leetown Science Center, 11649 Leetown Road, Kearneysville, WV 25430. 2Rutgers - The State University, Department of Biological Sciences, 195 University Avenue, Newark, NJ 07102. *Corresponding author - fpanek@usgs.gov. 218 Northeastern Naturalist Vol. 19, No. 2 the Hudson River, NY where the two species are sympatric (Schmidt and Daniels 2006). The Eastern Mudminnow has also been widely introduced in Europe and has become invasive in watersheds in France, Belgium, Germany, and the Netherlands (Verreycken et al. 2010). Both U. pygmaea and U. limi have been used as laboratory animals for the detection of cytogenetic effects of chemical pollutants in aquatic environments (Hooftman and Vink 1981) and both species have been suggested as possible agents to control Aedes mosquitoes in marshes and slowmoving vegetated streams (Slavin et al. 1977). There is little published life-history information on the Eastern Mudminnow within its native range. The purpose of this investigation was to document some age, growth, and reproductive characteristics of the Eastern Mudminnow at the Great Swamp National Wildlife Refuge (Great Swamp NWR) and to compare these to that of its close relative, the Central Mudminnow. Field Site Description This study focused on Eastern Mudminnow populations located at the US Fish and Wildlife Service Great Swamp NWR in Basking Ridge in north-central New Jersey. The refuge was established as an area to provide migration, nesting, and feeding habitat for migratory birds. The western half of the refuge is managed for a wide variety of wildlife, and the eastern portion was designated by the United States Congress as a National Wilderness Area in 1968. The refuge is within the lowland section of the Passaic River drainage of the Lower Appalachian Province in Morris County, NJ. The topography of the area is typically flat and comprised of woodlands, old fields, and marshes. Soils are moderately rich and contain a high clay component typical of Piedmont lowlands. Streams and marshes are primarily muck-detritus, and aquatic vegetation is moderately dense. Stream and marsh acidity varies from pH 6.1–6.8. Fish collections were made in the Black Brook system (see the refuge map at http://www.fws.gov/northeast/ greatswamp/). Methods During this investigation, we made every effort to minimize potential sample bias from gear selectivity. Jones (1973), in a study of the Central Mudminnow in Fish Lake, MN, discussed sampling and gear selectivity problems associated with the collection of umbrids. In this study, we used data of fish captured with a variety of sampling gear including minnow traps, seines, and hand/dip nets from June 1978 through May 1979. All fish were identified to species, counted, preserved in 10% formalin, and later transferred to a 30–40% ethanol solution for laboratory processing. Laboratory processing included determinations of total length (mm), weight (g), sex, and stage of maturity. Fish lengths were measured to the nearest tenth of a millimeter with a Helio caliper, and weights were measured to the hundredth of a gram on a Mettler analytical balance. Since scales of the genus Umbra do not lend themselves to age-growth studies (Peckham 1955), growth 2012 F.M. Panek and J.S. Weis 219 was determined by back-calculations of age using interpretation of annulus marks on the saccular otolith. Criteria for annuli formation followed that of Westman (1941) for the Central Mudminnow. Westman (1941) verified annulus marks by length-frequency methods, and Peckham (1955) did so using direct observations of annuli marks with fish of known age. Paired otoliths were dissected and cleared in 50% glycerine-water mixture for a minimum of two weeks, then examined with reflected light against a black background with a dissecting microscope. The face of the otolith rather than the cross section showed annuli markings most clearly, which permitted their use without polishing. In this confi guration, annuli appeared as clear bands, and periods of active growth were opaque. Standardized measurements were made at 30x magnification with an ocular micrometer. Distances from the origin to successive annuli and the otolith margin were recorded along with observations of total length