Evidence for an unrecognised blue whale foraging ground in New Zealand

Abstract

Blue whale distribution in the Southern Hemisphere is poorly understood. Their survival is dependent on the ability to reliably encounter large aggregations of euphausiid prey. Therefore, documenting and protecting blue whale foraging grounds is fundamental to enhancing their recovery. Various data sources are compiled here to support the hypothesis that the South Taranaki Bight, between the north and south islands of New Zealand, is used as a foraging ground by blue whales for a common euphausiid prey that aggregate as a function of a nearby coastal upwelling system. The distribution of blue whales is compared with ship traffic density and the distribution of seabed mining activities in the region, and reveals close proximity between whales and these potential threats. This paper presents evidence that the South Taranaki Bight is a blue whale foraging habitat and calls for a greater understanding of their habitat use patterns to manage anthropogenic activities effectively.

Keywords:

• blue whale
• distribution
• foraging ground
• New Zealand
• Nyctiphanes australis
• seabed mineral exploration and extraction
• ship traffic

Introduction

Blue whales Balaenoptera musculus were subject to intensive exploitation by whaling operations during the 20th century, reducing the Antarctic blue whale population to less than 1% of its original population size (Branch et al. 2004). Despite their massive size and once large population, the blue whale is fairly elusive and little is known about its distribution or habitat use. In fact, only four blue whale foraging grounds have been documented in the Southern Hemisphere outside Antarctic waters: the south and south western coasts of Australia (Gill 2002; Rennie et al. 2009), near the fjords of southern Chile (Hucke-Gaete et al. 2004), near the Crozet Islands in the Indian Ocean (Samaran et al. 2010) and on the Madagascar Plateau (Best et al. 2003).

In the Southern Hemisphere, two subspecies of blue whales are recognised based on genetics and morphology: the Antarctic (or true) blue whale (Balaenoptera musculus intermedia) and the pygmy blue whale (Balaenoptera musculus brevicauda). During the austral summer, the majority of pygmy blue whales do not migrate to Antarctica (Branch et al. 2007; Attard et al. 2012), but Antarctic blue whales are generally found south of 55°S (Ichihara 1966; Branch et al. 2007). However, there is recent evidence from a number of locations around the world, including New Zealand (McDonald 2006), that some Antarctic blue whales do not migrate south every winter (Branch et al. 2007; Samaran et al. 2010). The IUCN Red List of Threatened Species currently lists the Antarctic blue whale as Critically Endangered (Reilly et al. 2008) and the pygmy blue whale as Data Deficient (Cetacean Specialist Group 1996). The New Zealand Threat Classification System lists the blue whale as a Migrant (Baker et al. 2010) and therefore does not designate a threat status.

Due to their enormous size, fully aquatic lifestyle and zooplankton diet, blue whales have the highest prey demands of any predator that ever existed (Rice 1978; Williams et al. 2001), but they exhibit relatively short dive durations because of the high energetic demands of their lunge-feeding strategy (Acevedo-Gutierrez et al. 2002). Therefore, a critical life-history strategy for blue whales is to encounter and exploit dense aggregations of their preferred prey, euphausiids, which may be spatially and temporally ephemeral (Croll et al. 1998; Fiedler et al. 1998). Aggregations of foraging blue whales have been reported near cold water coastal upwelling systems in Australia and the USA where dense patches of their prey are concentrated. Along the southern Australian coast, pygmy blue whales aggregate each austral summer between the Great Australian Bight and Bass Strait to feed on the euphausiid Nyctiphanes australis (Gill 2002). Similarly, blue whales off California, USA predictably congregate to feed on dense euphausiid schools in the Channel Islands (Croll et al. 1998) and in Monterey Bay (Croll et al. 2005).

