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Using Ultrasound to Sex Catfish

Summary

Ultrasound images can be utilized to improve the accuracy of sex determination of catfish. Sexually immature female catfish can be identified with 80 percent to 100 percent accuracy using ultrasound after the first growing season. Ultrasound sex determination is a rapid method of sorting catfish for hybridization programs.

Introduction

Hybridization of the blue catfish (Ictalurus furcatus) male and the channel catfish (Ictalurus punctatus) female is becoming a viable commercial alternative to production of channel catfish as a foodfish. The current practice requires that a population of blue catfish be grown for five years or more to reach sexual maturity. The males are then killed to remove testicular tissue which is macerated to release sperm for in vitro fertilization of channel catfish eggs. New technology in this area is needed to allow conservation of animals and improved efficiency. Ultrasound has been utilized to improve reproductive management of a number of animal species. Past studies document its use to observe gonads noninvasively for fish species including striped bass, salmon, and herring. Catfish are usually sexed visually when they reach sexual maturity, four to five years for blue catfish and three to four years for channel catfish. However, with the commercialization of the blueXchannel hybrid, earlier gender determination would allow culling the female blue catfish or the male channel catfish at the optimum marketing age and thereby minimize maintenance costs.

Materials and Methods

Channel catfish and blue catfish of various ages were examined with a portable ultrasound unit to detect the presence of ovary or testes. Confirmation of fish sex was by visual observation of genital openings or by dissection. Sexually mature catfish of either species were not dissected since the ovary tissue was obvious by ultrasound. All fish were anesthetized with MS-222 prior to handling.

The ultrasound unit was an Aloka 500V model SSD500. Probes of 3.5MHz or 7.5MHz were utilized depending on the size of the catfish. The 3.5 MHZ probe was utilized for larger fish because of the thickness of abdominal muscling and the larger viewing area this probe provided.

Best ultrasound images were obtained when a plastic container was lined with foam rubber to reduce ultrasound echo from the sides of the container. The probe was held 1 to 2 cm above the catfish skin but still underwater to obtain an image that focused on the desired tissues. Images were studied starting at the pelvic fins and moving anteriorly.

Results and Discussion

Female catfish were identified by the presence of maturing eggs inside ovaries in sexually mature individuals. Sexually immature females were identified by paired masses of ovary-like tissue which extended anteriorly from the pelvic girdle. The ovary tissue images were approximately 1 cm diameter at the widest point in catfish of 1 year of age. Males had no paired ovary-like tissue images but showed a disorganized tissue mass which included the testes, lobes of visceral fat, and intestine. Testes tissue was observed as paired images lying on top of the swim bladder in certain individuals. However, a determination of female or not female was utilized to select males.

Experience with the technique indicated that two or three sessions are required to obtain a skill level that exceeds the accuracy of visual sex determination. Successful determination of 67 percent for males and 80 percent for female catfish at one year of age was obtained with ultrasound on the second date of examination but the accuracy of sex determination improved by the third date (Table 1 below). Visual methods were about 60 percent accurate due to the variation in primary and secondary sex characteristics. The best ultrasound images were obtained from catfish that had been fasted for 48 hours or more before examination.

Table 1. Sex determination of channel catfish and blue catfish using ultrasound imaging.

Date

Fish Age ; Length (cm)

Fish Weight (kg)

Ultrasound Sex Determination

% Accuracy

US Male /Male

US Female/ Female

Male

Female

1/10

Year 3-5 ; 54 to 64

2.0-4.0

3/3

3/3

100

100

1/10

Year 2-3 ; 27.5-45

.17-.84

4/9

4/8

44

50

1/10

Year 1 ; 13.5-16.5

.02-.04

0/0

3/3

--

100

2/5

Year 2-3 ; 29-46

.19-1.0

6/9

4/5

67

80

2/19

Year 2-3 ; 31-58

.23-2.2

2/2

3/3

100

100

5/12

Year 5-61 ; 71-87

4.6-8.4

6/6

8/8

100

100

5/12

Year 5-62 ; 66-80

3.6-6.8

0/0

6/6

--

100

1 Blue catfish sexed on 5/12. 2 Channel catfish sexed on 5/12.

Implications

Ultrasound can be utilized as a non-invasive method for sex determination of catfish as early as eight months of age. That would allow early sorting of female and male catfish for future breeding or hybridization and catfish which were not needed in the reproductive program could then be marketed when desired between 12 to 24 months of age. Since maintenance of catfish past the first growing season as brood stock costs about $1.25 per fish per year, approximately $5.00 per catfish could be saved by sorting brood stock at the end of the first growing season.

As the blueXchannel hybrid is commercialized, ultrasound should be considered as a means of brood stock selection. Gender determination by ultrasound would be more accurate than visual methods. Egg development within the ovary may be measured non-invasively with ultrasound by measuring egg diameter on the screen. Future research will compare egg diameter measured with ultrasound with in vitro fertilization success in catfish in order to properly time hormone-induced ovulation for hybridization.