Life-history Aspects of Moxostoma cervinum (Blacktip Jumprock) in the Roanoke River, Virginia

Abstract

Life-history aspects of Moxostoma cervinum (Blacktip Jumprock) were identified using specimens from recent collections and the Roanoke College Ichthyological Collection. The largest specimen examined was a female 161.27 mm SL and 66 months of age. Spawning appears to occur in May, with a mean of 2477.6 oocytes (SD = 2825.3) up to 1.54 mm diameter in gravid females. Sexual maturity appears to occur by 1-2 years of age in males and 2-3 years of age in females. Male to female ratio was not significantly different from 1:1. Chironomidae composed the bulk of the diet; while detritus, Trichoptera, Ephemeroptera, and Acari were important food items in multiple months. Weight of gut contents and proportion of Chironomidae as food items increased with size of specimens examined. INTRODUCTION Moxostoma cervinum (Cope) (Blacktip Jumprock) inhabits upland streams in the James, New, Roanoke, Tar, and Neuse river systems of Virginia and North Carolina (Jenkins and Burkhead 1994). Jenkins (1970), Buth (1978), and Smith (1992) all placed the species in the genus Scartomyzon with other small suckers inhabiting faster, shallower waters. However, most recent analyses embed the species within the genus Moxostoma (Harris et al. 2002, Doosey et al. 2010, Chen and Mayden 2012) with larger suckers often found in very different habitats. This phylogenetic placement means that understanding the biology and life-history of M. cervinum is important in identifying derived and ancestral character states, thus helping to interpret the substantial variation in the biology and life-history of the Moxostoma. Despite this importance, our understanding of this species’ life history is restricted to three paragraphs in the species account in Freshwater Fishes of Virginia, which gives limited details on aspects of diet, size and age at maturity, and timing of spawning (Jenkins and Burkhead 1994). The objective of this study is to document more detailed life-history aspects of M. cervinum from specimens collected throughout the year employing methods utilized in similar studies. MATERIALS AND METHODS Moxostoma cervinum were collected from the Roanoke River near Salem, VA (Roanoke County) between September 2010 and August 2011 by sampling daylight 1 Corresponding author: powers@roanoke.edu Virginia Journal of Science, Vol. 66, No. 4, 2015 http://digitalcommons.odu.edu/vjs/vol66/iss4 392 VIRGINIA JOURNAL OF SCIENCE hours near the end of each month using a Smith-Root LR-24 electrofisher and a 3.3-m x 1.3-m seine with 9.5-mm mesh. We supplemented our collections with specimens from the Roanoke College Ichthyological Collection (RC) for months when we collected few specimens (n < 15). Specimens were collected following Nickum et al. (2004) protocols, fixed in 10% formalin, rinsed with water and then stored in 45% isopropyl alcohol. A total of 154 specimens were examined in this study. Details on specimens examined (collection sites, collection dates, numbers of specimens taken, collector field numbers) are available from the authors upon request. Standard length (SL) of preserved specimens was measured to the nearest 0.01 mm using digital calipers. Total weight (TW) and eviscerated weight (EW) were measured by blotting the specimens dry and weighing to the nearest 0.001g on a digital analytical scale. Regressions by least sum of squares were performed for EW and SL to examine the relationship between length and weight. A two sample t-test was used to test for difference between male and female standard length. A chi-square test was used to detect a sex ratio different from 1:1. All statistical analyses were performed using Minitab 17 Statistical Software (Minitab, Inc., State College, PA) with alpha equal to 0.05. Specimens were aged using two methods. For all specimens, three scales were removed from the right dorsolateral portion of the body, mounted on a slide and examined under 40x magnification for annuli (see Bond 1996). If the three scales removed did not have the same number of annuli (e.g. regenerated scales that lack a focus), more scales were removed until a clear consensus number of annuli was identified. For specimens with a standard length >119 mm, a single opercle was removed, prepared and analyzed following Beckman and Hutson (2012). Each opercle was removed from the left portion of the body, set in boiling water for 10 minutes, then set in bleach for another 10 minutes to facilitate the removal of excess tissue and then allowed to air dry until annuli were clearly visible. Annuli were read by locating the presence of an opaque region near the edge of the opercle, and counting each opaque region as a single annulus; the number of annuli present was determined by two observers. This method, in comparison to scale annuli data, revealed that the number of annuli on the scales underestimated the age of specimens three years of age or greater and agreed with the scale data for specimens