There would be widespread congestion when vehicle accidents occur in city river-crossing tunnels, which are usually crucial parts in city road network. Great gradient change at the lowest point of the tunnel makes it more difficult to control the vehicles if the drivers have to make a turn at the same time. This paper researched on the safety of the lowest point of city river-crossing tunnels by building the simulated tunnel scenes through UC-win/Road software and exploring the heart rate of drivers in different conditions through the driving simulator experiments. Controlling variables method was adopted to establish the simplified models and the considered variables were longitudinal grade and radius of horizontal curve. Based on the heart rate data around the time vehicles passing through the lowest point, we defined two new physiological indexes in this paper——the variability of maximum heart rate and the variability of average heart rate to measure the drivers’ physiological load. The driving simulator experiment discovered that the two physiological indexes both had positive correlation with the longitudinal grade while they had negative correlation with the radius of horizontal curve. We also found that there was no big difference of the drivers’ heart rate between the situations with design speed of 60km/h and 40km/h. At last, we evaluated the South Xizang Road Tunnel and the conclusion was that its lowest point was safe for driving.
Both warmer water and longer net entanglement result in greater physiological disturbance in coho salmon and, for some variables, an extended recovery after simulated fisheries capture based on the data presented here. White muscle and plasma variables provided the strongest evidence of treatment effects (Fig 3). Overall, we found that most physiological variables had recovered or approached routine levels within 4 h (i.e., 4 h, Figs 3 and 4) but that many individuals took much longer to return to pre-stressor fH (Fig 5 and Table 2).
In a closely related population of coho salmon held at ~8°C, Donaldson et al.  found that fH took up to 16 h to recover from an exhaustive exercise stressor, while Anderson et al.  similarly found Atlantic salmon required 15 h to recover fH after angling (at 8 and 16.5°C). In contrast are centrarchids (Centrarchidae), which return to routine fH within 2–4 h of exercise and angling stressors [17,40,41]. Our data support previous findings that the relative increase in fH from fisheries-related stressors is not affected by the nature of the stressor or water temperature, with an approximate doubling of fH in all cases [28,40]. However, whereas the duration of air exposure has a strong effect on fH recovery time in rock bass (Ambloplites rupestris; ), we found the duration of net entanglement had no such effect in salmon, though our ability to detect such effects was somewhat limited by low statistical power. More notable in our study was that several fish did not return to resting fH within 24 h, and that recovery time varied widely even within treatments. As such, our data confirm that capture-related stressors can cause very prolonged fH elevation in free-swimming salmon. Accordingly, we suggest that fH may provide the best indication of whole-organism recovery from a stressor but not of the severity of the stressor. Future research could explore why salmon exposed to seemingly identical acute stressors can vary so widely in their recovery profiles—knowledge that could help explain why delayed mortality occurs. Such differences could potentially be explained by inter-individual variation in spontaneous activity levels, stress responsiveness (e.g., [42,43]), physiological or behavioural syndromes , prior experiences (e.g., “training”, , or “carry-over effects”, ), or pre-existing pathogen loads .
The biological importance of an extended elevation in fH (e.g., 15+ h) following capture remains unclear, particularly in light of the fact that ṀO2 appeared to return to baseline relatively quickly (Fig 4), meaning the direct energetic cost of recovery was modest. Based on the mean recovery profiles in ṀO2 in Fig 4, excess post-exercise oxygen consumption was 783 ± 284 and 501 ± 98 mg O2 kg-1 at 10 and 15°C, respectively. Those values translate to 2358 ± 920 and 1623 ± 317 calories of excess energy used during recovery from capture; energy that could otherwise be used to achieve 1.4 ± 0.2 km (15°C) to 2.2 ± 0.8 km (10°C) of upstream migration, or 1.1 ± 0.2 h (15°C) to 1.8 ± 0.6 h (10°C) of spawning activity (based on migration energetics data in ). Perhaps a more important energetic consideration was the mismatch between oxygen demand and availability during entanglement. Both in the actual fishery and our simulation, dissolved oxygen declined from 9–10 mg L-1 (~90–100%) to ~ 5–7 mg L-1 (50–70%) in the crowded seine within 10–15 min, while oxygen demand (Fig 4) ranged from ~ 8–24 mg min-1 per fish (depending on body size and water temperature), likely necessitating a significant shift towards anaerobic metabolism.
