National Marine 
Fisheries Service 
NOAA 
Fishery Bulletin 
@ established in 1881 «<s 
Spencer F. Baird 
First U.S. Commissioner 
of Fisheries and founder 
of Fishery Bulletin 
Abstract—Opah (Lampris spp.) are 
commercially valuable, non-target fish 
that are regularly landed in pelagic 
longline fisheries in the central and 
eastern North Pacific Ocean. Genetic 
studies have confirmed 2 species in the 
region: bigeye Pacific opah (L. megalop- 
sis) and smalleye Pacific opah (L. incog- 
nitus). Spatial distributions of each 
species are not well-defined, and little 
is known about species-specific catch 
distributions because the 2 species are 
difficult to distinguish visually. The 
objectives of this study were to charac- 
terize spatiotemporal patterns of opah 
landings by using catch per unit of effort 
(CPUE) from logbook and observer data, 
for the period 1995-2018, and genetics- 
based species identifications. Results 
from generalized additive models indi- 
cate that bigeye Pacific opah dominate 
west of 140°W and that smalleye Pacific 
opah dominate waters east of 130°W 
(dominance is defined as a proportion 
of catch 20.7). The deep-set pelagic 
longline fishery had higher opah CPUE 
than the shallow-set fishery across the 
geographic range of these fisheries. 
Opah CPUE increased from west to 
east on both longline gears, indicating 
higher overall opah CPUE in the east- 
ern regions of both fisheries, primarily 
dominated by smalleye Pacific opah. 
Opah CPUE also increased over time 
as the Hawaii-based longline fisheries 
expanded eastward. The results of this 
study highlight the need for species- 
specific catch data and will inform man- 
agement of opah species. 
Manuscript submitted 19 November 2021. 
Manuscript accepted 15 April 2022. 
Fish. Bull. 120:138-149 (2022). 
Online publication date: 29 April 2022. 
doi: 10.7755/FB.120.2.4 
The views and opinions expressed or 
implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Spatiotemporal catch patterns and 
population distributions of bigeye Pacific 
opah (Lampris megalopsis) and smalleye 
Pacific opah (L. incognitus) in the eastern 
North Pacific Ocean 
Ross Cooper (contact author)” 
Heidi Dewar” 
Barbara A. Muhling”? 
Steven L. H. Teo” 
John Hyde? 
Keith Bigelow’ 
Email address for contact author: r6cooper@ucsd.edu 
' Scripps Institution of Oceanography 
University of California San Diego 
8622 Kennel Way 
La Jolla, California 92037 
* Southwest Fisheries Science Center 
National Marine Fisheries 
Service, NOAA 
8901 La Jolla Shores Drive 
La Jolla, California 92037 
Opah (Lampris spp.), also known as 
moonfish, are large, laterally compressed 
teleosts that can reach upwards of 90 kg 
and 2 m fork length (Hawn and Collette, 
2012). Opah occur across a broad latitu- 
dinal range in tropical, temperate, and 
subpolar marine regions (Hyde et al., 
2014). Previous studies have found 
that opah have unique adaptations to 
retain heat created by their pectoral- 
fin swimming mode and distribute it 
throughout their entire body (Wegner 
et al., 2015). This full-body endothermy 
enhances capture of prey because opah 
can maintain a higher activity level and 
forage in cold deep water for long peri- 
ods before returning to warmer surface 
waters (Wegner et al., 2015). Despite 
these intriguing discoveries, much is 
still unknown about the life history, 
distributions, and populations of opah, 
especially at the species level. 
As recently as 2010, only 1 species of 
opah had been reported to reside in the 
North Pacific Ocean. Genetic studies, 
however, have confirmed the presence 
3 Institute of Marine Sciences 
University of California Santa Cruz 
1156 High Street 
Santa Cruz, California 95064 
4 Pacific Islands Fisheries Science Center 
National Marine Fisheries Service, NOAA 
1845 Wasp Boulevard 
Building 176 
Honolulu, Hawaii 96818 
of 2 distinct species: the bigeye Pacific 
opah (L. megalopsis) and the small- 
eye Pacific opah (L. incognitus) (Hyde 
et al., 2014; Underkoffler et al., 2018). 
Although most biological data and stud- 
ies in the North Pacific Ocean do not 
distinguish between the 2 species, some 
species-specific information is avail- 
able. Results from gut content studies 
indicate that bigeye Pacific opah forage 
deeper in the water column than small- 
eye Pacific opah because their diet was 
almost entirely composed of mesope- 
lagic species and the diet of smalleye 
Pacific opah included both epipelagic 
and mesopelagic species (Choy et al., 
2013). This finding is consistent with 
relationships between eye size and for- 
aging depth seen in other species (e.g., 
Kréger et al., 2009). In a study by Hyde 
et al. (2014), the spatial distribution of 
the 2 species were examined by using 
genetic analyses, and although results 
indicate that most smalleye Pacific 
opah were caught near the West Coast 
of the United States and that most 
