Science Friday: Steelhead invade the east!
While our Initiative is focused on the West Coast where steelhead are native, there is also interesting research being done elsewhere in areas where they are not native. For example, nearly every angler knows that we also have “steelhead” in numerous streams draining the Great Lakes in the Midwest and East Coast of the United States. Those fish in the Great Lakes have been there for decades, and are admired and loved by thousands of anglers.
Of course, technically speaking they are not a true steelhead. The definition of a steelhead is a rainbow trout that goes to the ocean. In the Great Lakes the rainbow trout are migrating to large freshwater lakes, which makes them – again, technically speaking – an adfluvial rainbow trout. Adfluvial simply means that the fish spawn and rear in streams but migrate to lakes to feed. Regardless, we refer to the Great Lakes fish as steelhead because it is simpler and they look and fight like steelhead.
What many anglers may not realize is that scientists are trying to determine whether any of those fish on the East Coast are actually making it to the Atlantic Ocean.
Here we review one such study (Here is a link to the abstract: http://onlinelibrary.wiley.com/doi/10.1111/j.1600-0633.2010.00417.x/abstract). It was conducted in the St. Lawrence River system in Eastern Canada, where rainbow trout have been stocked for several years. The stocking lead to the establishment of some self-sustaining populations of rainbow trout in Eastern Quebec. The question is: Did any of those fish make it to the sea?
To help answer that question a group of scientists sampled rainbow trout in 2005 and 2006 from several locations in the St. Lawrence River. Most fish were captured from June to August.
The scientists collected a tissue sample for DNA analyses to determine the origin of the fish and also extracted their otoliths. Otoliths are tiny ear bones, and over time, they accrete different chemical signatures based on where the fish live due to differences in water chemistry. This means we can determine whether a fish has been to salt-water by measuring the microchemistry of the otolith. To accomplish this, the authors looked at strontium:calcium ratios, which differ substantially between fresh- and salt-water. Basically, if a fish goes to the ocean there should be a higher strontium:calcium ratio than if the fish remained solely in freshwater. The patterns are so strong that we can even determine whether the fish’s mother went to the ocean. And that was a key part of this study: How many fish went to the ocean, and what proportion had mothers that went to the sea?
The results are interesting. First, they found that thirteen fish out of a couple hundred had been to the ocean and were anadromous – these are true Atlantic Ocean steelhead.
Second, about 30% of the fish had anadromous mothers, meaning that almost one-third of the fish came from steelhead moms. So, this provides even more evidence of anadromy.
Third, in terms of life histories, most fish that became smolts and migrated to the ocean did so at age-3, which is a fairly common smolt age for steelhead on the West Coast. In addition, three of the thirteen fish that were steelhead were also repeat spawners.
The study suggests that rainbow trout are invading new habitats in Quebec outside of those immediate areas where they were stocked. Given that anadromy requires an extensive migration, it is plausible that part of that range expansion is due to the onset and success of anadromy. Although steelhead are not native to the region, much can be learned about how anadromy is established by studying the species after they are introduced to new areas. This is certainly one such case.
Whatever the mechanisms, the fact is that there are now steelhead entering saltwater beyond the Great Lakes on the East Coast. While there are many specifics yet to be determined, such as where they go in the ocean, it will be interesting to see how the populations respond in the coming years and decades.