Extrapolating Growth Reductions in Fish to Changes in Population Extinction Risks: Copper and Chinook Salmon

ABSTRACT

Fish commonly respond to stress, including stress from chemical exposures, with reduced growth. However, the relevance to wild populations of subtle and sometimes transitory growth reductions may not be obvious. At low-level, sustained exposures, Cu is one substance that commonly causes reduced growth but little mortality in laboratory toxicity tests with fish. To explore the relevance of growth reductions under laboratory conditions to wild populations, we (1) estimated growth effects of low-level Cu exposures to juvenile Chinook salmon (Oncorhynchus tshawytscha), (2) related growth effects to reduced survival in downriver Chinook salmon migrations, (3) estimated population demographics, (4) constructed a demographically structured matrix population model, and (5) projected the influence of Cu-reduced growth on population size, extinction risks, and recovery chances. Reduced juvenile growth from Cu in the range of chronic criteria concentrations was projected to cause disproportionate reductions in survival of migrating juveniles, with a 7.5% length reduction predicting about a 23% to 52% reduction in survival from a headwaters trap to the next census point located 640 km downstream. Projecting reduced juvenile growth out through six generations (∼30 years) resulted in little increased extinction risk; however, population recovery times were delayed under scenarios where Cu-reduced growth was imposed.

Key Words:

• Cu
• extinction risk
• Chinook salmon
• population modeling
• size-selective mortality
• threatened species

ACKNOWLEDGMENTS

We thank the staff of the Idaho Department of Fish and Game, particularly Timothy Copeland of the Natural Production Program, for sharing of outmigrant data collected at the trap. We are grateful to Alec Maule and Christian Torgersen for their critical reviews of an early version of this article.

This article not subject to U.S. copyright law.

The National Marine Fisheries Service (NMFS) funded analyses on which this article is based; however, the views expressed are the authors’ and are not official policy views of NMFS or USGS. This article has been reviewed in accordance with USGS policy. Mention of trade, product, or firm names is for descriptive purposes and does not imply endorsement by the U.S. government.

Notes

*Significantly different from control, p = .05, Dunnett's Procedure. Water chemistry estimates: hardness (mg/l CaCO3) 25.4 ± 3.9, alkalinity (mg/l CaCO₃) 23.9 ± 2.3, pH 7.32 ± 0.07, T 12.2°C; dissolved organic carbon 1.2 mg/l, Ca 6.8 mg/l, Mg 1.8 mg/l, Na 5 mg/l, K 0.6 mg/l, sulfate 4.2 mg/l, chloride 6 mg/l (Chapman 1978, 1982; USEPA 2007).

^aValues not used because trap was operated for shorter periods than other years.

^bBecause no redds were detected by air or ground in the index reach, SAR must have been close to zero, although it could not be estimated in the same manner as years with detections.