Account of a mass aggregation of Port Jackson sharks Heterodontus portusjacksoni at Point Cooke Marine Sanctuary, Victoria, Australia. (2024)

Introduction

A mass aggregation of over 100 Port Jackson sharks (Heterodontusportusjacksoni) was noticed during a recreational snorkel at the PointCooke Marine Sanctuary over two consecutive days in January 2009. Thisaccount aims to describe what appears to be a relatively very unusualoccurrence, since in the literature there are virtually no recorded ordocumented instances of mass aggregations of this size and type.

Dates of sightings

The sharks were first observed at approximately 11:00 on Sunday, 4January 2009 during a routine recreational snorkel. The sharks werestill present during another snorkel at around 2.00 pm on the same day,and were also found at approximately 1:00 pm on Monday, 5 January 2009,but all had left the area by approximately 10.00 am on Tuesday,

6 January 2009. To summarise, four recreational free dives wereconducted, and the sharks were present during three of them. It isunknown how long these sharks had actually remained in this locationprior to the initial sighting on 4 January, and all observationsdescribed herein were conducted opportunistically and were not part of adefined sampling program.

Location and habitat of shark aggregation

The approximate location of this sighting at the Point Cooke MarineSanctuary was 37[degrees]55'24.52"S and144[degrees]47'58.62"E (Fig. 1).

The aggregation occurred off the south-eastern side of the shorenear emergency location area PCC506, near the Point Cooke Homestead,placing it within Altona Bay (Fig. 2). The area was between 50 and 80metres offshore, past a number of rocks in the shallow intertidal zonethat exhibit heavy coverage of the calcareous tubeworm Galeolariacaespitosa, and also features an intertidal seagrass meadow.

Access to this site was easily made by walking to the eastern side,thus avoiding trampling the seagrass beds as well as the boulders, whichare festooned with sea urchins and thus present a potential injury riskto waders.

The majority of sharks were found in water of 1.5 and 2 metresdepth, depending on bottom topography. The benthos in this area consistsof basalt reef and boulders, interspersed with sand patches andextensive colonisation by the green algae Ulva spp. (Fig. 3).

No ontogenetic (age) differences in habitat use were noted on thisoccasion, with the distribution of sharks according to size appearing tobe random.

The section of the Point Cooke Marine Sanctuary that the sharkswere residing in during their mass aggregation does not feature densecoverings of macroalgae; the dominant algal species in the area tend tobe the southern sea lettuce Ulva rigida and/or U. australis, andSargassum spp. is also common, while at the time of the sighting, theinvasive marine pest species Undaria pinnatifida (Japanese kelp orWakame) was in a state of dieback. As it stood, none of these macroalgalspecies would have provided appreciable levels of shelter. This,combined with the shallowness of the water, meant that all animals werevery exposed to the solar radiation that was present at the time.

Very high solar radiation readings were recorded on both days (34.8and 34 MJ/[m.sup.2] on Sunday and Monday respectively) (Data: Bureau ofMeteorology) that the sharks were sighted, due to the time of year(summer), the consequently long photoperiod, and the minimal cloud coverthat was present during the times the observations were made.

Prevailing conditions

Bureau of Meteorology data from the nearby Laverton RAAF base forthe days that the sharks were observed to be in situ and a day eitherside of the aggregation being noted is shown in Table 1.

The wind during all days in which observations were conducted wasrelatively light, with seas being calm and wave heights generally below0.5 m. Tidal heights varied between a low of 27 cm to a high of 88 cmwhile the sharks were present at the aggregation site.

The contention from several authors (including Last and Stevens,1994) that the species generally returns to rocky gullies and cavesduring the day appeared to be flouted on the days in question by theanimals in this aggregation, as the sharks stayed in the same area forat least 48 hours during day time and presumably also at night time,given the sheer size of the aggregation.

