Arica-Atacama is an important reproductive area for two shark species. Within the area, neonate, young-of-the-year, and juvenile Shortfin Mako and Blue Shark are reported (Bustamante and Bennett 2013; Doherty et al. 2014; IFOP 2018, 2019) from fishery-dependent data (catch-per-unit-effort [CPUE] and size-frequency data). Overall, along the coast of Chile, the highest captures (volume), fishing effort, and fishing yields for both species are within this area, and where 95–100% of captures

from these species are juveniles (IFOP 2018, 2019). In 2019, the smallest Blue Shark individual measured 79 cm total length (TL) and 92 cm TL for Shortfin Mako; thus, likely young-of-the-year animals for both species.

Between 2005–2010, 4,202 Blue Sharks (75% juveniles) and 1,748 Shortfin Makos (93% juveniles) were sampled (Doherty et al. 2014). Here, the mean CPUE (sharks/1,000 hook-hours) was 33.2 ± 35.6 SD (range: 0–295; total sets: 618) between March and November, and 3.0 ± 20.7 SD (range: 0–256; total sets: 402) from December to February revealing high seasonality of these catches. The mean size was 130.7 cm TL for Blue Shark and 132.6 cm TL for Shortfin Mako with smaller sizes around the size-at-birth for both species (Doherty et al. 2014). There was a significant effect of depth on Blue Shark and Shortfin Mako CPUE observed in this area with a restricted depth range between 6–12 m (Bustamante and Bennett 2013). This is similar to studies from the eastern North Atlantic (Maia et al. 2007) and northeast Pacific (Sepulveda et al. 2004; Nosal et al. 2019) where immature, including young-of-the-year, Shortfin Mako and Blue Shark mostly occupy the upper 40 m of the water column. In the north of Chile, the upper 30 m of the epipelagic zone is rich in small scombrid and carangid fishes (Alegría 1995; Zuleta 2005) that are a major component of the diets of small-sized Shortfin Makos (López et al. 2009) and Blue Sharks (López et al. 2010).

Similarly, in 2005 and 2010 between January and February, 1,153 Blue Sharks and 1,241 Shortfin Makos were collected from 178 longline sets, with a predominance of small immature sharks (Bustamante and Bennett 2013). Blue Shark and Shortfin Mako were not caught in 23% and 9% of sets, respectively. However, the CPUE (sharks/1,000 hook-hours) ranged from 0–230 for Shortfin Mako and 0–662 for Blue Shark. These values are high compared to similar studies in other regions for these species (Doherty et al. 2014). For example, in Mexican Pacific (Velez-Marin and Marquez-Farias 2009; Smith et al. 2009), and Papua New Guinean (Kumorum 2003) fisheries, less than one shark per 1,000 hook-hours are captured (Bizarro et al. 2009). For Blue Shark, the mean CPUE (sharks/1,000 hook-hours) was 18.4 in the North Atlantic (Campana et al. 2005), 5.5 in Australian waters (Stevens 1992), and 15 in New Zealand waters (Francis et al. 2001). For Shortfin Mako, in the Caribbean and the Gulf of Mexico values oscillated between 3.5 and 11.9 (Cramer 1996) while in the North Atlantic values were between 0.1 and 1.1 (Beerkircher 2005).

Overall, no gravid females were observed for Blue Shark with mature ova found in 8.4% of the female sharks caught (Bustamante and Bennett 2013). For Shortfin Mako, mature ova were found in a single specimen (Bustamante and Bennett 2013). Sex ratio had non-significant deviance from 1:1 for both species. For Shortfin Mako, size of males captured in 2005 presented a mean ± SD of 121.9 ± 23.7 cm TL, while for males was 122 ± 25.4 cm TL. The smallest individual measured 66 cm TL (size-at-birth of Shortfin Mako is 65–70 cm TL; Duffy and Francis 2001; Maia et al. 2007). There was also a predominance of the smaller size classes (70–100 cm TL) in both years for female Shortfin Makos. Sharks between 60 and 70 cm TL are likely to be young-of-the-month, while sharks between 100 and 120 cm TL are likely to be young-of-the-year. For Blue Shark, in 2005, the mean size was 133.1 ± 35.4 TL cm and 152.7 ± 48.6 TL cm for males. In 2010, mean size ranged 139.0 ±27.5 cm TL for females and 151.3 ± 43.3 cm TL for males. The smallest individual measured 52 cm TL (size-at-birth for Blue Shark is 35–60 cm TL; Clarke et al. 2015). Other less recent surveys also assessed the size-frequency of Blue Shark and Shortfin Mako from fisheries off these four political regions between November 2000 and August 2001 and found that most individuals were juveniles with the presence of neonates (Acuña et al. 2001).