Metabolic rates of juvenile scalloped hammerhead sharks (Sphyrna lewini)

Oxygen consumption of juvenile scalloped hammerhead sharks, Sphyrna lewini, was measured in a Brett-type flume (volume=635 l) to quantify metabolic rates over a range of aerobic swimming speeds and water temperatures. Oxygen consumption (log transformed) increased at a linear rate with increases in tailbeat frequency and swimming speed. Estimates of standard metabolic rate ranged between 161 mg O₂ kg^-1 h^-1 at 21°C and 203 mg O₂ kg^-1 h^-1 at 29°C (mean-SD: 189ᆣ mg O₂ kg^-1 h^-1 at 26°C). Total metabolic rates ranged from 275 mg O₂ kg^-1 h^-1 at swimming speeds of 0.5 body lengths per second (L s^-1) to a maximum aerobic metabolic rate of 501 mg O₂ kg^-1 h^-1 at 1.4 L s^-1. Net cost of transport was highest at slower swimming speeds (0.5-0.6 L s^-1) and was lowest between 0.75 and 0.9 L s^-1. Therefore, these sharks are most energy efficient at swimming speeds between 0.75 and 0.9 L s^-1. These data indicate that tailbeat frequency and swimming speed can be used as predictors of metabolic rate of free-swimming juvenile hammerhead sharks.     

Inter-island movements of scalloped hammerhead sharks (Sphyrna lewini) and seasonal connectivity in a marine protected area of the eastern tropical Pacific

Marine top predators are common at offshore bathymetric features such as islands, atolls, and seamounts, where most pelagic reef fish reside, while certain sharks perform inter-island movements between these formations. Scalloped hammerhead sharks are known to school in great numbers at small islands and seamounts in the eastern tropical Pacific (ETP) and are very susceptible to fisheries while moving into the open sea. It is, therefore, essential to understand hammerhead inter-island movements and environmental effects to provide baseline information for their conservation and management within and beyond an insular marine protected area. Movements of scalloped hammerheads were analyzed in the Galapagos Marine Reserve (GMR) and ETP, and environmental factors were linked to their movements. Hammerheads were tagged (N = 134) with V16 coded pingers (July 2006 to July 2010) in the northern Galapagos and detected at listening stations around four islands in the GMR and two isolated islands in the ETP, 700 and 1,200 km away. Hammerheads formed daytime schools at specific locations, but dispersed at night. Overall, more daytime than nighttime detections were recorded at all receivers in the northern Galapagos Islands, and more detections in the up-current sides of these islands. Hammerheads remained more days at the northern islands during part of the warm season (December–February) compared to the cool; however, fewer individuals were present in March–June. Movement modes were diel island excursions (24-h cycles) in the northern Galapagos and inter-island in the GMR and ETP at different scales: (1) short back-and-forth (<50 km, SBF), <5 days cycles, (2) medium distance (50–300 km, MDT), 5–20 days, and (3) long distance (>300 km, LDT), 15–52 days. The high degree of inter-island connectivity of hammerheads within the northern GMR is striking compared to the almost nil movement to the central GMR. A seasonal migratory pattern to locations offshore is indicated by (1) fewer hammerheads observed in the northern GMR during part of the warm season (March–June) and (2) evidence of LDT movements from the northern GMR to other islands in the ETP. LDT movements of mature female hammerheads are possibly associated with pupping areas. Our results indicate that currents, season, and individual behavior mainly drive inter-island movements of hammerheads at small (SBF) and medium (MDT) scales. These findings have important implications for the management of a highly mobile and endangered top predator within a marine protected area and beyond.     

Seasonal changes in movements and habitat preferences of the scalloped hammerhead shark (Sphyrna lewini) while refuging near an oceanic island

Movements and habitat preferences of sharks relative to a central location are widely documented for many species; however, the reasons for such behaviors are currently unknown. Do movements vary spatially or temporally or between individuals? Do sharks have seasonal habitat and environmental preferences or simply perform movements at random at any time of the year? To help understand requirements for the designation of critical habitats for an endangered top predator and to develop zoning and management plans for key habitats, we examined vertical and horizontal movements, and determined habitat and environmental preferences of scalloped hammerhead sharks (Sphyrna lewini). We tracked seven hammerheads for 19–96 h at Wolf Island (1.38ºN, 91.82ºW) between 2007 and 2009 using ultrasonic transmitters with depth and temperature sensors, and we profiled temperature through the water column. Movements of individual hammerheads fell in two classes: constrained (remaining near the island) and dispersive (moving offshore to pelagic environments). The central activity space or kernel off the southeast side of Wolf Island was small and common to most, but the area varied among individuals (mean ± SE 0.25 ± 0.2 km²), not exceeding 0.6 km² for any of the sharks, and not changing significantly between seasons. In general, hammerheads showed preference for the up-current habitat on the eastern side of Wolf Island in both the warm and cold seasons. However, the depth of sharks varied with season, apparently in response to seasonal changes in the vertical structure of temperature. Hammerheads performed frequent vertical excursions above the thermocline during offshore movements and, in general, were observed to prefer temperatures of 23–26 °C found above the thermocline. At times, though individuals moved into the thermocline and made brief dives below it. Our results provided evidence that hammerheads (1) are highly selective of location (i.e., habitat on up-current side of island) and depth (i.e., top of the thermocline) while refuging, where they may carry out essential activities such as cleaning and thermoregulation, and (2) perform exploratory vertical movements by diving the width of the mixed layer and occasionally diving below the thermocline while moving offshore, most likely for foraging.