Reliance on anaerobic metabolism can explain why the fisheries capture simulation caused changes in blood and muscle metrics that were modulated by temperature and the duration of net entanglement. The initial corralling and entanglement elicited ~ 1 min of exercise, which was followed by 2–15 min of crowding in very shallow water with declining oxygen content (e.g., 60% air saturation within 10 min). The protocol was more typical of a true fisheries net capture than those applied by exhaustive exercise studies (e.g., 5 min of manual chasing; ). Nevertheless, the rich physiological literature that exists on the latter  is relevant to understanding our results. Anaerobic exercise relies initially on using white muscle stores of PCr and ATP (whose concentrations remain unchanged during aerobic swimming; [48,49]), and thereafter shifts to greater consumption of glycogen (glycogenolysis), resulting in production of lactate and a drop in pH . Some lactate leaks out of muscle cells to blood plasma , while decreased muscle pH creates an osmotic pull of water from plasma to muscle cells, effectively concentrating plasma ions (i.e., heightened osmolality and plasma ion PC scores at 1 h; S1 Table and Fig 3). ATP and PCr in muscle typically recover to resting levels within 1–2 h of exhaustion [26,49], while muscle lactate conversion back to glycogen (glycogenesis; ), the primary fate of muscle lactate, typically occurs more slowly (e.g., 6–8 h), as does the restoration of osmotic balance . These processes were effectively integrated into a synthetic variable (metabolic PC score) for our experiment, which can be thought of as a robust measure of departure from metabolic homeostasis, where higher scores represent more exhausted fish and low scores represent a rested state (Table 1 and Fig 3). Although we did not measure muscle glycogen, we expect that it would have inversely tracked lactate . It was notable that although fish likely did not exercise maximally, evidenced by only reaching 50% of maximum attainable ṀO2, lactate reached maximal levels comparable to Atlantic salmon and rainbow trout in exhaustive exercise experiments (i.e., ~ 40 and ~20 mmol L-1 in muscle and blood plasma, respectively; [15,16]), but only in the 15 min duration entanglement (Fig 3; S1 Table). Entanglement time had a significant effect on metabolic disturbance, which helps explain why it was negatively correlated with reflex impairment in the field , and is supported by the catch-and-release literature where longer angling or air exposure have both been shown to cause greater lactate accumulation and longer recovery of cardiac variables [17,40,51,52].
Temperature had a significant effect on metabolic PC scores at 1 h, likely resulting from a combination of higher metabolic rate at 15°C (Fig 4) and to a lesser extent the lower dissolved oxygen content of warmer water. Although past studies on the effects of temperature on physiological responses to exhaustive exercise have found minimal differences in resulting lactate loads [15,16], the present study exposed fish to hypoxia which only followed brief burst swimming during netting. Therefore, exhaustion was likely to be a function of both entanglement time and temperature, given the role of the latter in determining metabolic rate (Fig 4). In fact, it appears fish were not fully exhausted in the 2-min duration treatments based on their lower muscle lactate loads (and metabolic PC scores) at 1 h (Fig 3). After exhaustive exercise, largemouth bass (Micropterus salmoides) accumulate more lactate and have more depressed white muscle energy stores if recovered in hypoxic or warmer water . Similar trends have been observed in bonefish (Albula vulpes, ). Importantly, resting levels of ATP, PCr, and glycogen are relatively independent of acclimation temperature in fish (), such that exposure to hypoxia should deplete energy stores more quickly at a higher temperature (Fig 4, ). In addition, the crowded fish would have depleted the oxygen content of the water more quickly, which is consistent with mortality occurring only in the 15°C/15 min treatment. The fish that survived the 15°C/15 min treatment exhibited the highest mean metabolic PC scores at 1 and 4 h, the longest heart rate recovery, and the highest number of excess post-stressor heartbeats (Table 2), although statistically significant differences did not occur in the latter two cases. Collectively, our data show that physiological disturbance in coho salmon is increased by longer entanglement time, particularly in warmer water.
