While alewives spend most of their life at sea, their early days in freshwater spawning grounds are some of the most influential of their lives. Environmental conditions during this time can set the lifetime growth trajectory of the fish, influencing their susceptibility to predators and long-term survival when they reach the ocean. These conditions can vary greatly among river systems and offer scientists a unique opportunity to identify markers that link a fish back to its spawning location.
Our Fisheries Ecologist, Lisa Kerr, is looking at alewives’ growth patterns during their first 30 days of life by examining their inner ear bones. These structures, called otoliths, contain growth rings like those found in trees. By viewing an otolith under a microscope, we can measure how much it grew each day and then calculate the growth rate of the fish.
If we can identify patterns in early growth among alewives, we might be able to use otoliths to identify the spawning location of an individual alewife, as well as understand the factors that might be driving differences in growth among river systems.
Alewives along the east coast of the United Sates have been dwindling for decades and were even recently considered for listing under the Endangered Species Act. While some rivers still support healthy runs of alewives, others only support a few thousand returning adults each year. Without a way to tell which populations alewives caught at sea are coming from, it’s impossible to tell whether some groups are being disproportionately caught as bycatch by fishermen targeting other species.
We’ve already been able to identify differences emerging in growth patterns, and we’re now trying to determine the factors that might be driving those differences. Otoliths from all 14 sampling locations show similar growth rates in the first few days after hatching, but we see much more variation by the end of their first month.
We’re currently looking for correlation with environmental differences between systems, such as temperatures, habitat availability, and fresh water flow. If we can understand the mechanisms underlying these differences, we’ll be able to better identify unique characteristics of populations.