Science Friday: Do it once, do it twice. The tradeoffs of repeat spawning in steelhead
We are back after a short break, after coming through a heavy dose of conservation work. That work, in part, helped convince the Washington Department of Fish and Wildlife and the National Marine Fisheries Service to re-open the iconic Skagit River for a catch-and-release season for wild steelhead. It feels like a new day has dawned for steelheaders in Puget Sound.
So, let’s get back to steelhead science. While winter steelhead season is drawing to a close in most rivers and many anglers face a lull in steelheading until the summer runs appear, that doesn’t mean we take any time off from delving into the complexity of these amazing fish.
In fact, just the opposite. It’s now time for the steelhead to spawn.
It is apropo then, that one of the ways in which steelhead differ from salmon is their ability, under the right conditions, to spawn multiple times. A new paper just published by Mark Christie and others on repeat spawning in steelhead (see an article on the paper here with a link to the study at the bottom: https://www.sciencedaily.com/releases/2018/04/180412102951.htm) offers a useful evaluation of the tradeoffs of being a repeat spawner compared to being a one-time spawner.
In this analysis, Christie and colleagues studied a population of winter-run steelhead in the Hood River in Oregon. The river used to have a dam at the mouth, which allowed scientists to collect all steelhead that moved upstream. Over a 16-year period they sampled a total of 12,579 steelhead. Of those, only 299 fish were repeat spawners. That’s only 2.3% of the fish, but this ratio may be influenced by the fact that the Hood River is a tributary of the Columbia River and is located about 150 miles upstream from the mouth. In other words, it’s not a coastal river with a short jaunt back to the ocean.
The study’s results are interesting.
The first finding was that 98% of the repeat spawners increased in length and 95% increased in weight after their first spawning trip. Basically, most fish grew bigger during their second trip to sea.
Second, while the sampled population had both wild and hatchery fish, the hatchery fish, despite being equal in numbers to wild fish, had a lower repeat spawning rate than their wild counterparts.
Third, repeat spawning females had 2.6 times greater lifetime reproductive success than single spawning females. Similarly, wild repeat spawning males had 2.5 times greater reproductive success than their single spawning counterparts. In contrast, hatchery-origin repeat spawning females had only 1.6 times greater reproductive success than single spawning hatchery females, and repeat spawning hatchery males had only 0.8 times the reproductive success of single spawning hatchery males. Overall then, spawning more than once increases the reproductive capacity of those fish that repeat spawn, at least for wild fish, largely because they get two shots at the bullseye.
Lastly, while doing it twice gets you more chances at producing offspring, the study found there was a price to pay. For example, during their first spawning, the repeat spawners were generally smaller in length and weight than other first time spawners. And perhaps consequently, those fish that eventually became repeat spawners produced fewer offspring during their first spawning compared to fish that were one time spawners.
The bottom line is the authors found a distinct tradeoff in spawning success with repeat spawners. On the one hand, repeat spawners have over twice the lifetime reproductive success of single-time spawners. However, during their first spawning they are not as successful as fish that only spawn once. The results suggest the repeat spawners may not commit as much of their resources to spawning that initial time, possibly because they are saving some of that energy to make a second attempt the next year?
Perhaps they are like the cagy prizefighter doing the rope-a-dope. The first round isn’t as important as the last.
The bottom line is, repeat spawning among this population of steelhead came with this tradeoff: be less successful on the first attempt, and make up for that disadvantage by having another go at it later.
It’s a risky tradeoff. A couple of studies have found that quite a few steelhead survive spawning and migrate back to the ocean. In fact, 70% of radio-tagged fish in some rivers have been found to emigrate back to sea and to try to spawn again. But because repeat spawning levels are generally below 10% in most populations, it appears that survival of those 70% is really low. There is no guarantee that a fish will survive to spawn again once they leave the river and head back to the sea.
At the end of the day, the study highlights that repeat spawning is an important feature in steelhead biology. Unfortunately, we have not really adapted our steelhead management strategies to better protect repeat spawners. This is something that fisheries managers will have to consider going forward if we want to rebuild wild steelhead populations and sustain our opportunity to fish for them.