Biology and management of deepwater snappers of Hawaiian Archipelago

Marine Fisheries Review, Spring, 1993 by Wayne R. Haight, Donald R. Kobayashi, Kurt E. Kawamoto

Introduction

Commercial exploitation of deepwater snappers in Hawaii began before the turn of the century (Cobb, 1903), and except for a hiatus during World War II, has continued to the present. The fishery operates throughout the Hawaiian archipelago, but the inhabited Main Hawaiian Islands (MHI) experience the highest exploitation by both commercial and recreational fishermen. The uninhabited islands and atolls northwest of Niihau, called the Northwestern Hawaiian Islands (NWHI) (Fig. 1), are fished mainly by commercial fishermen.

The deepwater handline fishery targets a multispecies group of fishes consisting of snappers (Lutjanidae), groupers (Serranidae), and jacks (Carangidae). However, snappers are the most important group in the fishery, both by numbers and revenue.(1) Most of the commercially important species have a relatively high age at maturity, long life span, and slow growth rate. These factors, combined with considerable variation in larval recruitment, make these fishes highly susceptible to overfishing. Recent evidence suggests that some of the species may be overfished, and several others are approaching a level of concern(1,2) (Ralston and Polovina, 1982; Ralston, 1984).

Historically, minimum size limits have been the only measure used to manage both the MHI and NWHI stocks. Since 1989, the NWHI have been managed on a limited entry system. Minimum size limits remain the only management tool currently in effect in the MHI. Several management measures are being evaluated to maintain adequate spawning stock biomass in the future. These include changing minimum size limits, creating closed refuge areas, and imposing bag limits and closed fishing seasons. This report presents a summary of available information regarding the biology, fishery, and management of these valuable species.

Species and Distribution

The commercially important deepwater snapper complex in Hawaii is composed of seven species of the lutjanid subfamily Etelinae: Pristipomoides filamentosus (opakapaka), P. seiboldii (kalekale), P. zonatus (gindai), Etelis carbunculus (ehu), E. coruscans (onaga), Aprion virescens (uku), and Aphareus rutilans (lehi). Some commercial landings of an introduced snapper, Lutjanus kasmira (taape), are also made, but this species usually occurs in shallower water than the other species. The commercially important deepwater lutjanids inhabit the deep slopes of island coasts and banks at depths of 100 to 400 m. These banks and deep slopes comprise an area over 6 times that of shallow water reefs in the state. Biological production in these deepwater areas is thought to be quite different from shallow-water coral reef areas (Agegian et al., 1988). Throughout their spatial and depth range, deepwater snappers in Hawaii are typically distributed in a clumped pattern, and are often associated with underwater headlands and areas of high relief. Four of the deepwater snapper species found in Hawaii were observed in situ from manned submersibles and an unmanned remotely operated vehicle (ROV) and were found to form large aggregations of up to 100 individuals near submerged promontories and areas of high relief (Brock and Chamberlain, 1968; Ralston et al., 1986; Haight, 1989). This clumped distribution pattern is also apparent when fishing from the surface.

A contributing factor in the distribution pattern of these fish may be that currents striking deepwater areas of high relief form localized zones of turbulent vertical water movement, increasing the availability of planktonic prey items (e.g. Brock and Chamberlain, 1968). In an ecological study of the bottomfish resources of Johnston Atoll, Ralston et al. (1986) found P. filamentosus in much higher densities on the upcurrent versus the downcurrent side of the atoll, and postulated that this was related to increased availability of allochthonous planktonic prey in the neritic upcurrent areas due to oceanic currents impacting the atoll. Although snappers throughout the world are generally thought of as top level carnivores, several snapper species in the Pacific are known to incorporate significant amounts of zooplankton, often gelatinous urochordates, in their diets (Parrish, 1987). Haight et al. (1993) found zooplankton to be an important prey item in four of the commercially important snappers in Hawaii. The same study found that the six snapper species studied were either primarily zooplanktivorous or primarily piscivorous and showed little overlap in diet composition between trophic guilds.

Reproduction

Relatively little is known about the reproduction and early life history of deepwater snappers in Hawaii. Size at maturity has been estimated for only two species in the MHI and two species in the NWHI. In the MHI, uku reaches sexual maturity at 47 cm fork length (FL), which is 46% of maximum size [L[infinity]. Onaga reaches sexual maturity at 61 cm FL (62% L[infinity]) (Everson et al., 1989). In the NWHI, ehu reaches maturity at about 30 cm FL (46% L[infinity] and opakapaka reaches maturity at around 43 cm FL (48% L[infinity] (Everson, 1984; Kikkawa, 1984; Grimes, 1987).