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Demography, movement patterns, and mating system of leopard sharks (Triakis semifasciata) aggregating along the open coast of southern California, USA

  • Author(s): Nosal, Andrew Phillip
  • et al.
Abstract

This dissertation describes the aggregation behavior of the leopard shark (Triakis semifasciata) along the open coasts of La Jolla (LJ; primary focus) and Del Mar (DM), California, which are 12 km apart. The male-to-female ratio was 0.0299 in LJ (n = 140) and 1.0450 in DM (n = 45). Twenty females and 13 males were surgically fitted with coded acoustic transmitters and passively tracked for 1,148 days by an array of seven underwater acoustic receivers spanning 120 km of coastline from San Clemente, California to the Mexican border, including LJ and DM. Female abundance at the LJ receiver was highest in late June - early December and was directly related to sea surface temperature and inversely related to swell height; seasonal arrival to and departure from LJ was highly synchronous and coincided with the summer and winter solstices, respectively. Eight additional sharks in LJ were externally fitted with continuous acoustic transmitters and actively tracked for up to 48 hours. These sharks were generally confined to a divergence zone of low wave energy, caused by wave refraction by an offshore submarine canyon. Sharks spent 80.0% of the daytime in shallow water (less than 2 m), with some dispersing at night to deeper (max: 53.9 m) and cooler (min: 12.7°C) water, likely to forage. Male abundance at the DM receiver was highest in late April - early October and was directly related to photoperiod and sea surface temperature. Lastly, sharks were seasonally philopatric; 50.0% of females and 60.0% of males returned annually to LJ and DM, respectively, throughout the three-year study period. These findings suggest the anomalously low water turbulence at the LJ site facilitates the invasion of warm shallow water by pregnant females to increase their body temperatures and accelerate gestation. The proximity of the LJ aggregation site to feeding grounds (e.g., submarine canyon) may further benefit females by increasing foraging efficiency. Finally, multiple paternity was detected in only 36.4% of genotyped litters sampled from mothers collected in LJ (n = 22). Thus, the predominance of singly fathered litters (63.6%) and scarcity of males at the LJ site suggest this aggregation may also function as a refuge from males by reducing harassment in the form of excessive mating attempts

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