less than three years of age. Therefore, the number of annuli on the opercle was solely used to estimate the age of specimens three years of age or greater. Specimens less than 12 months of age were counted as 0+, specimens 12-23 months were counted as age 1+, specimens 24-35 months were counted as age 2+, specimens 36-47 months were counted as age 3+, specimens 48-59 months were counted as age 4+, and specimens greater than 60 months were counted as age 5+. The proportion of all specimens examined represented by each age class was calculated to approximate the age-class distribution of the population. A t-test of age in months was used to test for differences in lifespan among sexes. Gonads were examined to determine sex, removed from each specimen, and weighed to the nearest 0.001 g. Gonadosomatic Index (GSI) was calculated for all specimens by dividing gonad weight by EW. One-way analysis of variance was performed to test for differences in GSI among specimens of the same sex collected from different months. In gravid females, fully yolked, mature oocytes were counted, and five representative oocytes were measured to provide an approximation of ova size Virginia Journal of Science, Vol. 66, No. 4, 2015 http://digitalcommons.odu.edu/vjs/vol66/iss4 LIFE HISTORY OF Moxostoma cervinum 393 and number (Heins and Baker 1988). Regression of SL as a predictor of number of mature oocytes was performed to test the influence of specimen size on fecundity. Due to GSI values peaking in May, declining in June, and reaching a minimum level in July, we used May as the month of spawning for estimating age of specimens. The anterior third of the gastrointestinal tract was opened and its contents were removed and weighed using a digital analytical balance and recorded to the nearest 0.001 g. Weight of gut contents for specimens with empty guts was recorded as 0. Food items were counted and identified to the lowest taxonomic category possible following Thorp and Covich (1991) and Merritt and Cummins (1996). Detritus was noted as being present or absent in an individual specimen. The number of identifiably different food items in each specimen was recorded as variety of food items. One-way analysis of variance was performed on weight of gut contents, variety of food items, and percent Chironomidae to test for differences in feeding throughout the year. Regressions by least sum of squares were performed for SL and weight of gut contents, SL and variety of gut contents, and SL and percent Chironomidae to test the influence of size on feeding. RESULTS Eviscerated weight increased with standard length (r = 88.78%, P < 0.0001) and is described by the model EW = (SL) x 0.4952 – 29.05. Females were larger than males (P < 0.0001) with the mean size of females 105.57 mm SL (SD = 33.26) and males 85.02 mm SL (SD = 25.46). The smallest specimen examined (37.94 mm SL) was collected in January, had zero annuli and appeared to be eight months of age. The largest specimen examined (161.27 mm SL) was a female collected in November, had five annuli, and appeared to be one of the oldest specimens examined at 66 months of age (Figure 1). All specimens examined for annuli had zero to five which corresponded to annuli forming near the end of winter or early spring in specimens up to 66 months of age. Mean lifespan was greater (P = 0.003) for females (24.76 months, SD = 16.11) than males (18.08 months, SD = 11.31). There was also a slightly skewed sex ratio of 1:1.69 in favor of females; however, the difference in the number of males and females was not significant (P = 0.938). Standard length increased with age in months (r = 83.99, P < 0.0001) and is described by the model LOGSL = (LOG age in months) x 0.4906 + 1.3465. Of the 154 collected specimens, 25.97% were age 0+, 35.06% were age 1+, 23.38% were age 2+, 6.49% were age 3+, 7.41% were age 4+, and 1.95% were age 5+ (Figure 2). Monthly GSI was not uniform for females (females P = 0.005), but did not differ significantly for males (P = 0.116). Individual GSI was highest in May for females (0.135) (Figure 3) and males (0.052) (Figure 4). Maximum GSI values declined in June to 0.002 for males, and for females reaching a minimum value of 0.00453 in July. Mean GSI values were lowest during June and July for both females (June = 0.01, SD = 0.013; July = 0.008, SD = 0.004) and males (June = 0.003, SD = 0.0005; July = 0.004, SD = 0.004). Mean GSI values were highest for both sexes during November (females = 0.05, SD = 0.04; males = 0.03, SD = 0.008). Elevated GSI values generally persisted from fall months through spring for both sexes (Figures 3 and 4). Mature oocytes were 0.6-1.54 (mean = 0.96, SD = 0.19) mm in diameter and numbered from 560 to 15,441 (mean = 2477.6, SD = 2825.3). The smallest female with mature oocytes was 24 months of age and had a SL of 93.3 mm. All females collected during spring Virginia Journal of Science, Vol. 66, No. 4, 2015 http://digitalcommons.odu.edu/vjs/vol66/iss4 394 VIRGINIA JOURNAL OF SCIENCE FIGURE 1. Standard length