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Using ultrasonography to identify late-stage maturity in rainbow trout


June 4, 2015
By David Scarratt

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Morphometric measurements by ultrasonography have been used to determine gonad and follicle size in many species of fish for purposes of sex identification and estimating the stage of maturation. However, Gregory M. Weber and Timothy D. Leeds of the National Center for Cool and Cold Water Aquaculture, Kearneysville, West Virginia, have been using a portable ultrasound system (SonoSite MicroMaxx, L25e/13-6 MHz transducer) to identify female rainbow trout (Oncorhynchus mykiss) that are close to ovulation and to assess testis size in neo-males treated with salmon pituitary extract to induce maturation at or shortly after 1-year of age.

In each situation, information needed to assess maturation status can be attained by examining a sonogram screen image, which allows for sorting fish in real time.

Ultrasonography can be used to identify female trout that are typically within about three days of ovulation, based on a dramatic decrease in echogenicity in which the eggs turn from light gray in the sonogram to black. Estimation of testis size by ultrasonography is hampered by the irregular shapes of testes in neo-males, but fish sufficiently mature to yield viable milt can usually be identified.

As part of an effort to induce maturation of male rainbow trout at one year of age, 230 neo-males at ~7 months post-hatch were given thrice weekly injections of salmon pituitary extract (SPE: 1.5 mg/kg). Approximately 95% survived until either their testes were removed or until the end of the experiment at month eleven. Ultrasound was used at month ten to identify potential breeders, and of these, over 60% had developed testes, and >85% of those selected had motile sperm.

Previously untreated 14-month old neo-males were also injected thrice weekly with SPE (1.5 mg/kg) to evaluate the effect on testis size. The testes of control (un-injected) fish remained below 6g in weight and had gonado-somatic index (GSI) of 0.6%. Testes of treated fish reached 23g with a GSI of 2.4% after four weeks of treatment, but did not exceed 31g testes weight and 3.1% GSI after eight weeks of treatment.

In subsequent correspondence with Hatchery International, Weber explained their particular interest in determining whether observed differences in the time from when the change in ovarian echogenicity takes place to when the fish ovulates reflect differences in egg quality. They are still waiting for egg quality data.

Their work on SPE and testis development combines what had already been shown years ago that: 1) SPE can induce maturation, and 2) ultrasonography can be used to assess testis size.

 For more information contact Dr. Weber at: greg.weber@ars.usda.gov

Reason for the study

The study with SPE and ultrasonography was designed to provide data that closely matched the interests of a particular hatchery that was constantly running short of neo-males. The hatchery wanted a better assessment of what to expect before committing to purchasing an ultrasound machine and pursue permits that would be required to administer SPE.

A second objective was to show the diversity of benefits that might be derived from having an ultrasound machine available in a hatchery. Although the SPE treatment might be too much to take on at this point, having an ultrasound machine might be worthwhile if hatchery efficiency could be increased. For example, knowing before they are sacrificed which fish have the largest testes would allow more efficient use of the neo-male population, similarly, having a more precise knowledge of when a female ovulates would help avoid over-ripening.

Using ultrasonography to help sort broodstock early in the reproductive season would give a better estimate of potential egg production and save labour by only spawn-checking those fish that would mature that same year, and possibly identifying females with poor egg quality before the eggs are pooled for fertilization.

These decisions have still to be made.

– DJS


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