No dedicated study of blue whales has ever been conducted in New Zealand. However, marine mammal observer records from recent seismic surveys in the South Taranaki Bight (STB; Blue Planet Marine 2011) suggested a potential concentration of blue whales in the region. This paper presents a compilation of data from published and unpublished sources demonstrating the consistent presence of blue whales in the STB. Also investigated is the potential for a nearby and prominent cold water coastal upwelling system off Kahurangi Point (Bowman et al. 1983; Shirtcliffe et al. 1990) to generate concentrations of blue whale prey.

Since 1979, the largest offshore natural gas and oil extraction operation in New Zealand has been located within the STB. This operation now includes seven production platforms and significant sea-floor pipelines. Despite extensive environmental assessments conducted in the 1970s and 1980s prior to and during the development of these extraction facilities, no consideration was given to potential impacts on marine mammals or other megafauna (Shell BP and Todd Oil Services Limited 1974; Kibblewhite et al. 1982; Office of the Parliamentary Commissioner for the Environment 1988). As a coastal area near multiple urban centres, shipping traffic is also prevalent throughout the STB. Both shipping traffic and seabed mining activities have been shown to impact blue whales directly, alter their behaviour and degrade their habitat (Zacharias & Gregr 2005; Van Waerebeek et al. 2007; Di lorio & Clark 2010; Melcon et al. 2012). As these activities continue to intensify in the STB, knowledge about ecosystem function and biodiversity is necessary if we are to exploit our marine environment sustainably and avoid deleterious impacts.

Materials and methods

The STB encompasses 55,835 km² and lies between the north and south islands of New Zealand (38°49'S to 40°53'S, 171°37'E to 175°13'E; Fig. 1). Within this region, blue whale sighting and stranding records were compiled to examine the frequency, seasonality and persistence of blue whale presence. Modern (since 1979) records of blue whale sightings within the STB from the following sources of cetacean sighting records were examined: (1) a database curated by the New Zealand Department of Conservation (DOC) in which each sighting is validated (Department of Conservation 2012a); (2) sightings recorded by trained observers aboard transiting ships between New Zealand and overseas ports collated between 1979 and 1999 (Cawthorn 2009); (3) sightings recorded by scientists and vessel captains at the National Institute of Water and Atmospheric Research Ltd (NIWA) and photo verified by the author (L. Torres unpubl. data); (4) sightings recorded during two recent seismic surveys with on-board marine mammal observers in the STB between 9 May and 22 July 2011, which consisted of 519.7 h of visual surveys (Blue Planet Marine 2011); and (5) anecdotal sightings by a tugboat captain and a DOC ranger in October 2007, which were followed-up with personal conversations. Additionally, the distribution and density of incidental blue whale sightings from the DOC and Cawthorn datasets were compared between the STB and all of New Zealand. Geographic locations for all these sightings were recorded on-site by the observers. Also investigated were stranding records of blue whales within the STB and all of New Zealand held in the DOC whale stranding database (Department of Conservation 2012b). It is difficult to distinguish between an Antarctic and pygmy blue whale without genetic analysis and therefore sightings and strandings in the STB region were not recorded to subspecies. Hereafter, the term ‘blue whale’ refers to both subspecies in the Southern Hemisphere unless otherwise described to subspecies.

Figure 1  Distribution of blue whale sightings and strandings within the South Taranaki Bight (STB), New Zealand relative to regional bathymetry, the location of the Kahurangi Point upwelling system, and sampled areas of high blue whale prey density. Incidental, survey and anecdotal sightings are symbolised by source. Inset map shows New Zealand with a black box around the STB that is enlarged; Wellington and the Cook Strait are denoted. Black lines indicate 50-m bathymetry isobaths. The centre of upwelling off Kahurangi Point is demarcated in grey; tongues of upwelled water extend as a plume to the north and northeast. The ellipses indicate the approximate areas of increased Nyctiphanes australis density sampled in March and April 1983 (green ellipses; Bradford & Chapman 1988; James & Wilkinson 1988) and February 1981 (blue ellips; Foster & Battaerd 1985). Note: No zooplankton sampling has been conducted in the STB north of c. 39'50°S.

