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Courbis & Timmel: Effects of vessels and swimmers on behavior of Hawaiian spinner dolphins…

Courbis & Timmel’s paper on the effects of vessels and swimmers on behavior of Hawaiian spinner dolphins (stenella longirostris) in Kealake‘akua, Honaunau, and Kauhako bays, Hawai‘i was published in 2009 in the journal, Marine Mammal Science. The purpose of their paper was to record the behaviors of Hawaiian spinner dolphins with respect to boat and swimmer traffic in three different bays in Hawaii.

Methods

They had three different study sites, Kealake‘akua Bay (where cliffside observations occurred), Honaunau Bay (where sea level observations occurred), and Kauhako Bay (where cliffside observations occurred). Kealake‘akua Bay is home to kayaking, swimming, snorkeling, and commercial tours. Honaunau Bay has swimmers, snorkelers, SCUBA divers, and a canoe club, but also has boat access on one end of the bay. Lastly, Kauhako Bay is used mainly by swimmers looking for dolphin encounters. Courbis & Timmel collected behavioral data from February 11, 2002 to May 29, 2002. They recorded number of aerial behaviors (through an all-occurrence sampling method) and number of vessels and swimmers (through an instantaneous scan sample method, every 5 minutes) from dawn to dusk. Additionally, the time when dolphin groups entered (or when first seen in) and exited (or when last seen in) the bays was recorded. Average total group size was estimated for each dolphin group.

Major Findings

The mean frequency of aerial behaviors was not significantly correlated to the mean frequency of vessels and swimmers in any of the three bays. However, specific instances of aerial behavior appeared to be closely correlated with approaches by vessels and swimmers. Mean aerial behaviors per hour were also not significantly different for any hour of the day in two of the bays, but were significantly higher later in the day at Kauhako Bay. Mean dolphin group sizes were not significantly different among the three bays.

Comparisons

When comparing results to earlier studies, the pattern of aerial behaviors during the day in Kealake‘akua Bay have changed over time from active, engaging in lots of spinning and aerial behaviors when entering the bay to  minimal if any aerial behaviors while entering Kealake‘akua or Honaunau Bay. Compared to Norris et al. (1994) the number of aerial behaviors during resting period of the midday has increased and/or the numbers of aerial behaviors occurring in the mornings or evenings have decreased over time. Activity levels of spinner dolphins have changed over time from 2.232 activities per hour in 1970 to 0.750 activities in 2002. There have been increases in traffic from prior to 1968 to present day.

Broader Impacts

Long term impacts are unknown and could be potentially high. Both vessel and swimmer traffic have reached a level that may be affecting the daily behavior patterns (aerial) of the dolphins. There is a potentially biological significant relationship between reactive aerial behaviors and vessel traffic. Likewise, swimming with dolphins may be disturbing enough to the dolphin to evoke strong reactions and behavioral changes ultimately affecting their ability to rest and to adequately prepare for evening hunts. These behavior changes could eventually lead to the dolphins abandoning these bays for bays with lesser traffic and therefore completely changing their “routine”. Changes in resting patterns are potentially biologically important and interruptions during rest by vessels and swimmers could have short and long term effects on the dolphins.

Conclusions

The effect (short and long term) of behavioral changes on the survival and fitness of the spinner dolphins is unknown. Further research is needed to determine whether there are impacts on spinner dolphins due to swimmer and vessel traffic in the Hawaiian Bays. Behavior should be monitored more consistently as new regulations come into place.

Courbis, S. & Timmel, G. (2009). Effects of vessels and swimmers on behavior of Hawaiian spinner dolphins (stenella longirostris) in Kealake‘akua, Honaunau, and Kauhako bays, Hawai‘i. Marine Mammal Science, 25(2), 430-440.

  2009  /  Science  /  Last Updated August 11, 2012 by admin  /