Between a Rock and a Hard Place: Habitat Selection in Female-Calf Humpback Whale (Megaptera novaeangliae) Pairs on the Hawaiian Breeding Grounds

The Au'au Channel between the islands of Maui and Lanai, Hawaii comprises critical breeding habitat for humpback whales (Megaptera novaeangliae) of the Central North Pacific stock. However, like many regions where marine mega-fauna gather, these waters are also the focus of a flourishing local eco-tourism and whale watching industry. Our aim was to establish current trends in habitat preference in female-calf humpback whale pairs within this region, focusing specifically on the busy, eastern portions of the channel. We used an equally-spaced zigzag transect survey design, compiled our results in a GIS model to identify spatial trends and calculated Neu's Indices to quantify levels of habitat use. Our study revealed that while mysticete female-calf pairs on breeding grounds typically favor shallow, inshore waters, female-calf pairs in the Au'au Channel avoided shallow waters (<20 m) and regions within 2 km of the shoreline. Preferred regions for female-calf pairs comprised water depths between 40–60 m, regions of rugged bottom topography and regions that lay between 4 and 6 km from a small boat harbor (Lahaina Harbor) that fell within the study area. In contrast to other humpback whale breeding grounds, there was only minimal evidence of typical patterns of stratification or segregation according to group composition. A review of habitat use by maternal females across Hawaiian waters indicates that maternal habitat choice varies between localities within the Hawaiian Islands, suggesting that maternal females alter their use of habitat according to locally varying pressures. This ability to respond to varying environments may be the key that allows wildlife species to persist in regions where human activity and critical habitat overlap

Between a Rock and a Hard Place: Habitat Selection in Female-Calf Humpback Whale (Megaptera novaeangliae) Pairs on the Hawaiian Breeding Grounds

The Au'au Channel between the islands of Maui and Lanai, Hawaii comprises critical breeding habitat for humpback whales (Megaptera novaeangliae) of the Central North Pacific stock. However, like many regions where marine mega-fauna gather, these waters are also the focus of a flourishing local eco-tourism and whale watching industry. Our aim was to establish current trends in habitat preference in female-calf humpback whale pairs within this region, focusing specifically on the busy, eastern portions of the channel. We used an equally-spaced zigzag transect survey design, compiled our results in a GIS model to identify spatial trends and calculated Neu's Indices to quantify levels of habitat use. Our study revealed that while mysticete female-calf pairs on breeding grounds typically favor shallow, inshore waters, female-calf pairs in the Au'au Channel avoided shallow waters (&lt;20 m) and regions within 2 km of the shoreline. Preferred regions for female-calf pairs comprised water depths between 40-60 m, regions of rugged bottom topography and regions that lay between 4 and 6 km from a small boat harbor (Lahaina Harbor) that fell within the study area. In contrast to other humpback whale breeding grounds, there was only minimal evidence of typical patterns of stratification or segregation according to group composition. A review of habitat use by maternal females across Hawaiian waters indicates that maternal habitat choice varies between localities within the Hawaiian Islands, suggesting that maternal females alter their use of habitat according to locally varying pressures. This ability to respond to varying environments may be the key that allows wildlife species to persist in regions where human activity and critical habitat overlap

Humpback whale sightings along the eastern shoreline of the Au'au Channel, Maui, Hawaii, classified by presence (female-calf groups) or absence (adult-only groups) of a calf in the group.

<p>Humpback whale sightings along the eastern shoreline of the Au'au Channel, Maui, Hawaii, classified by presence (female-calf groups) or absence (adult-only groups) of a calf in the group.</p

Habitat preference in adult-only humpback whale groups along the eastern shoreline of the Au'au Channel, Maui, Hawaii.

*<p>Totals slightly less than entire study region due to slight gaps in bathymetry data (Δ6.5 km²). Neu's indices provide standardized estimates of habitat use, based on habitat availability. Regions were classified as preferred (where 95% CI's of observed group counts (Oi) were entirely above the expected counts based on habitat availability (P_i)) and avoided (where 95% CI's of observed counts were entirely below the expected counts). In all other (neutral) regions, 95% CI of observed counts included the expected count. * borderline – neutral/preferred.</p

Habitat preference in female-calf humpback whale groups along the eastern shoreline of the Au'au Channel, Maui, Hawaii.

*<p>Totals slightly less than entire study region due to slight gaps in bathymetry data (Δ6.5 km²). Neu's indices provide standardized estimates of habitat use, based on habitat availability. Regions were classified as preferred (where 95% CI's of observed group counts (Oi) were entirely above the expected counts based on habitat availability (Pi)) and avoided (where 95% CI's of observed counts were entirely below the expected counts). In all other (neutral) regions, 95% CI of observed counts included the expected count.</p

Map of the Hawaiian Islands, Au'au Channel and the study area.

<p>Within the study area, zigzag transect lines were constructed between waypoints set at 1 minute intervals. Inner waypoints were set at 0.25 km from the shoreline and outer way points set at the deepest or mid-point of the channel, whichever lay furthest offshore. The final perimeter of the study area extended from the shoreline to 750 m beyond the outer waypoints.</p

AIC values for generalised additive models describing the influence of environmental factors on the occurrence of female-calf humpback whale groups in the Au'au Channel, Maui, Hawaii.

<p>Legends:</p><p>“s” identifies smoothed data.</p><p>“*”denotes terms representing interactions between variables.</p><p>AICc – second order Akaike's Inspection Criteria.</p><p>ΔAICc – relative change in AIC value, compared to the lowest value recorded.</p><p>Dev Exp – provides the estimated percentage of deviation that can be explained by the variable(s) in the model.</p

Locations of humpback groups classified according to the presence or absence of a calf.

<p>Mean (s.d.) values provided.</p><p>Differences were not significant, once corrections for multiple testing were taken into consideration (α/k = 0.016).</p

Encounter rates for March ′08, ′09 and ′10, Au′au Channel, Maui, Hawaii.

<p>Encounter rates for March ′08, ′09 and ′10, Au′au Channel, Maui, Hawaii.</p

AIC values for generalised additive models describing the influence of environmental factors on the occurrence of adult humpback whales in the Au'au Channel, Maui, Hawaii.

<p>Legends:</p><p>“s” identifies smoothed data.</p><p>“*”denotes terms representing interactions between variables.</p><p>AICc – second order Akaike's Inspection Criteria.</p><p>ΔAICc – relative change in AIC value, compared to the lowest value recorded.</p><p>Dev Exp – provides the estimated percentage of deviation that can be explained by the variable(s) in the model.</p