and sex. The relationship of fish length to otolith radius was determined by linear regression (McDonald 2009) and served as the basis for back-calculation of length at time of annulus formation, using procedures outlined in Ricker (1970). Age-specific mortality estimates were obtained by semi-graphical analysis of the catch-curve derived from the number of fish captured by age (Gallucci et al. 1996, Ricker 1975) and the application of linear regression techniques (McDonald 2009) to the descending right limb of the catch curve. A table of exponential functions was used to obtain instantaneous mortality and annual mortality for those age-classes fully recruited to the sampling gear (Ricker 1975). Lengthweight relationships were calculated for males and females by use of paired total length (TL) and weight (g) measurements taken from samples collected from November through January. Only specimens stored in formalin for less than one month were used in calculating log10 length and weight relationships. Guidelines for determination of sex and stages of gonadal maturation were modified from Nikolsky (1963). Five gross stages of gonadal condition were considered: immature (Stage I), resting (Stage II), mature (Stage III), ripe (Stage IV), and spent (Stage V). Measurements of egg diameters were recorded monthly and used to determine the maturation sequence and time of spawning. Fecundity was estimated by counting all Stage-III and Stage-IV eggs. Eggs were counted on a gridded Petri plate under a dissecting microscope at 10–30x magnification, and egg diameters were measured with an ocular micrometer. Results A total 336 fish of seven species was collected from June 1978 through May 1979. The Eastern Mudminnow was common, comprising 74% (248 individuals) of the catch (Table 1). Other species captured included Enneacanthus obesus (Banded Sunfish), Enneacanthus gloriosus (Bluespotted Sunfish), Ameiurus nebulosus (Brown Bullhead), Notemigonus crysoleucas (Golden Shiner), Lepomis gibbosus (Pumpkinseed), and Esox americanus americanus (Redfin Pickerel). The relationship of total length (mm) to a standardized measure of the saccular radius was established from a subsample of 88 of the 248 fish aged (Fig. 1) and used to estimate back-calculations of total length at annulus formation for 220 Northeastern Naturalist Vol. 19, No. 2 123 mudminnows. The relationship of otolith radius to total length was characterized by the linear equation ro = 0.42 (TL mm) + 2.21 (n = 88, r2= 0.81), where ro is the otolith radius and TLmm is total length in millimeters. These back-calculated lengths at annulus formation were compared to length-at-age observations made during the course of fish collections. There was little variation in length-at-age, Table 1. Species composition and abundance of fishes captured at the Great Swamp NWR. Species name Common name # captured % abundance Umbra pygmaea DeKay Eastern Mudminnow 248 73.8 Notemigonus crysoleucas Mitchell Golden Shiner 30 8.9 Esox americanus americanus Gmelin Redfin Pickerel 22 6.5 Ameiurus nebulosus Lesueur Brown Bullhead 16 4.8 Enneacanthus obesus Girard Banded Sunfish 9 2.7 Lepomis gibbosus L. Pumpkinseed 7 2.0 Enneacanthus gloriosus Holbrook Bluespotted Sunfish 4 1.3 Total 336 100.0 Figure 1. Relationship of standardized unit measurements of otolith radius as measured with an ocular micrometer to the total length (mm) of the Eastern Mudminnow at the Great Swamp NWR. Table 2. Mean total length (mm) at capture and back-calculated total length (TL mm) at annulus formation for the Eastern Mudminnow from the Great Swamp NWR. Mean TL mm Back-calculated TL mm Age Class n at capture n 1 2 3 4 5 0 4 37 ± 1.9 - - - - - - 1 158 53 ± 6.3 47 42 - - - - 2 45 70 ± 3.3 38 39 58 - - - 3 27 82 ± 3.0 27 39 57 72 - - 4 10 96 ± 3.6 7 38 57 73 84 - 5 4 109 ± 3.0 4 42 57 81 95 107 Weighted mean 40 58 73 88 107 ± s.d. 1.7 0.4 4.0 5.5 na 2012 F.M. Panek and J.S. Weis 221 which is consistent with the November through January collection times when growth in fishes is expected to be slow due to winter temperatures (Table 2). We noted that annulus formation at the Great Swamp NWR occurred in late May and early June at water temperatures of 16–17 °C. The average annual growth