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Item Understanding common dolphin and Australasian gannet feeding associations from nutritional and ethological perspectives(Oxford University Press on behalf of International Council for the Exploration of the Sea, 2022-09-01) Stockin KA; Amiot C; Meynier L; Purvin C; Machovsky-Capuska GEPrey detection and subsequent capture is considered a major hypothesis to explain feeding associations between common dolphins and Australasian gannets. However, a current lack of insight on nutritional strategies with respect to foraging behaviours of both species has until now, prevented any detailed understanding of this conspecific relationship. Here we combine stomach content analysis (SCA), nutritional composition of prey, a multidimensional nutritional niche framework (MNNF) and videography to provide a holistic dietary, nutritional, and behavioural assessment of the feeding association between dolphins and gannets in the Hauraki Gulf, New Zealand. Dolphins consumed ten prey species, including grey mullet (Mugil cephalus) as the most representative by wet mass (33.4%). Gannets preyed upon six species, with pilchards (Sardinops pilchardus) contributing most of the diet by wet mass (32.4%) to their diet. Both predators jointly preyed upon pilchard, jack mackerel (Trachurus spp.), arrow squid (genus Nototodarus), and anchovy (Engraulis australis). Accordingly, the MNNF revealed a moderate overlap in the prey composition niche (0.42) and realized nutritional niche (0.52) between dolphins and gannets. This suggests that both predators coexist in a similar nutritional space, while simultaneously reducing interspecific competition and maximizing the success of both encountering and exploiting patchily distributed prey. Behavioural analysis further indicated that dolphin and gannets feeding associations are likely to be mutually beneficial, with a carouselling foraging strategy and larger pod sizes of dolphins, influencing the diving altitude of gannets. Our approach provides a new, more holistic understanding of this iconic foraging relationship, which until now has been poorly understood.Item The influence of multi-species feeding associations on the foraging behaviour of Australasian gannets (Morus serrator) in the Hauraki Gulf, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Conservation Biology at Massey University, Auckland, New Zealand(Massey University, 2015) Purvin, Cameron MackenzieIn order to successfully capture prey in a challenging and physically demanding environment, Australasian gannets (Morus serrator) have developed a number of foraging strategies. Foremost among such strategies is the joining in formation of mixed species feeding associations (MSFA’s) and the use of specialised plunge diving behaviours. This study sought to determine how gannets vary individual and group plunge diving behaviours in order to maximise the benefits and mitigate the risks associated with feeding in densely packed, highly active MSFA’s. Specifically this study’s aims are to examine how variations in gannet dive heights, angles and completion rates are influenced by external stimuli, including weather parameters, oceanographic conditions and MSFA’s species composition and behaviour. This study additionally seeks to examine what mixed and/or con-‐specific cues are associated with the formation of synchronous diving bouts, and determine its role as a form of local enhancement for gannets foraging in MSFA’s. Between March 2013 and June 2014, 45 independent boat based surveys were conducted aboard Dolphin Explorer, a 20m tour boat based in New Zealand’s Hauraki Gulf. Using a Canon XH A1S high definition video camera, approximately 11.9hrs of MSFA video footage was collected, containing 5565 recorded gannet plunge dives. Variations in these dives, including their height, angle of entry, and synchronous timing were examined in context of environmental variables including wind speeds, sea surface temperatures, and Beaufort sea states as well as against MSFA variables including the abundance of other foraging gannets, the abundance and behaviour of common dolphins (Delphinus sp.), and the presence of other species including shearwaters (Puffinus spp.), petrels (Fregetta spp.), terns (Sterna spp.), and Bryde’s whales (Balaenoptera edeni). While weather influences were found to have minimal effects on gannet dive behaviours, higher winds were associated with lower altitude dives, and less frequent synchronous diving bouts. This is likely due to the decreased visibility of deeper prey caused by increased surface disturbances as a result of higher wind speeds, coupled with the inherent difficulties of performing highly coordinated aerial manoeuvres in high wind conditions. Increased gannet flock sizes were found to correlate with more frequent high-‐ altitude 90° dives, and increased rates of dive synchrony. This may be a result of the need to better coordinate dives in denser assemblages of con-‐specifics to avoid collisions, coupled with the increased foraging success associated with larger foraging groups, and local enhancement. Increased foraging dolphin pods were also associated with more frequent high-‐ altitude vertical dives. This may result from gannets capitalising on the increased bait-‐ball density and stability associated with larger foraging pods by taking deeper dives in order to make multiple prey captures with greater ease. Gannet synchrony, however, was only found to decrease in the smallest dolphin pods, again attributed to the decreased foraging abilities of smaller pods. Dolphin foraging behaviours most associated with the maintenance of dense stationary bait-‐balls were most heavily associated with high, vertical gannet dives, and increased gannet synchronous dives. This is likely the result of gannets capitalising on denser prey assemblages by taking deeper dives in pursuit of multiple prey capture. The presence of Bryde’s whales at MSFA’s was found to decrease the frequency of both high vertical dives and synchronous dives, as lunge feeding on bait-‐balls scatters fish and decreases