Took a short break from our Science Friday posts to do some actual science (on Olympic Peninsula winter steelhead) and to weigh in on some important policy issues. But now, like anglers looking for fresh winter chrome: we’re b-a-a-a-c-k.
As you know, in the Science Friday forum we discuss a wide range of topics important to the management and appreciation of steelhead. We have spent quite a bit of time, for example, talking about the value of diversity for steelhead, including diversity in size. Steelhead come in a great array of sizes, from small to large — and this size diversity has benefits for reproductive success and for strengthening the resiliency of a population.
We have shown videos highlighting the immense competition among males for females, and how smaller, non-dominant males can and do mate with females by being extra-sneaky. While many males may try to mate, ultimately, genetic information is needed to truly understand how many are actually successfully fertilizing eggs.
Given the number of potential dads (upwards of 6-10 in a single mating in some cases), it’s not hard to wonder, just like Maury Povich: Who is the father?
A panel of male mykiss waits with bated breath.
To better understand this question, this week we review a study (https://www.tandfonline.com/doi/abs/10.1577/T04-187.1?journalCode=utaf20) published in 2005 by David Kuligowski, Michael Ford and Barry Berejikian working out of the Northwest Fisheries Science Center (NMFS). The research was conducted as part of a captive brood program for winter steelhead in Hood Canal and took place in the Hamma Hamma River.
The authors of this study used a hydraulic pump to collect eggs from a total of nine redds, one of which didn’t have enough eggs for sampling. Why so few redds? Well, the populations had collapsed and there were hardly any redds remaining. As a result, the eggs were then transferred to a streamside incubator and eventually taken to redd-group-specific rearing tanks at the Lilliwaup Hatchery, which is operated by Long Live the Kings, in order to start a captive brood rearing program.
The results are in.
First, and as we might expect, fish from the same redd were more closely related to each other than they were to fish that came from different redds. This result makes sense intuitively. Mr. Smith’s family down the street is more closely related to the members in their own household than they are to Mrs. Duncan that lives next door.
Second, only two redds had eggs that came from a single pair of parents (the steelhead version of the American Dream — a white picket cobble fence with a mom and dad and some baby steelhead).
Third, the remaining redds fell into Maury territory. One redd had three parents, while the remaining five had at least four parents contributing to the eggs. So who made who?
Fourth the authors identified a total of 21 individual parents altogether that contributed DNA to the offspring in the eight redds that were sampled. Among those, five were females and 16 were male. Which means we had eight redds with DNA from five females and 16 males. The authors inferred that the strongly male-biased sex ratio was due to contributions from smaller resident males, either precocious parr or larger rainbow trout.
Lastly, the specific matings were quite diverse. For instance, in two cases females constructed two redds each on the same day but mated with a different set of males in each redd. Interestingly, one of those females dug two redds in the same day that were over 1 km apart. The other double-redd digging female did so on the same day also and they were over 700 meters apart. In another case, a female constructed one redd on May 13 and a second redd on May 16, and mated with two of the same three males each time.
The results have a number of potential implications. For example, they highlight how unique steelhead are among salmonids. A study on Chinook salmon found a roughly equal sex ratio when sampling 14 redds. Similar results have been found for Brown trout. In steelhead, the sex ratio of successful parents was skewed strongly towards males, suggesting that diversity is – as we know – important to the success of our favorite fish.
All the more reason to appreciate all the male steelhead, from small to large and anadromous to resident. We need to account for and conserve this diversity in order to successfully and sustainably manage our fishery and hatchery practices.
In addition, this work clearly indicates that females can and do dig multiple redds. Moreover, females may travel up to a kilometer or more to dig two redds in different locations in one day. In many watersheds we use redd counts to estimate abundance of steelhead. Converting redds to females is not easy, but the more we look, the more scientists are realizing that – unlike salmon – it is not a simple 1:1 ratio. It varies, and research on the topic suggests it can vary year-to-year and population-to-population, which is why it is hard to make generalizations about such a diverse species.
Ultimately, this study highlights that steelhead are not the typical nuclear family. Nope. Females mating with multiple males and building multiple homes for their eggs; males moving nilly-willy to find as many ladies as possible and frequently being sneaky about it.
In other words, it’s both the males and the females that are responsible for steelhead diversity in a population.
This type of complexity, and its implications for steelhead population and fishery management, is why we at TU and Wild Steelheaders United are always trying to educate anglers about the uniqueness of steelhead, and why we work hard to ensure that these data and analyses are incorporated into our steelhead management and habitat restoration strategies. Because there is one thing we know for certain about steelhead: they know what they need to make it (even if Maury can’t, in the end, reveal a single deadbeat dad). We just need to make sure they have the opportunity to make that happen.