The temporal distribution of blue whale sightings and strandings was examined to identify seasonal trends of blue whale presence in the STB. In this temporal examination, sightings recorded during seismic surveys were ignored due to the sampling bias of observational effort these data create in May, June and July when the surveys occurred. Monthly distribution maps of blue whales in the Southern Hemisphere based on catches, strandings, acoustic records and Discovery marks published by Branch et al. (2007; figs. 11 and 12) were also examined. Due to the lack of standardised survey effort across the study region, it is not possible to infer trends in spatial distribution of blue whales within the STB.

Additionally, literature on blue whale distribution within the Southern Hemisphere was investigated for information on occurrence patterns in the STB. Branch et al. (2007) provided low resolution maps (2° grid cells) that indicated the relative occurrence of blue whales at global and regional scales, including the distribution of (1) Soviet whaling effort between 1958 and 1973 and the proportion of which were blue whales, and (2) Japanese Scouting Vessels (JSV) survey effort between 1965 and 1987 with the corresponding sighting rate of blue whales.

In order to assess the potential association between blue whale distribution and their prey in the STB, the literature on the Kahurangi Point upwelling system and related zooplankton studies were explored. Group size data from the recent blue whale sightings were also examined as an indication of foraging behaviour because blue whales are typically solitary but are known to aggregate on foraging grounds (Sears & Perrin 2009; Gill et al. 2011). However, solitary, or pairs of, blue whales do not necessarily denote non-foraging behaviour.

To understand possible impacts to blue whales and their habitat in the STB, overlap was assessed between recent incidental and survey sightings of blue whales with two sources of potential anthropogenic threats: (1) seabed minerals exploration and extraction activities and (2) ship traffic density. Spatial data layers were acquired of production platform locations (Land Information New Zealand 2012), areas currently permitted for petroleum or mineral and coal extraction, and proposed blocks for oil and gas exploration (Ministry of Economic Development 2012), and a map of commercial shipping traffic that represents ship density per 1 km² grid cells (Halpern et al. 2008). These shipping traffic data were derived from 12 months (October 2004–September 2005) of ship location data, which represents roughly 11% of merchant ships >1000 gross tonnage at sea, and does not include fishing vessels. Therefore, these data likely under-represent ship traffic density in the STB.

Results

The DOC and Cawthorn sighting databases both have eight blue whale sightings recorded in the STB (Fig. 1 and Table S1). Additionally, two blue whale sightings in the STB were reported by NIWA scientists. In addition to these 18 incidental sightings of blue whales, two recent seismic surveys with on-board marine mammal observers in the STB region (Blue Planet Marine 2011) recorded 11 blue whale sightings (n=2 in May 2011; n=9 in June 2011). (An additional 16 sightings of unidentified large baleen whales likely to have been blue whales are also reported, but for this study only the 11 confirmed blue whale sightings are considered.) Recently, two anecdotal sightings of blue whales were described: on 29 October 2007, the captain of the tugboat Rupe en route to service an oil rig observed a group of approximately 20 blue whales in 100 m water depth (B. Govier pers. comm. 2012). The following day, a DOC ranger followed up this report and initially observed a single blue whale at 10:30 h in approximately 120 m of water (C. Lilley, DOC, pers. comm. 2012). Soon after, the Rupe captain reported observing an estimated 12 blue whales from the bridge within a few nautical miles of the sighting on the previous day. At 11:15 h the DOC ranger observed five more blue whales in about 100 m of water near the location of the Rupe. The DOC ranger notes,

There were possibly more [blue whales], but it was very difficult to obtain a count as they were distributed and spending long lengths of time under water. [The Rupe captain] could probably see more as he was much more elevated on the bridge of Rupe compared with us on a smaller (8 m) vessel. The whales were displaying feeding behaviour … They would surface, spout a few times over a minute or two, then dive again for about 5 or 6 minutes.

Photos taken at the sighting were provided to confirm individuals as blue whales.

Scattered incidental sightings of blue whales have been recorded broadly across New Zealand waters (Fig. 2), yet two clusters of sightings are evident: (1) in the STB and (2) off the east coast of Northland, between the Hauraki Gulf and the Bay of Islands.

