Polovina and Mitchum: Recruitment of Panulirus marginatus relative to sea level in NW Hawaiian Is 



485 



Since 1983, the lobster fleet 

 has been composed of 9-14 ves- 

 sels (20-30 m long), each averag- 

 ing 3 trips per year. The vessels 

 set about 800 traps per day and 

 remain at sea almost 2 months 

 per trip. Landings in recent 

 years have averaged almost 1 

 million lobsters, valued at about 

 US$6 million ex-vessel. Because 

 of heavy fishing since 1986, the 

 population has been fished down 

 to the point that 3-year-old re- 

 cruits comprised most of the 

 fishery catches (Polovina 1991). 

 Since 1988, about 80% of land- 

 ings have been spiny lobster 

 (Table 1). Two banks-Necker I. 

 at the southeast end of the 

 NWHI, and Maro Reef which is 

 670km northwest of Necker I.— account for over 60% 

 of the fishery's catches. There is no recreational lobster 

 fishery in the NWHI. 



Spiny lobster spawn over a broad spring, summer, 

 and fall period. After hatching, the eggs are planktonic; 

 the planktonic period for the larvae is estimated at 12 

 months based on spawning season and larval tow data 

 (NMFS Honolulu Lab., unpubl. data). Further, the 

 larval tow data suggest that mid- to late-stage spiny 

 lobster larvae are close to the surface at night and 

 move down to ~100m during the day (Polovina, pers. 

 observ.). Based on growth curves estimated from both 

 tagging (MacDonald 1984) and length-based methods 

 (Polovina and Moffitt 1989), spiny lobster reach the 

 minimum legal size (which is slightly larger than the 

 size at onset of sexual maturity) approximately 3 years 

 after they settle onto benthic habitat. After settlement, 

 the lobster probably do not move between banks since 

 interbank depths exceed 1000 m. 



Regional oceanography 



The Hawaiian Archipelago lies within the subtropical 

 gyre formed by the Kuroshio Current to the west and 

 the north, the California Current to the east, and the 

 North Equatorial Current to the south (Fig. 1). The 

 speed of the gyre in the vicinity of the archipelago is 

 slow (<5cm/s; Roden 1991). An eastward-flowing cur- 

 rent within the subtropical gyre, named the Subtropical 

 Counter Current (SCC), was predicted by Yoshida and 

 Kidokoro (1967) and subsequently confirmed by Robin- 

 son (1969) and Uda and Hasunuma (1969) (Fig. 1). More 

 recent work has shown that, in at least the western por- 

 tion, the interior of the subtropical gyre is composed 



of a quasi-stationary banded structure of easterly- and 

 westerly-flowing currents (White and Hasunuma 1982). 

 The SCC consists of two bands of eastward flow at 23° 

 and 28°N, with mean annual speeds of 8 and 6cm/s, 

 respectively (White and Hasunuma 1982). 



In addition to these large-scale features, the meso- 

 scale oceanography around the Hawaiian Archipelago 

 is a complex system of fronts and eddies resulting from 

 both interactions between alternating east and west 

 currents and interactions between current and the 

 topography of the archipelago. 



Data and analysis 



Research data 



Standardized trapping surveys, using the same traps 

 set at the same sites, were conducted at Necker I. and 

 Maro Reef during June and July of 1986-88 and 1990. 

 The size-frequency data were converted to age-fre- 

 quency data with a von Bertalanffy growth curve 

 (MacDonald 1984). The age-frequency distribution was 

 standardized for the number of traps deployed to 

 estimate the relative age-frequency distribution of the 

 population. 



Fishery data 



Although detailed catch and effort data were not avaO- 

 able until after the logbook regulations were estab- 

 lished in 1988, catch and effort were generally light 

 and were concentrated around Necker I. from the in- 

 ception of the fishery until 1984 (Fig. 2). The combined 

 CPUE for slipper and spiny lobsters in 1983-90 

 generally declined from 2.8 to ~0.7 lobster per trap- 



