I hear a lot of anglers repeat some pretty significant misconceptions about when trout spawn with most thinking that the peak spawn is much earlier than it truly is. Around our part of Wisconsin, the peak of spawning is around the beginning of deer season. Spawning, in earnest, begins the last few days of October or early in November. Part of this comes from the fact that they start staging for spawning earlier and the smaller trout "play around" with spawning as early as late September or early October. But the peak of spawning happens later - for a very good reason I will write about later in this post.
From Stefanik and Sandheinrich 1999, the spawn dates of hatchery and wild Brown Trout (Salmo trutta) in Southwest Wisconsin streams. For reference, the end of the trout season in Wisconsin - October 15th - was day 288 in 1995 when the study was done - which by chance, matches up with this year (2023). And day 300 = October 27th, 310 = November 6th, 320 = November 16th, 330 = November 26th, 340 = December 6th, and 350 - Dec. 16th. So the peak of the wild trout spawn in the Driftless is around Thanksgiving and almost all trout - even the hatchery ones - started spawning after the season closed. In 1995, the season closed 15 days earlier, September 30th (day 273) rather than October 15th (Day 288).
Brook Trout tend to spawn a bit earlier but certainly there is a good bit of overlap. I don't have good data on it here but Brook Trout peak maybe two weeks earlier. But as you can see from the Brown Trout figure from Stefanik and Sandheinrich (1999), the spawn occurs over about a two month (60 day) period of time. As you probably know, Brook and Brown Trout can combine to form a sterile "tiger trout" - more on that later...
Brown Trout eggs require about 850 degree days to emerge. In this case, a degree day is calculated by how much above freezing (0°C) the stream temperature is for that day, then they are summed to get cumulative degree days. For example, a day that the water temperature averages 5°C (41°F) is counted as 5 degree days. To do a bit of the math for you, the median date of redd formation was day 312 (November 8th) for wild trout and the median emergence date was day 110 (April 20th) which is 163 days later. Using the 850 degree days as a threshold, it means that the temperature has to average about 5.2°C (41.4°F) over that time. Streams with more spring flow and thus warmer temperatures need fewer days to hatch, colder streams need more days.
There is, of course, a good bit of variability in the temperatures of redds with the larger fish generally selecting areas of great upwelling and thus warmer winter temperatures. In the Stefanik and Sandheinrich Brown Trout study in the Driftless, the median date of emergence was day 110 (April 20th) and the date of 75% emergence was day 114 (April 24th). Most of the degree days are generally accumulated during the early and later part of egg development with fewer accumulated during January and February. One other caveat - the 850 degree days came from an English study (Elliot 1988); Brown Trout in the Driftless may have evolved a different life history and require more or fewer (likely more) accumulated degree days.
What About Brook Trout?
First, we know less about the timing of Brook Trout spawning and the accumulation of degree days needed in the Driftless Area. A study of Brook Trout strains from Quebec and upstate New York shows a lot of variation in degree day accumulation before they emerge (Baird et al. 2002). Degree days ranged from 457 to 672 in their study - noticeably shorter than 850 degree days the Brown Trout study (Elliott 1988). Though this is not surprising given the locations and amounts of spring flow. There is likely some relationship between when trout have evolved to spawn and the locations water temperature.
Hatchery Fishes and Spawning / Emergence Dates
Spawning dates are often quite flexible for many species. In the Brown Trout study by Stefanik and Sandheinrich (1999), the hatchery fish spawned much earlier - the median redd formation date was 10 days earlier and the median emergence was 17 days earlier. Eggs and milt to raise hatchery trout are often collected early in the season if no reason other than it is more pleasant to collect them then. However this may lead to an issue of the fishes hatching too early. Anyone that has some experience fishing in the spring understands that a few days often makes a huge difference in terms of water temperature.
A quick fishing tip - as this is mostly a fly fishing blog after all - small Brown Trout imitations (Three-quarters of an inch to an inch and a half in length) work really well. The Milwaukee Leech in olive is a good, simple choice for mid-April through early-May when there are an abundance of young Brown Trout. Fish them in the slower margins of streams - that tends to be where most of the young-of-the-year will be. Trout are cannibals!
