Science Friday: The life stages of juvenile steelhead

We have spent the last several weeks introducing and defining a variety of terms used in steelhead management. This week, we shift to looking at terms related to steelhead biology, specifically terms used to describe different life stages of juvenile steelhead.

 

Let’s start with eggs. These are the little round suckers that produce the fish. Seriously, everyone understands the egg life-stage. Sometimes eggs are referred to as being “eyed,” meaning that you – literally – see the fish’s eye in the egg. The eyed-egg stage indicates the fish is close to hatching.

 

 

Next, consider the alevin. An alevin is a newly emerged steelhead that is still carrying its yolk sac.  They are almost transparent.

 

Pretty straightforward. But beyond the alevin stage we start to get a little ambiguous, a little messy, a little confusing.

 

After the fish absorbs its egg sac and has to actively feed, it is technically a fry. The fish remains a fry for a few months, basically until it fully develops scales. This life stage for steelhead is essentially equivalent to the “infant” stage in humans.

 

Once a steelhead has fully developed scales and fins, it is referred to as a parr. However, sometimes people refer to them as fingerlings. In fact, this was at one time to most common term used by biologists for this phase (and some biologists still prefer this term over parr). The problem is, the term fingerling implies the fish is the size of a finger — which often the juvenile steelhead are not at this stage.

 

A juvenile steelhead remains a parr for one to five years (longer in rare cases) before eventually emigrating to the ocean. As a result, steelhead parr are often much larger than a typical human finger, reaching up to 7-9 inches in length depending on growing conditions and age. On the other hand, fry are much smaller than our fingers. Nonetheless both fry and parr at times have been referred to as fingerlings. Confusing to say the least. Stick with the terms fry and parr and you’re safe.

 

If lucky enough to survive beyond the parr stage, a juvenile steelhead will undergo a physiological transformation that allows it to enter the ocean. As part of this transformation the parr’s outward appearance changes from drab and densely spotted to very silvery with fewer spots. The tips of the fin and the tail become dark blue or black. The oval parr marks on the side disappear. The fins become translucent. Now this fish is a smolt, a fish whose physical appearance reflects the process of physiological changes required to prepare for and undertake a freshwater migration to the ocean. The smolt stage is brief, lasting weeks to a month or two at most.

 

So, that’s it. The life phases of a steelhead before it reaches adulthood are egg, alevin, fry, parr and smolt. Simple terms for a remarkably complex life history. This complex life history and the ability to remain in freshwater and then go to the ocean is one reason wild steelhead have, for hundreds of thousands of years, been able to overcome factors such as drought, unfavorable ocean conditions, even over-fishing, and rebound in numbers once habitat conditions become more favorable.