Acoustic Tracking of Deep Water Snapper

The Hawaiian bottom fish fishery is comprised of 12 species, the most important of which are seven deepwater snappers in the family Lutjanidae.

Two species, the onaga, Etelis coruscans, and the ehu, Etelis carbunculus, are locally depleted in the main Hawaiian Islands (MHI). In 1998, the Division of Aquatic Resources in the State of Hawaii Department of Land and Natural Resources (DLNR) developed a management plan that included the creation of 19 restricted fishing areas (RFAs) where bottom fishing is prohibited.

These RFAs are spread throughout the MHI and were designed to close approximately 20 percent of the designated essential fish habitat (EFH) for onaga and ehu. However, each RFA is unique in regards to the amount and type of suitable habitat that it protects. Furthermore, neither the migration patterns in and out of these areas nor the larval settlement rates in the areas are understood.

These factors suggest that the effectiveness of each RFA, in terms of the number of protected fish per unit area, is highly variable. It has also been suggested that even if the closures do succeed in increasing the MHI onaga and ehu populations, they could still result in decreased local catch rates for the fishery, due to the increased pressure on the areas that are not closed.

Therefore a need exists to demonstrate that bottom fish reserves can be an effective management tool and can have a positive effect on the fishery.

The Kahoolawe Island Reserve (KIR) is another area where bottom fishing is prohibited. KIR is not part of the state's plan to recover onaga and ehu stocks, but was created in 1993 as a marine protected area (MPA) to promote native Hawaiian traditions, environmental protection, and education.

With 91 square miles of Kahoolawe coastal waters within its boundaries, it is the largest MPA in the MHI. KIR is of interest to researchers working on the bottom fish problem since its eastern boundary borders the Alalakeiki Channel, one of the most productive grounds for onaga and ehu.

Furthermore, anecdotal reports from fishers indicate bottom fish yields are also high outside its western boundary on Kuia Shoals. Understanding if there is a relationship between the presence of the reserve and the fishing in these areas would be useful to both DLNR and the Kaho?olawe Island Reserve Commission (KIRC).

The objective of this research was to examine whether and to what degree spillover of bottom fish could be occurring from KIR to adjacent open fishing areas.

In this initial project, live opakapaka (Pristipomoides filamentosus) were caught and implanted with acoustic tags. Opakapaka were chosen for this study as they were the most abundant deep-water-living snapper in the southwestern portion of the KIR. Furthermore, live juveniles of this species have already been successfully captured, tagged with visual tags, and released.

An array of five passive acoustic receivers was installed on the seafloor just outside the western boundary of the KIR. The receivers were deployed in June 2004 and retrieved in August 2004, potentially detecting and recording signals from tags within signal range over a period of two months.

All five receivers were retrieved and the data they contained analyzed to determine daily and longer-term movement patterns of the tagged fish, and whether these fish across the KIR boundary.

The results of our acoustic tracking experiments show that opakapaka typically move from the deeper area of the shelf break during the day to the shallower shelf slope at night. The ranges of movement during the day also appear to be limited compared to those observed at night. Movement patterns such as this suggest that opakapaka are feeding most actively at night.

There was also evidence of movement out of or into the KIR from the adjacent open shelf. Many of the tagged fish showed frequent movement in a generally east to west direction, and in most cases the frequency of movement appeared generally balanced, i.e., as many eastward as westward bearings.

For those tagged fish that were within range of the acoustic receivers, movement patterns appeared primarily south to north at dusk and back at dawn, with small east-to-west movements that did not often take the opakapaka out of range of the receivers.