oa Body measurements and size estimation of whale sharks in Okinawa Churaumi Aquarium

Background Okinawa Churaumi Aquarium (formerly Okinawa Expo Aquarium) has been conducting research on the whale shark, both in captivity and in the wild since 1975. Proportional measurements, including total length, girth, and weight, are essential to examine the growth, maturity, and physical condition of the animal, as well as to estimate the required quantity of food and drug dose for medical treatment. However, measurement of the precise length of large aquatic animals is very difficult and often results in errors. We formulated an allometric equation used to estimate the total length (TL) and body weight (BW) by measuring appropriate smaller body parts. Approach We used morphometric data from 33 individuals stranded or accidentally caught by commercial fishery off Okinawa Island between 1979 and 2010. We examined 66 measured characters of each whale shark. Furthermore, we applied the conventional allometric function, and modified the function to be logarithmic to use a liner approximation. The reduced major axis (RMA) test was applied to all logarithmic morphometric data, and we chose to measure characters that were highly correlated with TL. Some of the characters showing high correlation with TL according to the RMA test were further analyzed for sex-based differences using the t-test. The most suitable morphometric characters that were measurable in free-swimming sharks were chosen from the set of characters that passed those tests. In addition, we analyzed weight data from 8 specimens (7 male and 1 female) that were considered to be in an almost normal condition. We applied the conventional allometric function to an estimation equation. Girth measurement at the posterior margin of the pectoral fins was directly performed on the free-swimming individuals. Results A high coefficient of determination (R2) was obtained for 8 morphometric characters: pre-first dorsal length (PD1), pre-second dorsal length (PD2), pre-caudal length (PRC), pre-pelvic length (PP2), pre-anal length (PAL), length of dorsal caudal margin (CDM), mouth width (MOW), and interorbital space (INO). We considered that the equation formulated using PD1 was the most suitable based on sex-based differences and the animal's swimming properties. The following equation estimates the TL in centimeters:

log TL = 0.964 log PD1 + 0.443 (R2 = 0.930, n = 33)

For BW estimation, the sex-based differences could not be tested because of the small sample size. The following equation estimates BW in kilograms:

BW = 4.510 TL3.280 (R2 = 0.958, n = 8)

The data of girth measurements was collected from captive (n = 3) and wild (n = 6) sharks. Conclusions Data on proportional measurements is useful as an index of growth condition. Our focus is to monitor the size of our females and track the physiological indicators of maturation to achieve the first captive reproduction of whale sharks.