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Fisheries Behavioral Ecology
Research Topic 3: Develop New Methods for Predicting Bycatch Mortality in the Field

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The fifth step is to develop new methods for predicting bycatch mortality in the field. Predictor relationships were calculated for delayed and total fish mortality based on correlations between measures of fish condition and mortality.  Fish condition was measured as injury, plasma constituents, and reflex actions.


Photographs of fish restraining device

Photographs of a fish restraining device for testing reflex actions.  The left side shows the restrainer open and the right side shows the restrainer rotated 90o and closed on a fish.  The fish is being tested for mouth closure.

Impairment of reflex actions was correlated with delayed and total mortality under a wide range of fishing conditions including: injury from capture or escape; environmental factors; and different fish sizes.  Fish reflexes may be readily measured in the field during fishing experiments to predict bycatch mortality.  Free swimming fish, such as escapees placed in sea cages after escape from fishing gear, can be tested for orientation and startle responses to light and sound stimuli.  Fish that are to be discarded can be restraining and then tested for reflex actions such as body flip, operculum closure, mouth closure, gag response, and vestibular-ocular response.  The proportion of reflex action impairment can be observed and is termed RAMP (reflex action mortality predictor).  RAMP can then be correlated with mortality and RAMP curves calculated for mortality prediction.  RAMP was related to mortality with sigmoid functions that are biphasic.  As fishing stressors increased in intensity, the first phase showed increase in RAMP without concomitant mortality.  In the second phase RAMP continued to increase while mortality became apparent and increased.

Graph showing relationship of reflex impairment and mortality in free swimming fish Graph showing relationship of reflex impairment and mortality in restrained fish

Relationship between RAMP (Reflex Action Mortality Predictor) measured for fish in tanks at 5 minutes after towing and total mortality (proportion) in walleye pollock, sablefish, northern rock sole, and Pacific halibut. Points represent replicate groups with five fish per group.

Relationship between RAMP measured for restrained fish at 5 minutes after towing and total mortality (proportion) in walleye pollock, coho salmon, northern rock sole, and Pacific halibut. Points represent replicate groups with five fish per group.

For further information on RAMP see Davis and Ottmar (2006).

Pictures of injury in walleye pollock and Pacific halibut that were towed in a net
Pictures of walleye pollock and Pacific halibut that were towed in a net, exposed to a fluorescein bath, and then illuminated with uv light.  The extent of injury was measured using computer analysis and expressed as proportion of coverage.

 

Injury to fish can be quantified by visual observation and by the use of fluorescein to visualize lesions under uv light.  Mortality may be correlated to the extent of lesions caused by abrasion, internal injury, or hooking injury.  However, because environmental factors can cause mortality without concomitant changes in physical injury, observing injury alone is not always sufficient to predict mortality.  See Davis and Ottmar (2006) for further information on fluorescein.

Changes in plasma constituents have been used to quantify physiological impairment in stressed fish.  However, changes in cortisol, lactate, glucose, sodium, and potassium were found to generally not be correlated with mortality in laboratory studies of the effects of fishing stressors on walleye pollock, sablefish, Pacific halibut and lingcod.  Plasma constituents became elevated after fish were exposed to sublethal stressors and did not increase further as fish began to show mortality.  See articles for further information on plasma constituents (Davis et al., 2001; Davis, 2002; Davis and Schreck, 2005; Milston et al., 2006).

 

Steps for Studying Bycatch Mortality in the Laboratory
[ Step 1] [ Step 2 ] [ Step 3 ] [ Step 4 ] [ Step 5 ] [ Step 6 ]

 

Research Topic 1: Fish and Environment

Research Topic 2: Fish Habitat

Research Topic 3: Bycatch Mortality

Research Topic 4: Fishing Gear

 

Last updated 27 March, 2009


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