prey density. The presence of hetero-‐specific avian species had various effects on gannet dive behaviours. Most notably, the presence of shearwaters and petrels was found to be associated with greater frequencies of low altitude gannet dives, thought to be a result of an increased collision risk in their presence. Terns meanwhile were associated with decreased dive synchrony, possibly as a result of false hetero-‐ specific signalling caused by their white plumage.Item Hunting between the air and the water : the Australasian gannet (Morus serrator) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Auckland, New Zealand(Massey University, 2012) Machovsky-Capuska, Gabriel E.Australasian gannets (Morus serrator) are the second rarest member of the seabird group Sulidae. Among the three species of gannets worldwide, they are the only species that regularly breeds in southeastern Australia and New Zealand. Like all gannets, M. serrator face considerable challenges in foraging, relying on sparsely and patchily distributed pelagic prey, which move in a 3D environment. Whereas most predators are specialise hunters in one media, gannets have to hunt within a complex air-water interface. The aim of the present thesis is to examine the hunting strategies of Australasian gannets, with particular emphasis on how these birds use both aerial and aquatic adaptations to locate and capture prey. The acquisition of information concerning food sources was analysed using GPS data loggers, field observations and high resolution video footage. I tested the hypothesis that gannets obtain information of food resources from their partners using bill fencing as referential signals analogous to the waggle dance in honeybees (Apis mellifera) (Chapter 2). Results did not support this hypothesis but suggested that Australasian gannets use a combination of strategies, probably including memory that facilitates their return to locations where prey was previously captured (Chapter 3) and local enhancement to locate active feeding sites (Chapter 2). The impact of intraspecific competition for local resources was studied between large (Cape Kidnappers, 7,300 breeding pairs) and small (Farewell Spit, 3,900 breeding pairs) colonies in New Zealand using GPS data loggers (Chapter 3). Results indicated that gannets from the larger colony invested more in foraging (greater foraging times and foraging distances). This is consistent with previous studies of other gannet species, suggesting that M. serrator experience intraspecific competition for food when living in large colonies. Pelagic prey are able to evade predation by descending to depths beyond the reach of diving birds. Among the adaptations evolved by gannets for dealing with this challenge is plunge-diving, where the bird uses gravity in the aerial phase of the hunt to gain speed and momentum for descending into the water column. I conducted a fine scaled analysis using videography of the aerial and aquatic phases of this highly specialised hunting strategy. Analysis of the aerial phase (Chapter 4) showed that the initiation of plunge dives are synchronised among members of foraging groups, suggesting a form of group-level behaviour in which gannets might benefit from the sensory experiences (prey detection) of conspecifics. The analysis also showed that gannets adapt the aerial phase of their dives in presence vs. absence of heterospecific predators. In the aquatic phase (Chapter 5), gannets perform short and shallow V-shaped dives and long and deep U-shaped dives in pursuit of pelagic fish and squid. My findings revealed that gannets adjusted their dive shape in relation to the depth of their prey rather than prey type, as previously hypothesised. Although the maximum number of prey captured per dive by the gannets was higher than previously reported, reaching up to five fish in a single U-shaped dive, the results presented herein suggest that the two dive profiles were equally profitable. To examine the role of underwater vision in prey capture, I used underwater video footage, photokeratometry and infrared video photorefraction (Chapter 6). Analysis of video footage confirmed that there are two distinct phases in the underwater component of plunge dives in Australasian gannets, an initial phase in which the bird is propelled through the water column by the momentum of the plunge (M phase) and a phase in which it is actively propelled by wing flapping (WF phase). The highest prey capture rate was observed during the WF phase, a result that suggests the use of vision in underwater prey pursuit. I therefore used photokeratometry and video photorefraction to test whether gannets are able to adapt optically in the transition from aerial to aquatic media. My measurements showed that underwater visual accommodation in the gannets was attained within 2 - 3 frames (80 - 120 ms) of submergence, a remarkably short timescale in relation to the optics of most vertebrate eyes. The preceding chapters demonstrate some highly effective behavioural and sensory capacities used by gannets in foraging. In Chapter 7 I demonstrate evidence of fatal injuries due to collision between conspecifics in plunge-diving Australasian and Cape gannets (M. capensis). The analysis also revealed a case of attempted underwater kleptoparasitism, in which a diving bird targeted a previously captured fish in the beak of another gannet. This novel observation suggests a further challenge for hunting gannets, namely to retain prey following the capture.Item Variability in the breeding ecology of Australasian gannets, Morus serrator, at Cape Kidnappers, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Ecology at Massey University, Palmerston North, New Zealand(Massey University, 2005) Stephenson, Brent MarkThe Australasian gannet, Morus serrator, a member of the Family Sulidae, primarily inhabits New Zealand and Australian coastal waters. It is considered to be closely related to the Atlantic gannet, Morus bassana, in the North Atlantic, and Cape gannet, Morus capensis, in South African waters, with which it shares similar ecological niches. Although