On 3 January illuminaytion from the moon was 37.7% of that from afull moon, increasing to 48.4% and then 59.5% on the two days that theaggregation was observed. This increased further to 70.5%(Timeanddate.com website, http://www.timeanddate.com/moon/australia/melbourne) on the day the sharks were no longer observed in the area.Sunday 4 January marked the start of the first quarter lunar phase, witha full moon appearing on 11 January, well after the sharks had left thearea (Data: Planetarium, Museum Victoria website, http://museumvictoria.com.au/planetarium/discoverycentre/moon-phases/)

Shark numbers

The school of sharks in this case was estimated to number well inexcess of 100 individuals, an estimate that was made easier based ontheir generally docile and stationary disposition, and the relativelysmall area that they were found to inhabit. One area that was observedto be barely 5 m x 5 m or 25 [m.sup.2] contained 27 sharks, with manystacked one on top of the other. The total area that the sharks occupiedwas difficult to estimate because of the patchy distribution of thesharks, though there was not much distance between sharks in adjacentareas. The aggregations on both days seemed to be of similar size.

Shark sex and size

The Port Jackson shark, like all elasmobranchs, features obvioussexual dimorphism in that males and females can be distinguished readilyupon examination of their genitalia. Claspers on the males of thisspecies, like those of many elasmobranchs, are prominent (Fig. 4), andnone were seen. In this instance, shallow water made lateralobservations possible; these are useful for providing additionalqualification of the sex of the sharks. Claspers on male animals may bedifficult to detect if the shark is stationary and the claspers sit inline with the body and are concealed by the shark's dorsal surface;objects that may be present in the benthos, such as rocks andmacroalgae, may also hinder observations. In this case it certainlyappeared that all animals observed were in fact female. While the authordid not survey every single shark in the mass aggregation, and it istherefore possible that males could have been concealed amongst theschool, it is certain that these males would have represented an extrememinority, if they were present at all.

Lengths of the sharks varied between 40 cm and 120 cm, with nospecific size segregation apparent.

Shark behaviour

All sharks were observed to be extremely docile, with very fewsharks actively swimming; their behavioural pattern can therefore bedescribed accurately as 'resting'. No foraging was observed tobe taking place.

On a couple of occasions some sharks were observed to swim awaywhen approached, but this was to be expected considering the size of theobserver (approximately 181 cm tall and 79 kg, and the additional lengthof the flippers) being considerably larger than any of the observedsharks. Sharks that did relocate generally swam only a very shortdistance (10 metres or less) before settling again on the benthos. At nostage did the author ever feel threatened by the sharks, with noaggressive or even investigative approaches instigated. No sudden orerratic movements were made by the author, and all movements towardssharks were very slow and deliberate, so as not to alarm the animals.

Other occurrences of Port Jackson sharks and other elasmobranchs atPoint Cooke Marine Sanctuary

Prior to this occurrence, the author had never seen a Port Jacksonshark at the Point Cooke Marine Sanctuary, despite snorkellingsporadically in the area between 1997 and November 2005, when the authormoved to the area and then snorkelled regularly, up until the presenttime. Only a few other Port Jackson sharks have been seen since the massaggregation, with another sighting of a female made on 24 October 2009.In this incident, the shark appeared to have her head down amongst somerocks and was making twisting motions with her body, which assumed anearly vertical attitude suspended in the water column. This could havebeen suggestive of either foraging behaviour or the planting of an eggcase, though no evidence of either was found upon examination of thebenthos where her head had been. The month of October fits withStevens' (1987) assertion that this period is when the laying ofeggs takes place. Another female (possibly gravid on account of asomewhat distended abdomen) was sighted during September of 2014 (Fig.5).

In additon to sightings of sharks, the occasional egg case has beenfound washed up on the beach (Fig. 6), even though these have generallynot been in very high numbers, with only a few sighted over the years.This would seem to indicate that Point Cooke Marine Sanctuary is not avery well frequented area, and certainly not a residential haunt, ofthis particular species.

The low number of sightings of Port Jackson sharks in the shallowsof the Point Cooke Marine Sanctuary must be considered a true indicationof their general lack of presence in this area, since experiencedsnorkellers and divers are able to spot similarly sized animals such asstingrays and stingarees even when only the eyes and spiracles(accessory breathing organs) are visible. It appears that Point CookeMarine Sanctuary is something of an elasmobranch haven, with at leastsix species of ray having been sighted within the boundaries of thesanctuary, including the Southern Fiddler Ray Trygonorrhina dumerilii,which is extremely common during the summer months in particular, andthe eastern shovelnose stingaree Trygonoptera imitata, and SparselySpotted Stingaree Urolophus paucimaculatus, which can also be found inconsiderable numbers in the sanctuary. Also encountered are the SouthernEagle Ray Myliobatus australis, the less common Spotted StingareeUrolophus gigas and the Smooth or Short-tailed Stingray Dasyatisbrevicaudata. Other shark species sighted in the Point Cooke MarineSanctuary include the Gummy Shark Mustelus antarcticus.

