The lobster and shrimp fisheries in Hawaii

Marine Fisheries Review, Spring, 1993 by Jeffrey J. Polovina

The Lobster Fishery

Introduction

The commercial lobster fishery in Hawaii is a trap fishery which harvests several lobster species in the Northwestern Hawaiian Islands (NWHI)--an isolated range of islands, islets, banks, and reefs extending 1,500 n.mi. northwest, from Nihoa Island to Kure Atoll (Fig. 1). The fishery targets two species: the endemic spiny lobster Panulirus marginatus (Quoy and Gaimard, 1825) and the common slipper lobster Scyllarides squammosus (Milne-Edwards, 1837) (Fig. 2,3). Two other species--the ridgeback slipper lobster S. haanii (de Haan, 1841) (Morin and MacDonald, 1984) and the Chinese slipper lobster Parribacus antarcticus (Lund, 1793)--are caught incidentally in low abundance.

Lobster concentrations in the NWHI were documented by research cruises in 1976, and commercial trapping began in 1977 (Uchida and Tagami, 1984). Since 1983, the lobster fleet has ranged from 9 to 16 vessels (15 to 35 m long), each averaging 3 trips per year. A typical vessels sets about 800 traps per day and remains at sea almost 2 months per trip. The NWHI lobster fishery is Hawaii's most valuable demersal fishery; in recent years, annual landings have averaged about 600 metric tons (t) (I million lobsters), valued at about $6 million U.S. ex-vessel (Fig. 4). Since 1988, about 80% of the landings have been spiny lobster (Polovina(1)).

A commercial shellfish trap made by Fathoms Plus2 iS used by all the fishermen. This is a dome-shaped, single-chambered trap made of molded black polyethylene which measures 980 x 770 x 295 mm, with a mesh size of 45 x 45 mm (inside dimensions). Each trap has two entrance cones located on opposite sides. Each trap also has two escape vent panels each consisting of four 67 mm diameter circular vents located on opposite sides to facilitate the escapement of sublegal lobsters (lobsters under minimum legal harvest size). The traps are typically baited with chopped mackerel (Scomber sp.) and fished in strings of several hundred traps per string most frequently set in depths from 20 to 70 m.

Synopsis of the Fishery

The historical landings from the lobster fishery exhibit a classical trend of a developing fishery with a period of low catches at the beginning of the fishery (1977-83) followed by a rapid increase in landings as more vessels entered the fishery and markets were developed (1984-86) and most recently a decline in landings as the population is reduced by overfishing (1987-91) (Fig. 4). In the early years of the fishery (1977-84) and since 1988, landings have been about 80% spiny and 20% slipper lobsters. However, for a three-year period from 1985 to 1987 the fishery targeted and largely depleted a previously lightly exploited population of slipper lobsters.

Stock assessment has used the annual catch of spiny and slipper lobsters and trapping effort data from the commercial logbooks since 1983 (Table 1).

[TABULAR DATA 1 OMITTED] Both spiny and slipper lobsters may be caught in the same trap but fishermen can alter the proportion of each species by selecting the trapping area and depth. Logbooks record only the number of traps hauled and do not specify when effort targets spiny or slipper lobster. Since 1983 when logbook reporting was in effect, the combined catch per unit effort (CPUE) for legal slipper and spiny lobsters has declined from 2.75 to 0.56 lobster per trap-haul (Fig. 4).

Stock assessment of the lobster resource is hindered by the relatively short catch and effort time series and our inability to age lobsters. A dynamic production model, fit to the combined spiny and slipper lobster catch and effort data for the entire NWHI, estimates an equilibrium production curve with a maximum sustainable yield of 900,000 lobsters/year from a fishing effort of 740,000 trap-hauls, resulting in a CPUE of 1.22 lobsters/trap-haul ([Polovina.sup.1]). A CPUE time-series model estimates the annual instantaneous natural mortality (M) at 0.7/year and catchability (q) at 1.0 x [10.sup.-6] ([Polovina.sup.1]). Thus the 1990 fishing effort of 1.2 million trap-hauls corresponds to a fishing mortality (F) of 1.2/year or 1.7 times M. Based on the minimum harvest sizes and this level of fishing mortality, the spawning stock biomass per recruit is estimated at 40% of the level in the absence of fishing.

Both the level of fishing mortality relative to natural mortality and the relative spawning biomass suggest that fishing effort alone was not sufficient to cause the decline in CPUE observed in 1990 and 1991. Current research suggests this decline is the result of poor recruitment (due to oceanographic conditions) at some banks which resulted in a concentration of fishing effort at the remaining banks where recruitment was strong.

Research

After the initial research cruises documented lobster concentrations in the NWHI in 1976, research focused on the biology of the spiny lobster Panulirus marginatus. Tagging studies at Kure Atoll and French Frigate Shoals estimated a von Bertalanffy growth curve for growth (in carapace length) to have a parameter K of 0.3/ year with an asymptotic carapace length of 13.2 cm, a mean natural mortality estimate of 0.37/year, and estimates for the ages at the onset of sexual maturity of 2.7 and 1.7 years for males and females, respectively (MacDonald, 1984). Trapping surveys mapped the spatial distribution of P. marginatus in the NWHI and indicated that the highest catch rates ranged from depths of 55-73 m in the southeastern portion of the NWHI to 19-54 m in the northwestern portion of the Hawaiian Archipelago (Uchida and Tagami, 1984). The settlement of post-larval lobster, puerulus, were monitored at Kure Atoll, French Frigate Shoals, and Oahu with surface collectors (MacDonald, 1984). Puerulus settlement appeared seasonal at the ends of the Hawaiian Archipelago; the greatest settlement occurred in the summer at Kure Atoll and in the winter at Oahu while at French Frigate Shoals, more centrally located, settlement appeared more uniformly throughout the year (MacDonald, 1986).