The Stream Doctor

October 27, 2008

Email YOUR Questions directly to the Stream Doctor. This is your opportunity to get an experts professional opinion on anything stream related.

Question: I've read some of your articles on Fly Anglers Online and I have decided to write you an email to see if you can help me with a stream ecology topic. My question regards this kind of a scenario: a typical trout stream, with a high angling pressure year round (most of the fish caught by local anglers are sacrificed for human consumption), a relatively low trout population (mainly rainbows but also browns), I would define the sector of the river which I'm trying to describe as an epipotamal section which flows with those characteristics for 30 miles aprox, a rainy mediterranean climate, etc. My concern regards the genetics of low population sizes, specifically stream trout, and have wondered of the genetic impacts that this has on the recuperation to a sustainable population size. I guess my question is, since trout are territorial, how do individual trout breed to ensure healthy gene frequencies (through wild/natural mechanisms of course) in small population sizes? Is it possible that in a section of a river with a low fish count per mile have problems in relation to breeding and gene pools? Would it need human intervention in relation to programs destined in fish population breeding/recovery? I have tried to look for literature, and found some books on conservation genetics, but haven't had the opportunity to purchase any and read them. I don't know where to look for information on trout populations and their genetics/breeding, and thought of writing to you. If my stream scenario is a bit confusing, I can write to you a more detailed writing. I hope you can write me soon.

I study Veterinary Medicine in Chile and plan on getting a postgraduate degree in wildlife veterinary medicine or freshwater fisheries. These kinds of topics are of interest to me, and I hope we can write each other so that I can learn some things from you. Hans Krarup Swenson, Universidad Austral de Chile, Valdivia, Chile

Response: This question is outside of my area of expertise, so I took the liberty of forwarding it to two colleagues knowledgeable in trout genetics. To date, only one has responded and rather than delaying any longer, I am submitting his reply below.

Dr. Robert Behnke states, "In nature, populations under natural selection are in no danger of suffering from "inbreeding depression." This is because negative alleles are purged from a population in Darwinian evolution. I've commented on this in some of my publications based on many examples where nonnative brook trout were chemically treated before attempting native cutthroat restorations. If one male and one female brook trout remain, they take over the stream again in a few generations. Why don't they suffer from inbreeding depression? I once reviewed a text on conservation biology. In the genetics chapter a standard cook book warning of dangers of inbreeding depression was given. The nonnative species chapter discussed 3 goats introduced on one of the Galapagos Is. Within about 10 years, 10,000 goats divested the island. All descendents came from one or two males and one or two females (3 goats started it all). Like the brook trout, the goats didn't suffer from inbreeding."

I hope this helps a bit, and if my second respondent replies, I will include his observations in a follow-up column.

Take care,

If you have a question, please feel free to contact me.
~ C. E. (Bert) Cushing, aka Streamdoctor
105 W. Cherokee Dr.
Estes Park, CO 80517
Phone: 970-577-1584

The 'Stream Doctor' is a retired professional stream ecologist and author, now living in the West and spending way too much time fly-fishing. You are invited to submit questions relating to anything stream related directly to him for use in this Q & A Feature at

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