An ecological perspective on inshore fisheries in the main Hawaiian Islands

Marine Fisheries Review, Spring, 1993 by M. Kimberly Smith

Introduction

Regional Geography and Fishing Pressure

The volcanic peaks and platforms that make up the Hawaiian Islands rise from the ocean floor between roughly lat. 19-28[degrees] N and long. 155-178[degrees] W, giving the archipelago a length of close to 1,500 miles. However, almost all of Hawaii's population and land mass (above sea level) is concentrated on eight islands, located within 300 miles of the southeastern tip of the island chain Fig. 1). These are the main Hawaiian Islands (MHI), which include Hawaii, Maui, Lanai, Kaho'olawe, Molokai, Oahu, Kauai and Ni'ihau. They are distinguished geologically and for management purposes from the submerged islands and atolls northwest of Kauai (beginning with Nihoa), known as the Northwestern Hawaiian Islands (NWHI).

Accessibility and rates of exploitation of Hawaiian inshore fisheries are determined largely by regional geography. Emergent portions of the NWHI are minimal, are exposed to treacherous northerly storms, and offer only limited freshwater and vegetation. These are some of the reasons the NWHI are largely uninhabited by humans. Travel from populated islands can take from days to weeks, depending on the size and condition of the vessel. Because of the distances involved, commercial fishermen with large vessels are essentially the only participants in NWHI fisheries.

The NWHI are an important breeding and resting ground for monk seals, green sea turtles, and various migratory seabirds whose natural habitat has been disturbed because of human activity in the MHI (Balazs, 1980; Gilmartin et al., 1980; Harrison and Hida, 1980). Most of the inshore area is part of the Hawaiian Islands National Wildlife Refuge (designated in 1909 by President Theodore Roosevelt as a bird refuge), managed by the U.S. Fish and Wildlife Service (USFWS). To maintain a less disturbed environment for threatened and endangered species, recreational and commercial activities (including fishing) are not allowed within the 10-20 fathom isobath of most islands northwest of Kauai (varying with location). Because of this, inshore fisheries in the NWHI are largely unexploited.

Inshore fish and invertebrate resources in the NWHI include many popular MHI species, such as a'ama crab, Grapsus grapsus; aholehole, Kuhlia sandvicensis; striped mullet, Mugil cephalus; and moi, Polydactylus sexfilis (Okamoto and Kanenaka, 1984). Spiny lobster, Panulirus marginatus, various eteline and lutjanid snappers, jacks, groupers, and large pelagic fishes are found slightly farther from shore (Uchida and Uchiyama, 1986), just as is seen in the MHI. These and other inshore fisheries in the MHI, where they are harvested, are the subject of this review.

Climate, Coastal Topography, and Inshore Fishery Habitats

Inshore fisheries will be defined for this review as those within the 100-fm contour. This arbitrary boundary is found within three miles of shore throughout most of the MHI. Its correspondence with the offshore limit of state waters is convenient, although many inshore species migrate freely across the three-mile boundary. Normally the continental shelf is used as a guideline for the limit of inshore fisheries; however, these volcanic islands have no continental shelf. Gosline and Brock (1976) also selected the 100-fm isobath as an outer boundary, justifying this in part because it was the maximum depth fished by traps and handlines at that time. Modern hydraulic gurdies have extended the depth limit for fishing somewhat, but 100 fm is still a reasonable limit for small boat inshore fisheries (Squire and Smith, 1977).

Depth profiles, climate, and terrestrial influences are important determinants of the distribution of inshore fisheries in the MHI. The importance of coastal topography and hydrography may be accentuated by Hawaii's relative isolation in the northern tropical Pacific. The attraction of some pelagic species toward land formations (Murphy and Shomura, 1972) may also enhance inshore fishing opportunities.

The climatic pattern, which affects the distribution of terrestrial and aquatic communities throughout the MHI and most of the northern tropical Pacific is determined by prevailing trade winds. Wind-born weather fronts lose some of their moisture in passing over the mountainous portions of islands in this region. Thus, windward (northeastern) slopes have higher rainfall than leeward (southwest-facing) slopes. Because of this, windward embayments tend to support more estuarine fisheries than leeward areas. Although high rainfall, erosion, and sedimentation are antagonistic to the survival of healthy corals, the coastal shelf also sustains fringing and patch reefs in windward regions. These habitats support rock- and crevice-dwelling organisms, such as octopus, crabs, and lobsters. The balance between the degree of protection from wind and waves, the amount of rainfall and sedimentation, and the availability of shallow shelf influences the extent of reef development in windward and leeward areas.

There are few stream-fed estuaries in Hawaii. The most important freshwater input to inshore areas may well be through groundwater (Carlquist, 1980). Wherever sources of freshwater meet the ocean (particularly in embayments), fish such as the Hawaiian anchovy or "nehu," Encrasicholina purpurea; round herring, Etrumeus micrdpus; and gold spot herring, Herklotsichthys quadrimaculatus; return seasonally to spawn (Williams and Clarke, 1983; Clarke, 1989). More commonly, Hawaiian fishes use estuaries as feeding and nursery areas, and may spawn offshore (Clarke, 1991). Fishes which feed in Hawaiian estuaries include mullet, Mugil cephalus; Hawaiian flagtail, Kuhlia sandvicensis; bigeye scad, Selar crumenopthalmus; and various species of snappers and trevally. Schools of adults and juveniles are targeted by fishermen as they enter and leave embayments.