Revitalizing Fisheries

stock_fisherman.jpgCultivation of Moi Larvae

Oceanic Institute's stock enhancement projects over the past 12 years have been aimed at replenishing Hawai?i's coastal fishery. The objective is to demonstrate that it is sound policy and economically feasible to dedicate resources to stock enhancement. The study considers the various kinds of technological questions that need to be addressed in implementing a responsible stock enhancement program.

The program began on O?ahu at Kaneohe Bay in 1990. 243,500 mullet fingerlings were raised at OI's Makapu?u facility. One of OI's areas of expertise is the ability to cultivate larvae from eggs at any time of year, not only at spawning time. This is accomplished with captured wild broodstock in a photoperiod room that mimicks spawning season through controlled light and temperature.

Mullet is a local, good-tasting fish that adapts well to coastal waters off Hawai?i. Between 1990 and 1996, 138.000 mullet were also released at Waiakea Pond, a protected fishery near Hilo, and 20,000 were released off the Kona coast.


Pacific threadfin (moi) was traditionally reserved for the ruling chiefs in ancient Hawai?i and is today considered one of the premier local food fishes. Despite catch limits, seasonal closures, and size limits, catches of moi have declined dramatically over time, particularly around the more populated coastal areas.

Moi was identified as the top candidate for the next phase of stock enhancement in Hawai?i, based partly on the decline in stocks, and also on its high market value, and importance in the recreational fishery.

First the fingerlings are tagged with microscopic coded-wire tags that record the fingerlings release size and other data.


The fingerlings are released into the coastal waters. Researchers time the release to coincide with a period when young moi normally would join nursery habitats and when the fishing season is closed. They are also released at low tide because the area is protected and free from predators.

Over 380,000 moi fingerlings of various sizes were tagged and released in sandy shoreline nursery habitats along the windward coast of O?ahu between 1993 and 1998, using a variety of stocking densities and release habitats.

In Kahana Bay more than 250,000 moi were released, with numbers per release ranging from ca. 15,000 to 97,000 fish at a single time.


Following release, fish are recovered monthly to conduct sampling to determine fish populations and growth rate in the new habitat. Nets, both large and small, have proven a non-invasive method to examine both wild and cultured fish. A magnetic sensor can tell whether a fish is carrying the coded wire tag. If a fish has the tag, it is taken back to the lab, where the tag is surgically removed to read the data. The stomach contents are examined to found out what it has been eating. The study will also determine how quickly the fish are maturing.

Results of recovery showed that when up to 40,000 fish were released, the catch rates of moi in Kahana Bay increased. If a larger number were released, the numbers declined, but adjacent sites increased. This supports the hypothesis that high stocking density resulted in emigration to nearby habitats.

In Kailua Bay, a much larger bay, release of ca. 97,000 fish did not appear to affect the growth of hatchery-reared moi during the 1997 year. Knowledge of the carrying capacity of nursery habitats has important implications for development of successful stock enhancement strategies. Small releases at many sites may be more productive than large releases at a few sites.


Because few moi (Pacific threadfin) were present in roving creel surveys conducted between 1994 and 1998, O?ahu fishermen were offered a $10 reward for each hatchery-reared or wild moi caught. A total of 1,882 moi were recovered from the reward program in just over a year, 163 of which were hatchery reared fish. It is notable that the percentage of hatchery-reared fish in the release sites was as high as 71%.

Cultured moi juveniles had survived and recruited successfully into the recreational fishery, accounting for nearly 10% of fishermen's catches. The presence of a few large, fully-developed females suggests that hatchery-reared fish can survive, grow and reproductively contribute to the population.

The next phase of the project continues on the Island of Hawai?i, sometimes referred to locally as the Big Island. On the coast near Kona, two gathering areas for moi that have traditionally been popular fishing spots have shown a sharp decline in the moi population over the past 5 ? 6 years.

OI's Stock Management Program, funded by the National Marine Fisheries Service (NMFS), made the commitment to restock the area with moi.


OI will be studying how well the fish survive, how fast they grow, how fast they move from the release spot to other beaches, what they are eating, and how quickly they become mature.

The stock enhancement program also assesses the habitat, existing fisheries management practice, and provides for the release of genetically diverse stocks to safeguard the habitat.

The success of the stocking experiments suggests that while it may take 20 years or longer to restore a fishery through closure and relying only on natural processes, a depleted fishery can be restored through restocking in a little as five to 10 years.