We describe here the horizontal and vertical movements of a female juvenile scalloped hammerhead at the time it is believed to have made an ontogenetic movement from shallow coastal waters, which they inhabit for the first 3 years of their life, to offshore locations. Prior studies suggest that adult hammerheads display a diel pattern in relation to their use of seamounts
[17, 18], forming schools around seamounts during the day and dispersing to forage in open water at night. This diel pattern has also been observed around Malpelo
 and Galapagos
 islands. It is assumed that they make offshore migrations into the pelagic environment at night to feed on pelagic fish and squid
[13, 21]. Little was known about the movements of juvenile hammerhead sharks in the pelagic realm, nevertheless, here we document such horizontal and vertical movements.
The female juvenile shark performed the shallowest dives from 23 May to 2 July and the deepest dives from 4 September to 7 November, indicative also of an ontogenetic migration offshore. This migration likely occurred after the first days of September when the dive oscillations became much greater with the shark making regular dives between 150 to 250 m (see Figure
2B). The shallowest dives were performed during the transitional months from May to July (spring to summer), when the depth of the thermocline is usually shallow. According to Ketchum et al.
, hammerhead sharks perform frequent vertical excursions above the thermocline during offshore movements and, in general, were observed to prefer temperatures of 23°C to 26°C, which are found above the thermocline. The deepest dives of the female hammerhead in this study were performed from late summer to late fall (from September to December), when the thermocline is deeper (see Figures
An alternative factor possibly influencing the vertical movements of the juvenile shark could be the stable oxygen minimum layer. In the southern Gulf of California, this layer is well pronounced between 250 and 800 m deep. In the case of our tagged juvenile shark, vertical movements ranged from the surface to 272 m in a 10-month period. In contrast, the vertical movements of an adult scalloped hammerhead shark tracked by Jorgensen et al.
 ranged from the surface to a depth of at least 980 m. Hence, adults can inhabit a highly expanded, vertical niche in the open ocean, tolerating large fluctuations in temperature and extremely low levels of dissolved oxygen.
One possible explanation for the offshore ontogenetic migration is that females move into open waters due to social factors. The social factor hypothesis states that sexual segregation may be maintained by aggression of one sex towards the other
. Therefore, sub-adult females could also leave inshore areas to avoid aggressive behavior of males
. Nevertheless, there is little information on the behavior of juvenile hammerheads in inshore areas of the Gulf of California that could support this hypothesis.
Another explanation for the offshore migration may be environmental. There was a cold episode during the second half of 2007
. However, the relationship between these climatic processes and trophic responses is often complex and may be delayed in response to the primary climate signal change. These effects may manifest in predator population distributions, movements, densities, phenology, behavior, and community interactions
. Although the behavior of the juvenile hammerhead could be influenced by these phenomena, the ontogenetic migration of a juvenile hammerhead coincides more with the offshore movements of females at an early age related to diet and growth differential as shown by previous studies in the Gulf of California
An additional explanation is fitness-related. Capture data indicate that female scalloped hammerhead sharks migrate offshore at a smaller size than males. Females in the size range of 74 to 125 cm were captured at a median depth of 50 m, whereas males with same size range were caught at a median depth of 25 m
. When the female shark in this study was tagged she had a size of 95 cm TL and remained in shallow waters above 50 m depth from January to August (Figure
2A). Ten months later, when the female was recaptured, she had a size of 123 cm TL. This size corresponds to a 4-year-old female
. Between the ages of 3 to 4 years, females grow at a more rapid rate than males in the Atlantic off the coast of North Carolina
. Similarly, an average female at an age of 4 years has a TL of 119 cm, 6 cm larger than a male of the same age, even larger than a male of 5 years, in the Gulf of California
. Thus, an offshore movement of females results in a growth differential
. It is possible, then, that the juvenile females are trying to get offshore as quickly as possible to jockey for position, assert dominance in the schools, and establish social rank. In a comparative study of teleost and elasmobranch schooling behavior, Klimley
 observed that scalloped hammerhead shark schools are comprised predominantly of females with larger individuals aggressively vying for central positions within the school. Schools of hammerheads at offshore seamounts and islands in the Gulf of California studied during late 1970’s and early 1980’s were composed of juveniles as well as larger individuals of both sexes
. This was evident from a frequency distribution of individual sharks measured using stereo-photography, where only females existed in the 113, 128, and 143 cm TL size classes
Moreover, this offshore movement of female scalloped hammerhead sharks at an earlier age than males results in different diets between them. As might be expected from their early offshore movement, according to Klimley
, females less than 160 cm TL fed on a higher percentage of pelagic prey than did males of similar sizes. Mesopelagic prey formed 27.5% of the female total Index of Relative Importance (IRI) and epipelagic prey formed only 5.5% of the female IRI, and only 18.1% and 3.6% of the male total IRI, respectively. Furthermore, the diet of the females consisted of only 15.1% benthic prey in comparison to 40.9% benthic prey of the IRI of male sharks. Scalloped hammerhead shark pups have high metabolic rates and frequently feed on prey of low caloric value while in nursery grounds. Thus, they require higher ration levels to fulfill their energetic needs. Those sharks that do not become successful in learning how to catch prey quickly may starve as the result of their high energetic requirements
, which would partially explain the early fledging of this juvenile female to offshore waters for richer food.
The recorded depths of the female hammerhead during a single day in October 2007, showed swimming depths above 50 m during the day and 120 to 250 m during the night (see Figure
3B), possibly indicating a diel change of lifestyle, from refuging in large schools at seamounts during the day to venturing into deep water to feed at nighttime
. And, vertical movements of the hammerhead during all months represent a seasonal shift of lifestyle after August 2007, from shallow to deeper waters (see Figure
4). The mass of the stomachs of female sharks larger than 100 cm TL and smaller than 160 cm TL exceed those of males of the same size
, likely representing greater foraging success for females as they make diel migrations into the deep waters surrounding the seamounts to feed on pelagic fishes and squid
. A possible adaptive advantage from this early offshore migration of females is increased growth despite greater exposure to risk of predation. Increased body size would not only provide more body cavity space for the female’s ova or embryos, but also more muscle and a larger liver for energy storage, later to be allocated to reproduction
. Supporting the assertion that greater growth increases reproductive success in females and not males is continued growth of the former upon reaching maturity and absence of such growth in the latter
. Hence, smaller female sharks trade-off the risks of an open ocean migration at an early age for the opportunity to get big quick and increase their reproductive potential.
The presence of neonates, gravid females, and small juveniles of scalloped hammerhead sharks in LPB and in the southeastern coast of the Gulf of California, indicates that these areas are used as a nursery for this species
[16, 17]. In addition, the presence of male and female adult hammerheads during the summer in seamounts in the southwestern Gulf of California
, in the mid-riff islands, and southeastern Gulf
, suggests that these places are important as mating aggregation sites for the scalloped hammerhead shark.