Figure 2  Distribution of all blue whale sightings (black dots) and stranding (open stars) records in New Zealand with the South Taranaki Bight indicated by the black box. The east coast of Northland, between the Hauraki Gulf and the Bay of Islands, is indicated by the dashed box. City centres of Auckland, New Plymouth and Wellington are denoted by crosses.

Twenty blue whale strandings have been recorded in New Zealand since 1893. Six of these strandings occurred in the STB and another four were within 45 km of the STB (Figs. 1 and 2, Table S2). These 10 strandings are not clustered, but rather are evenly spread along the coastlines surrounding the STB from just north of New Plymouth, through to Wellington and along the northern coast of South Island. The other 10 blue whale strandings in New Zealand are spread around the country's coastlines with a cluster of three near the densely populated city of Auckland (Fig. 2).

Examination of whaling records from Soviet catches between 1958 and1973 illustrates a substantial increase in catch density of blue whales within the STB region and to the immediate west relative to surrounding ocean basins (Figs 3a and 3b). Additionally, the STB is indicated as having high sighting rates of blue whales from JSV between 1965 and 1987 (Figs 3c and 3d).

Figure 3  Increased blue whale presence in the South Taranaki Bight (red boxes in B and D) from Soviet catches between 1958–73 in the Southern Hemisphere (A and B) and sightings from Japanese Scouting Vessels (JSV) between 1965 and 1987 (C and D). A, Number of Soviet catches of all large cetaceans in each 2° grid cell, to be used as a rough measure of effort compared with B, the proportion of large cetacean catches in each 2° grid cell that were blue whales. C, JSV survey effort in km to be compared with D, sightings of blue whales per unit effort in each 2° grid cell (no effort in the northern Indian Ocean). Figures taken from Branch et al. (2007); reprinted with permission from T. Branch, Mammal Review and John Wiley & Sons Ltd. Publishing.

The sightings derived from the seismic surveys indicate the presence of blue whales in the STB during May and June (Table 1). Without considering these seismic survey sightings, small peaks of blue whale presence in the STB are evident in May and October–November. The May peak is complemented by five recent blue whale strandings, which occurred in May and June (Table S2). One or more incidental blue whale sighting has been recorded in the STB in all months but February and July when no blue whale sighting has been reported. Similarly, year-round presence of blue whales along the west coast of North Island, New Zealand is indicated by monthly illustrations presented by Branch et al. (2007) of blue whale catches by whalers, sightings, stranding, acoustic recordings and Discovery marks. These monthly plots also show peaks in the number of records in the STB region between December and April, but this pattern could be due to increased observational effort during summer months.

Table 1  Monthly distribution of blue whale sightings in the South Taranaki Bight (STB).

	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec	Unknown	Total
Incidental and anecdotal sightings	1	0	1	1	3	1	0	1	1	3	6	1	1	20
Seismic survey sightings	0	0	0	0	2	9	0	0	0	0	0	0	0	11
Total	1	0	1	1	5	10	0	1	1	3	6	1	1	31
Notes: Blue whale sighting sources include incidental sightings derived from: (1) the New Zealand Department of Conservation (2012a); (2) Cawthorn (2009); and (3) the National Institute of Water and Atmospheric research (NIWA) cetacean sighting databases (see Methods); (4) anecdotal sightings provided by a tugboat captain (pers. comm., Barry Govier, April 2012, Tug Master, 341 South Road, Omata, New Plymouth, New Zealand) and Department of Conservation ranger (pers. comm., Callum Lilley, April 2012, Ranger/Technical Support—Marine, Department of Conservation, Taranaki Area Office, PO Box 462, 55A Rimu Street, New Plymouth 4310, New Zealand); (5) seismic survey sightings collected by marine mammal observers aboard seismic survey vessels in the STB (Blue Planet Marine 2011).

