Cooper et al.: Spatiotemporal catch patterns and population distributions of Lampris megalopsis and L. incognitus 139 
bigeye Pacific opah were caught near the main Hawaiian 
Islands (Hyde et al., 2014), there was a gap in sampling 
between ~140°W and 125°W. Consequently, the species 
distributions in the eastern North Pacific Ocean could not 
be determined. 
Both species of opah are a relatively common inciden- 
tal catch in the drift gill-net fishery targeting swordfish 
(Xiphias gladius) in the California Current system, a 
productive eastern boundary current system that is an 
important foraging ground for many highly migratory 
species (Block et al., 2011), and in pelagic longline fish- 
eries targeting swordfish and bigeye tuna (Thunnus obe- 
sus) in the North Pacific Ocean. Although opah have not 
been a primary target of commercial fisheries, seafood 
marketing efforts and new culinary uses for previously 
discarded cuts of these fish have recently increased the 
popularity of opah, and they are an increasingly valuable 
secondary target. Reported landings of opah in Hawaii 
increased from approximately 400 metric tons (t) per year 
in the early 2000s to more than 800 t by 2017 (National 
Marine Fisheries Service, Office of Science and Technol- 
ogy, commercial fisheries landings, available from website, 
accessed May 2020). Additionally, in the California large- 
mesh drift gill-net fisheries, nominal catch per unit of 
effort (CPUE) of opah increased roughly 4-fold from 1982 
to 2017 (Walker and Teo’). 
The regions where opah are landed by U.S. fleets fall under 
the jurisdiction of 2 management councils: the Western 
Pacific Regional Fishery Management Council and the 
Pacific Fishery Management Council. In the Western Pacific 
Regional Fishery Management Council, opah are listed as a 
“pelagic management unit” in the fishery ecosystem plan. 
In contrast, they are not listed in the fishery management 
plan of the Pacific Fishery Management Council. There is 
currently no stock assessment for opah in the Pacific Ocean, 
nor are there any management measures in place for these 
species (WPRFMC, 2021). 
Stock assessments are a key component of fisheries 
management, and any effort to assess or manage opah 
in the North Pacific Ocean will require species-specific 
information on population distributions and fisher- 
ies data such as CPUE. However, past difficulties with 
visual identification have led to both bigeye Pacific opah 
and smalleye Pacific opah being simply recorded as opah 
in fisheries records and most scientific studies. Infor- 
mation on the spatial distributions of bigeye and small- 
eye Pacific opah is limited (Hyde et al., 2014), and the 
species-specific CPUE is unknown. Fisheries observers 
are now able to more effectively identify opah in land- 
ings to species level using new visual identification keys 
that illustrate the differences between species in spot 
patterns and the trailing margin of the caudal fin. Dif- 
ferentiating between morphologically similar species is 
essential to effectively characterize the fishing mortality 
of each species and contribute to accurate stock assess- 
ments (Beerkircher et al., 2009). 
1 Walker, J. M., and S. L. H. Teo. In review. Distribution and rela- 
tive abundance trends of opah in California waters. 
The primary objectives of this study were to examine 
spatial and temporal patterns of catch rates of opah on 
pelagic longline fishing gear and to estimate the species- 
specific distributions and CPUE within the central and 
eastern North Pacific Ocean. First, we characterized 
the spatial and temporal patterns of fisheries effort 
and non-species-specific opah CPUE of the U.S. pelagic 
shallow- and deep-set longline fleets. We then combined 
the use of genetic analyses and generalized additive mod- 
els (GAMs) to estimate the proportions of bigeye Pacific 
opah and smalleye Pacific opah caught in different areas 
and to estimate CPUE in regions dominated by the 2 
species. Finally, patterns in species distributions and 
CPUE were linked to oceanography to gain insight into 
preferred habitat. 
Materials and methods 
Fisheries data 
Fisheries data were obtained from pelagic longline vessels 
based in California and Hawaii. Since 1990, captains of 
vessels in the U.S. pelagic longline fleet have been required 
to fill out logbooks with details describing fishery-specific 
gear and effort, latitude and longitude, and catch infor- 
mation for each set (Walsh and Brodziak, 2016). In the 
logbook data, the species of opah that was caught on each 
set is not specified; instead, catch taxa are listed as opah 
or moonfish. 
The pelagic longline vessels that operate in California 
and Hawaii are also required to use trained observers 
to document catch and bycatch, ensure regulatory com- 
pliance, and record exact catch locations. On Hawaii- 
based vessels, observers are present on 20% of deep-set 
longline trips and on 100% of shallow-set longline trips 
(Sippel et al.”). California-based vessels participate only 
in the deep-set fishery and have been required to have 
20% observer coverage since 2016 (NMFS, 2016). Logbook 
and observer data for vessels based in both Hawaii and 
California were provided by the NOAA Pacific Islands 
Fisheries Science Center (Sippel et al.?; PIFSC?). 
Both the logbook and observer data contained substan- 
tial fishing information. To standardize the data for anal- 
yses, the fields selected were number of opah, date of the 
set, effort (number of hooks), number of hooks per float 
(which affects fishing depth), set and haul times, and the 
longitude and latitude of the set. 
Opah are caught on both deep- and shallow-set longline 
gear configurations. Both fleets operate in the eastern 
and central Pacific Ocean but do so with different spatial 
extents, reflecting the habitat of their respective target 
species (Figs. 1 and 2). Deep-set gear is used primarily to 
? Sippel, T., N. Nasby-Lucas, and S. Kohin. 2014. Description of the 
Hawaii longline observer program. Int. Sci. Comm. Shark Work. 
Group ISC/14/SHARKWG-1/05, 6 p. [Available from website.] 
3 PIFSC (Pacific Island Fisheries Science Center). 2019. Hawaii 
longline logbook. [Available from website, accessed March 2019.] 
