Polovina et al. : Increases in the relative abundance of mid-trophic level fishes in the subtropical North Pacific 
527 
Figure 2 
Temporally and spatially adjusted monthly catch per 1000 hooks (CPUE) and linear trend 
lines from the generalized additive models for those species exhibiting a significantly 
increasing trend in the Hawaii deep-set longline fishery, 1996-2006: (A) mahimahi 
( Corypliaena hippurus ), and (B) sickle pomfret ( Taractichthys steindachneri). 
greater than or equal 1.0, has approximately doubled 
from about 20% to 40% (Fig. 5). 
Discussion 
The longline CPUE, like most fishery-dependent data, 
responds to a variety of factors that include changes in 
species targeted, gear changes that impact species catch- 
ability, changes in season, and area fished. It is likely 
that some of these factors have affected the Hawaii- 
based longline fishery. In an attempt to limit the effect 
of some of these factors we used only data from the 
deep-set fishery and from the core geographic region 
of the fishing ground. Further, for the 10 species for 
which we had logbook data we used a GAM to account 
for seasonal and spatial effects. However, in the case of 
albacore tuna, the 9.1% decline per year may be, at least 
in part, a result of a shift in targeting. On a basin-wide 
level the albacore stock, although reduced by fishing, has 
not exhibited the collapse seen in the deep-set fishery 
catches (Sibert et al., 2006). Albacore CPUE is greatest 
in the summer months, and since 2002 a summer fishery 
for large bigeye tunas has developed at 30°N latitude 
outside the core area covered in this study. This may 
have contributed to a shift in targeting from albacore 
to bigeye tuna. 
We observed declines in CPUE trends of large high- 
trophic-level and lower P/B species, including striped 
marlin, shortbill spearfish, bigeye tuna, albacore tuna, 
and blue shark. Increasing CPUE trends were observed 
for mahimahi, sickle pomfret, escolar, and snake mack- 
erel that are midtrophic-level species with higher P/B 
values. The increasing trends for mahimahi, escolar, 
and snake mackerel are most likely not due to increased 
targeting because snake mackerel has no commercial 
value, escolar has limited commercial value, and ma- 
himahi is not caught efficiently with deep-set gear (it is 
generally only caught when the gear is being recovered 
and hooks are at the surface). The increasing trend is 
also not likely a response to more hooks being available 
from the decline in apex species because total catch 
rates are in the range of 10-20 fish per 1000 hooks and 
hence hook saturation is not likely a cause. However, 
the observed increase in catch rates of midtrophic level 
species concurrent with a decrease in catch rates of 
apex species is consistent with top-down control where 
fishing has reduced the abundance of apex species and 
mid-trophic level species have increased in abundance 
in response to decreased predation. Mahimahi is an epi- 
