Importance of context for RAMP curves used to predict mortality and survival of stressed animals

Relationships between reflex/buoyancy impairment and post-capture mortality for Atlantic cod (Humborstad et al. 2016).

Humborstad et al. (2016) looked at the relationship between reflex/buoyancy impairment and post-capture mortality for Atlantic cod exposed to fishing stressors. RAMP curves were generated for (a) fish exposed to laboratory simulated forced swimming, air exposure, and net abrasion, (b) field longline capture, and (c) field pot capture. The authors concluded that:

“It appears that specific RAMP curves may be needed for gears that involve different stressors, including consideration of any additional stress associated with captive observation of delayed mortality. Differences in stressors and holding conditions certainly reduce the general applicability of RAMP across different stressors and fisheries. However, once a RAMP curve has been established for a specific set of stressors or gears, the strong relationship between reflex impairment and mortality shows the potential for predicting mortality outcomes, especially at high and low levels of impairment.”

“Reflex impairment could predict mortality among fish caught by pot and longline. However, different RAMP curves were observed between laboratory and field conditions, indicating that careful consideration must be given to the types of stressors present and captive-observation conditions for delayed mortality when comparing RAMP curves for different fisheries. The inclusion of buoyancy status in modelling greatly improved mortality predictability.”

Science and medicine generally do not know proximate and ultimate causes for why fish and other animals die. This lack of mechanistic knowledge precludes us from direct understanding and prediction of death. However, we can observe correlates with death; animal size, stressors, vitality impairment, and physiological impairment. These correlates can be used to identify risk factors and predict immediate and delayed mortality. 

Successful mortality and survival prediction requires that the context of animal exposure to stressor risk and recovery be included in any experimental analysis of this problem.  We cannot simply identify stressors, impairment, or physiological numbers and say that they will result in a particular mortality (Davis 2002). RAMP curves clearly show the importance of context for exposure to stressors and potential mortality or survival (Davis 2010). The question of interactions among stressors and their context has recently been elaborated for freshwater and marine systems (Jackson et al. in press).

What is RAMP: reflex action mortality predictor?

Reflex actions and injury traits in crab scored for impairment (Stoner 2012, Yochum et al. 2015).

Reflex actions and injury traits in sharks scored for impairment (Danylchuk et al. 2014).

Reflex actions and injury traits in fish scored for impairment (Davis 2010, McArley and Herbert 2014).

Reflex actions and injury traits in turtles scored for impairment (LeDain et al. 2013, Stoot et al. 2013).
Photos; crab - Farm to Market, shark -  Swell Brains, fish - DEEP, turtle - Aquatica.

Any animal has reflex actions and potential injury traits; see diamonds in figures. These fixed traits can be observed, scored present or absent, and summed to form an animal vitality impairment score. Animal vitality is a gestalt of reflex and injury traits that we can observe as a whole animal, active and responding to stimuli. Vitality impairment and mortality are correlated and this relationship is expressed as RAMP, reflex action mortality predictor.

Impairment of well-defined reflex actions and injury types may differ for each species, dependent upon their natural history and phylum.  These species traits of reflex actions and injury types can be scored and combined to express the percentage of whole animal impairment. No impairment represents a healthy animal with all actions present and all injury absent. Increasing absence of reflex actions and presence of injury types is increasing impairment and is correlated with mortality.

Sublethal and lethal zones associated with reflex action impairment scores (RAMP) in walleye pollock, rock sole, sablefish, and Pacific halibut (Davis and Ottmar 2006). For these species at specific transition impairment values, a rapid rise in mortality is observed after a small increase in reflex impairment. 

These curves illustrate the expression “you are alive until you are not”. Animals live in various states of vitality impairment that are correlated with stress. Above a quantifiable level of vitality impairment, animals begin to show mortality, correlated with continued increase for impairment. The distribution of reflex impairment and injury in a group of animals is a measure of population vitality. 

For fish species (Davis 2010, McArley and Herbert 2014), animals have several types of reflex actions which can be secondary or primary. One action group contains secondary peripheral actions that are part of swimming and defensive behavior (fin erection and startle). Impairment of these reflex actions generally indicates sublethal stress effects and is associated with increasing stressor intensity (duration or strength). A second action group contains primary body functions (orientation and coordinated breathing). Impairment of primary body functions generally indicates delayed mortality after stress induction. In the same way, for crustacean species (Stoner 2012, Yochum et al. 2015), loss of leg reflex actions are associated with sublethal stress effects. Loss of eyestalk and mouth actions are associated with delayed mortality after stress induction.