Chronic exposure to increased water temperature reveals few impacts on stress physiology and growth responses in juvenile Atlantic salmon

Highlights

• First study to associate chronic exposure to 16 °C to eye darkening and high cortisol levels in Atlantic salmon.

• Maintaining juvenile Atlantic salmon at 20 °C had no significant effect on body mass or length compared to 12 °C and 16 °C.

• Bimodal growth under a chronic thermal stress regime remained un-affected by temperature.

Abstract

Fish are subjected to a variety of stressors under common cage aquaculture conditions. While short-term exposure to a stressor often results in an adaptive response to cope with stress, repeated and/or chronic exposure to stress can result in negative impacts on fish welfare and production. In fish, little is known about the impact of long-term exposure to stressors, including elevated water temperature. In this study we examined and developed temporal response profiles of physiological indicators of stress and growth in juvenile Atlantic salmon (Salmo salar) exposed to 12 °C, 16 °C, and 20 °C for 99 days. Five times throughout the study we quantified plasma cortisol, glucose and cholesterol levels, and growth. Fish body mass and fork length were not significantly different amongst temperatures after 99 days. Plasma cortisol was significantly elevated at 16 °C when comparing day 8 with 99, while at 12 °C plasma cortisol increased from day 1 to day 8, then returned to initial levels (day 1 and 8) after 99 days. Plasma glucose and cholesterol were not significantly different amongst the temperatures throughout the experiment. In addition, at the end of the experiment we quantified eye darkening, and identified the development of a bimodal growth distribution in all temperatures. Fish with a fork length ≤ 240 mm were categorised as lower mode (LM) and those with a fork length > 240 mm as in the upper mode (UM) of growth. Plasma cholesterol was significantly lower in the LM group in all three temperatures, but plasma cortisol and glucose levels did not differ between modes. Eye darkening also did not differ between modes, but increased significantly in the groups exposed to 16 °C and 20 °C when compared with 12 °C. This study showed a clear physiological stress response (elevated levels of cortisol) and eye darkening in fish maintained at 16 °C but not at 12 °C or 20 °C, suggesting that some aspects of the physiological responses available to deal with chronic stress are affected by temperature.

Introduction

Atlantic salmon (Salmo salar) aquaculture is an established industry in several countries, including Norway, Scotland, Ireland, the Faroe Islands, Canada, USA, Chile and Australia (Tasmania) (“Aquaculture topics and activities. Aquaculture resources. In: FAO Fisheries and Aquaculture Department,”, 2015). The majority of the production areas are within latitudes 40–70° in the Northern Hemisphere, and 40–50° in the Southern Hemisphere (“Aquaculture topics and activities. Aquaculture resources. In: FAO Fisheries and Aquaculture Department,”, 2015). When reared in freshwater or seawater phases, Atlantic salmon, can be restricted in their movement, and therefore, be exposed to several environmental stressors, including seasonal or abrupt changes in water temperature.

It has been demonstrated that Juvenile Atlantic salmon are tolerant to a wide range of temperatures, where incipient (50% survival after 7 days) and ultimate lethal tolerance (survival for 10 min) range from 0 to 28 °C and −0.8–33 °C respectively (Elliott and Elliott, 2010). However, reports concerning the optimal temperature for growth in juvenile Atlantic salmon when maintained under a static regime are conflicting. In their review, Elliott and Elliott (2010) suggested that 16–20 °C provided maximum growth efficiency in juvenile Atlantic salmon. Similarly, Jensen et al. (2015) demonstrated improved growth in juvenile Atlantic salmon maintained in saltwater for four weeks at 16 °C compared to 4 °C and 10 °C, respectively. In contrast, Handeland et al. (2008) reported that the optimal temperature for growth in post-smolt juvenile Atlantic salmon was 12.8 °C when 70–150 g, and 14 °C when 150–300 g.

The temperature range for which the onset of chronic thermal stress occurs in juvenile Atlantic salmon is not well understood. Under commercial aquaculture conditions, Atlantic salmon are confined to particular areas of the water column, and therefore, can be unwillingly subjected to abrupt or seasonal changes in water temperature. It is well known that metabolism in fish is influenced by water temperature, and to some extent also health, stress response, growth and survival (Afonso et al., 2008; Dominguez et al., 2004; Pérez-Casanova et al., 2008a, Pérez-Casanova et al., 2008b; Tromp et al., 2016). Therefore, chronic thermal stress could have deleterious consequences on the overall fitness of fish. Most studies concerned with the metabolic effects of high water temperatures in Atlantic salmon were conducted for relatively short periods of time (Elliott and Elliott, 2010 (7 days); Hevrøy et al., 2012 (56 days); Olsvik et al., 2013 (45 days)), and therefore, information regarding the effects of chronic exposure to high water temperatures is poorly known in this species.