A cold upwelling plume, driven by prevailing strong westerly winds, is a persistent feature in the southwestern region of the STB (Bowman et al. 1983; Shirtcliffe et al. 1990; grey area in Fig. 1). Alongshore winds and coastal prominences result in the upwelling of nutrient-rich water from depths of about 100 m off Kahurangi Shoals. Rotating eddies are created that are transported downstream (north and northeast) with a life span of >2 weeks (Foster & Battaerd 1985; Shirtcliffe et al. 1990). Studies have documented large concentrations of zooplankton, including elevated biomass of N. australis, linked to these upwelling plumes due to enhanced primary productivity (Foster & Battaerd 1985; Bradford & Chapman 1988; James & Wilkinson 1988; Bradford-Grieve et al. 1993). All of these studies found N. australis concentrations most abundant downstream of the upwelling area to the north and east (Fig. 1).

Unfortunately, the majority of zooplankton sampling sites in these studies occurred between the upwelling area and south of 39′50°S, and no long-term records of the zooplankton community in the STB have been made. Therefore, no empirical data are available on the variation in zooplankton composition throughout the entire STB region or temporally. However, Bradford & Chapman (1988) found the greatest biomass of N. australis at the northern and eastern limits of their sampling area and illustrated increasing wet weight toward the north. Furthermore, the STB area has the most extensive zooplankton biomass of all coastal regions in New Zealand (Bradford & Roberts 1978) and a study of the euphausiid community in nearby Cook Strait found N. australis to be the dominant species with year-round presence (Bartle 1976). Bradford & Chapman (1988) postulated that a resident population of N. australis may exist within the STB region, but this remains unproven. Although more comprehensive data on the spatial and temporal distribution of the zooplankton community in the STB is needed, available evidence indicates that a common prey of the blue whale, N. australis, is abundant and does concentrate in the STB in response to upwelling plumes formed off Kahurangi Point.

Another indication that the STB may be a foraging ground for blue whales is the relatively large group sizes estimated at incidental sightings (Table S1). Although group size estimates from these sightings cannot be regarded as completely accurate, many observations recorded multiple animals including eight sightings with more than two individuals present. These larger groups could represent feeding aggregations. Furthermore, the anecdotal sighting on 30 October 2007 was reported by a DOC ranger as five foraging blue whales. Due to the large group size (20 individuals) reported by the tug boat captain on the previous day, this is also assumed to be a feeding aggregation.

Almost the entire northern portion of the STB (above 40°S) and in waters less than 150 m is permitted for either petroleum exploration and extraction or mineral and coal mining (Fig. 4). Twenty-four of the 31 blue whale sightings examined in this study (77%) occurred within a permitted seabed mineral extraction area. Additionally, a few of the incidental blue whale sightings were recorded from production platforms. Low-level ship traffic covers the entire STB (not illustrated in Fig. 1 to simplify figure), and a relatively dense shipping lane extends north–south near the 173°E meridian (Fig. 4). This shipping lane runs within 10 km of 14 blue whale sightings examined in this study.

Figure 4  Distribution of blue whale sightings within the South Taranaki Bight (STB), New Zealand overlaid on the distribution of potential anthropogenic threats in the region. Locations of production platforms and layers of permitted seabed mineral exploration and extraction areas are derived from Land Information New Zealand (2012) and the Ministry of Economic Development (2012). Ship traffic density is derived from Halpern et al. (2008). Grid cells with less than 284 ship tracks per km² cover almost the whole STB region and are not displayed here to simplify the plot. Inset map shows New Zealand with a black box around the STB that is enlarged; Wellington and the Cook Strait are denoted. The centre of upwelling off Kahurangi Point is demarcated in grey; tongues of upwelled water extend as a plume to the north and northeast.

Discussion

Multiple data sources have been examined here in an effort to understand the occurrence patterns of blue whales in the STB, New Zealand and their functional habitat use. This synthesis of data sources supports the hypothesis that the STB is a blue whale foraging ground where whales feed on N. australis that concentrate in response to Kahurangi Point upwelling plumes. Whaling records indicate that blue whales were common in this region, but the STB has never been recognised as a blue whale foraging ground. This potential oversight is reinforced by the listing of blue whales as a Migrant under the New Zealand Threat Classification System (Baker et al. 2010), which should be re-evaluated.

