Results
Adult Population
The adult steelhead population in Portage Creek has undergone significant fluctuations since the study began in the early 1990's (Figure 1). The estimated adult population size went from 500 in 1992 to over 2000 in 2004 and back down to 400 in 2013 (Figure 1). The initial increase in the adult population can be attributed to higher production of juveniles (e.g. 2000 year class), and better survival of adults following the reduction of harvest in 1994. The fluctuation in adult steelhead abundance from 2004 to 2013 can be attributed to environmental conditions (increased water temperature and reduced flow) in Portage Creek, and poor lake survival of juvenile steelhead in Lake Superior (Black Bay). Figure 2 illustrates the low recruitment of juveniles since 2004. Figure 3 shows the 2013 age structure in Portage Creek (based on an estimated population size of 400 adults). With limited recruitment from since 2004, the 2012 (Figure 2) population size (488 +/- 166) has been reduced to similar adult numbers that were observed in the early 1990's.
Note: The 400 adults calculated for the 2013 spawning migration is a preliminary estimate based on normal mortality and catch rates. A more precise estimated will be calculated in 2014 with a mark recapture "Petersen Estimate".
At three to five years of age, most Portage Creek steelhead had reached maturity and are at the most abundant level in the spawning population. With ~ 40% natural mortality following each annual spawning event (predation and the rigors of migration), few adults survive past eight years of age (in rivers that have harvest in addition to natural mortality). When the strong year classes of 2000, 2001 and 2004 were produced in Portage Creek, they maintained reasonable representation to older ages. The resilience of strong year classes is mainly due to the reduction of harvest. This is illustrated in Figure 3 showing the 2004 year class (age nine years) still contributes almost 20% to the 2013 spawning migration. At this age, they are for the most part large, fecund, multiple repeat spawners (five to seven spawning events) with high potential egg production, and the ability to spawn in the best stream locations.
Life history strategies
In Portage Creek, one year of stream life is a dominant life history strategy. Figure 4 illustrates the number of stream and lake years (by sex) expressed in the adult population. Once in Lake Superior, one to three years is required prior to the first spawning event. Males may reach maturity following one year in the lake, while females generally require two or three years in the lake before spawning for the first time (Figure 5).
Steelhead growth is slow while in the stream environment and rapid once in Lake Superior. Figure 6 illustrates the average fork length of first-time spawning adult steelhead with one stream year at smolting. In Portage Creek, a one-year-old stream steelhead is approximately 8 cm in length. After one year in the lake, the same fish will average 30 cm, 50 cm after two lake years, and 58 cm after three lake years. Following the first spawning event, growth slows as the spent adults must recover from spawning and replenish their gonads in order to reproduce again the following spring. The largest steelhead in Portage Creek (>70 cm) are often three and four lake years at maturity, rather than old individuals.
