337 
Abstract — Delayed mortality asso- 
ciated with discarded crabs and 
fishes has ordinarily been observed 
through tag and recovery studies or 
during prolonged holding in deck 
tanks, and there is need for a more 
efficient assessment method. Chi- 
onoecetes bairdi (Tanner crab) and 
C. opilio (snow crab) collected with 
bottom trawls in Bering Sea waters 
off Alaska were evaluated for reflexes 
and injuries and held onboard to track 
mortality. Presence or absence of six 
reflex actions was determined and 
combined to calculate a reflex impair- 
ment index for each species. Logis- 
tic regression revealed that reflex 
impairment provided an excellent 
predictor of delayed mortality in C. 
opilio (91% correct predictions). For 
C. bairdi , reflex impairment, along 
with injury score, resulted in 82.7% 
correct predictions of mortality, and 
reflex impairment alone resulted in 
79.5% correct predictions. The rela- 
tionships between reflex impairment 
score and mortality were indepen- 
dent of cr. b gender, size, and shell 
condition, and predicted mortality 
in crabs with no obvious external 
damage. These relationships provide 
substantial improvement over earlier 
predictors of mortality and will help 
to increase the scope and replication 
of fishing and handling experiments. 
The general approach of using reflex 
actions to predict mortality should be 
equally valuable for a wide range of 
crustacean species. 
Manuscript submitted 25 March 2008. 
Manuscript accepted 7 May 2008. 
Fish. Bull. 106:337-347 (2008). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
An assessment of discard mortality 
for two Alaskan crab species. 
Tanner crab ( Chionoecetes bairdi ) 
and snow crab (C. opilio), 
based on reflex impairment 
Allan W. Stoner (contact author ) 1 
Craig S. Rose 2 
J. Eric Munk 3 
Carwyn F. Hammond 2 
Michael W. Davis 1 
Email address for A. W. Stoner: AI.Stoner@noaa.gov 
1 Fisheries Behavioral Ecology Program 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
2030 Marine Science Dr 
Newport, Oregon 97365 
2 Conservation Engineering Program 
Alaska Fisheries Science Center 
National Marine Fisheries Service, NOAA 
7600 Sand Point Way NE, Seattle, Washington 98115 
3 Shellfish Assessment Program 
Alaska Fisheries Science Center, 
National Marine Fisheries Service, NOAA 
301 Research Court 
Kodiak, Alaska 99615 
Numerous species with economic and 
ecological significance are discarded 
from fishing operations because of 
harvest restrictions or low value com- 
pared with the target species or size 
groups. Although discarded catch can 
exceed that retained (Alverson et ah, 
1994; Witherell and Pautzke, 1997), 
discard mortality rates are rarely 
known. Mortality can occur on deck 
(immediate and observed mortality) or 
after the animal is released or escapes 
capture (delayed and unobserved mor- 
tality) (Stevens, 1990; Davis, 2002; 
Suuronen, 2005). Delayed mortal- 
ity can result directly from physical 
injury and physiological stress or indi- 
rectly by an increased susceptibility to 
disease and predation, and through an 
inability to feed. Although a signifi- 
cant effort has been made to reduce 
unwanted bycatch through improve- 
ments in gear selectivity and han- 
dling procedures, unobserved discard 
mortality continues to be an impor- 
tant source of uncertainty for fishery 
management (Harrington et al., 2005; 
Broadhurst et al., 2006; Coggins et 
ah, 2007). 
Directed fisheries for Chionoecetes 
spp. and Paralithodes camtschaticus 
(red king crab) in the North Atlan- 
tic, North Pacific, and Bering Sea 
are prosecuted with baited pots, the 
only legal commercial gear for these 
crabs in the United States and Cana- 
da. Harvesting is restricted to males 
within strict size limits; consequently 
all females and undersized males are 
discarded. Crabs are also captured 
and discarded or escape from trawl 
gear. Given the difficulties of deter- 
mining mortality for discarded crabs, 
fixed mortality rates have been as- 
sumed for specific fishing sectors in 
most fishery management models on 
the basis of a handful of experiments 
(see below). For example, fishery 
managers in Alaska have assumed 
fixed values of 20%, 50%, and 20%, 
respectively, for the discard mortality 
rates of Chionoecetes bairdi (southern 
