Response to Behnke on Pascual et al., First documented case of anadromy in a population of introduced rainbow trout in Patagonia, Argentina

PASCUAL, M.A.; KINNISON, M.; RIVA ROSSI, C.M.

TRANSACTIONS OF THE AMERICAN FISHERIES SOCIETY

The American Fisheries Society

Lugar: Bethesda, MD, United States; Año: 2002 vol. 131 p. 585 - 588

1548-8659

The comments raised by Dr. Behnke deal with two subjects: the origin of the Santa Cruz River fish and the interpretation of the genetic relationships between anadromous and resident fish. Fish introductions in Argentina began over a decade after the Baird Station on the McCloud River had closed, so rainbow trout eggs could not have come directly from this station. The Baird Station was a major source of rainbow trout used to found stocks at federal hatcheries around the United States, which led us to take it as the most likely source for Santa Cruz fish. However, it is likely that a mix of anadromous and resident fish probably constituted the parental stock of Santa Cruz fish, originating most likely in the Sacramento River and in other rivers of California and Oregon and these same stocks were planted elsewhere in Argentina and around the world. Population genetics analysis used to evaluate for the existence gene flow between life history forms and to search for the source origin revealed that there is little population structuring between the forms. However, we never claimed that they conform a panmictic population. This is a misquote by Dr. Behnke. We only indicated that the two forms appear to share common ancestry, appear to exchange migrants, and may arise within a single nominative population. This population may undergo a significant amount of assortative mating, or structuring in other ways. Whether one is dealing with alternate life histories within a single population or two sympatric populations with significant gene flow. The alternatives are just  “two sides of the same coin”. Dr. Behnke’s also suggests that because we refer to “life history forms” we attribute no genetic basis to the forms, which may in time, degenerate into to hybrid swarms and loss of life history uniqueness. This is an over-simplified characterization of gene flow and its interaction with selection and the inheritance of anadromy. These results show that steelhead cannot be managed as a single, separate population Further studies will provide us with crucial insight about the origin of the anadromous behavior and about what the management units should be.