Science Friday: Deschutes River (Oregon) rainbow and steelhead, how much do they overlap during spawning?

First Friday of the New Year, and we are serving up a fresh batch of steelhead science to help you further appreciate the remarkable relationship between resident and sea-going forms of O. mykiss.

 

In this post, we review a study published in 2000 by Christian Zimmerman and Gordie Reeves, two scientists that have contributed a great deal to our understanding of salmonids. The focus of this study was  resident rainbow trout and steelhead in Oregon’s Deschutes River in an attempt to determine if either or both forms were reproductively isolated during the mating season. In other words: Did the timing and location of their spawning overlap and, if so, how much opportunity was there for interbreeding?

 

This science may seem “old hat” nowadays, but this was truly a novel study at the time and has provided inspiration and baseline analysis for many subsequent studies.

 

We have previously reviewed at length how rainbow trout can provide important contributions to steelhead, both in terms of mates and genetics. However, most of those studies were focused on how male rainbow trout mate with steelhead. The Zimmerman-Reeves study, in contrast, evaluated the spawn timing and location of female rainbow trout and steelhead in one of the best rainbow trout and summer steelhead streams in the Pacific Northwest. The authors collected and analyzed otoliths – tiny ear bones in fish – to determine if female rainbow trout were producing steelhead and vice-versa for female steelhead.  As a fish grows, otoliths form a series of chemical layers, and differences in chemistry at the core of the otolith can tell us whether the fish’s mother went to the ocean or not.

 

Here’s what they found.

 

The study, conducted over three years, did find significant differences in spawn timing of female rainbow trout and female steelhead. Steelhead spawned from mid-March through May, while rainbow trout spawned from mid-March through August. Although there was overlap, the time at which 50% of steelhead spawned occurred 9-10 weeks earlier than the time at which 50% of the rainbow trout spawned.

 

The study also found that steelhead excavated their redds in deeper water with larger cobble and gravel. That makes sense. Larger-bodied fish can mobilize larger substrate. The combination of spawning at slightly different times and using slightly different micro-habitats (e.g. larger substrate) reduces the opportunity for interbreeding between the two life histories.

 

 

Analysis of the otolith chemistry was interesting. There was potential overlap for mating between the two life histories, but did steelhead actually produce rainbow trout and rainbow trout produce steelhead? The answer is no, at least not in the Deschutes samples collected by the authors. Female steelhead that were sampled were produced by female steelhead and female rainbows were produced by female rainbow trout.

 

In the mainstem Deschutes River, female rainbow trout and female steelhead spawn in slightly different habitats and times, and among a sample size of 20 steelhead all had steelhead mothers and among a sample size of 38 rainbow trout all had rainbow trout mothers. The evidence of this and subsequent studies suggests that the two life histories are – even though they may spawn in proximity at times – reproductively isolated.

 

Since this pioneering study was completed, other studies have also found that females of each life history tend to disproportionately give rise to the same life history: a female steelhead tends to produce another female steelhead. This makes sense. Life history is partly controlled by genetics and research on salmonids suggests that female age and size at maturity – which differs between smaller rainbow and larger steelhead – is strongly heritable.

 

Male O. mykiss also have a genetic component to their life history, but reflect a stronger environmental influence. Consequently, males tend to display a broader array of life histories – from small to large – than females within the same population. The Zimmerman-Reeves study  did not evaluate male behavior, but as we have discussed often in the Science Friday forum, we know those guys are sneaky, and there were plenty of opportunities in the Deschutes for a small trout to move over a few meters and try to fertilize a female steelhead. Indeed, genetic research cited in their study concluded the two life histories do share gene flow. Based on field observations from numerous scientists across the PNW, it seems unlikely that such gene flow is occurring when large male steelhead suddenly get eyes from a smaller female rainbow. Rather, it’s probably because male trout are being, well, like males of almost any species.

 

The Deschutes River as habitat for O. mykiss is unique by almost any measure. For instance, it is one of the rare large rivers where we see small-bodied female rainbow trout spawning in the main-stem. Try that on a river on the Olympic Peninsula or the Oregon Coast — the shallow redd will be scoured many times over by the end of the season. That is one likely reason we see steelhead-dominated populations on the west-side of the Cascades and coastal ranges: big flows require deeper redds to avoid scour.

 

 

The Deschutes’ flows, however, are among the most stable on the West Coast. Fed by substantial springs and underground aquifers, and being on the dry side of the mountains, this system rarely floods and consequently even a small-bodied female rainbow can dig a redd without having it wiped out by high flow events. In such environments it is possible, if there is enough food (and the Deschutes has food in spades), that both steelhead and rainbow trout can thrive. But we know, thanks in large part to the groundbreaking research of the Zimmerman-Reeves team, that to maintain their respective life histories the females of each form must be reproductively isolated to some degree.

 

Wishing all of you a Happy New Year and lots of luck in your fishing endeavors.