A RAPID ASSESSMENT APPROACH FOR EVALUATING SILVER CARP GENDER

WOLF Michael C.， PHELPS Quinton E.， SEIBERT Justin R. and TRIPP Sara J.

(1. Department of Natural Resources， Fish and Wildlife Division， 204 Main Street East Baudette， Minnesota 56623， USA;2. West Virginia University， Division of Forestry and Natural Resources， 307C Percival Hall， Morgantown， West Virginia 26505， USA; 3. Southeast Missouri State University， Biology Department 1 University Plaza Cape Girardeau，Missouri 63701， USA; 4. Big Rivers and Wetlands Field Station， Missouri Department of Conservation，3815 East Jackson Boulevard， Jackson， Missouri 63755， USA)

Abstract: Silver carp Hypophthalmichthys molitrix were introduced into the U.S. to control water quality in aquaculture ponds. From this point of origin， silver carp escaped into nearby rivers through multiple flood events. Because of their documented negative effects on native biota， silver carp have been labeled as problematic. Therefore， evaluating the biology and ecology of these non-indigenous species is critical. Multiple parameters are needed to evaluate silver carp populations (length， weight， age， and sex). Furthermore， developing methods for rapidly acquiring these data are needed. In relation to sex determination， sexual dimorphism was observed where males exhibit distinct pectoral fin ray features. Specifically， males have pronounced ridges or a “rough patch” on the dorsal surface of pectoral fins. Therefore， to test if this was an applicable way of determining silver carp gender; silver carp were collected from Midwestern U.S. rivers(N=2015) in the fall of 2011 (N=870)， spring of 2012 (N=645)， winter of 2013—2014 (N=202) and summer 2015 (N=323) via electrofishing. For each silver carp collected， presence (e.g.， rough patch) or absence (e.g.，smooth) of pronounced ridges on the top side of the pectoral fins was recorded， and an incision was made in the body cavity to identify gender. Based on the results of our evaluation， gender was correctly identified over 99% of the time (2006 out of 2015) based on the pectoral fin dimorphism. In the samples taken in the winter of 2013—2014 and summer of 2015， accuracy for silver carp shorter than 300 mm and longer than 800 mm was 53.7% (19 out of 41) while accuracy for silver carp between 300 mm and 800 mm total length was 98.9%(289 out of 292). This study provides a rapid assessment approach for evaluating silver carp gender.

Key words: Invasive species; Gender assessment

Aquatic invasive species are believed to be a major threat to native aquatic species in North America[1，2].Aquatic invasive species are known to have several negative impacts including depletion of resources like phytoplankton and zooplankton resulting in changes to lower trophic level communities[3—5]， compete with native species for similar resources[6—8]， cause native species extinctions[9]， and threats to human health by carrying parasites[10]. Due to the negative impacts invasive fish can have on native species， it has become essential for fisheries managers to attempt to manage invasive fish populations. Management of fish populations typically requires accurate population evaluation which in turn requires accurate assessment of multiple parameters (length， weight， age and sex)[11].Furthermore， developing methods for rapidly acquiring these data are needed and will be necessary for future efforts directed at managing these invasive species populations.

One major invasive species currently spreading through North America is the silver carp (Hypophthalmichthys molitrix). Silver carp were introduced into the U.S. to control water quality in aquaculture ponds[12]. From this point of origin in Louisiana， silver carp escaped into nearby rivers through multiple flood events[12]. Therefore， evaluating the biology and ecology of these non-indigenous species is critical for management decisions. Length and weight are obtainable simply by measuring the total length and weighing the entire fish on a simple scale but， age and sex can sometimes be more time consuming to determine.Aging structure validation was conducted for the lapilli otolith and a relatively quick method for removing this aging structure has been described[13]. In relation to sex determination， sexual dimorphism was observed where males exhibit distinct pectoral fin features. Specifically， males have pronounced ridges or a“rough patch” on the dorsal surface of pectoral fins，while female pectoral fins were smooth.

Use of external characteristics as indicators of sex in fishes is not a broadly used concept but one historically applied to other cyprinids[14]. Specifically，for some species external characteristics appear in males during spawning season. This can be a useful technique for individuals collectiong broodstock during spawning season but ineffective during the remainder of the year. To our knowledge， no quantitative evaluation has been made of pectoral fin ridges as sexually dimorphic characteristics in silver carp.Therefore， this was tested as an applicable technique for determining silver carp gender.