Observations of marine life during the summer months are generallylimited by the prevailing weather conditions. During hot and sunny days,the norm is for northerly winds to flatten out the surface of PortPhilip Bay in this location during the morning hours, thus allowinggreater visibility. During the afternoon, however, sea breezes from thesouth tend to prevail and cause greater wave swash and surge, reducingvisibility.

On the occasions of these sightings visibility was found to bebetween 4 and 6 metres, which is relatively good for the location, withabsolute maximum visibilities observed to be around 10 to 12 metres.Unfortunately, the best visibility in this section of Port Philip Bayoften occurs during the winter months, when elasmobranchs of any typeare relatively rare at Point Cooke Marine Sanctuary; they seem to bevery common from October to March, and numbers then start dwindling fromApril right through until September, when they start to return.

Possible reasons for this mass aggregation

There are myriad reasons why this aggregation may have occurred,and as is often the case with opportunistic observations these arenecessarily speculative; however, the aggregation of such a large numberof individuals is surely worth recording and discussing further.

Location--why Point Cooke Marine Sanctuary?

Ultimately, we can only speculate on why the Port Jackson sharksthat formed this massive school chose Point Cooke Marine Sanctuary astheir aggregation destination. Hutchins and Swainston (1996) noted thatthis species prefers protected reef and adjacent sand and weed areas,and noted that several may crowd together in one cave. Point CookeMarine Sanctuary does not feature caves in the shallow subtidal region,but instead features a mixture of seagrass meadows, sand patches, andbasalt reefs with boulder fields and urchin barrens. Boulders and thelike could be considered as potentially suitable areas for thelaying/planting of the spiralled egg cases that are characteristic ofthe species, since females tend to lay eggs and then plant them in rockfissures with their mouths on inshore reefs in areas less than 5 m deep(Last and Stevens 1994). Since no oviposition behaviour or the presenceof eggs were observed, and apparently all sharks were female, and giventhat the timing of these observations was outside the expectedparturition period for this species, egg laying and breeding can beruled out as reasons for this mass aggregation, as will be discussed inmore detail shortly.

The Port Jackson sharks of Ricketts Point Marine Sanctuary

Areas such as the Ricketts Point Marine Sanctuary, near Beaumarison the north-eastern side of Port Phillip Bay, are renowned haunts forPort Jackson sharks, and the caves and rock gutters in this areaevidently support a resident population of these animals. These sharksappear in the shallow subtidal zone between about July and August, andare present until about February before they move on (Mike Letch pers.comm.).

Anecdotal observations from members of the Marine Care RickettsPoint group noted that the usual population of Port Jackson sharks thatinhabit the caves and boulder fields in their area was conspicuouslyabsent during January of 2009 (Mike Letch pers. comm.); it is thereforepossible that many sharks from this area may have relocated temporarilyto Point Cooke, a distance of approximately 23 kilometres.

Last and Stevens (2009) noted that Port Jackson sharks commencedconsiderable migrations to southern waters in summer and returned northduring the winter. Whether this was part of a large migration ofindividuals from more northerly climes (and therefore an infiltration ofPort Phillip Bay) is unknown. Future residency studies (which could becarried out by a combination of marine friends groups and scientificresearch agencies, and involve various tagging methods) would be aworthwhile endeavour to shed more light on movements of the species.

Breeding and oviposition

While Tricas et al. (1997) noted that groups of adults moved in andout of shallow water depending on water temperature and breedingconditions, with females and some males moving into shallower water forthe purposes of mating, it appears that most of the Port Jackson sharkpopulation of Australia's southern waters commence ovulation andmating behaviour between late winter and early spring, while oviposition(egg-laying) tends to occur between late winter and spring (Tovar-Avilaet al. 2007). Stevens (1987) noted that females lay 10-16 eggs (meaningthe species is oviparous) from late July to early October, favouringtraditional sites. The eggs take 9-12 months to hatch after oviposition,and young are sustained by a large yolk sac. In New South Wales, thebreeding season tends to occur also between July and November. Thecurrent aggregation, therefore, seems to be outside the envelope forthese processes (even allowing for differences that may occur in theVictorian population), meaning that they can be all but ruled out asdirect reasons for the aggregation.

