Tongan Archipelago IMMA

Criterion A – Species or Population Vulnerability

The Oceania humpback whale population, of which Tonga forms a sub-population, is assessed as Endangered (EN A1) on the IUCN Red List. The Oceania abundance estimate was 4,329 (CV = 0.12, 3,345-5,313) whales in 2003 (Constantine et al. 2012). Models for the recovery of the Oceania sub-populations suggest this stock likely remain below 30% of pre-exploitation abundance and is recovering at a slower rate than other populations (Jackson et al. 2007). The abundance estimate for the Tongan humpback whales is now over a decade old and given the absence of direct threats, the population is likely to have increased.

Criterion B: Distribution and Abundance

Sub-criterion B2: Aggregations

The nearshore waters of all island groups in the Tongan Archipelago are critical habitat where humpback whales aggregate for breeding during the winter months. In Vava’u this habitat is important for all sex- and age-class groups of whales (Lindsay et al. 2016). Similar aggregations of whales occur in the Ha’apai group (Kessler & Harcourt 2012) and in Eua (Constantine et al. 2014).

There is an aggregation of spinner dolphins that has been sighted over multiple years in the white cliffs area of Vava’u. There is little information to infer the residency pattern of individuals within and between years, but the consistency with which spinner dolphins are observed in the region suggests its importance to the species. Spinner dolphins have been reported at Tongatapu (D. Donnelly unpub. data) and it is possible that further populations of this species exist in other areas throughout the Tongan Archipelago. Sightings records are from reliable sources, but effort is lacking.

Criterion C: Key Life Cycle Activities

Sub-criterion C1: Reproductive Areas

The Tongan islands form critical annual winter calving and mating habitat for humpback whales (Constantine et al. 2012, Lindsay et al. 2016). Stenella longirostris Calves have been observed in Vava’u which is expected for a resident population of dolphin (South Pacific Whale Research Consortium unpub. data). There are no data on the calving rates or any other demographic information on the spinner dolphins in Vava’u that allow another classification at this time.

Criterion D: Special Attributes  

Sub-criterion D1: Distinctiveness

There are movements of humpback whales between island groups by some individuals and low levels of interchange with other breeding grounds throughout Oceania (Garrigue et al. 2002, 2011). Overall, there is strong natal site fidelity with mitochondrial DNA haplotypes distinctive to Tongan whales (Olavarría et al. 2007).

Available genetic samples of spinner dolphins from Vava’u show significant differentiation from other South Pacific Island populations but the sample size is small (Baker 2015). Further samples from Vava’u and other regions in the Tongan Archipelago would be useful to confirm distinctiveness.

Sub-criterion D2: Diversity

In addition to humpback whales and spinner dolphins, there are a number of secondary species reported in the Tongan Archipelago IMMA. There are seven other species of whale reported to date; pygmy blue, sperm, dwarf and Antarctic minke, kogia, sei and Bryde’s-like whales and one report of a stranded Cuvier’s beaked whale. In addition, there are nine other species of delphinid; short-finned pilot, false killer, pygmy killer, melon-headed and killer whales, pan-tropical spotted, rough-toothed, common bottlenose and Risso’s, dolphins (Miller 2009, SPREP 2014, South Pacific Whale Research Consortium and D. Donnelly unpub. data).