increment in total length was 15.3 ± 2.06 mm, with age-1 fish averaging 40 mm TL and age-5 fish, the oldest collected, averaging 107 mm TL (Table 2). The descending right limb of the catch curve (Fig. 2) was described by the linear regression equation y = - 0.3487x + 2.5311, where x = age in years and y = log10 abundance. The coefficient of determination (R2) for the relationship was 0.998 with 3 degrees of freedom. Using the table of exponential functions (Ricker 1975) the annual mortality for age 2–5 fish ranged from 40.0–71.6% with a mean of 58.6% ± 13.4% (Table 3). Length-weight relationships were determined for 97 males and 84 females. Males ranged from 42.8–107.4 mm TL, and females from 41.1–107.7 mm. Allometric growth of males and females was characterized by logarithmic transformations of weight (W) in grams and total length (TLmm). The length-weight relationship was log10W = -5.291 + 3.182 log10 TLmm for males and log10W = -4.999 Figure 2. A standard catch-curve of log10 abundance by age group for the Eastern Mudminnow at the Great Swamp NWR. Table 3. Estimates of rates of annual mortality (%) and annual survival (%) derived from a catch curve of the Eastern Mudminnow collected at the Great Swamp NWR. Age n Log10n % annual mortality % annual survival 0 4 0.602 - - 1 158 2.199 - - 2 45 1.653 71.6 28.4 3 27 1.431 40.0 60.0 4 10 1.000 62.9 37.1 5 4 0.602 60.0 40.0 Mean ± s.d. 58.6 ± 13.4 41.4 ± 13.4 222 Northeastern Naturalist Vol. 19, No. 2 + 3.032 log10 TLmm for females. We observed no statistically significant sexually dimorphic differences in length-weight relationships in this population. Of the 248 individuals collected, 221 were sufficiently mature for sex determination, and of these, 53 females were staged for gonadal maturity. We found that 96.4% of the males and 90.0% of the females were sexually mature at age-1. The smallest sexually mature female and male collected were 42.0 mm and 40.0 mm TL, respectively. The ratio of females to males increased from a low of 0.6 (predominance of male fish) at age-1 to a high of 4.6 (predominance of females) at age-5 (Fig. 3). This ratio increased linearly with age (R2 = 0.96, P = 0.004). We measured the cycle of egg maturation by monitoring monthly egg diameters and ambient water temperatures (Fig. 4). Two distinct groups of ova were present. One group ranged from 0.1–0.2 mm in diameter and was clearly oogonia reserved for future spawnings. The second group, which developed into mature ova, ranged from 0.1–0.2 mm in diameter in June and July and averaged 1.41 ± 0.1 mm (range = 1.29–1.62 mm) in early February prior to spawning (Fig. 5). Ripe fish were collected with minnow traps under 10–15 cm of clear ice in early March. By late March, swamp waters had warmed to 5–7 °C, and fish in spawning condition could be easily captured with hand nets in 12–20 cm of water. Male fish generally extruded milt spontaneously with handling, and 25% of the females collected in late March were spent. There was no evidence of batch spawning in the Eastern Mudminnow at the Great Swamp NWR. At two collection sites, eggs were found deposited on aquatic vegetation and leaf litter concurrently with fish in spawning condition. Peak spawning occurred in mid-April at water temperatures of 9–12 °C, and by late April all females were spent. Water temperatures in late April ranged from 13–15 °C. Although considerable effort was expended, no direct observations of spawning were made. If nests were present, they were indistinguishable from the normal bottom substrates and were poorly Figure 3. Sex ratios (females:males) by age for the Eastern Mudminnow at the Great Swamp NWR. 2012 F.M. Panek and J.S. Weis 223 constructed. Lateral movements of fish from stream channels into shallow and warmer flooded marshes were the only movements observed during spawning. Fecundity was determined from 53 sexually mature females of known total length and characterized by the equation F = log10 0.014 TLmm + 1.86 (R2= 0.76), where F = fecundity or number of stage III and IV ova and TLmm is total length in millimeters. Mature eggs used in these fecundity estimates were derived from prespawning adults collected from mid-December through mid-April. During this period, mature