With over 300,000 salmon growing to parr in our hatchery down in East Machias, we do our best to create a healthy environment for salmon to thrive as they grow into the little athletes they need to be to survive out in the wild.
Still, mortality is a normal part of rearing Atlantic Salmon (Salmo salar). While we work to keep mortality low in our hatcheries, raising Atlantic salmon at Downeast Salmon Federation (DSF), we have seen a lot of variance in the mortality rates of our hatchery reared salmon throughout the years. However, in the case of high mortality events, it’s easier for us to control certain factors (e.g. water flow) that impact those rates in a hatchery setting compared to in the wild.
At the Peter Gray Hatchery, we use unfiltered river water from the East Machias river, hoping that introducing the salmon to the native water will help ease their transition upon stocking. While we can’t control the temperatures of the river, we can control the cleanliness of the tanks and how much fish are fed. This ensures the salmon in our facility have a good quality of life and the opportunity to grow and thrive before releasing.
Research conducted by Ian Russell, with support from International Council for the Exploration of the Sea, investigated biological characteristics as predictors of salmon abundance. According to their research, smolt survival rate is dependent on multiple contributing factors including habitat availability, feeding opportunities, water temperature, and river current. These factors may influence juvenile salmon in freshwater and be reflected in smolt quality. The authors then go on to explain that smaller smolts are more likely to experience higher mortality rates compared to larger smolts, as smaller fish are more vulnerable to predation. Russell also believes wild smolts survive better at sea than their hatchery counterparts.
Raising our hatchery fish to be “little athletes” and then releasing them as parr may moderate this difference. Because we see better return rates on rivers stocked with parr than rivers only stocked with fry, preliminary numbers suggest that hatchery parr could offset the survival advantages wild Atlantic salmon have over hatchery raised fry. Further research needs to be done on the topic to support what data we collect at DSF.
Once Atlantic salmon venture into the ocean, of course, mortality remains an issue. Unsurprisingly, studies show that wild salmon populations have their highest mortality rates during their first year at sea. According to researcher Gerard Chaput, mortality can also be high during the second year, with monthly mortality rates estimated to be 10 –15% during that time.
The Peter Gray Hatchery in East Machias releases Atlantic salmon into the rivers during the parr stage, unlike other agencies in the region that only stock fry. DSF is hoping to see hatchery-raised Atlantic salmon improve their chances of survival along with their increase in size. As Ian Russell researched, wild smolts have a better survival rate than those from hatcheries, however, hatchery parr could help skew this survival difference.
The mortality rate in our hatchery is still up and down at times, especially on hotter days when river water warms up. (That's what makes our habitat conservation work so crucial: well protected rivers with ample shade from trees keep water cool and salmon happy!) And of course, more research on smolt mortality in wild salmon populations can help us understand how to better curb incidences in our hatchery and prepare our stock as best as possible to survive in the wild. As put by Dr. Gérald Chaput, this kind of research is crucial to improving our understanding of the key factors "that define salmon population abundance and regulation" to help us better manage and support wild Atlantic salmon's survival in Maine rivers.
With help from our community, staff, and partners, DSF can continue to give Atlantic salmon a fighting chance to survive and overcome their endangered species status. DSF continues to be the best last hope of Atlantic salmon in the state of Maine.
This post was researched and written in collaboration with our Aquaculture Research Institute Industry Extern, Athena Fretz. Athena is a student at Bowling Green State University in Ohio studying Marine Biology.