Science Friday: Survival of hatchery and wild steelhead smolts: lots of mortality, just not always where you think

 

November has arrived, and here on the Olympic Peninsula, the first frost of the year. Further south in steelhead range, however, things are a lot warmer, and not in a good way. We send our best wishes for everyone affected by the recent wildfires. 

 

The impact of the extreme fire events we are experiencing over the past few years on steelhead habitats will be something to watch closely. But for now, we bring you a Science Friday on another important topic: early marine survival of steelhead smolts.

 

Smolt-to-adult survival is an important consideration as resource managers try to recover and sustain steelhead populations and fisheries. One of the biggest challenges in estimating smolt-to-adult survival in steelhead is determining how much mortality occurs in the ocean compared to the extent of mortality that occurs in freshwater. In fact, very few studies have traditionally asked this question.  

 

First, a definition: smolt-to-adult survival is the proportion of smolts that emigrate to the ocean and then return to spawn as an adult. That survival for wild steelhead typically is somewhere between 1%-10%, depending on the year and population. This survival tends to be lower for hatchery steelhead. But how much of that mortality occurs in the ocean, rather than in freshwater when smolts are migrating downstream but have not yet reached the salt?

 

A study by Michael C. Melnychuk and several colleagues that investigated the survival of hatchery and wild steelhead smolts in the Cheakamus River, British Columbia, over three years (2004-2005,2008) sought to answer that question. They accomplished this research using acoustic telemetry. Smolts from each group, hatchery and wild, were implanted with acoustic tags. During and after release the fish were tracked and counted when they swam by several fixed acoustic receivers located in freshwater and out through the Strait of Georgia, which is a narrow body of saltwater between Vancouver Island and mainland British Columbia.

 

The acoustic data allowed the research team to determine how many smolts stayed in the river, how many migrated, and how many survived the migration. These data were then used to estimate smolt-to-adult survival for hatchery and wild origin fish.

 

Their results are interesting.

 

Overall, average smolt-to-adult survival was 8.0% for wild steelhead and 4.1% for hatchery steelhead. Basically, wild steelhead survived at about twice the rate of hatchery fish. While differences in survival between wild and hatchery steelhead can be greater in some places than in others, these values fall within the range that has been previously reported through other analyses.

 

This study found a large survival difference between hatchery and wild steelhead throughout the entire migration period, but perhaps most surprising – and this was one of the first studies to find such a result – was the ecological cleaver hatchery smolts encountered immediately upon release. Melnychuk et al found 30%-40% of the hatchery smolts died within the first 3-km of their migration. Yikes. A rather harsh introduction to being kicked out of the cozy comforts of the raceway.

 

Wild fish paid a steep price immediately after being released, too. Their mortality in the first 3-km after release was 7%-13%. 

 

The first phase of downstream migration for steelhead smolts is no joke. Birds, mammals and other fish, they know it’s feeding time. And not all fish are strong or fit enough to handle the physiological process of smolting.

 

In this study, after the 3-km mark the fish had to migrate about 15 km to hit the ocean. The authors estimated survival from release to ocean entry was 26%-40% for hatchery smolts and 71%-84% for wild smolts. Survival from release to the end of the Strait of Georgia was 22%-33% for wild smolts and 3.5%-6.7% for hatchery fish.   

 

It’s clear that far more hatchery smolts died in freshwater than wild smolts. This makes sense. Hatchery smolts are total newbies, like freshmen on the first day of high school. Except, the hazing they received was to get eaten. 

 

Wild smolts, in contrast, were statistically much less likely to be “hazed.” In particular, the travel speeds of wild smolts were especially fast, likely improving their chances at survival. For example, wild smolts took only a few days to migrate over 30km and out into the ocean. Some groups of hatchery smolts migrated at a similarly rapid rate, but it took an average of more than a week for other groups of hatchery smolts to cover that distance.  

 

Lollygagging can be costly, if you’re a smolt.

 

What is most striking about these results is the amount of mortality that hatchery smolts incurred within the first day or two after being released into the river. When we think of smolt-to-adult survival, we tend to think of marine mortality as being the biggest driver. But this study indicates there is more to the equation. 

 

In fact, poor hatchery smolt survival is one reason smolt-barging began in the Snake and Columbia Rivers. Unfortunately, this means we are losing a big return on our investment in hatchery fish almost immediately — like seeing your new car depreciate by 10-20% as soon as you drive it off the lot. 

 

There may not be any way to improve survival of hatchery smolts during those first few days after release. When you produce large numbers of fish that have never been exposed to nature’s inherent threats, opportunities, checks and balances, this kind of response is both predictable and intractable.