Progress in Stock Assessment of Opah

Opah
Opah

Opah (Lampris spp) are caught in pelagic longline fisheries world-wide. The population status of opah in the North Pacific was recently investigated by PIFSC scientist Pierre Kleiber and colleague Donald Hawn, then of the University of Hawai‘i Joint Institute for Marine and Atmospheric Research (now with the NOAA Fisheries Pacific Islands Regional Office). They analyzed operational catch and effort statistics from the Hawai‘i-based pelagic longline fishery along with ancillary market and environmental data and evaluated opah catch-per-unit-effort (CPUE) using data from the deep-set fishery targeting tuna.

They found that nominal annual opah CPUE by the Hawai‘i-based deep-set longline fleet dropped steeply in year 2000 from its maximum observed value, recorded the previous year. They found that opah CPUE was stable or increasing in subsequent years despite an increasing annual catch of the species by the Hawaii longline fleet. Standardized CPUE showed a less steep, but proportionally greater, drop from years 1999 to 2004 and a stable level thereafter. Kleiber and Hawn roughly estimated that foreign-flag longline fleets operating in the neighborhood of Hawai‘i caught more opah than the Hawai‘i-based fleet did, but comprehensive catch statistics from these other fleets do not exist. Therefore, a formal stock assessment was not possible, and the true status and outlook for opah are unknown. A more reliable determination of opah stock status would require that comprehensive fishery data be assembled from all longline fleets operating within the Hawai‘i region.

Figure 1.  Changes in proportion of fishing operations using various types of bait in the Hawai‘i-based deep-set longline fleet since 1995.
Figure 1. Changes in proportion of fishing operations using various types of bait in the Hawai‘i-based deep-set longline fleet since 1995.

While examining factors that might influence opah CPUE in the Hawai‘i-based deep-set longline fishery, Kleiber and Hawn found interesting patterns in bait usage. The bait type has been recorded in logbooks of daily fishing activity since the mid-1990s. Three kinds of bait were most common: saury, sardines, and a combination of saury and sardines. The proportion of bait types used changed over time (Figure 1) with a large shift around year 2000, when vessels changed from exclusive use of saury to strategies using all three bait types. This finding was of particular interest, because 2000 is the year of the dramatic change in opah CPUE. However, CPUE calculated for longline sets using just saury as bait showed the same temporal pattern as the nominal CPUE calculated using all sets. This result would not be expected if the change in bait type explained the drop in opah CPUE. So analyses of the effect of bait are possible, but in this case the correspondence of a bait change with a CPUE change may be coincidental.