THE RELATIONSHIP BETWEEN WHITE STURGEON CAVIAR YIELD AND STAGE OF MATURITY
Farming white sturgeon for caviar began in 1995 with approximately 47 females which yielded 152 kg of caviar. During the past fifteen years the harvest of caviar has gradually increased to about 10,000 kg/year, from over 3,000 processed fish. There are currently three major farms, both in California and Idaho, that culture white sturgeon for caviar. Preliminary data from processed females of multiple age classes (6-14 yrs of age) revealed that processing caviar during the late stages of ovarian maturity (close to spawning) but prior to the initiation of atresia, would produce the greatest yields of large eggs and high quality caviar. Stage of maturity is currently determined by the measurement of the oocyte polarization index (PI), requiring sampling of eggs through a small (8-10mm) abdominal incision. Oocyte PI is a ratio of the distance of the germinal vesicle from the animal pole to the oocyte maximum diameter, and is an indicator of spawning readiness. The focus of this study was to evaluate variation of caviar yield in relation to oocyte PI and egg diameter. During 2008, one hundred fish of the 2001 year class, representing an early maturing cohort, were processed for caviar. Twenty randomly chosen fish were sampled each month from February through June. As expected, mean PI decreased and the egg size increased over time. However, mean caviar yield did not change over the five month period due to high individual variation in any given month. Larger females produced more caviar but the correlation between caviar yield and fish size was low (r = 0.29). Caviar yield expressed as a percent body weight, was not correlated with either egg size or PI (r < 0.02). The highly variable yield and the lack of correlation with oocyte PI were likely caused by high ovarian adiposity, particularly in larger fish. Large amounts of adipose tissue surrounding individual eggs in the ovigerous folds and large lobes of fat attached to the ovary increased processing time and decreased yield. The largest females (38 5 kg; mean SD) tended to have the larger, fattier, and lower yield ovaries, with less than 50% of the ovary weight as tinned caviar and a mean yield of 2.2 kg per fish. The smaller females (30 6 kg) had leaner ovaries with higher yields of tinned caviar per weight of ovaries (> 65%) and per individual fish (2.9 kg). The ovarian fattiness was a major factor affecting caviar yield in the early maturing females, likely due to the energy budget on high energy diets favoring adiposity.
Selecting for females with the largest egg size, during any given harvest month, is one plausible method for optimizing caviar yield. Several minimally invasive sampling techniques, such as short wave near infrared (SWNIR) and fourier transform infrared (FTIR) spectroscopy and plasma steroid profiles, are currently being tested and seem to be just as accurate. A research program focusing on production of fish with leaner ovaries would improve caviar yield and profitability in white sturgeon farming.
Selecting for females with the largest egg size, during any given harvest month, is one plausible method for optimizing caviar yield. Several minimally invasive sampling techniques, such as short wave near infrared (SWNIR) and fourier transform infrared (FTIR) spectroscopy and plasma steroid profiles, are currently being tested and seem to be just as accurate. A research program focusing on production of fish with leaner ovaries would improve caviar yield and profitability in white sturgeon farming.