Summary of Sibship Method of Determining Young-of-year passage through
Crossings
By Jason Coombs, Post-doc research assistant, Univ of Massachusetts, Amherst
June 2010
Barriers restricting, or even eliminating, individual movement among freshwater habitat patches
has become a common occurrence. This reduction, or loss, of movement potential results in population
fragmentation, which in turn increases rates of population inbreeding and extirpation. Thus, barrier
remediation within freshwater systems has become a priority for both conservationists and managers alike.
However, barrier remediation efforts face two key challenges. The first involves ranking the overwhelming
number of potential barriers in freshwater systems to determine which remediation effort would provide
the greatest impact to affected populations. The second challenge is determining if the remediation effort
was successful.
Both of these challenges require addressing the basic question “Are individuals moving across the
barrier, and if so in which direction?”. Traditionally, this would require the use of a capture-mark-recapture
(CMR) experimental design. Outlined briefly, individuals would be initially captured, marked with an
individual or habitat specific identifier, and returned before executing a second capture event after an
elapsed period of time to recapture marked individuals and assess movement rates and directions.
However, the advent and advancement of molecular techniques and relationship reconstruction algorithms
has made another alternative possible. Like CMR, the populations adjacent to the barrier would be
sampled. Unlike CMR, individuals would not be tagged, but instead donate a tissue sample as a source of
DNA. Molecular techniques would then acquire individual genotypes which, when run through a
relationship reconstruction algorithm, would delineate individuals into full-sibling families. Movement
across a barrier would then be determined by whether all members of a full-sibling family were on one side
or both sides of the barrier. Directionality of movement could be attained through use of a majority-rule
approach, where the side with the greatest proportion of family members is assumed to be the natal patch,
and direction of movement would be away from this side. In addition to this key assumption, a second
assumes that a parental pair reproduces on only one side of a barrier. Initial data for brook trout (Salvelinus
fontinalis) shows these assumptions to be likely, but requires further assessment.
Simulations assessing the accuracy of this method for a variety of population differentiation (DST =
0, 0.1, 0.25) and movement restriction (two-way, one-way, none) scenarios have been positive, with
greater than 95% accuracy for assigned movers and directionality achieved for all scenarios. Additionally,
the use of this method does not preclude the use of traditional CMR techniques in that individual
genotypes can be used as individual identifiers. Furthermore, unlike traditional CMR techniques, this
method is able to use information on parent locations during spawning/birth to determine natal patch
origin, lessening the dependence on the majority-rule assumption. Parents can either be known directly or
assigned genetically through relationship reconstruction algorithms. In summary, we feel that this method
holds great promise for barrier assessment in that it is able to provide estimates of both movement rate
and directionality without the time lag of traditional CMR techniques.