Chronic effects of high water temperature are better known in other species (Pérez-Casanova et al., 2008b; Tromp et al., 2016). Acute exposure to stressors reveal that the immediate response to stress (increased plasma cortisol and glucose levels) helps the animal to cope with the stressor (Iwama et al., 2005). Information on the effects of chronic exposure to high temperatures on cortisol are important for understanding physiological processes, given its many effects on fish performance and production (Mommsen et al., 1999). For example, a chronic regime of increasing temperature led to elevated plasma cortisol, mortality, and expression of immune-related genes in Atlantic cod (Gadus morhua) when the temperature reached 16 °C, but this was only evident approximately 30 days after the beginning of the experiment (Pérez-Casanova et al., 2008b). On the other hand, in Atlantic salmon, constant exposure to 19 °C for 56 days led to reduced growth, but no significant change in plasma cortisol (Hevrøy et al., 2012). Similarly, culture of hapuku (Polyprion oxygeneios) at 22 °C for 98 days suppressed their gain in body mass, reduced their condition factor and specific growth rate, but did not significantly change plasma cortisol levels (Tromp et al., 2016). One explanation for the incidence of elevated cortisol in some studies where fish were subjected to chronic exposure to high water temperatures, yet not in others, may be an ability of some fish to acclimate to chronic stress, and thereby reduce elevated plasma cortisol levels back to a pre-stressed state (Barton and Schreck, 1987; Pickering and Pottinger, 1987).

In addition to a lack of studies on the effects long-term exposure to thermal stress on plasma cortisol levels, there is limited information about cholesterol levels in fish during stress. Cholesterol plays a central role in steroid hormone biosynthesis (Mommsen et al., 1999; Tokarz et al., 2015), including cortisol. The only study on plasma cholesterol levels in salmonids during exposure to high temperatures (from 10 °C to 20 °C) have shown decline in cholesterol values (Wedemeyer, 1973). In humans, low plasma cholesterol levels are described as hypocholesterolaemia, and affect immune and inflammation functions (Vyroubal et al., 2008). Obtaining information about cholesterol and cortisol levels during chronic exposure to high temperatures is important considering that cortisol is the principal corticosteroid in fish and exerts significant physiological roles in metabolic regulation, osmoregulation, growth and reproduction (Mommsen et al., 1999).

Throughout the last 30 years there has been considerable interest in studying the development of bimodal growth in juvenile Atlantic salmon during the freshwater phase. Most of these studies have been carried out in hatchery and laboratory-raised fish, and they have demonstrated that in their first year of growth, Atlantic salmon can be separated into two classes based on their length: upper mode (UM) and lower mode (LM) (Heggenes and Metcalfe, 1991; Kristinsson et al., 1985; Nicieza et al., 1994; Simpson and Thorpe, 1976; Zydlewski et al., 2014) The larger fish (UM) have the potential to become smolts earlier than the LM fish. Most of these studies have examined the development of bimodal growth under normal or ambient temperatures for growth (Imsland et al., 2016; Kristinsson et al., 1985; Metcalfe et al., 1988; Shrimpton et al., 2000; Shrimpton and McCormick, 1998). There are no studies that have investigated the effects of long-term exposure to elevated temperature on the bimodal growth. In addition a few studies have examined differences in physiological indicators between these two classes. For example, it has been shown that under normal hatchery conditions fish in the UM showed increased levels of classical indicators of smoltification (plasma cortisol, growth hormone, and gill Na⁺, K⁺-ATPase) well in advance than fish in the LM (Shrimpton et al., 2000; Shrimpton and McCormick, 1998).

Recently there have been few studies that investigate the development of rapid, non-invasive and reliable techniques for determining stress. The measurement of eye sclera colour changes, termed eye darkening (ED) has shown the potential to be used as an non-invasive indicator for stress in fish (Freitas et al., 2014; Suter and Huntingford, 2002; Vera Cruz and Brown, 2007; Volpato et al., 2003). The physiological processes that induce ED in fish are not well understood (Nilsson Sköld et al., 2013). However, there is evidence in sand goby (Pomatoschistus minutes) via an in vitro study that ED may be controlled by changes in the dispersal of eye chromophores due to melanin concentrating hormone (MCH) and the adrenocorticotrophic hormone (ACTH) (Sköld et al., 2015). Given the role of ACTH in cortisol production (Barton and Iwama, 1991), these findings suggest that ED may be controlled by hormones involved in the generalised stress response in fish.

Considering the paucity of studies investigating the long-term exposure of juvenile Atlantic salmon to elevated temperatures, we examined the effects of a chronic increase in water temperature for 99 days on this species stress response and growth. The chronic physiological responses were studied temporally by measuring total plasma cortisol, glucose, and cholesterol levels and eye darkening at the last sampling time. As we were able, at the end of the study, to identify bimodal growth, and therefore separate fish by size into upper and lower modes, we also measured the physiological and growth indicators in these groups. Investigation of the effects of long-term exposure to high temperature, including in fish categorised as fast or slow growers, may lead to a better understanding of their physiology, and identification of phenotypes that are better prepared to deal with elevated temperatures.