Populations of several species of odontocete have recently been split by NMFS into one or more island‐associated stocks near Hawaii, including false killer whales (Pseudorca crassidens), common bottlenose dolphins (Tursiops truncatus), and spinner dolphins (Stenella longirostris). The unique oceanographic environment of the Hawaiian Archipelago, including relatively high near‐shore productivity, together with significant isolation from other island ecosystems, may over evolutionary time‐scales encourage separation of populations into wide‐ranging pelagic stocks and island‐associated stocks that are better suited to exploit near shore resources. In the case of spinner and bottlenose dolphins, variation between stocks exists at individual and multiple island‐scales, both in genetic characteristics and group behavior, resulting in several island‐associated stocks within Hawaii.
As part of an overall review of odontocete stock structure near Hawaii, we evaluated data on the occurrence, abundance, population structure, movements and behavior of odontocete
species found near‐shore in the main Hawaiian Islands. Our review indicates that there may be additional species that could be split into one or more island‐associated stocks. It is also
apparent that the pace of data collection varies by species due to detectability, behavior, or density, and that pace may provide some guidance on the level of data required for providing information to information splitting stocks. Where significant new data continues to inform questions about stock structure, it may be best to hold decisions about specific stock boundaries as incoming datasets may reveal that those boundaries should be defined differently or that some stocks were created with incomplete data and now should be
combined or further subdivided. Where the pace of data collection is very slow, the conservative approach would be to propose new island‐associated stocks now based on what evidence is available, acknowledging that boundaries or stock designations may change should new data become available.
At least 23 species of cetacean occur in Hawaiian waters. Many of these are considered broadly oceanic with population distribution and movements not clearly influenced by island processes. Although these species are occasionally seen during nearshore surveys, there is no indication that species such as Risso’s (Grampus griseus), striped (S. coeruleoalba), and Fraser’s dolphin (Lagenodelphis hosei), sperm (Physeter macrocephalus) and pygmy sperm whale (Kogia breviceps), or Longman’s beaked whale (Indopacetus pacificus) associate strongly with the island ecosystem. Most baleen whales are migratory from productive North Pacific waters, residing in Hawaiian waters only during the fall and winter. One exception is Bryde’s whales (Balaenoptera edeni), a species for which preliminary genetics evidence suggests there may be multiple populations throughout the Pacific but which is rarely seen during nearshore surveys, such that it is currently not possible to evaluate island association. Here we review data from the main Hawaiian Islands on eight species of odontocetes ‐ melon‐headed whale (Peponocephala electra), short‐finned pilot whale (Globicephala macrohynchus), rough‐toothed dolphin (Steno bredanensis), pantropical spotted dolphin (Stenella attenuata), Blainville’s (Mesoplodon densirostris) and Cuvier’s (Ziphius cavirostris) beaked whales, pygmy killer whale (Feresa attenuata) and dwarf sperm whales (Kogia sima) ‐ species for which there is some evidence for separation of at least one island‐associated stock from the broader pelagic population in the central Pacific around Hawaii. To these reviews we applied three criteria for choosing whether to propose new stocks and their boundaries, criteria based upon the quality of data available now, the likelihood of obtaining new information that may significantly refine our knowledge of stock structure in the near‐term, and the overall pace of data collection for each individual species. From this we have developed three categories:
1. Data Rich Stocks: Stocks for which ample data are currently available to support or reject the recognition of new stocks and the placement of new stock boundaries. Although we attempt to base this judgment primarily on peer‐reviewed published science, high‐quality, well‐documented datasets that have been partially reviewed are also considered.
2. Data Poor Stocks: Stocks for which there is some evidence for the existence of an island‐associated stock, but for which data will not be available to specifically define the range
and boundaries of that stock in the foreseeable future.
3. Stocks with Increasing Data: Stocks for which the pace of data collection is high, such that there may be significant additional information within the next few years with which to better define stock structure. Although some peer‐reviewed data may be available which supports formation of new stocks, the range and boundary of those stocks may be much better informed by on‐going genetic, sighting history, or ecological analysis, such that postponing designation of new stocks for a few years will result in better defined structure with fewer adjustments to stock assessments over time.
Each of the eight species reviewed have been placed into one of these categories for subsequent action for either 1) officially recognizing new island‐associated stocks (Data Rich Stocks), 2) proposing designation of Prospective Stocks (Data Poor Stocks), or 3) withholding designation of any new stocks until on‐going analyses are complete (Stocks with Increasing Data). Melon‐headed whales and pantropical spotted dolphins are considered data rich stocks, Blainville’s and Cuvier’s beaked whale, dwarf sperm whales, and pygmy killer whale are considered data poor stocks, and rough‐toothed dolphins and short‐finned pilot whales are a species for which new data and analyses are expected within the next few years that will refine any designation of new stocks in the Hawaiian Archipelago. Available data by species are reviewed here, providing the rationale for designation of new stocks, prospective stocks, or no new stocks at this time. Species for which island‐associated stocks have already been designated and those for which the available data does not suggest island‐association are not discussed as part of this review.
Citation:
Oleson, E.M., R.W. Baird, K.K. Martien, and B.L. Taylor. 2013. Island-Associated Stocks of Odontocetes in the Main Hawaiian Islands: A Synthesis of Available Information to Facilitate Evaluation of Stock Structure. Document PSRG-2013-16 submitted to the Pacific Scientific Review Group.
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