Cooper et al.: Spatiotemporal catch patterns and population distributions of Lampris megalopsis and L. incognitus 147 
variability (Bigelow et al., 1999; Howell and Kobayashi, 
2006; Choy et al., 2013; Sculley and Brodziak, 2020). 
Second, evidence indicates that the abundance of some 
large pelagic predators, including opah, has decreased 
on the high seas. Results from both our study and that of 
Polovina and Woodworth-Jefcoats (2013) indicate a decline 
in CPUE in areas where bigeye Pacific opah are expected 
to dominate landings. Polovina and Woodworth-Jefcoats 
(2013) used longline data collected through 2011, prior 
to the eastward expansion of the deep-set fishery. Con- 
_ versely, CPUE of opah has increased in coastal waters 
off California (Walker and Teo’). The likely influences of 
natural climate variability, long-term climate change, and 
apparent changes in abundance highlight the potential for 
shifts in distribution of opah and the importance of long- 
term monitoring of species ranges. 
Opah and oceanography 
Although the ecological drivers for the spatial distribu- 
tions of both opah species are unknown, insights were 
gained by examining patterns in CPUE. Caution should 
be used when interpreting patterns in CPUE, but stan- 
dardized CPUE trends are commonly used as indices of 
relative abundance for both target and non-target species 
(Bigelow et al., 2002; Woodworth-Jefcoats et al., 2018; 
Ducharme-Barth and Vincent’). For both pelagic long- 
line fisheries, the highest CPUE of opah was recorded in 
the northeastern extent of the fishery ranges, starting at 
approximately 140°W. 
Quantification of the relative contribution of the 
2 species to the overall opah CPUE is beyond the scope of 
this paper; however, a number of factors may contribute 
to this pattern, with insights coming from the relatively 
high CPUE of bigeye tuna in the same region (Hanamoto, 
1987; Woodworth-Jefcoats et al., 2018; Ducharme-Barth 
and Vincent”). East of ~130°W, dissolved oxygen at depth 
declines rapidly because of the presence of the eastern 
Pacific oxygen minimum zone (Karstensen et al., 2008). 
It has been hypothesized that, where low-oxygen waters 
encompass the deep scattering layer, bigeye tuna are less 
able to access mesopelagic prey (Hanamoto, 1987). Given 
that the diets and vertical habitats of bigeye tuna and opah 
overlap (Nakano et al., 1997; Polovina et al., 2008, Choy 
et al., 2013), the same forces may influence abundance 
of opah. East of this region, productivity is enhanced by 
the California Current (Rykaczewski and Checkley, 2008). 
Although additional research on species-specific essen- 
tial habitat and gear vulnerability is needed, the appar- 
ent increase in abundance of opah in the eastern Pacific 
Ocean is likely linked to regional oceanography, produc- 
tivity, and prey availability, as has been observed for other 
highly migratory species. This study did not incorporate 
° Ducharme-Barth, N., and M. Vincent. 2020. Analysis of Pacific- 
wide operational longline dataset for bigeye and yellowfin tuna 
catch-per-unit-effort (CPUE). Sci. Comm. sixteenth regular ses- 
sion; online, 11—20 August. West. Cent. Pac. Fish. Comm., Inf. Pap. 
WCPFC-SC16-2020/SA-IP-07, 55 p. [Available from website.] 
seasonality as a predictor in the GAMs, but seasonal shifts 
in oceanographic conditions, and therefore potentially in 
distributions of opah species, could be occurring and could 
be revealed with additional sampling. 
Insights into species-specific habitats are gained by 
comparing the distribution of the 2 species, particularly in 
regions that are dominated by a single species. Although 
examining the broad range of factors that influence dis- 
tributions is beyond the scope of this paper, insights can 
come from examining distributions in the context of ocean- 
ographic conditions and prey availability. 
Off the coast of California, where smalleye Pacific opah 
are dominant, both dissolved oxygen and temperature 
decline rapidly with depth, and these decreases may limit 
access for smalleye Pacific opah to favorable thermal 
habitat and mesopelagic prey species (Karstensen et al., 
2008; Choy et al., 2013; Aksnes et al., 2017). At the same 
time, the shallow oxygen minimum zone may compress 
the habitat of the epipelagic species that are important 
prey of the smalleye Pacific opah (Prince and Goodyear, 
2006; Choy et al., 2013). In contrast, in oligotrophic gyre 
waters south of Hawaii, where bigeye Pacific opah domi- 
nate, productivity in surface waters and epipelagic prey 
biomass are low (Signorini et al., 2015) and the deep scat- 
tering layer is deeper. For example, Tont (1976) listed a 
difference of ~100 m in the depth of the top of the deep 
scattering layer between the California Current and the 
central Pacific Ocean (an average of 282 m versus 394 m). 
In addition, sea-surface temperatures are warmer in 
the central Pacific Ocean compared with those in com- 
parable latitudes in the California Current, and winter 
mixed layer depths are deeper (de Boyer Montégut et al., 
2004; Deser et al., 2010). Bigeye Pacific opah tagged with 
pop-up archival tags north of the main Hawaiian Islands 
frequently occupied depths as deep as 400 m during the 
day, potentially reflecting targeting of mesopelagic prey 
(Polovina et al., 2008). Similar information on the physio- 
logical capabilities and habitat use of bigeye and smalleye 
Pacific opah, and their prey, will help resolve niche sepa- 
ration and essential habitat. 
Implications of sympatric or cryptic species for fisheries 
management 
The presence of morphologically similar species in land- 
ings can cause issues with fisheries management. For 
example, the population status of white marlin (Kajikia 
albida) had to be reevaluated when it was discovered that 
pelagic longline fisheries in the North Atlantic Ocean were 
unknowingly landing significant amounts of the similar- 
looking roundscale spearfish (Tetrapturus georgii) without 
separating them in the catch records (Beerkircher et al., 
2009). Although we have studied the spatial distributions 
of both opah species, they are still listed as a single spe- 
cies in logbooks and landings data. It will, therefore, be 
important to collect fisheries data and conduct scientific 
studies for specific species. Establishing distinct, species- 
specific management strategies and techniques is import- 
ant because changes to the fishery range, gear, or landing 
