Tag Archives: post-smolt

Why are the salmon “Lost at Sea?”


As I have written about in an earlier post, the purpose of this research project, SALSEA-Merge, is to understand the life history of the Atlantic Salmon, and to the figure out why they are not returning to the river after their migration out of the rivers and into the sea.  This expedition is seeking to understand what is happening to these smolts along their journey through genetic testing, etc.  With the knowledge or lack there of about what is actually happening, some of the problems facing these salmon are known.  

Some of the problems facing these salmon include poor water quality in the rivers, over-harvest, discards of post-smolts as bycatch, sea lice, and lack of food when they reach the sea…to name a few.   The first two are clear, the fish need a good habitat to live in and with development, etc, water quality in the river goes down.  This needs to be improved so that the salmon don’t have to battle poor conditions in the beginning (and end) of their lives. 

Overharvest is also straight-forward.  If you take too many fish, there won’t be any spawners left to replenish the populations.  Overharvest is one of the largest problems facing the fisheries of the world and the salmon are right there in this losing battle. 


Two Russian trawlers in the distance


Bycatch is another issue that is key to the decline of many fisheries, but with the salmon, it was a new idea to me.  The post-smolts follow a similar migratory path to the shoals of mackerel that are targeted by the large Russian factory trawlers using massive surface trawls to harvest the mackerel in large numbers.  While the actual impact of these trawlers is unknown, it is hard to imagine that thousands of post-smolts are not captured in these trawls and processed with the mackerel.  The mackerel and post-smolt salmon occupy the same water and having seen the number of mackerel we captured in the sampling trawls, it is hard to imagine that with the size of the trawlers, they are not impacting the population of salmon. 

Mackerel bycatch from the sampling trawl…100 post-smolts were captured in this trawl…can you imagine how many are taken in the large factory trawls?  it must be thousands!

The other problems are distinct possibilities as well.  Many of the post-smolts we collected had sea lice on them.  I learned on the voyage that if a post-smolt has a certain number of sea lice on it, it will certainly die.  The sea lice attach themselves to the smolts on their way out of the river past fish farms.  Fish farms are the culprit in this game.  As the smolts pass, the nauplii (1st stages of development of the sea lice which are copepods, small crustaceans) attach themselves to the smolt and develop into adult sea lice which will remain on the salmon for the remainder of its life.  The level of infestation of the sea lice is tremendous and is one of the major problems arising from the use of fish farms to rear salmon. 

This salmon is infested with sea lice giving way to poor body condition.

Finally, the populations of the small pelagic fish, herring, mackerel, blue whiting, have boomed in the past years impacting the density of zooplankton in the Norwegian Sea.  With the increasing populations due to good management of the stocks for maximum abundance, the zooplankton has been grazed heavily leaving a sea nearly devoid of zooplankton.  Without zooplankton, our post-smolts have nothing to eat so their body condition is not good and they will not be able to survive without food.  This may be a cycle but it is largely unknown at this point…more research must be done.

With all of the potential problems outlined above, it may be something entirely different, but it is hard to imagine how one of our tiny post-smolts can survive its year-long journey in the sea to return to the river to spawn.  In fact, given all of the human obstacles we have put in front of them, it is remarkable that any return at all.  


June 26: Northernmost Point of the Cruise (or is it?)- Sampling Begins


In the early morning (0330), We reached the anticipated northernmost point of the cruise, 67.5°N, the starting point for the sampling.  The cruise plan was to head to this point and see what we find.  If we found a large number of salmon, we would continue north.  So with a CTD done and a plankton sample collected, the first trawl of the cruise was put out for one hour to test the waters.  In this first haul, we found only one post-smolt so it was determined that we should turn southward to continue our sampling for the day in as Southerly direction.  Our second haul was far more productive yielding 50+ salmon. 

In addition to the post-smolt salmon, the net captured a few other species that were sampled as well: mackerel, herring and lumpsuckers.  Lumpsuckers are one of my favorite fish.  I don’t know much about them but they are absolutely beautiful with a crazy turquoise coloration and textured body.  The mackerel are in large numbers as we see them on the surface.  The population of mackerel and herring in this region is healthy and almost over-abundant due to the intense management of the stocks in the past years. 

A small lumpsucker

A Large Mackerel

Herring (these made their way into the galley for breakfast)


We are in international waters.  As we headed south, we encountered the Russian trawler fleet, targeting the large mackerel shoals we are seeing on the surface of the sea.  These ships are massive, well over 100 meters.  One possibility for the loss of the salmon at sea is that large numbers are being discarded as bycatch.



Our sampling trawls stay out for about 3 hours.  In that time, there is ample time to spend on the bridge looking for passing wildlife.  There were some sperm whales cruising by the boat and someone sighted some orca as well.  I missed the orca but saw the sperm whales.  Apparently because of the ridge here, we should see quite a few whales and the sperm whales seem to like this area very much.  I hope at least one of the whales wants to check out this big green boat so that I can have a closer look at him. 

With the day of sampling completed, we will turn and head back north to begin sampling in the morning at a spot close to where we began today.  The smolts are moving at approximately 15 nautical miles per day so we must anticipate this movement when choosing where to sample.  Essentially, we are looking for the highest concentration of post-smolts along our journey.