Abstract

Ultrasonography has gained widespread use for diagnostic and research purposes in the medical and veterinary medical fields. Its possible use in aquaculture, however, is only beginning to be explored. The application of ultrasound to salmon conservation practice has great potential. In the United States, salmon captive breeding is a sophisticated practice that relies on advanced fish culture methods to rear valuable races of salmon under authorization from the National Marine Fisheries Service through the application of the Endangered Species Act. One of the challenges of working with these wild and rare fish is the risk of mortality associated with handling. Conventional methods of evaluating relative reproductive state range from the simple assessment of phenotypic characteristics to more invasive and costly serum hormone analyses. As a noninvasive, relatively non-stressful procedure, ultrasonography has been adopted as a critical tool in assessing sexual maturation in the endangered Sacramento River winter-run chinook salmon (Oncorhynchus tshawytscha) reared in a captive broodstock program.

Descriptions of ultrasonic anatomy of coho salmon (O. kisutch)³, rainbow trout (O. mykiss) and Atlantic salmon (Salmo salar)⁴, Pacific herring (Clupea harengus pallasi)², Atlantic halibut (Hippoglossus hippoglossus)⁵, and striped bass (Morone saxatilis)1 have been reported. These efforts have demonstrated the feasibility of using ultrasonography to identify gonads in fish. Yet few researchers have used this technique to follow reproductive development in multiple age and size classes of fish over time¹. Captive rearing of multiple year classes of fish throughout their entire life history provides a unique opportunity to evaluate sexual maturation over time in populations. Moreover, anadromous, semelparous fish such as the chinook salmon are particularly suited for such monitoring efforts since they spawn only once and sexual development is promoted by their reintroduction to fresh water.

The winter-run chinook salmon comprise a distinct population of chinook salmon in the Sacramento River, California USA. Adults generally leave the ocean and migrate through the Sacramento-San Joaquin Delta to the upper Sacramento River from December through June. Their spawning season extends from mid-April to August. Males typically return to spawn at 2-4 years of age, while females reach reproductive maturity between 3-4 years. In a captive broodstock environment where fish undergo smoltification and are reared in seawater, environmental cues are virtually absent and fish cannot volitionally return to fresh water. Rather, the fish culturist must determine when individual animals are approaching maturation and transfer them into fresh water.

In this study, we have used ultrasonography to evaluate relative reproductive development in the captive winter-run chinook salmon. Fish reared in two locations, the Bodega Marine Laboratory (Bodega Bay, California USA) and the Steinhart Aquarium (California Academy of Sciences, San Francisco, California USA), were included. Over 400 winter-run chinook salmon, representing 2 brood years (1993, 1994), were monitored in 1997. Beginning in January, fish were examined monthly while still in seawater and segregated based on gonadal size. By March, spawning candidates were transferred to freshwater systems. Ultrasonographic images were collected until time of spawning. Nearly 2,000 traverse scans were collected using a Pie Medical Scanner 200 with a 7.5 MHz probe, saved to disk using an external hard drive, and exported to a relational database. Various morphometric measurements, including gonadal width, thickness, and area circumference were taken from thermal prints of the images. Using these values, along with length and weight measurements of the fish, we are trying to develop a predictive model for determining time to spawning based on quantitative measures of gonadal size.

References

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2.  Bonar SA, Thomas GL, Pauley GB, RW Martin. 1989. Use of ultrasonic images for rapid nonlethal determination of sex and maturity of Pacific herring. North American Journal of Fisheries Management 9:364-366.

3.  Martin RW, Myers J, Sower SA, Phillips DJ, C McCauley. 1983. Ultrasonic imaging, a potential tool for sex determination of live fish. North American Journal of Fisheries Management 3:258-264.

4.  Reimers E, Landmark P. Sorsdal T, Bohmer E, T Solum. 1987. Determination of salmonids' sex, maturation and size: an ultrasound and photocell approach. Aquaculture Magazine 13:41-44.

5.  Shields RJ, Davenport J, Young C, PL Smith. 1993. Oocyte maturation and ovulation in the Atlantic halibut, Hippoglossus hippoglossus (L.), examined using ultrasonagraphy. Aquaculture and Fisheries Management 24:181-186.