Pink and Chum Salmon reflex impairment at spawning grounds

A study by Raby et al. 2013 has shown remarkable resilience by Pink and Chum Salmon to simulated fisheries capture stress incurred upon arrival at spawning grounds.  These salmon species were observed to have low mortality and successful spawning after being stressed by exposure to exhaustive exercise, air, and injury.  In the authors words:

"Our study results provide evidence that, after reaching spawning areas, both Pink and Chum Salmon may be resilient to certain forms of capture-related exhaustion stress. Short of producing immediate mortality through extended anoxia, Pink and Chum Salmon are apparently able to recover from substantial physiological disturbance related to capture and ultimately spawn. Natural prespawn mortality rates for Pink Salmon (6.5%) and Chum Salmon (3.2%) in the channel during the study year (R. Stitt, Fisheries and Oceans Canada, personal communication) were nearly identical to the prespawn mortality rates for fish subjected to our capture and tagging procedures."

One of the objectives of the study was to test for possible relationships between reflex impairment (RAMP) and mortality.  However because of the general lack of mortality in these salmon species just prior to spawning, increasing reflex impairment as RAMP was related to increasing intensity of capture stressors and not to mortality or spawning success.   

Patterns of impairment are shown by the authors:

"The pattern of impairment of individual reflexes with successively increasing levels of overall reflex impairment (RAMP scores) was largely consistent. Tail grab and body flex were by far the two most easily impaired reflexes (Tables 2, 3). However, in Chum Salmon, tail grab impairment predominated body flex impairment at low RAMP scores (0.2, 0.4) compared with Pink Salmon. Orientation was typically the third reflex to become impaired for both species when overall RAMP score increased beyond 0.4 (Tables 2, 3). We did not commonly observe VOR impairment, which was almost always the last reflex impaired."

The authors suggest that radical shifts in metabolism during preparation for spawning may be responsible for the observed resilience to simulated capture:

"We hypothesize that Pink and Chum Salmon are resilient to capture-related exhaustion upon reaching spawning areas because of a combination of low water temperature (about 12^◦C in this study) and a physiological shift towards increased use of anaerobic pathways during their final weeks of life. The capture and release of fish arriving at the spawning ground does not appear to influence survival, in contradiction to the results of other studies, which focused on earlier components of Pacific salmon spawning migrations."

The possible shifting of metabolic pathways in prespawning Pink and Chum Salmon, associated with lack of mortality at high levels of reflex impairment is a cautionary tale for use of RAMP to predict vitality, mortality, and spawning fitness. The relationships between reflex impairment, metabolic pathways, and life history traits are important subjects for future research and validation.