Estimating Natural Reproduction By Atlantic Salmon in Maine through Sibship Analysis

Atlantic salmon in the Gulf of Maine Distinct Population Segment are currently listed as endangered, and the population is maintained through hatchery supplementation.  Natural reproduction by Atlantic salmon in Maine rivers is limited, but evaluation of the number of Atlantic salmon reproducing in the wild is critical to assess progress towards recovery.   The number of reproducing adults is estimated through redd counts, which may not accurately reflect the number of individuals spawning due to reproduction by precocious parr or the contribution of adults to multiple redds.  To obtain broodstock from six of the seven captively spawned populations, parr are collected from the wild. For each of the six populations, the majority of each parr collection generally represents juvenile salmon stocked as fry (44-80% hatchery origin, for the 2008-2011 parr collection years based on genetic analysis of parentage).  However, it is assumed that natural reproduction contributes to the remainder of the parr collected.  To estimate the number of parents contributing to the parr not assigned to hatchery parents, sibship analyses were used.  Estimates of the number of parents were compared to linkage-disequilibrium based effective population size estimates to obtain Ne/N ratios and to redd counts to assess accuracy.