This is the third of four posts on the nuts and bolts of estimating wild steelhead populations, spawning success, and other key management variables.
First, we covered the concepts of carrying capacity and density dependence and how habitat can be used to estimate carrying capacity. Last week’s post shifted gears to review studies that found the spatial distribution of spawning fish can expand and contract depending on how many adults there are. That distribution can directly influence the habitat’s carrying capacity.
It turns out that we can’t talk about space in this context without also thinking about time.
We know the spatial distribution of spawning steelhead and their offspring is important because there can be tremendous competition among juveniles just after they are born, resulting in high levels of negative density dependence. In essence, spreading fish out across the watershed can improve productivity (if the habitat is good enough to allow it) and in turn increase the capacity of the population. However, fish don’t necessarily have to be separated by space to increase their productivity. They can also be separated by time.
A study by Gharrett et al. (2013) examined a pink salmon population in a stream in Alaska. The population included two sub-groups, one that entered in mid to late August and another that entered in early September. The early-entering fish spawned, on average, a couple weeks earlier than the later-arriving individuals. Both returning adults and out-migrating juveniles were counted each year at a permanent weir. The data were collected from 1971-2009 and run sizes ranged from 1,000 up to 27,000 adults. The authors also collected genetic data on adults and juveniles. The combined information allowed scientists to examine how differences in spawn timing influenced the production of pink salmon smolts.
There were several important results. First, the differences in run timing was heritable, meaning the timing of their return was genetically controlled.
Second, while later arriving fish may superimpose their nests on the early ones and dislodge their embryos, the differences in run timing were sufficient to allow early-spawned embryos to develop to a disturbance resistant stage, which helped reduce effects of superimposition.
Third, early-run embryos experienced warmer stream temperatures than those spawned later, and those offspring were different, at the genetic level, in rate of development. As a result, the early-run embryos developed more quickly and emerged about two weeks before the late-run embryos — almost like shift-work, one crew coming in as another clocks out.
Lastly, owing to the differences in spawn timing of adults and emergence timing of juveniles, a population that was experiencing density dependence was able to pack more spawners into limited spawning habitat. And that is kind of the Holy Grail for recovery of wild salmonid populations: finding ways to maximize use of available habitat by increasing diversity.
In sum, pink salmon were essentially staggering the use of the habitat over time. While the juvenile pink salmon were not feeding in freshwater, the main point is that even small differences in spawn timing can increase the productivity of a population.
So, how does this apply to steelhead? Frankly, we are not sure because this type of work, including both evaluation of both genetics, adult spawners and juvenile productivity, has not been published for a population of steelhead.
Nonetheless, if just a few weeks’ difference in timing can influence the capacity of pink salmon, there could be larger effects for a species like steelhead that spawn over a more protracted period. For example, steelhead commonly spawn over a four-month period ranging from February to May, and some populations may spawn over a period of 6-7 months. That broad range of spawn timing may allow steelhead to spawn in the same place, months apart.
Indeed, research I conducted in the Quillayute watershed found that the earliest-spawning steelhead occurred high in the watersheds while later-spawning fish were most common lower in the watershed (Click here for the paper). The caveat is that the very latest returning individuals did not spawn lower in the watershed, rather, they went back up high to spawn in areas where the earliest fish did. Based on the pink salmon work, the earlier-spawned steelhead embryos would have 2-4 month advantage in growth over the later-spawned individuals, and as a result, they are likely to disperse further away from the redd, in turn, leaving behind vacant habitat for the late bloomers. This would help alleviate that density dependent crunch that occurs around redds when all juveniles emerge at the same time and produce a similar pattern to the pinks where differences in run timing result in the staggered use of habitat by the offspring.
Ultimately, it’s not just about spawning in different places and expanding spatial distribution. The timing of spawning also matters for wild steelhead and differences in spawn timing can help fit more fish into a given amount of habitat by staggering habitat use of the offspring. This is an area that is ripe for research specific to steelhead and is one reason why Wild Steelheaders United has partnered with resource agencies to better understand how differences in spawning location and spawn timing influence the carrying capacity of watersheds for the iconic freshwater sport fish of the West Coast of North America.