Coastal Cutthroat Trout as Sentinels of Lower Mainland Watershed ...

33Overall, estimates of parr-to-adult (sea-run) survival are likely to be more useful formanagement purposes because parr densities are typically estimated rather than smoltyields. Furthermore, parr densities are more suitable as a benchmark of stock statusbecause the nursery area for smolts is rarely known. In addition, pre-smolt cutthroat maymove downstream from small nursery habitats into mainstem reaches for final rearing tosmolt stage, and the nursery area sets capacity (R. Ptolemy pers. comm. 2005).Mean percent fry-to-1+ parr survival, can also be roughly estimated from comparingdifferent year classes, using fry and parr abundances from DeLeeuw and Stuart (1982),which was 35.6 % (+ or - 1 SD = 0.27). By sub-region, apparent survivals were 31.5,33.3 and 50.1 % for Lower Fraser agricultural-suburban, urbanized Fraser and NorthShore, and Mainland Coast streams, respectively. However, up to 50 % of cutthroat cansmolt at age 1 in the more productive streams of the Lower Mainland Region, includingNathan Creek, Salmon River and Little Campbell River (Rempel et al. 1984). Thus,these estimates can be considered to be high. Comparative estimates for steelhead troutfrom the mainstem Keogh River (under oligotrophic conditions with moderate frydensities) are 7.5 % egg-to-fry survival (to mid-summer fry stage), 12.5 % fry-to-parrsurvivals, and 40 % age 1+ and 2+ parr-to smolt survival (50 % from age 1 parr and 80 %from age 2 parr, or 40 % in composite) (Ward and Slaney 1993).Given additional research on salmonid survival rates in the Pacific Northwest since 1980,the age specific survival rates of coastal cutthroat trout employed by DeLeeuw and Stuartas “biostandards” should be revised using age-specific survivals of:• 10 % egg-to-fry;• 20 % fry-to-parr;• 40 % parr-to-smolt; and• 25 % smolt-to-adult under the past ocean regime (likely 15 % currently).Thus, instead of survival “biostandards” of 1.1 % fry-to-adult and 8 % parr-to-adult asassumed by DeLeeuw and Stuart (1982), under the current ocean regime, 1.2 % fry-toadultand 6 % parr-to-adult are considered more applicable to small cutthroat streams.However, as with steelhead trout, fry-to-adult survivals have limited application owing todensity dependence that occurs at the fry stage. Furthermore, age-specific survivals canvary with complexity of habitat and stream productivity.As a density dependent mechanism, the amount of habitat and food abundance sets aceiling or asymptote to the abundance of parr and thereby smolt production of sea-runcutthroat trout (Figure 9). As the juvenile densities in Figure 9 are not from the samecohort, this relationship is only a crude approximation; however, one generated fromsteelhead using the same cohort data is similar, albeit “flatter” at the asymptote (Wardand Slaney 1993). As extrapolated from more intensive studies of steelhead trout, theinflection point of the asymtote occurs at 20 fry per 100 m 2 and thus 20 fry per 100 m 2 isan adequate recruitment of fry in oligotrophic streams. A recruitment relation forsteelhead is available in Ward and Slaney (1993). Minimum fry densities required “toachieve capacity” (the asymptotic average level) will vary with stream productivity andtemperature as documented for steelhead streams. Thus, as is also evident from Figure 7,it requires about twice the fry to saturate a productive or fertile stream to capacity than an