egg diameters ranged from 1.00–1.62 mm. As noted above, our observation of time of annulus formation in late May or early June coincides with post spawning of the Eastern Mudminnow. Age-specific fecundity estimates ranged from 250 eggs/female for age-1 to 2168 eggs/female at age-5 (Fig. 6). The relationship of number of mature ova to age was best described (R2 = 0.998) by the exponential function y = 149.29e0.5287x, where y = age-specific fecundity and x = age in years. Figure 4. Cycle of egg maturation in the Eastern Mudminnow at the Great Swamp NWR as related to ambient water temperatures and time of year. Figure 5. Relationship (log10) of the number of mature eggs to total length in the Eastern Mudminnow at the Great Swamp NWR. 224 Northeastern Naturalist Vol. 19, No. 2 Discussion There is surprisingly little information in the literature on the life history of the Eastern Mudminnow within its native range along the Atlantic coastal slope of North America. Most of the accounts and information available is scattered in descriptive studies or regional fish identification guides (Jenkins and Burkhead 1994, Murdy et al. 1997, Smith 1985) or in basic natural history accounts and observations of spawning and reproduction, some of which date to 1890 (Breder and Rosen 1966). As noted by Breder and Rosen (1966), it was also unclear from some of the earliest naturalist observations whether the authors were describing the Eastern Mudminnow or a closely related species, the Central Mudminnow. The Eastern Mudminnow at the Great Swamp NWR represents a significant component of the fish community and comprised 74% of our catch in flooded marshes. This result is similar to the findings from the New Jersey Pinelands where the Eastern Mudminnow, along with the Banded Sunfish, are the most frequently occurring species (Bunnell 2006) and likely are ecologically important to the functioning of these systems. Like the Central Mudminnow, the Eastern Mudminnow is hardy and often lives in marginal habitats and withstands extreme environmental temperatures, periods of hypoxia, and low pH. Early observations by naturalists assumed that mudminnows hibernate in mud during the winter (Abbott 1870). However, this assumption was later refuted by Peckham and Dineen (1957) and was certainly refuted during the course of our investigation, where active Eastern Mudminnow were trapped under ice. In the Netherlands, where it is an introduced species, the Eastern Mudminnow survives low alkalinity, low pH, and relatively high metal concentrations (Dederen et al. 1986). The fish survives in hypoxic conditions by utilizing its swim bladder as an accessory respiratory organ (den Hartog 1990, Tonn and Paszkowski 1987). Evidence suggests that mudminnow abundance is inversely related to the presence of predatory species (Dederen et al. 1986) and to the number of fish species present (Panek 1981). Figure 6. Age-specific fecundity of the Eastern Mudminnow at the Great Swamp NWR. 2012 F.M. Panek and J.S. Weis 225 Observations of growth in length and/or weight at age for the Eastern Mudminnow within its native range are mostly limited to field observations. Jenkins and Burkhead (1994) reported specimens in Virginia attaining 91mm TL, Pardue and Huish (1981) reported a maximum length of 149 mm TL in North Carolina, while Mansueti and Hardy (1967) reported a maximum TL of 152 mm in Maryland tributaries to the Chesapeake Bay. Our study is the first reported that provides back-calculated and age-at-capture growth data for the Eastern Mudminnow within its native range. Mudminnows at the Great Swamp NWR ranged from 24 mm TL (age-0) to 123 mm TL (age-5). The results agree with the observations made by other investigators for the Eastern Mudminnow (Jenkins and Burkhead 1994, Mansueti and Hardy 1967, Pardue and Huish 1981) and are consistent with growth observations for the Central Mudminnow in Indiana (Peckham 1955), New York (Robinson et al. 2010, Westman 1941), and Minnesota (Jones 1973) (Table 4). While sexually dimorphic growth was documented for the Eastern Mudminnow in the Netherlands (Dederen et al. 1986) with females growing faster than males, we did not observe any statistically significant growth differences, and growth