Woodworth-Jefcoats et al.: Oceanographic variability, fishery expansion, and longline catches in the North Pacific 
233 
Table 1 
Significant linear trends (P<0.05) in the median depth of the preferred thermal habitat of 
bigeye tuna (Thunnus obesus) (8-14 °C) based on logbook records of the Hawaii-based longline 
fishery and ocean temperatures obtained from the Global Ocean Data Assimilation System 
for 1995 through 2015. These records were transformed into a grid, and the values in this 
table were determined by using all grid cells with fishing effort at any time (any effort) and 
by weighting grid cells by total quarterly effort (effort-weighted). A dash denotes the lack of a 
significant trend. Each trend value is followed by the depth for 2005 from the linear regression 
or, in the absence of a significant trend, by the mean depth of the time series. Results are pre¬ 
sented for the full fishing ground, as well as for the northeast (NE), northwest (NW), central 
west (CW), and southwest (SW) regions individually. 
Any effort Effort-weighted 
Region 
Quarter 
Trend (m/y) 
Depth (m) 
Trend (m/y) 
Depth (m) 
NE 
Qi 
-1.36 
255.46 
_ 
302.59 
Q2 
-1.38 
256.66 
- 
301.29 
Q3 
-1.17 
254.00 
-2.64 
291.71 
Q4 
-0.96 
251.55 
- 
292.45 
NW 
Qi 
- 
264.21 
- 
346.42 
Q2 
- 
263.20 
-3.49 
328.70 
Q3 
- 
262.85 
-3.69 
290.24 
Q4 
- 
262.92 
-1.71 
336.40 
CW 
Qi 
-1.52 
367.35 
-0.85 
358.42 
Q2 
-1.64 
369.48 
-1.15 
357.56 
Q3 
-1.76 
365.06 
-1.63 
352.98 
Q4 
-1.75 
361.89 
-2.07 
348.46 
SW 
QI 
- 
265.83 
- 
292.62 
Q2 
- 
265.43 
-1.47 
292.82 
Q3 
-0.48 
266.78 
- 
302.72 
Q4 
-0.69 
264.76 
-2.14 
320.98 
Full fishing ground 
QI 
-0.55 
272.21 
- 
319.58 
Q2 
-0.71 
272.67 
-1.78 
303.25 
Q3 
-0.63 
271.92 
-3.11 
300.54 
Q4 
-0.70 
269.91 
-2.23 
336.08 
overall, shoaling from roughly 280 to 265 m between 
1995 and 2015 (Table 1). Shoaling was significant and 
greater when the depths of these temperatures were 
weighted by total quarterly effort (1.78-3.11 m/year or 
37-65 m overall in the second-fourth quarters, shoal¬ 
ing from depths of 320-340 m to depths of 270-315 m 
over the course of the study period; Table 1). Signifi¬ 
cant shoaling within each region is presented in Table 
1 . 
We lack sufficient data to examine variability in 
oxygen concentration over time. However, given that 
the oxygen-concentration threshold of bigeye tuna was 
found below 500 m across much of the area where the 
fishery operates, it is unlikely that low oxygen concen¬ 
trations affected bigeye tuna in the study area. 
Catch variability 
Catch rates The annual catch rates of bigeye tuna de¬ 
clined until 2009, but have increased in subsequent 
years (Fig. 3A). Catch rates of longnose lancetfish, on 
the other hand, have increased over the past 2 decades, 
especially after 2004. For the past decade, catch rates 
of longnose lancetfish have exceeded those of bigeye 
tuna (Fig. 3A). 
Quarterly catch rates of bigeye tuna and longnose 
lancetfish were considerably variable across the 4 re¬ 
gions of the fishery included in our analysis (Fig. 3B). 
The variability in catch rates of bigeye tuna was most 
striking in the third quarter, when the rates were no¬ 
tably higher in the NW and NE regions than in the 
SW and CW regions. Catch rates of longnose lancetfish 
were highest in the NW region and lowest in the SW 
region throughout the year. The quarterly and regional 
catch rates of mahi mahi, skipjack tuna (Katsuwotius 
pelamis), yellowfin tuna, and striped marlin are also 
presented in Figure 3B. The highest catch rates for 
these species, with the exception of mahi mahi, gener¬ 
ally occurred in the SW and CW regions. 
Catch composition The contribution of bigeye tuna to 
total catch varied by both quarter and region as did 