The fact that no males were observed in this particular aggregationis consistent with the assertion by Whitley (1981) that Port Jacksonsharks tend to breed on shallow reefs during winter months, with malesmigrating seaward during the summer months.

In the aggregation described, it is apparent that the sharks(varying in size between 40 cm and 120 cm total length) did notsegregate by size (and therefore, presumably, age). These observationsdiffer from those reported in numerous other studies, which haveindicated that size-based segregation (and therefore segregation bymaturity level) tends to occur (Tricas et al. 1997). The spatialseparation of adults and juveniles in many species of elasmobranchs iswell recognised, and it must be assumed that this could be due to thepotential for cannibalism. Based on its dental morphology andbehavioural characteristics, it is highly unlikely that Port Jacksonsharks would indulge in cannibalism to any significant degree, and itcan be speculated that there is therefore possibly more'leeway' in the size groupings of this species.

Last and Stevens (1994) noted that this species hatches at a sizeof 23 cm, with females maturing upon reaching 80-95 cm. Without havingattempted to conduct measurements and collect length frequencydistribution data, it is difficult to express in percentage terms howmany individuals could have been expected to be sexually mature, thoughthe suspicion of the author is that the majority of sharks were longerthan 80 cm, meaning that they could have been mature or approachingmaturity, while some were considerably smaller than this and weretherefore presumably immature.

Feeding

Due to the fact that no feeding was observed at all during threeseparate observational periods, it is unlikely that feeding played adirect role in this particular aggregation. It is uncertain if sharksreverted to feeding during the night, as all observations were madeduring daylight hours. Edgar (2008) suggested that Port Jackson sharksare predominantly nocturnal feeders that forage for and prey oninvertebrates during the night (an assertion supported by Hodgson (1987)and Whitley (1981)), which may explain why no feeding behaviour wasobserved throughout the entire aggregation.

It is sometimes suggested that sharks and fish are attractedtowards the mouths of rivers with the promise of food that sometimesappears to follow on from high rainfall events. January 2009 was thesecond driest January on record in Melbourne. In this instance, becauseof the extremely low rainfall totals that were experienced for the localarea, attraction of sharks to the area due to freshwater discharges forthe period considered can be ruled out. While the prevailing northerlywinds and current systems operating at the time the observations weremade could have transported scents and various olfactory cues offshoreand potentially drawn sharks to the area, it seems that this is a highlyunlikely reason for this particular aggregation.

The Point Cooke Marine Sanctuary contains a wide variety of faunathat have been identified by various authors as being standard in thediet of Port Jackson sharks. Whitley (1981) mentioned that this sharkprefers crustaceans, molluscs and echinoderms, particularly sea urchins.All of these species groups are in abundance at Point Cooke, with SeaUrchins Heliocidaris erythrogramma being dominant to the extent ofcreating numerous large urchin barrens (areas that have been stripped ofmacroalgae and are covered in urchins, even in daylight hours). Powteret al. (2010), however, implied that urchins were not as important inthe diet as previously thought for this particular species. Dingerkus(1987) noted that the range of Port Jackson sharks was necessarilyrestricted by their diet, which featured items that were typically foundrelatively close to shore in temperate and tropical waters. Compagno(1987) noted that the diet of this shark consisted mainly ofinvertebrates, including seastars, crabs, shrimps, barnacles, marineworms, sea snails and other hard prey items, which are crushed withpavement-like molars towards the rear of the jaws; small fish are alsocaptured and eaten. All of these prey items are in abundance at thePoint Cooke Marine Sanctuary, which would therefore seemingly make anexcellent foraging and feeding location for this species.