A prediction from the literature is that physiological recovery from exhaustion in fish is more rapid in warmer water . The data from our experiment generally did not support that prediction, with the exception of our small ṀO2 dataset. However, the focus of our sampling design on a small number of time points, in the case of blood and muscle variables, likely precluded our ability to detect such effects. Interestingly, there was an effect of temperature on plasma cortisol recovery, with cortisol remaining particularly high in females in the 15°C treatment 24 h after release (~ 300 ng mL-1; Fig 3). Though we have little pre-capture control data (plasma cortisol was 15.9 and 131.2 ng mL-1 in two 15°C female controls [among four 15°C controls]; S1 Table), the data suggest either a) female salmon may have impaired recovery of cortisol after capture in warm water, or b) cortisol is maintained at higher routine levels in warmer water, though this is not the case in sockeye or pink salmon . It has recently been established that mortality is exceptionally high in upriver migrating female sockeye salmon exposed to warm water and capture stressors [6,38,54,55], and our cortisol data here may help explain why those trends occur. Integrating knowledge about sex-specific consequences of fisheries capture is particularly relevant to salmon populations in light of warming river temperatures because females are usually the limiting sex to spawning ground productivity . Further experiments are required to establish whether impaired cortisol recovery is a mechanism for delayed sex-specific mortality of salmon caught and released in warm water.
Relevance to conservation and management
Our data are directly relevant to understanding bycatch of endangered coho salmon in Fraser River beach seine fisheries. Beach seine fisheries that encounter coho salmon typically occur in mid-September (targeting pink and sockeye salmon) when water temperatures are 14–16°C , but the fishery also sometimes re-opens in late October when water temperatures are 8–10°C (to target chum salmon). It has been proposed that upriver migrating salmon are adapted to the modal water temperatures they historically experience at the time of river entry  and in our case we used a population of salmon for which 15°C would represent the upper limit of their lifetime experience, with 10°C being closer to their modal upriver migration temperature (water was 8–10°C in the Chilliwack River at the time of the experiment). In the fishery, where entanglement times ranged from 5 s to 56 min (median = 3.3 min; ) plasma lactate averaged 12.3mmol L-1, ~8–10 min after release from the net . This is only slightly lower than our 1 h samples, which were taken closer to when plasma lactate peaks during recovery [25,58]. Thus, although in an experimental setting and using a surrogate population, our data are relevant to informing best handling practices for the beach seine fishery, and help highlight the importance of releasing bycatch from nets as rapidly as possible , particularly at elevated temperatures.
In a human dimensions survey of fishery participants, rapid release of coho salmon was the most commonly suggested method for reducing bycatch mortality. Many fishers (35%), however, presented no ideas for reducing mortality, suggesting there is potential to increase awareness of the importance of rapid release . Our experiment illustrates that the mismatch between oxygen demand and supply during crowding results in additional and substantial physiological disturbances (Fig 3); clear evidence of why, if rapid release is not possible, fishers should be urged to maximize oxygen availability by leaving the net in deeper water for sorting. Most fishers that participated in the survey  indicated a willingness to leave their nets in knee-deep water, although the depth required to ensure crowding does not deplete dissolved oxygen would likely depend on a variety of factors, such as catch size and local water flow.
In addition to informing bycatch management for an endangered population of salmon, the trends relating to handling time and temperature in our experiment are likely applicable to other fisheries. For example, similar physiological data and recommendations exist in rainbow trout recreational fisheries  and fyke net fisheries that bycatch northern pike . Our maximal and recovered muscle lactate data are comparable to those that occur in coho salmon captured in marine gillnet and troll fisheries [25,34], further emphasizing that our data are likely replicable and relevant in real fisheries.
Estimates of global marine fisheries bycatch range from 6.8 to 38.5 million tonnes [60,61]—a conservation problem that has caused population declines (e.g., [62,63]) and drawn considerable research effort in the last 20 years . In Canada’s Pacific fisheries, a policy to move towards “selective fishing” has been in place for more than ten years, which states that non-target fish should be released “unharmed” if bycatch cannot be avoided . Our experiment is relevant in this context, given that physiological data are objective measures of fish welfare . To date, there is sparse use of terms relating to stress, welfare, or the sublethal effects of bycatch in IUCN documents on imperiled species that are captured in commercial fisheries . Nevertheless, well-controlled experiments with physiological assessments can help provide mechanisms needed to facilitate evidence-based implementation of best practices , especially when complemented by field and human dimensions data (e.g., ). We hope the present study provides a helpful addition to a growing physiological literature (for reviews see [24,31,68]) that can be used by conservation practitioners to understand trends in fish impairment and mortality while moving towards methods of live release that benefit the welfare and survival of bycatch.