Effects of parasites from salmon farms on productivity of wild salmon

Abstract

The ecological risks of salmon aquaculture have motivated changes to management and policy designed to protect wild salmon populations and habitats in several countries. In Canada, much attention has focused on outbreaks of parasitic copepods, sea lice (Lepeophtheirus salmonis), on farmed and wild salmon in the Broughton Archipelago, British Columbia. Several recent studies have reached contradictory conclusions on whether the spread of lice from salmon farms affects the productivity of sympatric wild salmon populations. We analyzed recently available sea lice data on farms and spawner-recruit data for pink (Oncorhynchus gorbuscha) and coho (Oncorhynchus kisutch) salmon populations in the Broughton Archipelago and nearby regions where farms are not present. Our results show that sea lice abundance on farms is negatively associated with productivity of both pink and coho salmon in the Broughton Archipelago. These results reconcile the contradictory findings of previous studies and suggest that management and policy measures designed to protect wild salmon from sea lice should yield conservation and fishery benefits.

Fig. 1.

Map of Broughton Archipelago showing the study area, salmon farms (squares), and salmon spawning rivers (pink salmon, Embly, Wakeman, Kingcome, Viner, Ahta, Kakweiken, Lull, Glendale, and Ahnuhati; coho salmon, Embly, Wakeman, Kingcome, Viner, Shoal, Ahta (two rivers), Kakweiken, Lull, Glendale, Kwalate, Ahnuhati, and Klinaklini). Also shown are the migration routes considered in the infection scenarios. The northwestern unit of the Broughton Archipelago consists of the Embly River and the farms represented by open squares. The southeastern unit consists of the other 12 rivers and farms represented by solid squares. See Data in Methods for infection scenarios.

Fig. 2.

Estimated percentage of mortality, [1 − exp(cL_t+τ)] × 100 (±95% credible intervals) (Methods), per cohort of pink salmon and coho salmon due to sea lice from salmon farms over brood years 1999–2007. (A–D) Results of infection scenarios 1–4. See Data in Methods for infection scenarios. Numerical values for the estimates are in Table S2.