Prior documentation of mass aggregation behaviour in this species

To date there is limited information in the literature regardingvery large aggregations (>100 individuals) of this species. Compagno(1984) noted that this species often has 'rest areas', whichmay be used by as many as 16 sharks at a time, and that the animalscould range as much as 850 km from breeding areas. Powter (pers. comm,2010) indicated that the largest aggregation size he had observedpersonally after eight years of research was 42 individuals, andacknowledged that aggregations in water depths greater than 30 metresthat exceeded 100 individuals occasionally occurred, with some videoevidence said to exist that verifies these claims. Rocky gutters havebeen cited as male avoidance structures by Powter and Gladstone (2008a),and sometimes contain 15 to 20 individuals in a relatively small area(such as a gutter 6 m long). This particular aggregation occurred in anarea with topography that could hardly be considered ideal for thepurposes of communal male avoidance, as sharks could be relativelyeasily accessed by interested potential suitors.

While it is possible that some refuge from strong water movementswould have been conferred by moving to the shallow subtidal region, itis noteworthy that most currents in the northern section of Port PhillipBay are relatively minimal when compared to the conditions encounteredin the open sea and coastal areas, and any longshore drift and othermovement patterns would have been minimal.

Powter and Gladstone (2008a) stated that juveniles often occupy aseagrass nursery in a large coastal embayment, but the aggregationobserved on this occasion was not in an area dominated by seagrass, andwhile some were juveniles it was clear from the total lengths observedthat a considerable percentage of the sharks present were adults or wereat least approaching sexual maturity.

Surely the most interesting question surrounding these observationsis what actually caused the sharks to choose Point Cooke MarineSanctuary in the numbers that they did, and how the sharks all managedto rendezvous at the same location and 'find' each other. Anydiscussion here is speculative, but it is likely that a combination ofsenses (olfactory, electro-sensory and possibly even visual) could havebeen used to get the sharks to their shallow water destination. Hodgson(1987) noted that Port Jackson sharks were bottom dwellers withrelatively small eyes, and that vision did not play as important a partas olfaction (smell) and electroreception in feeding and socialbehaviour. Did sharks respond to olfactory cues that were released uponan increasing number of sharks arriving in the area? It appears certainthat at present, we simply do not have answers to this question, butfuture observations of such aggregations could well yield importantclues as we learn more of the biology and behaviour of this fascinatingspecies.

Another question concerns what caused the sharks to move away fromthe area, presumably en masse. It is interesting that when all sharkshad left the area the day was fine, sunny and warm, but the followingday was considerably cooler and overcast. It is possible that theanimals detected an approaching low pressure system and decided to movefrom the shallows into deeper water, though this is only speculation.

Future work and documentation--will they return?

Even during the summer months, the beaches around Point CookeMarine Sanctuary are not very popular with people, and are relativelypoorly attended despite the idyllic location and pleasant surrounds. Itis therefore entirely possible that aggregations of these animals mayhave been occurring regularly and gone undetected, which is made morepossible by the ban on fishing that has been in force since the early1990s (though illegal recreational fishing has been observed on areasonably regular basis within the limits of the Point Cooke MarineSanctuary). It is hoped that with the formation of a new communitygroup, Marine Care Point Cooke, such aggregations will be documented inthe future in greater detail than has been provided here. Equipmentprocured by this group in recent times includes items such as digitalcameras with underwater housings, handheld GPS instrumentation andkayaks, all of which could prove invaluable in documenting futureaggregations of this and other species in this particular location. Thatsaid, in the six years since the mass aggregation occurred, no othersuch aggregations have been observed, with only a relatively smallnumber of sharks seen in the area. When considering the patronage of themarine sanctuary and the relatively small area that is covered during arecreational snorkel or SCUBA dive, it is not unlikely that aggregationscontinue to happen and simply go unnoticed. A number of authors(Whitley, 1981; Dingerkus, 1987) have noted that migration to the samefavoured sites (including reefs and even crevices) year after year forthe purposes of resting, mating and oviposition could be expected;O'Gower (1995) noted that this was possible on account of thespecies' outstanding spatial memory.

As noted by Powter and Gladstone (2008a), quantitative studiesaddressing habitat preferences are required to gain a full understandingof the selection of various habitats by elasmobranchs. Likewise, Powterand Gladstone (2008b) noted that significant aspects of the reproductivebiology and ecology are qualitative (as this study is), incomplete orunknown. It seems that this current record of a single mass aggregationof the Port Jackson shark reveals that their behaviour may be even morecomplex than previously thought.

