Is Overwinter Mortality of Age-1 Alewife Size-Selective?

Alewife (Alosa pseudoharengus) are the primary prey species supporting recreational fisheries for salmonids in Lake Ontario, yet wide fluctuations in both recruitment and survival complicate population projections.  Overwinter mortality affects age-0 alewife, but it is not clear if additional size-selective mortality occurs on age-1 fish during their second winter of life.  We evaluated this question using data from 2005 and 2006 year classes of alewife collected during standard assessments on Lake Ontario.  We used image analysis software to measure otolith radii at formation of the first annulus for alewife of known length.  A positive relationship existed between age-1 alewife total length and otolith radius (R²=0.84), which we used to back-calculate the size at first annulus formation for age-2 fish (fish that had survived the second winter of life).  We compared size at first annulus formation for age-1 alewife to the predicted size at first annulus formation for alewife that survived to age-2 to determine if size selective mortality occurred, and if so, what length was reached by alewife that survived over-winter.  Our results showed evidence for size-selective mortality for the 2006 year class (minimum size threshold at age-1 of 75 mm), but not the 2005 year class.  Year classes differed markedly in size (2005 large, 2006 small), indicating that density dependent factors or interactions of these factors with climate contribute to over-winter mortality of age-1 alewife.