1 Methods

Silver carp were collected from Midwestern U.S.rivers (i.e.， Illinois， Missouri， Mississippi and Ohio rivers) in the fall of 2011， spring of 2012， winter of 2013—2014 and summer 2015 via electrofishing. For each silver carp collected， presence (e.g.， rough patch)or absence (e.g.， smooth) of pronounced ridges on the top side of the pectoral fins was recorded， and an incision was made in the body cavity to verify gender.The data collected were analyzed with a Chi-square test for independence to determine if rough patch presence (observed) significantly differs from actual sex (expected) at an alpha level (α=0.99)[15]. To further explore the accuracy and effective size range of this method， total length was also recorded to the nearest mm for silver carp sampled in 2015.

2 Results

A total of 2015 silver carp were collected from Midwestern U.S. rivers (i.e.， Illinois， Missouri， Mississippi and Ohio rivers)， 870 in the fall of 2011， 645 in the spring of 2012， 202 in the winter of 2013—2014 and 323 in the summer of 2015 via electrofishing. Male and female fish were correctly identified among all seasons over 99% of the time (Fig. 1) for fish between 300 mm and 800 mm total length. Male silver carp in the spring of 2012 were correctly identified with presence of a rough patch 354 out of 355(99.72%). Male silver carp in the summer of 2015 were correctly identified with presence of a rough patch 180 out of 181 (99.44%). Male silver carp in the fall of 2011 were correctly identified with the presence of a rough patch 520 out of 522 (99.62%).Male silver carp in the winter of 2013—2014 were correctly identified with the presence of a rough patch 127 out of 127 (100%). Female silver carp in the spring of 2012 were correctly identified with the absence of a rough patch 289 out of 290 (99.65%). Female silver carp in the summer of 2015 were correctly identified with the absence of a rough patch 114 out of 117 (97.44%). Female silver carp in the fall of 2011 were correctly identified with the absence of a rough patch 347 out of 348 (99.71%). Female silver carp in the winter of 2013—2014 were correctly identified with the absence of a rough patch 75 out of 75 (100%).

This technique was accurate for over 99% of the sample (spring males=99.72%， summer males=99.44% fall males=99.62%， winter males=100%，spring females=99.65%， summer females=97.44%fall females=99.71% and winter females=100%). The pattern of rough patch presence did not differ from the pattern of actual gender (χ2=0.0909， α=0.99 withDF=3 and Critical χ2=0.115).

Fig. 1 Accuracy of predicting the gender of silver carp based on the presence or absence of a rough patch on the dorsal surface of the pectoral fin ray

Chi-square test for independence revealed the presence of a rough patch did not differ from the expected pattern given the true fish gender (χ2=0.0992，α=0.99 withDF=3 and Critical χ2=0.115). In the samples taken in the winter of 2013—2014 and summer of 2015， accuracy was 53.7% (19 out of 41) for silver carp shorter than 300 mm and longer than 800 mm while accuracy was 98.9% (289 out of 292) for silver carp between 300 mm and 800 mm total length.Therefore， the optimal range of accuracy for this method appears to be between 300 mm and 800 mm.

3 Discussion

Given the results of this study the presence or absence of a rough patch on the dorsal surface of pectoral fins is an effective method (especially between 300 and 800 mm) for quickly and accurately determining the sex of silver carp. The speed and accuracy of this rapid assessment technique will aid in the efficiency of studies that analyze silver carp populations.We believe that silver carp shorter than 300 mm have not developed enough of a rough patch for accurate assessment and the silver carp longer than 800 mm have a rough patch that has been smoothed out by time and female pectoral fins may become wavy and therefore are difficult to delineate male from female.Silver carp less than 300 mm in total length could be difficult to determine gender regardless of method because gonads may be underdeveloped. This trait does appear to be present in bighead carpHypophthalmichthys nobilisbut was not observed in other similar cyprinids (grass carpCtenopharyngodon idellaand common carpCyprinus carpio). Additional studies need to be done to better understand this characteristic’s viability for bighead carp.

We recommend that identification of gender(i.e.， dissection) along with presence or absence of ridges should be conducted repeatedly to eliminate a slight learning curve. Overall， this study provides researchers and managers with a rapid technique for identifying silver carp gender. Gender is vital for assessing silver carp populations and is used in more complex models to assess silver carp populations. Access to more complex models and larger datasets may prove to be a vital tool in the future management of silver carp.

Acknowledgements:

We would like to thank Levi Solomon from the Illinois Natural History Survey with collection of silver carp.