often described as relatively well studied, much of our current knowledge of Australasian gannets has been derived from anecdotal observations, and irregular visits to breeding colonies. Few studies have derived information from continuous observations of known individuals over consecutive breeding seasons. Conversely, studies of Atlantic gannets, in particular, have been conducted on a far more rigorous basis. Thus, parallels drawn between the three species, and particularly Atlantic and Australasian gannets, may or may not be accurate. This study is the first to document three complete and consecutive breeding seasons (1999-00, 2000-01, and 2001-02), using marked nests at the Plateau colony, Cape Kidnappers, New Zealand. Birds were individually marked and I investigated their breeding ecology, and in particular the annual variability in success and other breeding variables. Contrasting with previous studies, I found that the onset of egg laying differed little between years, and although laying was less synchronous in one season, it was highly synchronous in the other two seasons. This is similar to the Atlantic gannet in which the onset of laying is similar year to year, and laying is also highly synchronised. At least for the Australasian gannet, the timing of egg laying does not appear to be linked to sea surface temperature near the colony. However, both species appear to time egg laying to allow chick rearing to coincide with a predictably timed peak in prey availability. Further in depth study is required to confirm this for the Australasian gannet breeding in New Zealand. The use of back-dating to estimate the onset of egg laying from hatching dates and chick ages was also tested and found to provide a useful estimate. Although previous studies of Australasian gannets have suggested highly variable breeding success, with almost complete breeding failures in some years, this study is the first to document this, and explore possible reasons. Previous studies have suggested human disturbance at breeding colonies as being the cause, with little evidence provided. However, I found in 1999-00, there was considerable egg and chick losses as a result of environmental conditions causing adult and chick starvation and desertion of eggs and young chicks, and productivity was only 9%. A similar pattern, although less severe, occurred in the following season, but conditions improved during the chick rearing stage resulting in 55% productivity. In 2001-02, environmental conditions were stable, resulting in high hatching success, however, an unseasonal storm resulted in considerable chick mortality, and productivity was again low at only 13%. Thus, productivity during the three seasons studied varied markedly, being very low in two of the three seasons as a consequence of environmental conditions, with no evidence of human disturbance. This is the first study which has linked the environment to variable breeding success in this species, and its findings contrast considerably with the invariably high breeding success of the Atlantic gannet. Retrospective analysis of previous Australasian gannet studies suggests this natural link between the environment and breeding success has been apparent, but not recognised, since the 1940s. Links between specific environmental factors and breeding failures are yet to be determined. I established that calculated egg volume is a good predictor of fresh egg mass, allowing analysis of eggs through the use of linear measurements, when fresh egg mass is not known. For all seasons combined mean egg volume was 89.2 mm3, whilst a mean incubation period of 45 days is similar to other Australasian gannet studies. Changes in specific gravity during egg development did not allow accurate determination of egg laying dates, with 12.7% of fresh egg mass being lost during development. Eggs laid both within and between seasons by individual females were highly correlated in all measures (length, width, shape, and volume). These egg measures generally showed a negative correlation with laying date, at least in the last two breeding seasons. Egg volume was positively correlated with chick mass at hatching, although chick growth rates did not seem correlated with egg volume. However, in 2000-01, chicks that survived to fledging were significantly heavier on day-one, despite hatching from similar volume eggs. For chicks that fledged successfully, those that hatched later in the season increased in mass more slowly than chicks that hatched early. However, the reverse trend was found for wing length, possibly as a consequence of wing development being more important than mass increase. Linked to the breeding failures in 1999-00, were low nest attendance rates by adults and the lowest recorded body mass for adult Australasian gannets in this or previous studies. Similarly, first egg volumes in this first season were significantly lower, and fewer lost eggs were replaced. Chick growth rates also varied between breeding seasons. Early development of chicks (0-11 days) was slower in 1999-00 than in the other two seasons (slower even than chicks that did not fledge in the other seasons). Growth rates of older chicks (20+ days) that survived to fledge, however, were actually greatest in that season. This suggests that older or more experienced adults, who were better able to provision chicks through poor foraging conditions, were then able to raise faster developing chicks once conditions improved. The evidence presented in this thesis demonstrates that the 1999-00 breeding season was a difficult one for breeding Australasian gannets at Cape Kidnappers. Furthermore, information derived from this three season study, along with analysis of historical studies, suggests that the Australasian gannet differs markedly to the Atlantic gannet in many respects. Most notable is the occurrence of pronounced breeding failures in some years, apparently influenced by environmental conditions. This seems to be a consequence of a more variable environment, involving variability in weather, oceanographic conditions, and prey availability and abundance.