Predatory Interactions and Niche Overlap between Mako Shark, Isurus Oxyrinchus, and Jumbo Squid, Dosidicus Gigas, in the California Current

Recent scientific and anecdotal observations have documented a range expansion of jumbo squid, Dosidicus gigas, into the Southern California Bight (SCB) and northward in the California Current Large Marine Ecosystem. The increase in squid abundance at higher latitudes has generated hypotheses concerning the ecosystem changes that may have permitted this expansion. Top-down explanations suggest that overharvest of higher trophic level species such as tunas and billfishes creates a trophic cascade that increases survivorship of jumbo squid. Bottom-up explanations suggest that changes in ocean climate, including temperature and hypoxia, may favor an expanded range for jumbo squid.Here we present information on: (1) predatory interactions between the mako shark and jumbo squid in the SCB, (2) vertical niche of mako sharks and potential for a hypoxic refuge for jumbo squid, and (3) changes in the oxygen minimum zone (OMZ) and trends in epiand mesopelagic prey.Mako sharks examined during the 2006 and 2007 NOAA Fisheries Southwest Fisheries Science Center Juvenile Shark Longline Survey had a high incidence of scars inflicted by jumbo squid. Diet studies based on the California Drift Gillnet Fishery, 2002–07 indicate that jumbo squid accounted for a substantial portion of the mako diet. Bioenergetic calculations suggest that the average 18 kg mako taken in the fishery would need 56–113 kg of squid to meet its annual dietary requirements.The high-resolution diel activity records of two representative animals indicate makos remained near the surface at night and were able to exploit the water column from the surface to a maximum depth of 300 m during the day. The maximum depth of dives corresponded to oxygen concentrations as low as 1.25 ml/L. Previous studies indicate that jumbo squid inhabit a depth range from the surface to the upper bounds of the OMZ where oxygen concentration is 0.5 ml/L or less. Jumbo squid in the SCB may have a deepwater refuge from mako sharks below 1.25 ml O2/L, but are clearly available to mako during diel vertical migrations. Examination of the CalCOFI database for changes in oxygen content and larval fish counts over the previous 56 years indicates a shoaling of the OMZ and periodic changes in abundance of epiand mesopelagic prey species, but did not reveal a simple relationship between oxygen, prey availability, and range expansion. Better estimates of squid and mako population size and mako removal rates are needed to fully understand the impact of mako sharks on jumbo squid abundance. INTRODUCTION The jumbo squid,Dosidicus gigas, is a dominant midtrophic link in the pelagic food webs of the Eastern Tropical Pacific (ETP;Olson and Young 2007). Recent studies originating in the Gulf of California (GOC), where jumbo squid are permanent residents, have described their age and growth, reproductive biology, and trophic niche (Markaida and Sosa-Nishizaki 2001, 2003; Markaida et al. 2004). Electronic tagging studies have provided new insights into the physical niche including horizontal and vertical movements, diel activity patterns, preferred temperatures, and a previously unappreciated tolerance of low-oxygen, mesopelagic habitats (Gilly et al. 2006). The mesopelagic oxygen minimum zone (OMZ) is a persistent feature of the ETP (Stramma et al. 2008) and extends northward throughout the California Current Large Marine Ecosystem (CCLME) usually at depths of 600 to 1000 m (Vetter and Lynn 1997; Chan et al. 2008). Two recent studies (Field et al. 2007;Zeidberg and Robison 2007) have reported a range extension of jumbo squid into the colder waters of the central California Current where they have been observed to a depth of 2000 m.Historically, the occurrence of the jumbo squid off California was restricted to periodic outbreaks, often associated with El Niño-Southern Oscillation (ENSO) events (Rodhouse, this volume). Interest by scientists,management agencies, and the general public has driven speculation on how the CCLME may be changing to allow this persistent northern range expansion. Proposed causes can be grouped into two categories: (1) top-down effects, such as removal of squid predators and competitors by fishing, which might increase jumbo squid survival and (2) bottom-up effects, such as changes in oceanography and prey fish abundance, that might increase favorable jumbo squid habitat. These speculations are not without controversy 142 142-156 Vetter final:• CALCOFI SETUP 11/8/08 9:58 PM Page 142