Mortality events resulting from Australia's catastrophic fires threaten aquatic biota

Abstract

The consequences of the 2019–2020 bushfires in Australia were also devastating for the aquatic biota. Following abnormal rainfall events in burnt areas, widespread mortality events including fish and invertebrates were recorded in estuarine and freshwater systems. Such negative impacts on aquatic resources highlight the need to include these ecosystems in bushfire recovery plans. Management should prioritise catchments at higher risks of further negative impacts and research must be conducted to understand the efficacy of actions post-fire. The size and intensity of bushfires during the 2019–2020 Australian season has been unprecedented (Nolan et al., 2020). The fires in south-eastern Australia were extraordinary in terms of the land area burnt (7.2 million ha; Figure 1); four times that of the 2019 Brazilian Amazon and 1.8 times the 2017 United States' fires (Bladon, 2018). The frequency and intensity of fires are predicted to increase over coming years as the Australian climate becomes warmer and drier (Leigh et al., 2015). Source of decile maps: Bureau of Meteorology, 2020; http://www.bom.gov.au/climate/austmaps/about-rain-maps.shtml#deciles Prolonged drought conditions have resulted in water scarcity and spatially continuous dry fuel loads that have both been linked to the intensity and extent of the fires (Nolan et al., 2020). Other fire events have had catastrophic impacts on terrestrial and aquatic ecosystems (Emelko et al., 2016) and the 2019–2020 season was predicted to have negative impacts across catchments within Australia (Alexandra & Finlayson, 2020). Over 43 catchments were burnt across a diversity of landscapes, and there is the high likelihood that downstream cumulative watershed effects will result (Emelko et al., 2016). Most studies focus on localized (e.g. single catchment) impacts, such as degraded water quality (Emelko et al., 2016), direct (Lyon & O'Connor, 2008) or indirect (Bozek & Young, 1994) mortality of aquatic species, or altered food-web structure (Carvalho et al., 2019) in freshwater systems (Bixby et al., 2015). By contrast with previous fires, the scale of those in 2019–2020 challenges our current knowledge and ability to effectively manage and respond to the impacts. Following a period of above average rainfall in the majority of the fire-impacted catchments (see Figure 1 for details), there were noticeable deaths of aquatic fauna, however, the full extent of these events remains undocumented. Rainfall records indicate abnormal events pre-fires (severe dry conditions) and post-fires (significant rainfall), related to the mortality events (Figure 1). Incidents reported in the media, state fisheries agency records and direct field observations have been used here to inform the extent of impacts to aquatic biota. Public outcry was precipitated by the incidence of large dead fish, including the threatened trout cod (Maccullochella macquariensis) and Murray cod (Maccullochella peelii; Table 1). In total, the deaths of ca. 27 species of freshwater and estuarine fish, along with four species of crustaceans, some of which are endemic, have been reported from 15 waterways and 17 locations across New South Wales (NSW) and Victoria (Vic.; Table 1). The highest number of species killed was documented in the Macleay River in northeast NSW (eight), including one event on its estuarine reach, and in the Upper Murray River region (NSW/Vic. border; also eight; Table 1). To our knowledge, this is the first record, globally, of fire events extending to, and impacting estuaries through the mortality of obligate estuarine species (11) in seven locations across south-eastern Australia (Table 1; Figure 1). Almost all estuarine sites with records of fish kills in this fire season were located downstream of burnt areas (Figure 1), posing previously unknown threats to these highly productive coastal systems. It is noticeable that no mortality was recorded from other burnt areas, such as the Hawkesbury-Nepean catchment, west of Sydney (Figure 1), perhaps because of the remoteness of many locations, and so a lack of reporting. The extension of the observed effects of fire to freshwater and estuarine systems is more far-reaching downstream than hitherto thought (e.g. ≈54 km in the Macleay River). Inland reaches were also impacted. For instance, the Upper Murray River also had a major mortality event extending approximately 70 km downstream (Figure 1) from the burnt area, with records of both large- and small-bodied fish (e.g. Australian smelt; Retropinna semoni). The processes by which fires impact freshwater ecosystems are inherently complex (Bixby et al., 2015) because they are influenced by the characteristics of the fire event (e.g. size), watershed (e.g. slopes, lentic or lotic waters) and hydrological events pre- and post-fire (e.g. drought, rainfall). Most of the fires in this 2019–2020 season burnt areas ranging from 800 to 2,000 m in elevation (Figure 1). The combination of fire severity, high elevation in the affected areas and thus steep slopes, followed by rainfall immediately after the fire, likely increased runoff and washed sediment, ash and debris to waterways (McInerney, Rees, & Joehnk, 2020). Dissolved oxygen concentration could have been rapidly depleted following increased carbon, and fell below tolerance ranges for fish and other aquatic biota. Other elements resulting from bushfires washed into waterways (e.g. nitrogen and phosphorus, methyl-mercury, burnt soils activated anions [nitrate and chloride] and cations [zinc and copper], and polycyclic aromatic hydrocarbons and polychlorinated biphenyls from ash and sediments) cannot be ruled-out as having detrimental impacts on aquatic biota (Harper et al., 2019). On the other hand, fire retardants used to extinguish fires across Australia such as Phos-Chek® D75 and WD881, which have been classified as ‘practically nontoxic’ and readily biodegradable in ecological toxicity tests, respectively (see Phos-Chek safety data sheets), are unlikely to cause significant or widespread mortality of adults, although the effects on aquatic invertebrates, larval and juvenile stages are unknown. Australia's freshwater fauna is characterized by a high level of endemism and low fish diversity (Collen et al., 2014), so any loss is of great concern. Current bushfire recovery plans must include freshwater resources and urgently (a) identify immediate and long-term actions to mitigate impacts; and (b) develop medium- and long-term research plans based on sound science to advance knowledge aimed at enhancing freshwater ecosystem resilience. While immediate responses have focused on rescuing threatened fish and listing species as a national priority for urgent management, research plans still need to be developed to understand the efficacy of such actions. Management units have been prioritized based on aquatic biota and catchments at greater risk post-fire, given that such events are predicted to be more common in future. Freshwater ecosystems are recognized as the most threatened globally (Vorosmarty et al., 2010), yet fire impacts are rarely acknowledged as a threat in the scientific literature and in water management policies. We acknowledge the Institute for Land, Water and Society, Charles Sturt University, for the funding provided, Cameron McGregor for valuable contribution collecting field data and Nathan Ning for assisting with the initial database.

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