Data from Honaunau and Honokohau Bays
The amount of data collected varied between the different bays included in this study. The biggest effort, in terms of observations and data collection, took place in Kealakekua Bay. Enough observations were also made in Honaunau and Honokohau bays for some statistical analysis. It compares well with information from Kealakekua bay. However, some of the differences are also very instructive.
The stars (*) in the figures below indicate statistical significance levels between the No Dolphin (dolphins absent from the bay) and Dolphins Present bars for each zone:
* - P<0.05
** - P<0.01
N.S. - No significant difference.
For more information on the statistical methods used to analyse the data see the methods section in the Final Report.
Figure 1. On average, the daily occupancy rate (proportion of days dolphins were sighted within each bay) was the lowest (14%) for Honaunau Bay and the highest (58%) for Honokohau Bay. Since the coverage of each bay varied from two to six hours, it is instructive to look at the hourly occupancy rate. This was lower than the daily rate for all bays but showed a similar relationship between the bays
Comparing Location and size of Core Resting Areas in the 3 bays.
Figure 2. a) Kealakekua bay, is a larger bay, with a much larger Core Resting Area than b) Honokohau bay (near right) and c) Honaunau bay (far right). See comparison to scale to the left. The double headed arrows in all 3 illustrations span 250m.
Honokohau Bay Honaunau Bay
Figure 3. Zone use by spinner dolphin schools in three resting bays, as measured during 5- minute activity scans for a) Honaunau, b) Honokohau , and c) Kealakekua Bays. As each scan was five minutes long, spinner dolphin schools could be recorded in more than one zone per scan. The total percentage per bay therefore exceeded 100% in all bays.
Dolphin zone use
The zone designed to encompass most of the core resting area was zone “B” for Honaunau and Kealakekua Bays, and zones “D” – “F” in Honokohau Bay (Figure 2), where the dolphins were recorded during 65.8%, 73.6% and 85.5% of the activity scans respectively. In Honaunau Bay zone “A” was also a very important part of the core resting area where dolphins were recorded during 47.4% of all activity scans. For Honokohau Bay it is particularly noteworthy that the dolphins spent time in the main boat channel to the Harbor (zone E) during 43.5% of the activity scans.
Human use of Honaunau and Honokohau Bays
Use by Zone
The main human presence in Honaunau bay was in the form of swimmers. When dolphins were present the number of swimmers increased in zone “B” and decreased in the surrounding zones “D” and “E” (Figure 2c). On average, there was a 9-fold increase in swimmers in zone “B” when dolphins were present, while the number of swimmers dropped by 18% in zone “D” and over 70% in zone “E”.
There were fewer swimmers in the area in front of the harbor mouth in Honokohau Bay when dolphins were present as compared to when dolphins were absent. When dolphins were absent, swimmers were primarily observed in zones “C”, “F”, and “H”, the zones where boat moorings are available and with easy beach-access into the water from land (Figure 5a). When dolphins were present in the bay, the number of swimmers decreased significantly in zones “C” and “H” to 34% and 6% respectively of the numbers when no dolphins were present. There were no records of swimmers in zone “E”, the boat channel, during a regular scan although swimmers were observed there on a few occasions when dolphins were present during non-scan times.
By combining the data on all vessel types it was possible to test the vessel use of zone “E” statistically. The number of vessels in this zone increased 7-fold when dolphins were present. By further combining the vessel data, it could be shown that the number of vessels using the three zones encompassing the core resting area (“D”, “E”, “F”) increased 4-fold overall when dolphins were present, while the use of the three remaining inshore zones surrounding the core resting area (“C”, “G”, “H”) decreased to 80% of the numbers when dolphins were not present (Figure 5b).
Figure 4. Human zone use in Honaunau Bay, comparing snapshots when spinner dolphins were present versus absent in the bay for a) swimmers and, b) all vessel-data combined.
Figure 5. Human zone use in Honokohau Bay, comparing snapshots when spinner dolphins were present versus absent in the bay for a) swimmers and, b) all vessels comparing the three zones (D-F) encompassing the spinner dolphin core resting area with the remaining inshore zones (C,G,H). Zone E was that boat channel.
Figure 6. Distribution of people relative to the dolphins during 5-min activity scans in a) Honaunau Bay (n=21) and b) Honokohau Bay (n=81), as compared to c) Kealakekua Bay (n=171).
Human Distribution Relative to the Dolphins
In Honaunau Bay, only swimmers were observed close to the dolphins. On average, 3.0 swimmers were within 10m of the dolphins actively trying to interact with them. Another 3.2 swimmers were within 50m, and a total of 9.5 swimmers were within 200m of the dolphins during each of the 21 activity scans that were done (Figure 6a).
The picture was quite different in Honokohau Bay where on average only 0.4 swimmers and a total of 3 vessels were within 200m of the dolphins (Figure 6b). This number includes vessels that were entering and exiting the harbor through the boat channel (zone “E”). The majority (64%) of all vessels transited straight through the dolphin resting area without changing speed or direction.
The Spinner Dolphin Population's Response to constant Disturbances of their Rest Period
Hawaiian spinner dolphins have also started to change where and how they rest. They were sighted less frequently in Makako Bay in 2003 than between 1989 and 1993. In 2003 they had started to use another nearby area to rest along a part of the coastline that was not used by any of the 135 spinner schools followed between 1989 and 1993. Several of the volunteers participating in this project who also kayak regularly described similar observations along different parts of the Kona Coast – finding schools of spinner dolphins milling along parts of the coastline where they never used to be found.
Comparing the occupancy rates, i.e. the proportion of days spinner dolphins were using each resting area, in various published reports provides another look at the possible long-term effects on the dolphins. Interestingly the timing of the changes is mirroring the spread of the swim-with-wild-dolphin activities along the Kona Coast. The occupancy rate in Kealakekua Bay was much lower (58%) during a 1993-94 study compared to previous findings (74% of 113 days 1968-72, 79% of 364 days-1989-92). The drop in occupancy rate corresponded with the time in the late 1980’s to early 1990’s when swim-with-wild-dolphin activities expanded rapidly in Kealakekua Bay. The earlier occupancy rate information was based on a years worth of data (May 1979 - May 1980) when the bay was covered from sunrise to 1800 hours for 364 days, only missing Christmas day. The coverage for the 1993- 94 data was similarly extensive. Thus the drop in occupancy rate from 79% to 58% is highly significant both statistically (P < 0.001) and biologically.
Swim-with-wild-dolphin activities began to spread from Kealakekua to the rest of the Kona Coast in the early 1990s, including Honaunau and Kauhako bays to the south and Honokohau and Makako bays to the north. By 1995 it was well established north of Honokohau Harbor. This is when several resting areas off Kailua-Kona and further north were affected, including Honokohau and Makako Bays, as well as other areas farther north. Indeed the occupancy rates provided in this and other studies over the years suggest that the overall trend in spinner dolphin occupancy rates of traditional resting bays on the Kona Coast dropped (Figure 18). Unfortunately, there are no available data between 1994 and 2002.
The only exception to this trend seems to be the resting area in front of Honokohau Harbor. The monitoring of this area suggest that it may be due to the vessel traffic in and out of the harbor, primarily charter fishing boats quickly going right through the core resting area on their way to the offshore fishing grounds, deterring swim-with activities and lingering kayaks in the resting area. Even the number of vessels directed towards the dolphins was much smaller than in any of the other resting areas.