Match / Mismatch Hypothesis
The match / mismatch hypothesis states that there is a strong correlation between the survival of young of one species with the right conditions when they are "born". The birth / emergence of one species has evolved to match the abundance of their prey. Think about the fledging of American Robins or Eastern Bluebirds and their prey or the timing of the hatching of birds of prey and the abundance of prey to feed them. For trout, this means emerging at a time when water temperatures allow for sufficient growth (see my post on bioenergetics) and consequently there is abundance of zooplankton prey for the trout once they pass the yolk-sac stage. Spawning too early likely means emerging too early - before the water has warmed and zooplankton and later aquatic insects are abundant. This likely leads to poor early survival and a poor year class.
Over the generations, evolution has shaped emergence dates to coincide with high prey and relatively warmer water temperatures. Hatchery fish are, in part, unsuccessful because they hatch too early when the water is too cold for them to grow during this critical time in their life history. Survivorship (the percentage of individuals that survive from one age to another) is often the lowest during those first couple of weeks of life for many species - trout included. A lack of food, a freak storm and snow melt, a flood, etc. after emergence often leads to a poor year class. Matching the correct water temperatures and prey densities leads to bumper year classes. Evolution drives species to emerge at "the correct time." Of course with climate change, this relationship has been altered and mismatches are probably more likely.
And About Those Tiger Trout?
For many anglers, the tiger trout is their white whale. I have caught two in my life - but it has been over two decades since I caught my last. I know where they are - just ask your local fisheries biologist and they'll tell you where they catch them more commonly. However, they are really rare and catching one involves a lot of luck - being in the right place at the right time. Many people have hit it right more often than I have - and certainly more recently. I've seen the shocking data, where they are "common", they are still very rare.
As you likely already know, tiger trout are a hybrid between a Brook and a Brown Trout. What makes them rather unique are a few things. First, these two species of trout are in quite different genera. Brook Trout are in the genus Salvelinus which are char (but char are trout, damn it!) and the genus has a circumpolar distribution meaning they occur in the northern part of the northern hemisphere. Brown Trout belong to the genus Salmo which are native to "the old world" - mostly in the Atlantic Ocean drainage of Europe, northern Africa, and western Asia. More interestingly, they have a different number of chromosomes - which generally makes hybridization exceedingly rare. They occur only when female Brown Trout eggs are fertilized by male Brook Trout milt (sperm; yes, fish fuck in water - thanks W.C. Fields for putting that image in our minds). Thus tiger trout are quite rare and sterile.
Because tiger trout are so rare, we do not know a ton about them and what conditions they are most likely to occur in. They seem to occur when one or the other population is low. Essentially, the rare species has a low likelihood of finding another of its species is quite low. Other than that, we don't know a heck of a lot about why tiger trout are formed. A colleague had a student looking at their distribution but tigers are so rare that they found out little about where they occurred. We do know that rarely are they never common and I would assume in most streams, they are the result of a single mating or maybe two.
While we generally do not think of tiger trout as a significant issue here (but see Grant et al. 2002), in come European streams, they are quite concerned about this hybridization (Cucherousset 2008). Here, they are mostly a cool novelty. In other places, they may be a significant issue that is causing the decline of native Brown Trout. I always find it interesting how in different places, species interactions are often quite different.
Thanks for the tiger trout photos:
Dieterman, D. J., & Mitro, M. G. (2019). Stream habitat needs for brook trout and brown trout in the Driftless Area. In A look back at driftless area science to plan for resiliency in an uncertain future. Special Publication of the 11th Annual Driftless Symposium, La Crosse, Wisconsin (pp. 29-44).
Grant, G. C., Vondracek, B., & Sorensen, P. W. (2002). Spawning interactions between sympatric brown and brook trout may contribute to species replacement. Transactions of the American Fisheries Society, 131(3), 569-576.
Stefanik, E. L., & Sandheinrich, M. B. (1999). Differences in spawning and emergence phenology between stocked and wild populations of brown trout in southwestern Wisconsin streams. North American Journal of Fisheries Management, 19(4), 1112-1116.