in both sexes was allometric in this study. At the Great Swamp NWR, we documented peak spawning of the Eastern Mudminnow in mid-April at water temperatures of 9 –12 °C, and by late April all females were spent. This finding agrees well with the observations summarized by Breder and Rosen (1966) and Jenkins and Burkhead (1994). While ill-defined nests have been described by others (Breder and Rosen 1966), we did not make such an observation despite considerable efforts to locate nests. The number of mature eggs produced by the Eastern Mudminnow was related to total length (Fig. 5), with age-specific fecundity from 250 eggs/female at age-1 (42 mm TL) to 2168 eggs/female at age-5 (107 mm TL) and with mature eggs averaging 1.41 mm in diameter. These observations were similar to those made by Jones et al. (1978), where fecundity was estimated at 31–2566 eggs/female for the Central Mudminnow (Peckham and Dineen 1957). We utilized a catch-curve derived from our sampling and age determinations (Table 2) to estimate age-specific survival and mortality. Survival estimates were Table 4. Total length (mm) at age for Umbra pygmaea (Eastern Mudminnow) at the Great Swamp NWR, and growth of Umbra limi (Central Mudminnow) at age from locations in Indiana, New York, and Minnesota (C = both sexes combined). Total length (mm) at age Locality Sex 1 2 3 4 5 Source Umbra pygmaea Great Swamp NWR, NJ C 53 70 82 96 109 Present study Umbra limi Judy Creek, IN C 69 80 86 - - Peckham and Dineen 1957 Red Creek, NY C 51 64 76 85 - Westman 1941 Fish Lake, MN M 64 89 102 120 - Jones 1973 Fish Lake, MN F 64 86 128 142 - Jones 1973 First Bisby Lake, NY C 41 55 75 85 90 Robinson et al. 2010 Little Moose Lake Outlet, NY C 43 56 68 68 71 Robinson et al. 2010 226 Northeastern Naturalist Vol. 19, No. 2 variable (28–60%) and averaged 41 ± 13%, which is a crude estimate based on the level of sampling, capture efficiencies, and relative abundance of the fish. However, it is likely indicative of survival of fishes in harsh environments such as the shallow, highly vegetated marshes where this fish is found. Pardue (1993) found survival of Acantharchus pomotis Baird (Mud Sunfish) to range from 20–28% in similar habitats in swamp streams in North Carolina. Quist and Guy (2001) found survival of Semotilus atromaculatus Mitchell (Creek Chub) in prairie streams to be about 28%, and Sammons and Maceina (2009) found a 38% annual survival of Lepomis auritus L. (Redbreast Sunfish) and a 29% annual survival of Lepomis macrochirus Rafinesque (Bluegill) in southern Georgia streams. The Eastern Mudminnow is a common and important member of the fish community at the Great Swamp NWR and in similar habitats along the Atlantic slope of North America. This paper provides some life-history information on this species. More research on the Eastern Mudminnow in shallow-water marsh habitats and swamp streams and in acid-water pineland streams, bogs, and swamps is needed to better understand its ecological significance. Acknowledgments Sincere appreciation is expressed to the late Dr. James D. Anderson for his support and guidance during the early stages of this work and to Claire E. Ryan (Panek) for her support in the field work. The cooperation of the New Jersey Division of Fish and Wildlife, the US Fish and Wildlife Service, and the staff at the Great Swamp NWR is greatly appreciated. We also thank the editor and several reviewers for their helpful comments on manuscript drafts. This work was supported in part by a Grant-in-Aid of Research from Sigma Xi - The Scientific Research Society. Disclaimer: Use of trade, product, or firm names does not imply endorsement by the US Government. Literature Cited Abbott, C.C. 1870. Mud-loving fishes. American Naturalist 4:385–391. Breder, C.M., Jr., and D.E. Rosen. 1966. Modes of Reproduction in Fishes. American Museum of Natural History, Natural History Press, New York, NY. Bunnell, J.F. 2006. Fish assemblages in pinelands streams draining active cranberry bogs, abandoned cranberry bogs, and forestland. Pp. 113–135, In R.A. Zampella, J.F. Bunnell, K.J. Laidig, and N.A. Procopia (Eds.). Monitoring the Ecological Integrity of Pinelands Wetlands. 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