Laser photogrammetry improves size and demographic estimates for whale sharks

Abstract

Whale sharks Rhincodon typus are globally threatened, but a lack of biological and demographic information hampers an accurate assessment of their vulnerability to further decline or capacity to recover. We used laser photogrammetry at two aggregation sites to obtain more accurate size estimates of free-swimming whale sharks compared to visual estimates, allowing improved estimates of biological parameters. Individual whale sharks ranged from 432-917 cm total length (TL) (mean ± SD = 673 ± 118.8 cm, N = 122) in southern Mozambique and from 420-990 cm TL (mean ± SD = 641 ± 133 cm, N = 46) in Tanzania. By combining measurements of stranded individuals with photogrammetry measurements of free-swimming sharks, we calculated length at 50% maturity for males in Mozambique at 916 cm TL. Repeat measurements of individual whale sharks measured over periods from 347-1,068 days yielded implausible growth rates, suggesting that the growth increment over this period was not large enough to be detected using laser photogrammetry, and that the method is best applied to estimating growth rates over longer (decadal) time periods. The sex ratio of both populations was biased towards males (74% in Mozambique, 89% in Tanzania), the majority of which were immature (98% in Mozambique, 94% in Tanzania). The population structure for these two aggregations was similar to most other documented whale shark aggregations around the world. Information on small (<400 cm) whale sharks, mature individuals, and females in this region is lacking, but necessary to inform conservation initiatives for this globally threatened species.

Long-Term Photo-Identification Reveals the Population Dynamics and Strong Site Fidelity of Adult Whale Sharks to the Coastal Waters of Donsol, Philippines

“…There was significant male bias (87%) for those individuals whose sex could be confirmed, though it could not be determined for 62.5% of individuals. Such male bias is consistent with coastal aggregations from across the globe Rowat et al, 2007Rowat et al, , 2009Riley et al, 2010;Ramírez-Macías et al, 2012a,b;Himawan et al, 2015;Rohner et al, 2015b) and with others from within the Philippines (Araujo et al, 2014. Adult-dominated sites remain a rarity, namely at Darwin's Arch in the Galapagos Islands (ECU), Gorda Banks in Baja California (MEX), at an offshore aggregation in Qatar (QAT), and at St Helena Island (GBR) in the South Atlantic, and at a newly identified area in the mid-equatorial Atlantic off Brazil (Ramírez-Macías et al, 2012b;Acuña-Marrero et al, 2014;Clingham et al, 2016;Macena and Hazin, 2016;Robinson et al, 2016).…”

“…The size at which 50% of the population reached maturity was c. 6.8 m, considerably smaller than that previously reported in the western Indian Ocean using laser photogrammetry (9.2 m, Rohner et al, 2015b) and visually estimated at Ningaloo Reef, Western Australia (8.1 m, Norman and Stevens, 2007). However, it is similar to the size at maturity from the Gulf of Mexico and Qatar, where 50% maturity in males was estimated at 7.0 m and 7.3 m, respectively (Ramírez-Macías et al, 2012b;Robinson et al, 2016).…”

“…Photographs of the pelvic region were used to identify the sex based on the presence (male) or absence (female) of claspers, and sex was only assigned to an individual when a clear photo of the pelvic region was available. Males were considered mature when claspers extended beyond the pelvic fins and had a cauliflower appearance (Norman and Stevens, 2007;Rohner et al, 2015b). The total length at which 50% of males were considered mature (LT 50 ) was calculated using a Generalized Linear Model (GLM) with a binary logit function.…”

“…Some evidence exists that adults spend most of their time in the open ocean (Ramírez-Macías et al, 2017), which might explain why coastal sites tend to be juvenile dominated. Norman and Stevens (2007) reported size at maturity (TL 50 ) of 8.1 m at Ningaloo Reef in Western Australia using a rope of known length, whereas Rohner et al (2015b) reported 9.2 m in Mozambique using paired-laser photogrammetry. Other studies have also used photogrammetry and visual estimates to determine maturity in males (Qatar, 7.3 m, Robinson et al, 2016;Gulf of Mexico, c. 7.0 m, Ramírez-Macías et al, 2012b, respectively), yet concerns remain about the inaccuracy of such approaches (see Sequeira et al, 2016).…”

“…Whale shark aggregations worldwide are mostly dominated by juvenile males (e.g., Meekan et al, 2006;Rowat et al, 2007Rowat et al, , 2009Riley et al, 2010;Ramírez-Macías et al, 2012a,b;Araujo et al, 2014Araujo et al, , 2016Himawan et al, 2015;Rohner et al, 2015b;McKinney et al, 2017). Few aggregations dominated by adults have been identified to date, namely the Galapagos Islands (Acuña-Marrero et al, 2014), St. Helena Island (Clingham et al, 2016) and the Revillagigedo Islands off Baja California (Ramírez-Macías et al, 2012b), and a large proportion of adults has been reported at an offshore aggregation in Qatar (Robinson et al, 2016) and in the mid-equatorial Atlantic off Brazil (Macena and Hazin, 2016).…”