Evidence of elevated blue whale presence in the STB includes increased density of blue whale sighting records in the STB relative to other areas around New Zealand. Elevated blue whale sightings along the east coast of Northland may indicate that this area is a migration corridor and may be due to increased observational effort from relatively extensive fishing effort and recreational boater activity in the area. Additionally, examination of the Soviet whaling and JSV blue whale data demonstrates increased blue whale presence in the STB region relative to other areas globally and within New Zealand, and a persistent use of the STB region by blue whales over time. The stranding record also indicates a history of blue whales in the STB with four of the nine strandings recorded between 1893 and 1951.

Due to unstandardised observer coverage across the STB and the various and limited data sources explored, a seasonal pattern of blue whale presence in the STB cannot be determined. This lack of seasonal pattern may be due to (1) the relatively few available blue whale sighting (n=31) and stranding (n=10) records in the STB, which are temporally biased due to the lack of standardised survey effort; (2) blue whales feeding in the STB during winter months based on evidence that winter blue whale distribution can be influenced by foraging opportunities (Croll et al. 2005; Branch et al. 2007); (3) a life history strategy that entails blue whale foraging outside the Antarctic in summer (e.g., Hucke-Gaete et al. 2004), which is a typical behaviour for pygmy blue whales (Gill 2002; Best et al. 2003); (4) different distribution patterns between blue whale subspecies where the presence of subspecies in the STB alternates temporally; or (5) a combination of these factors. Despite a lack of evidence for a seasonal pattern of sightings, the presence of blue whales in the STB during summer months (November–April), when the majority of Antarctic blue whales are expected to be foraging in southern Antarctic waters, indicates that these sightings may be pygmy blue whales. However, sympatric blue whale subspecies on foraging grounds has been documented, as well as the year round presence of Antarctic blue whales north of the Polar Front (Samaran et al. 2010). Therefore, in order to resolve occupancy patterns in the STB by blue whale population, individuals must be identified to subspecies.

Multiple publications on zooplankton composition in the STB offer evidence that N. australis is found in dense concentrations that could attract foraging blue whales. The large distances between the Kahurangi Point upwelling area and the majority of blue whale sightings presented here (50–150 km) does not weaken this hypothesis. A spatial disconnect between upwelling centres and locations of euphausiid blooms with high rates of blue whale sightings has been documented in studies of blue whales off southern Australia (Gill et al. 2011), Channel Islands, California (Croll et al. 1998) and Monterey Bay, California (Croll et al. 2005). All three of these studies reported increased sightings of foraging blue whales in association with dense concentrations of euphausiids that formed downstream of cold water coastal upwelling systems. This spatial lag is due to wind-forcing, currents and temporal lags in zooplankton growth. Based on concentrations of a main blue whale prey, N. australis, toward the distal end of the Kahurangi Point upwelling system, it is possible that blue whales use this habitat to feed on dense aggregations of their prey. Of course, the paucity of blue whale sightings south of 40°S where high densities of N. australis have been documented (ellipses in Fig. 1) cannot be interpreted as a lack of habitat use by blue whales because all these sightings data were collected without standardised observer effort across the region. Therefore, the absence of sightings does not indicate the absence of blue whales.

