Ocean climate effects on the relative abundance of short-finned (Illex illecebrosus) and long-finned (Loligo pealeii) squid in the northwest Atlantic Ocean

An unusually high abundance of long‐finned squid (Loligo pealeii) was observed around southern Newfoundland in August–September 2000. The prevalence of maturing females and mature males, along with the collection of a single viable egg mop, provide the first evidence of spawning of this species at the northern limit of its geographic range of distribution. Northward expansion of the long‐finned squid population may be related to a general warming trend in Newfoundland near‐shore waters. However, trends in size and abundance of short‐finned squid (Illex illecebrosus) suggest that this expansion may also be related to reduced competition. We hypothesize that these two squid species share, to a large extent, a common niche on the eastern USA shelf and that opposing responses to ecosystem variation affect their relative abundance. We address this hypothesis by applying time‐series analysis of species‐specific fishery‐ and survey‐based abundance indices with biologic and environmental input variables. Our models indicate that direct competition is not important, but that variation in atmospheric forcing, as well as latitudinal position of the Shelf‐Slope Front (SSF), are closely related to direct oceanographic processes that exert opposing effects on these two species. While the direct oceanographic mechanisms that regulate year‐class strength remain unknown, we present a hypothesis to account for opposing population responses to oceanographic variation that operates early in the life history of both species. For the oceanic and highly migratory short‐finned squid, variation in the latitudinal position of the SSF is related to efficiency of downstream dispersal by the Gulf Stream and survival of young stages. For the neritic long‐finned squid, variation in local inshore temperature affects the time exposed to intense predation through its effect on rates of embryonic development and growth. The expansion of long‐finned squid population abundance in 2000 was associated with both warm local water temperatures and an unusual eastward displacement of the atmospheric features associated with the North Atlantic Oscillation (NAO). These conditions may have resulted in an unfavorable oceanic regime for short‐finned squid but a favorable regime for long‐finned squid on the continental shelf as far north as southern Newfoundland.