Factors contributing to low embryo survival in Atlantic salmon (Salmo salar)

Embryo mortality of Atlantic salmon, Salmo salar, has been increasing for more than a decade in the State of Maine, a leading producer of this species in the U.S. Increasing embryo mortality not only creates a financial bottleneck for farms, but also prevents the sale of surplus eggs as an additional source of revenue. Blood and egg samples were collected at three Maine Atlantic salmon farms from female broodstock at the time of spawning over two spawning seasons. Correlative factors for reduced embryo survival were investigated by measuring egg and maternal plasma concentrations of 17β-estradiol (E2), 11-ketotestosterone (11-KT), testosterone (T) and calcium, as well as maternal hepatic ethoxyresorufin-O-deethylase (EROD) activity and fork length. Significant positive correlations (P≥0.001) were found between maternal plasma concentrations of E2 and 11-KT and embryo survival. Interestingly, there was no correlation with egg concentrations of sex steroids and embryo survival, suggesting that embryo mortality does not likely rest with the maternal deposition of sex steroids into the egg, but with another hormone regulated process related to egg assembly, ovulation or post-ovulatory aging.

Introduction:

The United States (U.S.) currently imports more than 90% of its seafood, resulting in a seafood trade deficit of more than 10 billion dollars; this is the greatest agricultural trade deficit in the U.S.. With capture fisheries at or exceeding their maximum sustainable yield for nearly all commercially important species, the growing trade deficit has encouraged the growth of domestic aquaculture. Aquaculture currently accounts for approximately 50% of U.S. seafood imports, with Atlantic salmon (Salmo salar) being the second leading import. Imports of Atlantic salmon were valued at more than $1 billion dollars in 2013, highlighting a tremendous opportunity for domestic growth. In the U.S., Atlantic salmon are listed under the Endangered Species Act, and consequently all of its production is from aquaculture sources. Two decades ago, hatchery managers in Maine could rely on greater than 80% embryo survival, but today, survival rates have dropped by nearly half. Embryo and fry production is labor intensive and costly, and reductions in embryo survival have caused a considerable financial bottleneck in the hatchery process in New England. In addition, hatchery operations can no longer rely on the sale of surplus eggs as an additional source of revenue.

In New England, a leading producer of Atlantic salmon in the U.S., commercial aquaculture production has dropped more than 35% since 2000, due in large part to reduced embryo survival. A principal goal of commercial hatchery operations is to maximize the production of high quality eggs from each broodfish; the full potential of a hatchery can only be realized if it can provide a predictable and reliable supply of high quality eggs and fry at set times of the year. The purpose of this study is to investigate correlations between embryo survival and critical factors that are involved in the reproductive success of Atlantic salmon, to highlight potential causes of declining embryo survival.