Located within the Skagit River tidal delta, the Fir Island Farms Restoration project restored approximately 53 hectares (131 acres) of tidal marsh and tidal channels. We monitored habitat conditions and fish use of the restored area and reference sites during the juvenile Chinook salmon outmigration period for two years pre- (2015 and 2016) and post-restoration (2017 and 2018).
Prior to the dike setback, the existing gravity operated tide gate was expected to prevent fish and salt water from entering pre-existing channels ‘inside’ of Fir Island Farms. However, we observed the tide gate leaking and measured water surface elevation fluctuations synchronous with daily tidal cycles suggesting Fir Island Farms ‘inside’ habitat areas before restoration were subject to some tidal influence and potential passage of estuarine fish species into ‘inside’ areas of Fir Island Farms prior to restoration.
Over the four years of beach seine sampling, we caught over 80,000 fish comprised of 21 different species. Juvenile Chinook were caught upstream of the tide gate in 2015 and 2016 before restoration occurred confirming that the leaking tide gate did allow upstream fish passage. After restoration, in 2017 and 2018, juvenile salmon and estuarine fish species catches increased upstream of the removed tide gate, while three-spine stickleback catches declined. Prior to restoration in years 2015 and 2016, wild juvenile Chinook abundance for ‘inside’ habitat of Fir Island Farms was estimated at 118 and 566 fish per year. Following restoration, we estimated a total annual Chinook abundance in the ‘inside’ habitat areas of 50,522 and 11,124 in 2017 and 2018. We hypothesize that the large increase in juvenile Chinook abundance following restoration was due to unobstructed fish access to newly restored habitat and a ~30-fold increase in potential rearing habitat in the form of newly wetted area. Analysis of seasonal density of juvenile Chinook salmon at Fir Island Farms compared to long term monitored reference sites in the Skagit tidal delta suggests that the restored habitat of Fir Island Farms is utilized by juvenile Chinook consistent with levels of other areas within the Skagit tidal delta. Predicted juvenile Chinook carrying capacity for the Fir Island Farms restored area varies by tens of thousands of fish per year as a function of predicted habitat amount, from 8,000 to 50,000 juvenile Chinook. The 50,000 fish estimate is most consistent with habitat conditions observed in 2017 and 2018. Predictions of channel allometry suggests tidal channel area at Fir Island Farms will decline from the 7+ hectares observed immediately after restoration to a sustainable equilibrium amount of 2.22 hectares (0.89-5.52 80% CI) hectares. Consistent with projected declines in channel area, we hypothesize that use of the restored area by juvenile Chinook will also decline. Prior to restoration, the magnitude in difference in annual growth rates of juvenile Chinook salmon upstream of the tide gate compared to reference sites, as inferred by seasonal changes in fork length, was highest compared to juvenile Chinook encountered in the ‘inside’ area post- restoration. Presumably the low densities of juvenile Chinook in the habitat upstream of the tide gate prior to restoration resulted in low competition for food and space. After restoration, the size difference between fish in the restoration site and those in the adjacent reference sites decreased. Although many mechanisms may have influenced this observation, we hypothesize that this inferred reduction in growth may be due to increased competition for food as a result of increased fish abundance, despite the increase in juvenile rearing habitat.
We suggest periodic monitoring of habitat (channels, hydrology, vegetation) coupled with juvenile fish monitoring to determine the trajectory of fish response to habitat as they move toward their sustainable equilibrium conditions. The frequency of monitoring can be determined based on the expected rate of change in habitat trajectory.
Beamer, E.M., Henderson, R., Wolf, K., Demma, J., Hood, W.G., 2018. Juvenile Chinook salmon response to dike setback restoration at Fir Island Farms in the Skagit Riber tidal delta, 2015-2018. Skagit River System Cooperative, La Conner, WA. pp. 68.
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