Nisqually River Juvenile Salmonid Monitoring Report: 2009 -2015

The Washington Department of Fish and Wildlife (WDFW) installed an eight-foot diameter rotary screw trap in the Nisqually River at river mile 12.8, near the Centralia City Light Yelm Hydro Powerhouse in 2009 to monitor outmigrating juvenile salmonids. The primary objectives of this study were to estimate abundance and document total catch, run timing, size and age composition of outmigrating juvenile Chinook salmon and steelhead. Our secondary objectives were to (a) estimate the abundance of juvenile Coho, Chum and Pink salmon; (b) document total catch, run timing, size and age composition of outmigrating of juvenile Coho, Chum, and Pink salmon; and (c) document species and total catch of non-target fish captured in the trap. Information collected at the Nisqually River rotary screw trap monitoring project provides critical abundance and life history information for the salmon and steelhead stocks within the river. These data coupled with adult return data can be used to measure key survival and productivity metrics to assess management practices and evaluate recovery efforts. This report describes the results of the monitoring efforts during the 2009-2015 field seasons.

We estimated that 34,745-408,158-Chinook sub-yearlings, 467-15,240 Chinook yearlings, 20,178-94,704 steelhead smolts, 80,048-254,456 Coho smolts, 17,197-862,328 Chum fry, and 2.1 million-27.5 million Pink fry outmigrated past the trap annually from 2009 to 2015. Smolt recruits per spawner time series data was too sparse to fit a formal stock-recruit curve (e.g., Ricker or Beverton-Holt) for all species. However, Chinook smolt recruits per spawner productivity shows positive linear relationships for fry (defined as sub-yearlings . 45 mm), parr (subyearlings > 45 mm), and total juvenile outmigrations, indicating that there is no evidence that density dependence is limiting freshwater population productivity. These data suggest that the Nisqually River has ample high quality rearing habitat for the juvenile Chinook abundances we have observed thus far. However, in years of low abundance, outmigrations were dominated by parr outmigrants, with very few fry outmigrants, suggesting that the fry migration strategy was rare when rearing territories were likely occupied at low densities by juvenile Chinook. It was premature to discern any initial trends in productivity for Steelhead, Chum, and Pink and we were unable to calculate productivity for Coho because of uncertainty associated with estimates of spawner escapement. Sub-yearling Chinook had a protracted outmigration timing relative to the other salmon and steelhead captured at the trap.

Sub-yearling Chinook outmigrated continuously throughout the trapping season from January through August. We did not account for migration before and after the trapping period in the abundance estimate. The outmigration was typically bimodal and composed of recently emerged fry from January through mid-May followed by river reared parr from mid-May through August, a general pattern similar to other Puget Sound systems. However, in 2014 and 2015, when sub-yearling Chinook abundance was low, the migrationtiming curve was unimodal, consisting entirely of parr outmigrants. Steelhead Coho, Chum and Pink all had distinct unimodal migration timings occurring within the trapping season. Steelhead and Coho outmigrated during the lull between the two modes of the sub-yearling Chinook from April through June. Chum and Pink outmigrated from late March through early June.

Lengths of salmon and steelhead were collected systematically though their outmigrations to accurately characterize size over time. The mean fork length of sub-yearling Chinook fry ranged from 39.1 to 40.7 mm with little intra-annual variation. The mean fork length of sub-yearling Chinook parr ranged from 88.3 to 100.5 mm with greater intra-annual variation than the fry. Nisqually sub-yearling Chinook parr tended to migrate later and attain a larger body size than other populations monitored by WDFW in the Cedar, Green and Skagit rivers. Yearling Coho mean fork length ranged from 105.6 to 116.4 mm. Chum and Pink mean fork lengths ranged from 35.3 to 42.9 mm and 33.7 to 34.2 mm, respectively. Similar to Chinook fry, pink and chum typically outmigrate as newly emerged fry with little river rearing.

Steelhead smolt scales were collected in conjunction with fork lengths from 2011 to 2015 to describe annual age composition of the outmigrants, reconstruct broods, and estimate productivity. Nisqually steelhead smolts were relatively young with the age composition made up of predominately age-1 (range, 10.9% to 41.9%), age-2 (43.0% to 78.2%), and age-3 (11.9% to 28.6%) smolts. Age-4 smolts were rarely observed: three in 2011 (1.4%), two in 2014 (0.5%), and one in 2015 (0.3%). One age-5 (1.1%) and one age-6 (1.1%) were present in 2012. Steelhead smolts were relatively large at a given age compared to other populations monitored by WDFW (Skagit, Green, Dungeness, Big Beef, Duckabush), with substantial overlap among size ranges; age-1 (range, 142 to 227 mm), age-2 (150 to 350 mm), age-3 (162 to 241 mm). Length ranges of age-4 (190 to 246 mm), age-5 (315 mm), and age-6 (196 mm) overlapped those of the younger age classes. However, sample sizes were too small to make any inferences about the population.

The trap was used to opportunistically collect samples to study estuary usage, document Pacific lamprey presence, assess early marine survival of outmigrating Chinook, steelhead and Coho, collect and archive tissue samples for future genetic mark-recapture estimates of Chinook escapement, monitor freshwater health of outmigrating steelhead and investigate resident rainbow contribution to anadromous steelhead outmigrants.