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.
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.
- Determine if maternal hormone concentrations correlate with embryo survival.
- Determine if maternal deposition of hormones into the egg correlates with embryo survival.
- Determine if egg calcium concentrations correlate with embryo survival.
- Determine if maternal deposition of calcium into the egg correlates with embryo survival.
- Convey our findings to salmon hatcheries throughout the Northeast.
- Determine if the expression of key genes involved in the production of steroid hormones correlates with embryo survival: This objective is partially complete. I have extracted RNA from all of the tissue samples I have collected in the field and now I need to complete the process of measuring the expression of my target genes.
- Analyze two biomarkers of contaminant exposure and determine if they correlate with embryo survival: This objective is incomplete because I have only analyzed one biomarker of contaminant exposure, the EROD assay, however I still need to measure the expression of certain hepatic and ovarian genes involved with toxin clearance.
The aim of this study was to investigate trends that span across all sampling seasons and all hatchery conditions, therefore, data amongst all hatcheries (3) and sampling seasons were pooled for each regression. The three Maine hatcheries that were visited in this study for sample collection were:
National Cold Water Marine Aquaculture Center: The Agricultural Research Service (ARS) is a scientific organization of the United States Department of Agriculture (USDA) which runs the National Cold Water Marine Aquaculture Center (NCWMAC), an Atlantic salmon research facility that focuses on developing a selective breeding program in order to provide more efficient and sustainable farming of Atlantic salmon. The NCWMAC facility raises their broodstock in a recirculating aquaculture system (RAS) that uses water from either Hog Bay (for seawater) or well water (for freshwater) depending on the strength of salinity the facility demands. Broodstock are photo-thermally manipulated throughout the year and are grown in fiberglass tanks.
Cooke Aquaculture’s Bingham Site: Cooke Aquaculture’s Bingham hatchery is a RAS that runs only on fresh well-water. The broodstock are raised in covered outdoor cement tanks with photo-manipulation.
Cooke Aquaculture’s Eastport Site: Cooke Aquaculture’s Eastport site, known as Deep Brook Cove, is a sea-cage site in which broodstock are exposed to full-strength seawater as well as ambient photoperiod and temperature.
At the time of egg stripping, female broodfish were euthanized and tissue samples were collected for future analysis.
There were no significant correlations between female plasma T (Fig.1.A) and embryo survival to the eyed-stage. Significant positive correlations (P≥0.001) were observed between female plasma 11-KT (Fig. 1.B) and E2 (Fig.1.C) with embryo survival to the eyed stage. There was no significant correlation between egg hormones (Fig.2.A,B,C) with survival of embryos to the eyed-stage.
No significant correlations were observed between female plasma calcium (Fig.1.D) and egg calcium (Fig.2.D) concentrations with survival of embryos to the eyed stage. Female hepatic EROD activity was not statistically significant (P≥0.05) between females compared to controls (Fig.3).
This is the first study to show that concentrations of maternal plasma E2 and 11-KT positively correlate with embryo survival in farm-raised Atlantic salmon. Interestingly, egg concentrations of sex steroids did not correlate with embryo survival. This indicates that embryo mortality does not likely rest with maternal deposition of sex steroids into the egg, but with another hormone regulated process related to egg assembly, ovulation or post-ovulatory aging. As of now, this project has not contributed to agricultural sustainability however, there is a lot of potential to do so with additional research that will help understand causes for embryo mortality in order to enhance Atlantic salmon production in Maine.
Education & Outreach Activities and Participation Summary
Findings have been presented at the Bi-annual Salmonid Hatchery Roundtable in Belfast, Maine on February 19, 2014 and will also be presented upon at the Northeast Aquaculture Research Exhibition (NACE) on January 14, 2015 in Portland, Maine. At the Roundtable in Belfast there were about fifty attendees that work directly with salmon and the discussion of my results was very positive and in depth; there was a lot of communicating about past experiences and possible triggers for high embryo mortality events. Famers were open to my interpretation of the results and they many wanted to help with my future research goals of obtaining more data.
The response I have received from farmers when they hear the results of my study is positive and that they want to help me collect more data in order to explain the exact causation(s) for increasing embryo mortality of farm raised Atlantic salmon. Famers are waiting for a remedy and they are willing to take the required steps in order for their egg production to be successful again. This study has allowed me to scratch the surface of finding answers to the problem at hand and only with more research can we realize the solution.
Areas needing additional study
This study showed that egg hormone concentration is not a good predictor of embryo survival, while maternal plasma concentrations of E2 and 11-KT at the time of spawning are positively associated with increased embryo survival. More work is needed to understand the mechanisms involved, including the role of post-ovulatory aging on egg competence in the commercial culture of Atlantic salmon. Also, knowing and understanding the annual hormone cycle of the broodfish will be critical to this project in order to verify that the fish did in fact go through the necessary cycle and that the hormone level at the time of spawning is optimal.