Stoner: Discard mortality for Alaskan crabs after exposure to freezing temperatures 
453 
of the total catch (NPFMC, 2008). Currently, a rate of 
50% mortality is applied to the crab discarded from pot 
fisheries in Alaska (Turnock and Rugulo, 2008), but it 
is widely recognized that more research is needed to 
refine this value and to improve handling methods to 
reduce discard mortality. 
The goal of this study was to determine if the RAMP 
approach can be applied to thermal stress experienced 
by C. bairdi and C. opilio in Alaska fisheries prosecut- 
ed during winter months. In a shipboard experiment, 
males of both species at submarket size were subjected 
to different subfreezing temperatures and exposure 
times to test for possible relationships between simple 
reflex actions, righting behavior, autotomy, and subse- 
quent immediate mortality and delayed mortality. 
Materials and methods 
Field collections and experimental animals 
Crabs for this study were collected with the 50-m fish- 
ing vessel Pacific Explorer in waters east of St Paul 
Island (Pribilof Islands), in the Bering Sea near 57°03'N, 
168°20'W, at 80 m depth. During the August 2008 field 
work, bottom water temperature was 1.1° to 1.5°C. Crabs 
were collected in various locations around a commercial 
trawl with a recapture net previously described for stud- 
ies of crab bycatch injury (Rose, 1999; Stoner et al., 
2008). The main trawl was a two-seam Alfredo bottom 
trawl (with headrope and footrope lengths of 36 m and 
54.6 m, respectively) similar to that used by many ves- 
sels in the Bering Sea. This trawl was towed with an 
open codend. The recapture net was a small 2-seam 
trawl with a longer headrope than footrope (14.3 m and 
12.0 m, respectively). The long headrope maximized 
escape of fish, and a small-diameter (5 cm) footrope was 
used to enhance crab capture. For the broader role of 
the research cruise, evaluating crab injury caused by 
various gear components, crabs were collected at various 
locations around the primary net (e.g., behind the main 
net sweeps, wings, or footrope). As a control for damage 
in the recapture nets, crabs were also fished directly 
ahead of the main trawl, so that they were captured 
with no previous damage. Crabs from this net position 
were used for the study of freeze-related injury reported 
here. Tows were short (15 minutes) so that crab stress 
was minimized. 
Once a recapture net was on deck, C. bairdi and C. 
opilio were sorted quickly from the catch and tested 
individually (in air) for losses of six previously studied 
reflexes (Table 1) (Stoner et al., 2008). For this study, 
experiments were limited to crabs meeting the follow- 
ing criteria: 1) males below market size (i.e. , C. bairdi 
= 80-100 mm carapace width [CW]; C. opilio - 71-100 
mm CW); 2) crabs without apparent physical injuries 
or limb loss; 3) crabs with carapaces in full hardness; 
and 4) crabs in perfect condition as revealed by re- 
flex actions. Crabs in the above prescribed size classes 
represent the primary discards observed in Alaska 
pot fisheries (see Warrenchuk and Shirley, 2002). Air 
temperature during the crab handling process ranged 
from 5° to 10°C, and sorting normally took <15 minutes. 
Crabs meeting the criteria specified were immediately 
moved to one of 12 large fish totes (98x110x85 cm deep; 
-900 liters) secured on the trawl deck. Each tote was 
supplied with a constant flow of seawater (>20 L/min). 
These systems were identical to those used in an ear- 
lier study of crab mortality (Stoner et al., 2008). Water 
temperature during the pre- and post-testing holding 
period ranged from 9.1° to 9.6°C, salinity was nearly 
constant at 27.2 psu, and oxygen (monitored morning 
and evening in every tote) never fell below 100% satura- 
tion. The holding temperature was higher than desired, 
but no control crab (see below) died or showed signs of 
stress in the form of impaired reflexes. 
Experimental systems and rationale 
for temperature exposures 
Crabs were exposed to freezing temperatures in a stan- 
dard chest freezer (internal dimensions 76x46x71 cm 
deep) secured below deck on the FV Pacific Explorer. Tem- 
perature was monitored with a digital instrument and 
cabled resistance temperature detector (RTD) platinum 
probe (0.1°C resolution, Fisher Scientific, Pittsburgh, 
PA) fastened at the bottom of the freezer compartment 
near the subject crabs. This digital thermometer was 
calibrated against a standard mercury thermometer. 
A 5-cm-thick layer of ice in the bottom of the freezer 
helped to stabilize the air temperature on the bottom 
of the compartment where crabs were placed -3 mm off 
the bottom surface in an open mesh plastic rack. Pre- 
liminary experimentation with a small fan inside the 
freezer compartment showed that temperature stability 
at the bottom of the freezer was greatest without air 
circulation, and this eliminated the complicating factor 
of wind chill. Although crabs in the fishing conditions 
would rarely be exposed to freezing temperatures with- 
out at least some wind, the primary objective of this 
study was to determine how well reflex actions would 
reflect thermal stress in crabs and predict mortality, 
not to provide absolute values of mortality under specific 
outdoor conditions. 
The strategy for thermal exposures was guided by 
the results of earlier cold-exposure experiments con- 
ducted by Carls and O’Clair (1990, 1995). They found 
that responses to cold air in C. bairdi and Paralithodes 
camtschaticus were best described when the units of 
exposure were considered as the product of temperature 
and duration of exposure (h), over a range of tempera- 
tures from -20° to 5°C. For example, short exposures 
at low temperature caused the same effects as long 
exposures at a higher temperature when the units of 
exposure (degree hours, °h) were equal. The same units 
of measurement were used in this study. 
The objective of this investigation was to determine 
whether stress (and mortality) caused by cold expo- 
sure can be predicted from a reflex impairment index; 
therefore, it was important to generate variable levels 
