Stevens' Survival of king and tanner crabs captured incidentally in the Bering Sea 



743 



In an attempt to simulate the effects of culling crabs 

 in the directed, winter-season pot fisheries, Carls and 

 O'Clair (1990) documented the effects of exposure of 

 ovigerous female king and Tanner crabs to various 

 combinations of time and air temperature (max. 40 min. 

 at - 19°C). They found that both mortality and right- 

 ing time were inversely proportional to exposure, ex- 

 pressed in terms of degree-hours, but neither response 

 occurred in air temperatures above 0°C. Their condi- 

 tions contrasted with this experiment; although water 

 and air temperatures were not recorded, 30-year mean 

 August seawater temperatures in the area where this 

 experiment was conducted have been 2-5 °C on bottom 

 and 8-9°C at surface (Ingraham 1983), and air tem- 

 peratures were probably above 10°C, so no negative 

 degree exposures occurred. However, during the 

 winter trawl fishery such exposures might occur, al- 

 though most crabs would be insulated by the sur- 

 rounding catch in the net or processing bins. 



Carls and O'Clair (1990) also found that exposure- 

 related mortality of king crabs was delayed, occurring 

 16-128 days after exposure, and was usually associated 

 with ecdysis; in contrast, 50% of their Tanner crab mor- 

 tality occurred within the first 24 hours. Although 

 crabs were not retained over 48 hours in the present 

 experiment, and no ecdysis occurred, exposures were 

 not as severe either. 



Location of leg breaks was found to be related to 

 mortality of stone crabs Menippe merceyiaria by Simon- 

 son and Hochberg (1986). They studied the effects of 

 exposure and claw breakage on survival of stone crabs 

 and found that breakage on either side of the breakage 

 plane resulted in significantly higher mortality than 

 breaks on the plane. Mortality also increased with 

 severity of the break, and length of exposure, from 2 

 to 6 hours. Severity of breaks also increased with ex- 

 posure time prior to breakage. In their study, claw 

 breakage was intentional rather than accidental, as 

 fishermen remove the claws for sale and return the live, 

 clawless crab to the sea. 



Crabs subjected to trawl capture and subsequent 

 release probably also suffer some sublethal effects, but 

 they could not be documented in this study. Brown and 

 Caputi (1985) studied the effects of exposure and leg 

 loss on growth of sublegal rock lobsters Panulirus 

 cygyius, and found that exposure for periods of 15, 30, 

 and 60 minutes resulted in decreased growth at the 

 next molt of 0.8-2.3 mm, or 1.2-3.3% of carapace 

 length. In addition, the growth increment was reduced 

 by 0.48 mm, or about 0.7%, for each appendage miss- 

 ing. Carls and O'Clair (1990) also found that female 

 king crabs exposed to subzero air temperatures showed 

 decreased growth at subsequent molts. Other sublethal 

 effects included reduced feeding rates and increased 

 limb autotomy by female Tanner crabs. 



The recapture of a tagged and released crab indicates 

 that such multiple captures of discarded crabs may be 

 common in this type of intensive fishery. Recaptured 

 crabs would probably have lower survival rates than 

 crabs captured and released only once, due to the 

 cumulative effects of stress and injuries. However, 

 there is no evidence suggesting that the rate of recap- 

 ture during the experiment differed from the normal 

 conditions of the commercial fishery, so the calculated 

 survival rates may adequately incorporate any effects 

 of recapture. 



Smith and Howell (1987) studied the effects of trawl 

 capture on American lobsters Homarus americanus in 

 Long Island Sound and found that the incidence of 

 damage varied seasonally from to 14%, and was 

 greater during months when molting was occurring. 

 Immediate plus delayed mortality ranged from 1% to 

 21%, depending upon season also. In contrast to this 

 report, they found that uninjured lobsters, and those 

 with autotomized claws rarely experienced delayed 

 mortality, and that seasonally warm seawater temper- 

 atures were associated with increased delayed mortal- 

 ity independent of damage. The conditions to which 

 lobsters were exposed differed greatly from those en- 

 countered by king crabs, however, as the lobsters were 

 commonly sorted out of the catch in less than an hour, 

 as opposed to times of 3-17 hours for crabs in this 

 study. 



The effect of discarding crab via conveyor belts and 

 discard chutes from the factory was not examined, and 

 could inflict further injury upon crabs via entanglement 

 in machinery or damage upon impact with the water 

 surface, 3-6 m below the discard chute. 



The estimated catch of 88000 red king crabs and 

 751000 Tanner crabs C. bairdi by the Bering Sea JV 

 fishery in 1988 (Berger and Weikart 1989) amounted 

 to 0.22% and 0.11% of the total estimated abundance 

 of those two species, respectively, in the Bering Sea 

 that year (Stevens et al. 1988). Even though the 

 amount of bycatch in the domestic fishery is unknown, 

 it is unlikely that the total bycatch exceeded 0.5% of 

 the population for either species, largely as a result of 

 restrictions on bycatch. Removals of this magnitude are 

 well below the ability of the NMFS crab survey to 

 detect, and probably have no significant biological im- 

 pact. The possibility of additional, unseen mortality of 

 crabs due to trawling has largely been discounted by 

 submersible studies (West 1987, Highliners Association 

 1988) in which the observed trawls made little or no 

 contact with the sea bottom, caused no observable in- 

 juries to crabs contacted, and no injured or dead 

 animals were observed in the wake of the net. 



Nevertheless, the issue of bycatch mortality is of 

 great concern to the fishing industry, and the North 

 Pacific Fishery Management Council has developed a 



