Steelhead Parr Maturation as an Indicator of Life History Pathway

Both steelhead and resident rainbow trout can produce resident and anadromous offspring.  Regardless of parental origin, a high level of uncertainty exists regarding the process by which a juvenile Oncorhynchus mykiss decides to follow an anadromous pathway or remain in freshwater and become a resident (or precocial male)?  We investigated the relative abundance of juvenile O. mykiss that appear to be sexually maturing in freshwater as age-1 and age-2 parr in Washington’s Hood Canal.  Fish sex was determined using microscopy.  Gonad development was visually classified as immature, developing or mature based on the proportion of the body cavity that was occupied by gonads, and calculated Gonad Somatic Index (GSI) value.  Based on the assumption that juvenile O. mykiss preparing to out-migrate will not expend unnecessary energy into premature gonad development, we predict that parr on an anadromous pathway will be classified as immature.  We assumed parr with developing gonads are likely going to become resident fish because of their early reproductive investment.  We also assume any fish containing mature gonads are likely precocial males or resident pre- or post-spawn females.  O. mykiss parr measuring 100-170mm were sampled from seven different Hood Canal rivers including Big Beef Creek, Dewatto, Tahuya, Skokomish, Hamma Hamma, Duckabush and Dosewallips Rivers (n ≈ 240/yr).  Parr sampling occurred in 2006 – 2008 during August and September in a lower, middle and upper reach on each drainage downstream of the upper limit of anadromy.  O. mykiss populations upstream of impassable barriers (i.e., falls) on three of the study rivers were also sampled (n = 90).  All samples were stored frozen (-30°C) and dissected in a laboratory setting.  Male O. mykiss parr GSI values ranged from 0.001 – 9.780, while female values ranged from 0.059 – 1.183.  Immature, developing and mature steelhead male parr were all observed in different ratios in the Hood Canal Rivers.  Only immature and developing female parr were found.  Interannual variation of O. mykiss developmental stages observed in each individual river was minimal.  The overall proportion of parr developmental stages sampled in the anadromous portion of all rivers was similar between years, averaging 69-percent immature, 30-percent developing and 1-percent precocial males.  Approximately 80-percent of the parr collected upstream of impassable falls were classified as developing (i.e., likely residents).  Future work will look at combining otolith microchemistry and parr maturation data to gain greater perspective on resident fish contribution to the anadromous form.