The impact of temperature stress and pesticide exposure on mortality and disease susceptibility of endangered Pacific salmon q Joseph P. Dietrich ⇑ , Ahna L. Van Gaest 1 , Stacy A. Strickland 2 , Mary R. Arkoosh Environmental & Fisheries Sciences Division, Northwest Fisheries Science Center, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, 2032 SE OSU Drive, Newport, OR 97365, USA highlights  Lethal concentration curves for salmon exposed to malathion are determined at two temperatures.  Malathion toxicity increases with increasing temperature.  The effects of multiple stressors (temperature and malathion) on disease in salmon are examined.  Disease mortality increased 11.2% in salmon exposed to malathion at the elevated temperature. article info Article history: Received 8 November 2013 Received in revised form 18 January 2014 Accepted 30 January 2014 Available online xxxx Keywords: Salmon Malathion Disease susceptibility Temperature Multiple stressors abstract Anthropogenic stressors, including chemical contamination and temperature stress, may contribute to increased disease susceptibility in aquatic animals. Specifically, the organophosphate pesticide malathion has been detected in surface waters inhabited by threatened and endangered salmon. In the presence of increasing water temperatures, malathion may increase susceptibility to disease and ultimately threaten salmon survival. This work examines the effect of acute and sublethal exposures to malathion on ocean- type subyearling Chinook salmon held under two temperature regimes. Chinook salmon were exposed to malathion at optimal (11 °C) or elevated (19 and 20 °C) temperatures. The influence of temperature on the acute toxicity of malathion was determined by generating 96-h lethal concentration (LC) curves. A disease challenge assay was also used to assess the effects of sublethal malathion exposure. The mala- thion concentration that resulted in 50% mortality (LC 50 ; 274.1 lgL 1 ) of the Chinook salmon at 19 °C was significantly less than the LC 50 at 11 °C (364.2 lgL 1 ). Mortality increased 11.2% in Chinook salmon exposed to malathion at the elevated temperature and challenged with Aeromonas salmonicida compared to fish held at the optimal temperature and exposed to malathion or the carrier control. No difference in disease challenge mortality was observed among malathion-exposed and unexposed fish at the optimal temperature. The interaction of co-occurring stressors may have a greater impact on salmon than if they occur in isolation. Ecological risk assessments considering the effects of an individual stressor on threa- tened and endangered salmon may underestimate risk when additional stressors are present in the environment. Published by Elsevier Ltd. 1. Introduction A properly functioning immune system is critical to salmonid health and population conservation. Twenty-eight Pacific salmon stocks in the western US are currently listed under the Endangered Species Act (ESA) as either threatened or endangered [http:// www.nwr.noaa.gov/publications/protected_species/salmon_steel- head/status_of_esa_salmon_listings_and_ch_designations_map. pdf; accessed April 5, 2013]. Infectious disease has contributed to the extinction of species that were already endangered (Smith et al., 2009). Natural and anthropogenic stressors can impact pathogen virulence and host immune systems. A number of studies have http://dx.doi.org/10.1016/j.chemosphere.2014.01.079 0045-6535/Published by Elsevier Ltd. q Funds were provided internally by the National Oceanic and Atmospheric Administration, Office of Protected Resources. Individuals at the Office of Protected Resources provided input into study design and objectives, but did not participate in the collection, analysis, interpretation of the data, preparation of the manuscript, or decision to submit the manuscript. All experiments with the lab-raised salmon were in accordance with the U.S. Government Principles for the Utilization and Care of Vertebrate Animals Used in Testing, Research, and Training for the humane treatment of fish to alleviate suffering. ⇑ Corresponding author. Tel.: +1 541 867 0264. E-mail addresses: joseph.dietrich@noaa.gov (J.P. Dietrich), vangaest@gmail.com (A.L. Van Gaest), sas70@me.com (S.A. Strickland), mary.arkoosh@noaa.gov (M.R. Arkoosh). 1 Present address: Portland, OR, USA. 2 Present address: Newport, OR, USA. Chemosphere xxx (2014) xxx–xxx Contents lists available at ScienceDirect Chemosphere journal homepage: www.elsevier.com/locate/chemosphere Please cite this article in press as: Dietrich, J.P., et al. The impact of temperature stress and pesticide exposure on mortality and disease susceptibility of endangered Pacific salmon. Chemosphere (2014), http://dx.doi.org/10.1016/j.chemosphere.2014.01.079