Assessment of spatial overlap between existing records of blue whale sightings in the STB and potential threats illustrates close proximity between whales and seabed mineral exploration and extraction activities and shipping traffic. This study suggests that a blue whale foraging ground in the STB has gone unrecognised for decades amidst these and other sources of anthropogenic threats. The impacts of these threats are unknown, but anthropogenic noise from seismic activity and shipping traffic has been shown to alter blue whale acoustic behaviour (Di lorio & Clark 2010; Melcon et al. 2012) and ship strikes of baleen whales is a growing threat globally (Van Waerebeek et al. 2007), including in New Zealand where one reported blue whale stranding death near Auckland was caused by a ship strike (Table S2). If human activities in the STB do affect blue whales the impacts could cause population declines of an endangered species and ecosystem changes due to the removal of top-down forcing and subsequent trophic cascades (Estes et al. 2011). Moreover, marine ecosystems and animals can be simultaneously under pressure from multiple sources of anthropogenic impacts (Greene & Pershing 2004; Rosa & Seibel 2008). Therefore, despite apparent low-level impacts from individual sources, we must be cognisant of cumulative effects and manage these threats with a coordinated approach.

To gain a more comprehensive understanding of blue whale distribution throughout the STB, two complementary methods, aerial surveys and acoustic loggers, can be employed. Systematic aerial surveys, conducted at regular temporal intervals (e.g., monthly) can collect standardised data that will elucidate the spatial (how big) and temporal (how frequent and persistent) scale of blue whale distribution in the region (e.g., Gill et al. 2011). The production of high intensity, low frequency and long duration acoustic calls is a common and important aspect of blue whale behaviour worldwide (McDonald et al. 2006). Therefore, an array of acoustic loggers deployed in the STB would detect the presence of calling blue whales and clarify temporal patterns of occupancy (e.g., Stafford et al. 2001), and identify habitat use to sub-species (e.g., Samaran et al. 2010), based on previously described unique calls (McDonald et al. 2006). Data collected from these methods should be examined relative to oceanographic data, either remotely sensed or collected in situ, to synoptically link blue whale presence and absence with upwelling events and plume location dynamics (e.g., Rennie et al. 2009). Boat-based surveys can complement these methods through photo-identification of individuals, collection of biopsy samples to genetically distinguish between Antarctic and pygmy blue whale subspecies, and the collection of continuous hydroacoustic data and conductivity–temperature–depth (CTD) casts to investigate synoptically the links between blue whales, prey and habitat.

Conclusion

This study presents evidence that the STB is a blue whale foraging ground and is an example of how extraction of marine resources can be sanctioned without a complete understanding of potential impacts on the environment or biodiversity. Human activities have already dramatically reduced blue whale numbers, leaving only a remnant population (Branch et al. 2004; Branch 2007). Foraging is a critical life history component of any animal, and for blue whales, the ability to encounter reliable, dense prey aggregations can dramatically impact survival and reproductive rates. Outside of the Antarctic there are few documented blue whale foraging grounds in the Southern Hemisphere. It is therefore essential that a firm understanding of blue whale distribution and habitat use patterns within the STB is gained before anthropogenic activities escalate to levels of disturbance that could cause displacement of blue whales from a potentially critical foraging ground.

Supplementary files

Supplementary file 1: Table S1. Details of incidental, anecdotal and survey sightings of blue whales in the South Taranaki Bight examined in this study.

Supplementary file 2: Table S2. Details of blue whale strandings in the South Taranaki Bight (STB) and all of New Zealand examined in this study.

Acknowledgements

I would like to thank OMV NZ Ltd and NIWA for funding this research and M. Patrick for facilitating the project. I thank M. Cawthorn and the New Zealand Department of Conservation for contributing incidental blue whale sightings and strandings data, and C. Lilley for supplying details of blue whale foraging sightings. Plots of Soviet catch and JSV sighting data of blue whales are reproduced here with permission from T. Branch, Mammal Review and John Wiley & Sons Ltd. Publishing. I am grateful to J. Bradford-Grieve, R. Constantine, A. Rowden, D. Thompson and two reviewers for insightful comments on this manuscript.

Notes

Supplementary data available online at www.tandfonline.com/10.1080/00288330.2013.773919

Supplementary file 1: Table S1. Details of incidental, anecdotal and survey sightings of blue whales in the South Taranaki Bight examined in this study; Supplementary file 2: Table S2. Details of blue whale strandings in the South Taranaki Bight (STB) and all of New Zealand examined in this study.