AIMS

The long-term objective of this project is to define the age structure of the spinner dolphin population off the Kona coastline (Hawaii) to provide a means by which to assess this population trajectory and health status. The finals steps of this project (link to previous page) will be to collect in-situ images of spinner dolphins and assess the age structure of this population. Here we present the last two phases of the four-phase project (see figure 1 below).

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Figure 1: Schematic overview of the last two components of the study to assess the feasibility of using multi- rotor Unoccupied Aerial Systems (UAS) to determine free-ranging delphinid group age composition and population age-structure.

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The aims of the first phase of this project are to (Figure 1, phase 3):

• Collect aerial images of the Kona (Hawaii) population of spinner dolphins.

Whilst the ultimate goal of this project is to develop a generalizable method for remotely assessing delphinid population health, we will achieve this by focusing on a localised, well-studied population, the Hawaiian spinner dolphin (Stenella longirostris). In 2014, the abundance of the Hawaii Island spinner stock was estimated to about 650 individuals, a smaller estimate than anticipated from previous studies (around 1,000 individuals). Hawaiian spinner dolphins exhibit a specific behavior pattern foraging offshore at night, returning to preferred sheltered bays to socialize and rest during the day. The resting bays have become popular touristic destinations which have raised concerns for their long-term population viability and has become a management priority for NOAA.

 

Aerial videos of the spinner dolphin groups will be obtained via UAS. Optimal videos of the animals consist of animals swimming in cohesive groups and surfacing to breath. The UAS will be launched either from a research vessel at sea or from land (i.e, cliffs, parks). Once a group is encountered, the UAS will hover it at altitudes ranging from 16 meters to 50 meters, for up to 18 minutes at one time (depending on weather conditions, animals’ behavior and data quality obtained).

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• Estimate the length of the spinner dolphins.

Videos will then be processed to extract still images, from which length measurement and age-class will be estimated (see project phase 1 for more information). Briefly, measuring the distance between the center of the blowhole of the animal to the insertion of its dorsal fin allows for the estimation of its total length, and therefore age-class. An example of a still image used for measurement estimates can be seen in figure 2. However, additional information about the population (e.g., age-at-length growth curve, life history) are required prior to estimating age. For more information, see next paragraph.

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Figure 2: Unoccupied Aerial System (UAS) image of a surfacing spotted dolphin. UAS-Photogrammetry is used to estimate the length (red dotted line) of the distance between the blowhole and the dorsal fin of the animal.

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​​The aims of the fourth and final phase of this project are to (Figure 1, phase 4):

• Develop age-length growth curves for Hawaiian spinner dolphins.

In order to utilize photogrammetric methods to assign animals to age classes, it is important to have auxiliary information, i.e. length-at-age growth curves and life history information such age of weaning and sexual maturity. It is then possible to classify animals into age stages according to their length. Length-at-age growth curves and life history information are available for Eastern spinner dolphins (Stenella longirostris orientalis) which provides an initial solid inference database.

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Growth curves will be informed for Hawaiian spinner dolphins from two available data sources:

a) published literature on by-caught eastern spinner dolphins from the Eastern Tropical Pacific (ETP) tuna fishery (figure 3a);

b) available stranding specimens of Hawaiian spinner dolphins (n=30; in collaboration with Dr Kristi West).

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The former (a) will rely on data on eastern spinner dolphins which are considered to be morphometrically similar to Hawaiian spinner and will provide detailed growth curves (data on length and age of >2,000 individuals). The latter (b) will based on 30 dead specimens of Hawaiian spinner dolphins where information is available on body morphometrics – and teeth from which age will be inferred, by counting growth layer groups (GLGs) in the dentine and cementum of the teeth (see figure 3b). Aging a tooth relies on the same principles than aging a tree (i.e.; counting the growth rings).

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Figure 3: a) example of an age-at-length growth curve for Eastern female spinner dolphins (Stenella longirostris orientalis; Taken from Larese and Chivers 2009) and b) counting the Growth Layer Groups (GLGs) of a Southern elephant seal (Mirouga leonina; taken from Martin et al. 2011)

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• Assess the age structure of the Kona population of spinner dolphins.

Age-class group composition will be assessed across a range of spinner dolphin group sizes, as it is likely that composition is group-size dependent. Subsequently, we will assess population age-structure for the Hawaii Island spinner stock. The age-structuration will be based on knowledge of the stock size (i.e., approximately 650 individuals) coupled with information on age-class group composition across a range of group sizes and average group size estimates (from previous studies).

Research findings will be shared with NOAA, and form baseline information as to whether implement further conservation actions for the management of this Hawaiian stock of spinner dolphins.

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