Polovina et al.: Increases in the relative abundance of mid-trophic level fishes in the subtropical North Pacific 
525 
Table 1 
The percentage of the observed catch, the annual percent change in catch per unit of effort (CPUE) from the linear trends, 
categorical values of production to biomass (P/B), and trophic level for each of the top 13 most abundant species in the observed 
deep-set longline catch in Hawaii, listed in order of increased annual percent change in CPUE. Trophic level and P/B values are 
taken from the Ecopath model of Kitchell et al. (2002). 
Species 
Percentage of 
total catch 
Annual 
percent change 
in CPUE 
Ratio of 
production to 
biomass 
(P/B) 
Trophic level 
1996 
2006 
Albacore ( Thunnus alalunga ) 
12 
2 
-9.1 
0.6 
4.0 
Striped marlin ( Tetrapturus audax) 
5 
4 
-4.8 
0.5 
4.3 
Bigeye tuna (Thunnus obesus) 
17 
17 
-3.4 
0.8 
4.0 
Shortbill spearfish ( Tetrapturus angustirostris) 
3 
2 
-3.3 
0.5 
4.3 
Blue shark ( Prionace glauca ) 
12 
10 
-2.6 
0.3 
4.0 
Skipjack tuna ( Katsuwonus pelamis ) 
4 
4 
0.0 
1.9 
3.9 
Yellowfin tuna (Thunnus albacares) 
4 
4 
0.0 
1.2 
4.0 
Ono (Acanthocbium solandri) 
1 
4 
0.0 
2.0 
3.9 
Longnose lancetfish (Alepisaurus ferox ) 
10 
20 
0.0 
0.3 
3.2 
Sickle pomfret (Taractichthys steindachneri ) 
5 
9 
6.0 
1.5 
3.2 
Mahimahi (Coryphaena hippurus) 
3 
7 
6.6 
3.0 
3.9 
Escolar ( Lepidocybium flavobrunneum) 
1 
4 
10.6 
0.8 
3.2 
Snake mackerel (Gempylus serpens) 
2 
6 
17.9 
1.0 
3.2 
for those commercial species where we had both types of 
data, the observer-based estimates of CPUE compared 
well with logbook-based estimates. For example, cor- 
relations between CPUE trends for commercial species 
computed from both logbooks and observer data were 
very similar — 0.93 or greater for albacore, striped mar- 
lin, shortbill spearfish, bigeye tuna, and sickle pomfret, 
between 0.80 and 0.89 for mahimahi, ono, and yellowfin 
tuna, 0.78 for skipjack, and 0.76 for blue shark. 
Based on the linear trend derived from either the 
GAM fitted to monthly CPUE data or the regression 
line fitted to annual observer CPUE, the annual per- 
cent change in CPUE of each species was computed as 
the slope divided by the intercept multiplied by 12 to 
convert from monthly to annual values, if necessary, 
and multiplied by 100 to convert to a percentage. For 
those species with linear slopes that were not statisti- 
cally different from zero the annual percent change 
was set at zero. 
From the catch data for the most abundant 13 species, 
we computed time trends of the annual mean trophic 
level of the catch, the annual proportion of the catch 
composed of apex predators (those with trophic level at 
least 4.0), and the annual proportion of the catch with 
moderate or high production to biomass (P/B) ratio 
(defined as at least 1.0). Here we define trophic level 
1.0 as primary producers (e.g. phytoplankton), level 
2.0 as secondary producers (e.g. zooplankton), level 3.0 
as mid-level consumers, and level 4.0 and above as the 
apex predators. The estimates of trophic level and P/B 
ratio for most of the 13 species came from a central 
North Pacific pelagic ecosystem EwE model (Kitchell et 
al., 2002). The annual trophic level, annual percentage 
of the catch with trophic level at least 4.0, and annual 
percentage of catch with P/B ratio of at least 1.0 were 
computed as a mean weighted by the relative catch in 
numbers as follows: 
13 
(i) 
7=1 
where M ] 
X: 
C u 
CCJ 
annual trophic level, annual percent of 
the catch with trophic level at least 4.0, or 
annual percentage of catch with P/B ratio 
of at least 1.0 in year j; 
trophic level of species i or binomial variable 
with value 0 if trophic level < 4.0 or P/B < 
1.0 and 100 otherwise; 
catch in number of species i in year j; and 
combined catch of top 13 species in year j. 
Three species — sickle pomfret, escolar, and snake 
mackerel — are not represented as species groups in 
the Kitchell et al. (2002) model. However, longnose 
lancetfish is assigned a trophic level of 3.2 and a P/B 
ratio of 0.3 in the model. Lancetfish is found from the 
surface to below 1000 m and feeds on a diverse assem- 
blage of fishes, cephalopods, tunicates, and crustaceans 
that occupy the scattering layers (Post, 1984). Sickle 
pomfret, escolar, and snake mackerel all appear to feed 
on much of the same prey as the longnose lancetfish so 
it seems reasonable to assign them all to a trophic level 
of 3.2 (Nakamura and Parin, 1993). For the P/B ratio, 
