Hawaiian bottomfish play hide and seek in ancient reefs

NURP's Hawaii Undersea Research Laboratory plays a key role in helping define Hawaiian bottomfish habitat

by Christopher Kelley,
Hawaii Undersea Research Laboratory at the University of Hawaii,
NOAA's Undersea Research Program Center for Hawaii and the Western Pacific

Beginning in the late 1980's, concern regarding the status of the Hawaiian Bottomfish Fishery began to escalate over a decrease in commercial landings. Fishermen were fishing more and yet catching less fish. At the urging of the Western Pacific Fishery Management Council (i.e., the Council responsible for managing federal fisheries in Hawaii and the Western Pacific), the Hawaii Department of Land and Natural Resources implemented Administrative Rule Chapter 13-94 in 1998 to set regulations to conserve remaining bottomfish stocks.

During the process of creating these rules, the realization was made that although the Hawaiian Bottomfish Fishery had been in existence for hundreds of years, there was relatively little known about their habitat requirements. Among fishermen, there was general consensus that there existed some connection between the bottom and these fish because time and time again one could find them at the same locations.

In an effort to better understand what was so special about these fishing sites, a partnership was formed in 1998 between the Hawaii Department of Land and Natural Resources and scientists at the Hawaii Undersea Research Laboratory (the NOAA Undersea Research Program (NURP) Center for Hawaii and the Western Pacific), NOAA Fisheries, NOAA Oceans & Coasts, and the University of Hawaii's Hawaii Institute of Marine Biology to answer the following question:

What are the important physical and biological features that define (or characterize) bottomfish habitat?

After six years, this study has just begun to close in on the answer.

Hawaiian Bottomfish -- What are they?

Hawaiian bottomfish are not what you would expect. If you were to look up the definition in a scientific textbook, you would find the following definition:

bottomfish n. Fish that live on the sea bottom, especially commercially important gadoid fishes like cod and haddock or flatfish like flounder.

However, this definition does not describe a Hawaiian bottomfish. The fishery is actually made up of an odd assortment of 12 species grouped together because they have some connection with the seafloor (i.e., bottom). The membership includes seven deepwater snappers (family Lutjanidae), two jacks (family Carangidae), one grouper (family Serranidae), one scorpionfish (family scorpaenidae, and one alfonsin (family Berycidae).

These fish typically live in deep water (i.e., 250 to 1000 + feet) and cannot be studied "in situ" using snorkel or SCUBA gear. Thus, to investigate these fish and their habitats, we have used the Pisces' submersibles to visit twenty-two different sites since 1998 and record the difference between bottom characteristics of locations where bottom fish existed and did not exist. The Pisces submersibles are operated and maintained by the Hawaii Undersea Research Laboratory and are capable of diving to depths greater than 6,000 feet.

Here is what we found...

Each fishing site was part of an ancient coral reef that "drowned" between 10,000 to 100,000 years ago. By drowning, I mean that the living part of the reefs sunk too deep, where it was too cold and dark for reef-building corals to survive.

As you may know from snorkeling - a typical shallow water coral reef consists of numerous holes, nooks, crannies, cracks, crevices, and ledges for fish and invertebrates to hide from predators or to ambush prey.

Like a shallow water reef, the dead reefs also contained these types of cavities which brought up another question - Were the bottomfish using them to hide from predators or to ambush prey just like shallow coral reef fish and invertebrates do?

The answer was yes. During our submersible dives we found that smaller snapper species, such as the ehu (Etelis carbunculus) and the kalekale (Pristipomoides sieboldii) used the cavities as places to hide. Both of these snappers are voracious predators, but were clearly attacked themselves by another bottomfish species, the amberjack (Seriola Dumerili) and to a lesser extent by sharks. The cavities were also being used by larger snapper species, such as the onaga (Etelis coruscans) in their juvenile and post-juvenile (up to 1 ft long) stages, when that species is most susceptible to predation.

In addition to serving as shelter for bottomfish, we found that the dead reef cavities also served as shelter for their favored prey. For example, cracks and holes on these habitat sites contained small fish such as Symphysanodon maunaloae and several anthiid groupers, as well as small invertebrates such as squat lobsters (family Galatheidae), glass shrimp, and octopi. This was verified by looking at the stomach contents of several bottomfish species caught during fishing surveys conducted by us and other researchers.

Interestingly, we found that the presence of these prey species at rocky locations with cavities was key to the presence of the bottomfish. If a rocky location was void of cavities, it was also void of prey species and bottomfish. We also found that the presence of other invertebrates such as, gorgonians, seastars, and urchins, were not critical elements for bottomfish habitat, since we found them at some habitats, but not at others.

The mystery begins to unfold...

After six years, we have just begun to answer the question:

What are the important physical and biological features of bottomfish habitats?

Prior to this study, little was known about the habitat preferences for the various species in this fishery other than the fact that bottomfish could consistently be found in the same locations. This study has established that these fish gravitate towards a seafloor that has a lot of cavities such as an old drowned reef, which provide bottomfish with food, as well as shelter from predators. It furthermore has helped to define the fish and invertebrate communities found in bottomfish habitats.

A more in-depth report of the findings is presently being prepared for publication. This study, of course, has also identified to both scientists and resource managers that we still have a lot to learn about the Hawaiian bottomfish, if we are to conserve them for generations to come.

For more information:
http://www.hawaii.gov/dlnr/dar/bottomfish/bf_research.htm