Polovina et al Increases in the relative abundance of mid-trophic level fishes in the subtropical North Pacific 
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Striped marlin 
Shortbill spearfish 
Blue shark 
Albacore 
Bigeye tuna 
Yellowfin tuna 
Skipjack tuna 
Ono 
Longnose lancetfish 
Mahimahi 
Sickle pomfret 
Escolar 
Snake mackerel 
-10 -5 0 5 10 15 20 
Percentage 
Figure 4 
Annual percent change in catch per 1000 hooks (CPUE) (declines in catch 
are represented by negative values and increases in catch are represented by 
positive values) from the Hawaii deep-set fishery, over the period 1996—2006, 
based on the linear trends presented in Table 1 for each species arranged 
in descending order of its trophic level. 
the biomass of mahimahi, flying squid, and lancetfish 
increased (Kitchell et al., 2002; Fig. 3). The pattern 
reversed when fishing was eliminated: the biomass of 
mahimahi, flying squid, and lancetfish all decreased as 
their predators increased (Kitchell et al., 2002; Fig. 3). 
Lancetfish CPUE in our analysis showed an increasing 
trend, but because of its large interannual variation, it 
was not statistically significant (Fig. 3). Flying squid is 
not caught in the longline gear. However, as previously 
discussed, sickle pomfret, escolar, and snake mackerel, 
although not specifically identified in the Kitchell et al. 
(2002) model, appear to occupy a very similar prey role 
in the food web as lancetfish and flying squids. Hence, 
the observed increase in CPUE for mahimahi, sickle 
pomfret, escolar, and snake mackerel is consistent with 
the top-down control seen in the Kitchell et al. (2002) 
model simulation. Considering an earlier and somewhat 
different central North Pacific Ecopath model (Kitchell 
et al., 1999) we concluded that there is no single species 
that serves as a keystone species in this ecosystem but 
rather the longline fishery may function as a keystone 
species. 
One additional piece of evidence supporting top-down 
control for sickle pomfret is that this species was ab- 
sent in the longline sets of the 1950s but present in the 
1990s (Ward and Myers, 2005a). This finding was inter- 
preted as a possible population response to a reduction 
in predators that included tunas, billfishes and sharks 
(Ward and Myers, 2005a). 
Top-down controls have been observed in temperate 
ocean ecosystems. A meta-analysis showed shrimp pop- 
ulation abundance was controlled by the abundance of 
its predator, the Atlantic cod ( Gadus morhua), in eight 
regions in the North Pacific (Worm and Myers, 2003). 
Further, at least in one ocean system, the eastern Sco- 
tian Shelf, removal of the top predator, the Atlantic cod, 
resulted in a trophic cascade impacting four trophic 
levels (Frank et al., 2005). In our pelagic ecosystem, if 
we considered the longline fleet functioning at the top 
trophic level, then we have top-down controls spanning 
three trophic levels: the longline fleet, the apex fishes, 
and the midtrophic level fishes. Our knowledge of the 
feeding ecology of many of the midtrophic level fishes 
that appear to have increased — sickle pomfret, escolar, 
and snake mackerel — is very limited and therefore we 
do not know the impacts on the ecosystem from their in- 
creased abundance. The juveniles of many of the tunas 
also occupy the midtrophic level but whether they ben- 
efit from the reduction in apex predators or suffer from 
increased competition or an increase in other predators 
is unknown but is a critical question for fisheries man- 
agement. For example, if juvenile tunas are adversely 
impacted by the increase in other midtrophic level spe- 
cies then a reduction in fishing effort may not result in 
an increase in adult tunas. Lastly, we do not have data 
on changes in cetacean abundance and cetaceans have 
not been included in previous central Pacific models. 
However, cetaceans are apex predators and if they are 