Acknowledgements

I would like to thank those with whom I have consulted during thepreparation of this record, including the late Mike Letch of Marine CareRicketts Point, Dr. David Powter of the School of Environmental and LifeSciences, Central Coast Campus, University of Newcastle, as well as DrBarry Bruce of the Division of Marine and Atmospheric Research at theCSIRO in Hobart, Tasmania, and Dr John Stevens (retired, formerly of theCSIRO in Hobart, Tasmania). I would also like to thank the volunteers ofMarine Care Point Cooke, and Melbourne Polytechnic.

References

Bureau of Meterology, National Tidal Centre (2008) Tide Guide.(Great Outdoors Publications: Hervey Bay, Queensland)

Compagno LJV (1987) Kinds of Sharks. In Sharks, pp. 18-33. Ed JDStevens. (Golden Press Pty Ltd: Drummoyne, New South Wales)

Compagno LjV (1984) Sharks of the World. United Nations Food andAgriculture Organisation, Rome. In Sharks: Silent Hunters of the Deep. :Anon. (1986). Reader's Digest, Sydney, New South Wales.

Dingerkus G (1987) Shark Distribution. In Sharks, pp. 36-45. Ed JDStevens. (Golden Press Pty Ltd: Drummoyne, New South Wales)

Edgar GJ (2008) Australian Marine Life. 2 Edn. (Reed New Holland:Sydney)

Hodgson ES (1987) The Shark's Senses. In Sharks, pp. 76-83. EdJD Stevens. (Golden Press Pty Ltd: Drummoyne, New South Wales)

Last PR and Stevens JD (2009) Sharks and Rays of Australia. 2 edn.(CSIRO Publishing: Collingwood, Victoria)

Last PR and Stevens JD (1994). Sharks and Rays of Australia. (CSIROPublishing: Collingwood, Victoria)

O'Gower AK (1995) Speculations on a spatial memory for thePort Jackson shark (Heterodontus portusjacksoni) (Meyer)(Heterodontidae). Marine and Freshwater Research 46, 861-871.

Parks Victoria (2004) Park Notes: Point Cooke Marine Sanctuary.(Victorian Government: Melbourne)

Powter DM, Gladstone W and Platell M (2010) The influence of sexand maturity on the diet, mouth morphology and dentition of the PortJackson shark, Heterodontus portusjacksoni. Marine and FreshwaterResearch 61, 74-85.

Powter DM and Gladstone W (2008a). Habitat preferences of PortJackson sharks, Heterodontus portusjacksoni, in the coastal waters ofeastern Australia. Proceedings of the Linnean Society of New South Wales129, 151-165.

Powter, DM and Gladstone, W (2008b) The reproductive biology andecology of the Port Jackson shark, Heterodontus portusjacksoni, in thecoastal waters of eastern Australia. Journal of Fish Biology 72,2615-2633.

Stevens JD (1987) Shark Biology. In Sharks pp. 50-75. Ed JD Stevens(Golden Press Pty Ltd: Drummoyne, New South Wales)

Tovar-Avila J, Walker TI and Day RW (2007) Reproduction ofHeterodontus portusjacksoni in Victoria, Australia: evidence of twopopulations and reproductive parameters for the eastern population.Marine and Freshwater Research 58, 956-965.

Tricas TC, McCosker JE and Walker TI (1997) Sharks Field Guide. InSharks and Rays. pp. 143. Ed TC Tricas, K Deacon, P Last, JE McCosker,TI Walker and L Taylor (Reader's Digest: Surry Hills, New SouthWales)

Whitley GP (1981) Sharks of Australia. (Jack Pollard Publishing,North Sydney)

Received 9 February 2012; accepted 21 May 2015

Andrew Christie

Melbourne Polytechnic, Cnr Cooper St and Dalton Rd, Epping,Victoria 3076

Email: [emailprotected]

Table 1: Daily maximum and minimum temperatures and rainfall totalsfor the period around the sighting of the shark aggregation (Source:Bureau of Meteorology).Date Day Maximum air Minimum air Rainfall temperature temperature (mm) ([degrees]C) ([degrees]C)3 January 2009 Saturday 19.2 6.5 04 January 2009 Sunday 24.7 7.2 05 January 2009 Monday 25.6 9.4 06 January 2009 Tuesday 29.8 12.4 0