Range expansion of the Humboldt squid was not caused by tuna fishing

Zeidberg and Robison (1) infer that the range expansion of Dosidicus gigas resulted from a top-down cascade caused by fishing for tunas and billfishes in the eastern equatorial Pacific (EEP). We offer an alternative perspective supported on three grounds. First, the thermal tolerance and plastic life history of D. gigas provide a parsimonious explanation of both the squid's historical visits to waters off central California and its range expansion, without requiring a top-down mechanism. Second, Zeidberg and Robison base their inference on the false tenet that the biomasses of tunas in the EEP have “recently experienced drastic depletions” (p. 12950) by fishing. In reality, fishing-induced reductions of tunas and billfishes had been occurring for at least three decades before the squid's range expansion, and the expansion occurred while tuna biomasses in the EEP were at contemporarily high levels (2, 3). Contemporary trends in tuna biomass are most relevant to the expansion given the short generation time (1–2 years) of D. gigas. Finally, published results from dynamic food-web models do not suggest top-down control of ommastrephid squids by tunas and billfishes; simulations demonstrate that historical trends in tuna and billfish abundance did not cause trends in squid abundance (4). In fact, results from a model for the EEP suggest that squids are controlled more by bottom-up forcing than by top-down cascades from tuna fishing (5).