Alaska Salmon Travels - Post 5

Juvenile Chinook salmon at different stages of the parr-smolt transformation. The bottom picture is a pre-smolt Chinook that’s still adapted to freshwater, the middle Chinook is undergoing smoltification, and the Chinook in the top picture has completed smoltification and is prepared for saltwater conditions. From “A Handy Field Guide to the Nearshore Marine Fish of Alaska” by Johnson, Neff, and Lindeberg, 2015. NOAA Tech. Memo NMFS-AFSC-293.

We’re done marking the fish!!! It was a long haul, but we processed all the juvenile Chinook salmon that we’ll be releasing this year from Little Port Walter. The final count was just under 195,000 fish. As astute readers, you may notice that the final number of fish is fewer than we expected; this stems from how we estimated the number of fish that we put into each vertical raceway over a year earlier. I won’t bore you with those details…the important thing is that we’re done!

Now that the marking process is complete, we will be holding the fish in net pens for a few weeks before they are released into the wild. We’re doing this for three reasons. First, we want the fish to recover from the stress associated with handling and marking. Second, we want to take a small subset of fish and see how many have lost their coded wire tags. Much like how your finger can push out a splinter over time, the tags in some fish may be lost. We won’t be able to identify those fish as coming from Little Port Walter or from the correct group in our marking study if that happens. So, we want to determine the proportion of tag loss (typically less than 1%) to ensure that we obtain accurate estimates of marine survival for our three experimental groups. This also will help us better understand the actual contribution of Little Port Walter fish to the commercial fishery.

The view of Chatham Strait at sunrise from the Little Port Walter Marine Research Station. Chatham is the first stop for fish as they begin their oceanic migration.

The third and final reason why we are holding the fish in net pens is to aid their transition from freshwater to saltwater. Juvenile salmon undergo a transformation prior to their oceanic migration called smoltification. During smoltification, their bodies become more streamlined (for swimming in the open ocean) and their coloration becomes more silvery (for predator avoidance). Fish physiology (the way the fish’s body functions) also changes in a way that enables them to survive in saltwater rather than in freshwater. We hold the fish in saltwater in the net pens, but the top part (about one meter deep) is covered with vinyl material. With this vinyl material, we are able to create a freshwater layer at the surface by pumping freshwater into the nets. This combination of layered fresh- and saltwater facilitates the process of smoltification. Soon, all the fish will be ready for the oceanic stage of their life cycle, and we’ll release them into the wild. After three to five years, the survivors will return as mature adults. We will then find out if marking negatively affects their survival. Let the wait begin!