All participants in the Co-operative Angler Program have received training on how to properly handle and sample live Steelhead.
Currently, on five tributaries, the sampling process involves marking Steelhead by clipping a selected fin. A different fin is selected each year to be marked. The purpose of marking fish is to generate a population estimate by using a Peterson estimate. If a fish is marked and recaptured during the same spawning migration, it is not sampled again. The formula of the modified Peterson estimate is as follows:
Number of fish marked in year 1 x Number of repeat spawners in year 2, DIVIDED BY
Number of marked fish from year 1 caught in year 2
The following is an example for how we sample and mark Steelhead to generate a population estimate using a revised Peterson estimate:
Bob catches and marks 250 Steelhead on River A in 2020 with an adipose clip. In 2021, Bob returns to River A and catches 300 Steelhead. Of the 300 Steelhead he catches in 2021, 12 of them have an adipose mark left from the previous year. After taking scale samples from each of those 300 fish and aging them, Bob determined that in 2021 the repeat spawning rate was 60% (this means that of the 300 Steelhead that Bob caught in 2021, 60% of them were present in the spawning population in 2020, 0.60 x 300=180). Using the modified Peterson estimate, here is Bob’s population estimate for River A in 2020:
250 x 180 = 45000 /12 = 3750
Based on Bob’s data, the population of adult Steelhead in River A was 3750 in 2020. To generate a population estimate, it is imperative that fish are marked in consecutive years as the formula does not allow for years to be skipped.
There are 7 different fins that have been used to mark Steelhead since the inception of the program in 1991. These fins include: the dorsal fin (only the first few rays), adipose fin, left/right pectoral fins, left/right ventral fins and the anal fin (no longer used). A common misconception is that there are Steelhead swimming around that have no fins left as a result of being marked for many years in a row. For this to be true, a Steelhead would have to have been sampled by a volunteer for seven years in a row. Most Lake Superior Steelhead die before reaching age 8 and often only spawn four times. Additionally, without any angling or harvest mortality, Steelhead populations in tributaries along the North Shore have an annual average mortality of around 30%. This means that without ANY mortality caused by human intervention, approximately 30% of a given adult Steelhead population dies annually. Therefore, the likelihood of sampling and marking a Steelhead for 7 years in a row is (astronomically) small. To illustrate this, there have been 14,967 different Steelhead marked across many different tributaries since 1991. Over this 30-year time period there has been 1 Steelhead out of 14,967 that was caught and marked for 6 consecutive years. This fish was marked every year for 6 consecutive years, in Portage Creek. Out of the 14,967 Steelhead that been marked, 3167 of them were caught in subsequent years and marked a second time. Therefore, 79% of marked Steelhead have only been marked a single time. Only 990 of 14,967 Steelhead have ever been marked more than two times in the 30 years that Steelhead have been marked along the North Shore.
Consequences of marking Steelhead
Marking fish with a fin clip is a common technique used around the world to create an easily identifiable mark on a fish. In addition to mark-recapture studies, another common use of marking fish is to clip fins on stocked fish so that anglers can easily distinguish between stocked and wild fish. Some people may feel as though marking Steelhead may lead to increased mortality. However, results collected from the Co-operative Angler Program do not support his notion. If our results showed that marking Steelhead caused increased mortality, we would not continue to do it. Furthermore, many Steelhead partially regenerate their fins after being marked (see images below).
As we can clearly see in Figure 1., the annual mortality rate of Steelhead in the McIntyre River has not increased since the McIntyre Steelhead Population Assessment Study started in 2008. This plot also shows that the population in the McIntyre River has increased substantially since the study began. The reason why the McIntyre River population has increased is likely because of the harvest restriction of 1 over 69cm along with catch-and-release practices of legal sized Steelhead that many anglers have adopted.
