523 
Abstract — Catch rates for the 13 
most abundant species caught in the 
deep-set Hawaii-based longline fish- 
ery over the past decade (1996-2006) 
provide evidence of a change among 
the top North Pacific subtropical 
predators. Catch rates for apex pred- 
ators such as blue shark (Prionace 
glauca), bigeye ( Thunnus obesus ) and 
albacore (Thunnus alalunga ) tunas, 
shortbill spearfish (Tetrapturus 
angustirostris), and striped marlin 
(Tetrapturus audax ) declined by 3% 
to 9% per year and catch rates for 
four midtrophic species, mahimahi 
(Coryphaena hippurus), sickle pomfret 
(Taractichthys steindachneri) , escolar 
(Lepidocybium flavobrunneum), and 
snake mackerel (Gempylus serpens), 
increased by 6% to 18% per year. The 
mean trophic level of the catch for 
these 13 species declined 5%, from 
3.85 to 3.66. A shift in the ecosystem 
to an increase in midtrophic-level, 
fast-growing and short-lived species 
is indicated by the decline in apex 
predators in the catch (from 70% to 
40%) and the increase in species with 
production to biomass values of 1.0 or 
larger in the catch (from 20% to 40%). 
This altered ecosystem may exhibit 
more temporal variation in response 
to climate variability. 
Manuscript submitted 7 May 2009. 
Manuscript accepted 4 September 2009. 
Fish. Bull. 107:523-531 (2009). 
The views and opinions expressed 
or implied in this article are those 
of the author and do not necessarily 
reflect the position of the National 
Marine Fisheries Service, NOAA. 
Increases in the relative abundance 
of mid-trophic level fishes concurrent 
with declines in apex predators 
in the subtropical North Pacific, 1996-2006 
Jeffrey J. Polovina (contact author ) 1 
Melanie Abecassis 2 
Evan A. Howell 1 
Phoebe Woodworth 2 
Email address for contact author: Jeffrey.Polovina@noaa.gov 
1 Pacific Islands Fisheries Science Center 
NOAA Fisheries 
2570 Dole St 
Honolulu, Hawaii 96822-2396 
2 Joint Institute for Marine and Atmospheric Research 
University of Hawaii 
1000 Pope Rd. 
Honolulu, Hawaii 96822 
The North Pacific subtropical gyre 
is a large oceanic gyre bounded on 
the south by the North Equatorial 
Current, on the west by the Kuroshio 
Current, on the north by the Kuroshio 
Extension Current and the North 
Pacific Current, and on the east by 
the California Current (Pickard and 
Emery, 1990). Although low in pri- 
mary productivity, the warm, verti- 
cally stratified oligotrophic waters of 
the subtropical gyre contain a highly 
diverse food web populated by tunas, 
sharks, and billfishes at the top tro- 
phic levels (Seki and Polovina, 2001; 
Kitchell et ah, 2002). Since the 1950s, 
the tunas, billfishes, and other large 
predators in this ecosystem have 
been targeted by large-scale fisher- 
ies. Several studies have indicated 
possible ecosystem impacts from fish- 
ing (Ward and Myers, 2005a; Kitchell 
et al., 1999; 2002). A comparison of 
catch, size, and species composition 
between a research longline survey 
in the 1950s and observer data from 
commercial longliners in the 1990s 
indicated a substantial decline in the 
abundance of large predators, in the 
mean size of these predators, and gave 
some evidence of an increased abun- 
dance of formerly rare species (Ward 
and Myers, 2005a). Models of the 
North Pacific subtropical gyre were 
generated with Ecopath with Ecosim, 
vers. 5 (EwE) modeling software 
(available from http://www.ecopath. 
org/index.php) to investigate whether 
the ecosystem contained any keystone 
species (Kitchell et ah, 1999; 2002). 
The results indicated that there was 
not any single species group that func- 
tioned as a keystone, but that a broad 
reduction of apex predators due to fish- 
ing might result in an increase in prey 
(Kitchell et ah, 1999; 2002). In effect, 
the fishing fleet is the keystone preda- 
tor (Kitchell et al., 1999). However 
another modeling effort with an EwE 
model that incorporated some size- 
class structure revealed that although 
fishing decreased predator abundance, 
there was limited evidence (based on 
the decline in predators) of trophic 
cascades or other ecosystem impacts 
(Cox et al., 2002). 
The longline logbook and observer 
data from the Hawaii-based fishery 
are valuable data sources for inves- 
tigating the spatial and temporal dy- 
namics of the exploited subtropical 
ecosystem. The fishery operates over 
a large portion of the central North 
Pacific, from the equator to 40°N 
latitude and from 140°W longitude 
to the International Date Line. Fed- 
erally mandated logbooks completed 
by fishermen provide catch and effort 
