The influence of abiotic incubation conditions on the winter mortality of wild salmonid embryos

Abstract

Embryos of many valued salmonid species incubate in the hyporheic zone of boreal streams over winter. Influence of individual winter‐related environmental variables on salmonid embryo success has been previously investigated. However, how multiple variables act together to influence embryo incubation remains poorly understood. Using a naturally spawning population of Atlantic salmon (Salmo salar) in the Miramichi River basin (New Brunswick, Canada), we related variation in the abiotic embryo incubation habitat in different streams (spatial) and over the course of two winters (temporal) to embryo mortality between fertilisation and hatch. Over two years (2013–2014 and 2014–2015), we introduced fertilised eggs to six simulated salmon redds in each of three riffles in each of five active spawning reaches (nredds = 90) with a range of hyporheic conditions. Embryo mortality was quantified at an early sampling event (March; pre‐freshet and during late embryonic development) and a late sampling event (May; post‐freshet and post‐hatch). We extracted 22 abiotic predictor variables for statistical analyses from continuous records of hyporheic environmental conditions, collected for the duration of the incubation period in each study reach. Through partial least squares regression analyses, 37.6% of the total variation in mortality was explained by the predictor variables. Each group of predictor variables explained similar proportions of variation (water temperature: 8.4%, water level: 7.4%, dissolved oxygen: 7.1%, ice conditions: 7.2%, and substrate characteristics: 7.5%), which suggests that mortality is influenced by multiple interacting abiotic conditions, rather than a single variable in isolation, and that the factors contributing to ideal salmonid incubation habitats are complex and interconnected. Our research highlights the value of a multi‐faceted research perspective and provides a baseline from which future changes in threatened salmonid populations can be measured and compared in an effort to identify relevant species‐ or population‐specific differences.

A rapid classification tool for deformities in Atlantic salmon (Salmo salar) embryos

“…For example in the hatchery, quantification of embryo deformities within specific populations or crosses can be valuable for informing population recovery or breeding strategies; our classification scheme makes this work better by providing consistent, clear deformity types that can be compared between stocks, years, hatcheries, etc . Furthermore, current efforts to examine sublethal effects on salmonid embryos in the laboratory and field rely heavily on measurements of survival at various stages ( e.g ., Lavery & Cunjak, 2019; Sear et al ., 2016): this classification tool facilitates examination of such effects using deformities, in addition to survival.…”

“…The original field studies were conducted with approval from Fisheries and Oceans Canada (licences SG‐RHQ‐13‐188 and SG‐RHQ‐14‐145) and the University of New Brunswick's Animal Care Committee (AUP 12044, 13045 and 14044). The embryos in this collection were incubated in plastic Jordan–Scotty incubator trays buried in the gravel (to simulate natural incubation as closely as possible; e.g ., Lavery, 2017, Lavery & Cunjak, 2019, Howell, 2021) of four rivers in the Tobique River basin (the Serpentine, Wapske and Gulquac Rivers and the River Dee: 527 embryos collected) and three rivers in the Miramichi River basin (Catamaran, Otter and Rocky Brooks: 1475 embryos collected), as well as in incubation trays in a hatchery setting (1224 embryos collected). Embryos in the collection were sampled at approximately the ‘eyed’ stage of development ( i.e ., after the development of eye pigmentation but before hatching: ~118–159 days since fertilization, depending on incubation temperature).…”

“…Further physical descriptions of the study rivers and details of fertilization, transport and incubation can be found in Nafziger et al . (2017), Lavery and Cunjak (2019) and Flanagan (2003).…”

“…Salmonid embryos incubate within freshwater sediments, often over winter, and can be subjected to highly variable and potentially harmful conditions within the lake or stream bed ( e.g ., Lavery & Cunjak, 2019). As a result, they may suffer sublethal damage that can affect their fitness in later life stages ( e.g ., Johnston et al ., 2000; Nislow et al ., 2004).…”

Elevated temperature and deposited sediment jointly affect early life history traits in southernmost Arctic char populations

“…This further suggests other physiological or behavioral alterations that would be relevant to investigate, particularly in regard to how early life exposure to temperature and fine sediment may carry effects into later life stages. However, although oxygen limitation can have severe consequences on embryonic traits of salmonids (Côte et al 2012;Del Rio et al 2019;Lavery and Cunjak 2019) and induce immediate and long-lasting alterations in the cardiovascular system of adults (Miller et al 2011;Johnston et al 2013;Anttila et al 2015), evidence regarding consequences of early life hypoxia on adult performance is mixed (Wood et al 2017), and further studies are needed to characterize the long-term effects of developmental environment on salmonid physiology.…”