128 



Estimates of lobster-handling mortality 

 associated with the Northwestern Hawaiian 

 Islands lobster-trap fishery 



Gerard T DiNardo 



Edward E. DeMartini 



Honolulu Laboratory, Southwest Fisheries Science Center 



National Marine Fisheries SeiA/ice, NOAA 



2570 Dole Street 



Honolulu, Hawaii 96822 



E-mail address (for Gerard T DiNardo) gdinardoShonlab nmfs hawaiiedu 



Wayne R. Haight 



Joint Institute of Marine and Atmosphenc Research 

 School of Ocean and Earth Science and Technology 

 University of Hawaii, 1000 Pope Road 

 Honolulu, Hawaii 96822 



The commercial lobster fishery in the 

 Northwestern Hawaiian Islands ( NWHI ) 

 is a distant-water trap fishery that tar- 

 gets the Hawaiian spiny lobster (Pan- 

 ulirus marginatus) and slipper lobster 

 (Scyllarides squammosus). The ISTWHI 

 are an isolated group of islands, atolls, 

 islets, reefs, and banks that extend 1500 

 nmi west-northwest of the main Hawai- 

 ian Islands from Nihoa Island to Kure 

 Atoll (Fig, 1). Reported landings in the 

 NWHI peaked at about 2,000.000 lob- 

 sters (spiny and slipper combined) in 

 1985, and then declined to about 38.000 

 lobsters from 1986 to 1995 (Fig, 2). 



The NWHI lobster fishery is man- 

 aged under the Fishery Management 

 Plan for the Crustaceans of the West- 

 ern Pacific Region (Crustaceans FMP) 

 implemented in 1983 and developed 

 by the Western Pacific Regional Fish- 

 ery Management Council (WPRFMC), 

 The National Marine Fisheries Sei-vice 

 (NMFS) is responsible for stewardship 

 of the resource and review and im- 

 plementation of proposed management 

 measures, A variety of management 

 measures have been adopted in re- 

 sponse to declining catches: a limited- 

 entry fishing regime that limited the 

 number of permit holders to 15; a prohi- 

 bition on fishing from January through 

 June when lobsters spawn; an annual 

 catch quota system; a minimum legal 

 tail width (TW) of 50 mm for spiny lob- 

 ster and 56 mm TW for slipper lobster. 



which are close to the sizes at first ma- 

 turity for these species in the NWHI; 

 a prohibition on landing berried (ovig- 

 erous) females; and a requirement that 

 traps be equipped with escape vents 

 to reduce capture of undersize lobsters 

 (WPRFMCM, Prior to 1996, fishermen 

 were required to discard all berried 

 and undersize lobsters, which were not 

 counted against the catch quota. 



The management plan assumed that 

 escape vents allowed substantial num- 

 bers of undersize lobster to escape cap- 

 ture and that undersize and berried 

 lobsters do not die during the discard 

 process. Although research on lobster 

 fisheries has found that escape vents 

 effectively reduce the capture of un- 

 dersize lobsters (Ki'ouse. 1978; Fogarty 

 and Borden, 1980; Harris, 1980; Ever- 

 son et al.. 1992; Skillman et al.-), con- 

 siderable numbers of undersize (hence- 

 forth termed "sublegal") and berried 

 lobsters are caught in the NWHI lob- 

 ster fishery Between 1983 and 1995 

 the reported lobster discard rate in- 

 creased from 2Q"c to 62^?^ (Fig. 3), re- 

 sulting from changes in the size- and 

 age-structures of the populations and 

 in the areas fished. The average size 

 of spiny lobsters generally increased 

 northwestward from Nihoa along the 

 Hawaiian Archipelago (Uchida et al., 

 1980). Although as many as 16 banks 

 within the NWHI have been fished, the 

 spatial distribution of fishing effort has 



shifted to banks in the southeast of the 

 Hawaiian Archipelago where there is a 

 higher concentration of spiny lobsters. 

 Qualitative data collected during the 

 early days of the fishery suggested that 

 mortality associated with the handling 

 and discarding practices of the NWHI 

 commercial lobster-trap fishery might 

 be high (Gooding. 1985; Gooding'^). Un- 

 less discard mortality is explicitly con- 

 sidered, fishing policy decisions can be 

 suboptimal, or worse. Where catch quo- 

 tas are used, the total fishing-induced 

 mortality of the population is greater 

 than expected and can even result in 

 recruitment overfishing. Using an equi- 

 librium yield-per-recruit (YPR) model, 

 Kobayashi^ found that the reproduc- 

 tive potential of the NWHI lobster 

 population more than doubled, and 

 mean weight per individual increased 

 by 22% in a retain-all fishery (all lob- 

 sters brought on deck were retained as 

 catch) if the mortality rate of discard- 

 ed lobsters was high (>75'7f ). Based on 

 these results, the observed high discard 

 rate of sublegal and berried lobsters 

 (62%), and the presumption that the 



1 Western Pacific Regional Management 

 Council. 199.5. Fisheiy management plan 

 for the crustacean fisheries of the Western 

 Pacific region, amendment 9. Western Pa- 

 cific Regional Fishery Management Coun- 

 cil. Honolulu. Hawaii. 227 p. 



- Skillman. R. A.. A. R. Everson, and G. L. 

 Ki-amer. 1984. Prospectus escape vent 

 experimental procedure for the spiny lob- 

 ster fishery under management of the 

 Magnuson Fishery Conservation and Man- 

 agement Act. Southwest Fish. Sci. Cent, 

 Admin. Rep. H-84-1.3, unpubl. report, lip. 

 Honolulu Lab.. Southwest Fish. Sci. Cent., 

 Natl. Mar Fish Serv., NOAA. Honolulu. 

 HI 96822-2396. 



* Gooding, R. M. 1979. Obsorv-ations on 

 surface-released, sublegal spiny lobsters, 

 and potential spiny lobster predators near 

 Necker and Nihoa. Southwest Fish. Sci. 

 Cent. Admin. Rep. H-79-16. unpubl. report, 

 8 p. Honolulu Lab.. Southwest Fish. Sci. 

 Cent., Natl. Mar Fish Sei-v., NOAA, Hono- 

 lulu, HI 96822-2396. 



■t Kobayaslii, D. R. 2001. Southwest Fish. 

 Sci. Cent. Admin. Rep., in prep. Simu- 

 lated effects of discard mortality on spiny 

 lobster iPanulirus mai-ginatus) sustain- 

 able yield and spawning stock biomass 

 per recruit in the Northwestern Hawaiian 

 Islands. Honolulu Lab., Southwest Fish. 

 Sci. Cent., Natl. Mar Fish Serv., NOAA. 

 Honolulu. HI 96822-2396. 



Manuscript accepted 11 Mav 2001. 

 Fish. Bull. 100:128-133 (2002). 



