By: Category: Science, Date: 29 Nov 2013
What makes Sydney’s harbour the most biodiverse in the world? Plunge beneath its murky waters with Museum scientists to find out.
Sydney Harbour has always been central to the lives of Sydneysiders – from the first inhabitants, the Gadigal people, who had long occupied the area; to the first European settlers, arriving by ship; and today’s bustling crowds of residents, tourists and workers.
It’s hardly surprising that the Harbour (also known as Port Jackson) and its environs have seen dramatic changes, starting with the displacement of the original Aboriginal inhabitants, then a history of residential and commercial development, fisheries, shipping and trade, and all types of industry from salt production to whaling and chemical manufacture taking place on its shores.
Today, the iconic Harbour Bridge and Opera House dominate Australia’s recently declared 16th National Landscape, and 90% of the catchment foreshore has been fully developed.
Although it’s the busiest port in the country, what lies below the water’s surface is little known among Sydneysiders. We know Sydney Harbour has more species than any other harbour, but just how many is a question that intrigues marine scientists at the Australian Museum. So we decided to find out.
The first port of call for such a question is the Museum’s own collection database. It’s an invaluable scientific and historical baseline for investigating biodiversity, with records dating back to 1850. Each record relates to a specimen, and each specimen has a name with the date and location of its collection, among other useful information.
Using these data, we mapped the distributions of five major marine animal groups in the Harbour – fishes, crustaceans, polychaetes, molluscs and echinoderms – and examined the historical trends of their discovery.
In total, we have documented an astonishing 3000 marine species in the Harbour, which is two to three times greater than neighbouring Botany Bay, Port Hacking or the Hawkesbury River. And of course this figure doesn’t include the many birds (including penguins), marine mammals (whales, seals and dolphins), turtles and many invertebrate groups (especially sponges and corals) that also inhabit or visit its waters.
Molluscs broadly include slugs and snails, mussels and clams, and squids and octopuses. They range in size from tiny micro-molluscs of just a millimetre or two in size to much larger creatures, like the common Sydney Octopus, up to 60 centimetres in length. Like crustaceans, some molluscs live in all kinds of habitats including fresh, brackish and salty water.
Of the animals examined in the study, molluscs comprise the greatest number of species in Sydney Harbour, with 1339 species from 224 families recorded since 1860. Many new mollusc species were first described from specimens collected in and around the Harbour, and diversity is greatest in the northern and eastern (seaward-most) areas.
We found ‘peaks’ over time in the number of species documented that correspond to the focused research and collection activities of various Museum curators, most recently Drs Bill Rudman and Winston Ponder, or to the acquisition of large donations of molluscs (mainly shells) by amateur and professional collectors. However, the Museum currently has no researchers investigating marine shelled molluscs, despite this group being among the largest in the Museum's natural history collections.
Fishes are some of the most familiar and iconic animals of Sydney Harbour. Species such as the Common Sea-dragon and the Eastern Blue Groper (the fish emblem for New South Wales) are well known, but numerous cryptic species and vagrants such as tropical juveniles sometimes call the Harbour home.
Archaeological records show that people have fished the Harbour for thousands of years. In the two centuries since European settlement you might think that we would now know all about its fish fauna. As it turns out, we don’t. New records are still found and sometimes new species discovered.
Sydney Harbour has a rich fish fauna, with 586 species of fishes from 160 families recorded – more than the 540 species known to occur in the entire Mediterranean Sea. We expect this number will continue to climb.
The Museum’s earliest fish records date from 1878, when 15 specimens (representing 11 species) were collected. We have more than 4300 records of fishes in the Australian Museum database. Fishes are found in all areas of the Harbour but are most numerous in the eastern region.
Collecting has been spasmodic since 1878, with the occasional big jump in the number of species known, most recently in the 1970s and corresponding to the employment of two new Museum ichthyologists, Drs John Paxton and Doug Hoese. At this time, scuba diving became widely used and the newly employed researchers started programs of Harbour-focused research.
Crustaceans include the familiar Blue Swimmer, Mud Crab, Eastern Rock Lobster and School Prawn, as well as the less familiar but more numerous isopods and amphipods. So far, we’ve recorded 672 species of crustacean in 163 families in the Harbour. They live in all types of habitat – mangrove muds in the upper reaches, weedy wharf pilings, sandy beaches, and wave-washed boulders of the Harbour mouth. Overall, crustaceans can be very resilient in the face of changing water salinities. Most crustaceans live in the eastern Harbour where the water quality is best.
When the history of their discovery is considered, it is clear that the number of new species has grown in steps following the appointment of each new crustacean researcher. The current research scientists are Drs Shane Ahyong and Jim Lowry.
Echinoderms, which include sea urchins, sea-stars and sea cucumbers, can be very common on rocky reefs and among seaweeds. At least 118 species in 45 families of echinoderms live in the Harbour. Unlike the crustaceans, most echinoderms cannot cope with fluctuating salinity, so almost all records are from the eastern Harbour.
As with the Crustacea, the rate of discovery follows a stepwise pattern, with the largest jump in the late 1960s to 1970s following expansion of the Museum’s research staff, among them echinoderm specialist Dr Frank Rowe.
Polychaetes, or marine worms, range in size from less than one millimetre to a metre or more in length. Over 300 species are found in the Harbour, with the area east of the Harbour Bridge home to the greatest diversity. Like echinoderms, polychaetes prefer the more stable oceanic salinity of the eastern part of the Harbour. In addition, a greater number have been recorded from this region because it has been subjected to intensive sampling.
Worms occur in all of the Harbour’s numerous habitat types, but they specifically live within deep crevices and holes or as encrusting species on coral, rocks, pylons and boats. One of these, Hydroides elegans, is a common encrusting species that is a pest in many areas around the world, but we don’t yet know precisely where it originated. It might have been introduced accidentally to the Harbour, or it may have originated here and been carried to other parts of the world on the hulls of ships.
Our knowledge of this group has increased considerably since the appointment of Pat Hutchings in 1970. An intensive survey of the Harbour around the commercial shipping areas in 2001 to look for introduced species greatly increased our knowledge of worms and some other groups.
The history of discovery for each of the major groups reveals some interesting features. First, our knowledge of Harbour species has not grown evenly and gradually but in distinct jumps that happen whenever marine scientists join the Museum staff.
Second, after more than two centuries, we are still discovering new species in the Harbour, yet many areas of the Harbour have not been scientifically surveyed.
Third, the largest proportion of species occurs in the eastern region of the Harbour, probably because of the wider variety of habitats there as well as more stable salinities and better water quality due to massive tidal flushing.
We are now seeing that Sydney Harbour is remarkable, not only above the water line, but also below. Of the major city-ports around the world, Sydney’s is the most biologically diverse. As part of our natural heritage, we have a responsibility to ensure that this valuable legacy remains.
The reasons for such diversity are many, but primarily Nature has been good to us by providing relatively good water quality, thanks to the high volume of tidal flushing (the entire volume of the main Harbour is replaced every 20 days). Without this, the Harbour’s health would have been far more seriously impacted by all the changes that have occurred along its shores and in its catchments.
Related to water quality is the combination of many different habitat types, each supporting different animal communities in seagrass beds, mangrove swamps, high-energy exposed rocky reefs, sandy beaches, intertidal areas and sheltered bays. Other habitats are out of sight below the surface – unless you’re one of the many snorkellers and scuba divers who take to the Harbour each year to enjoy sights such as fluorescent corals, rare lobsters and, each summer, the many tropical reef fishes carried south by the East Australian Current.
Harbour health has improved significantly over the last three decades. Offshore deep ocean outfalls built in the late 1980s divert sewage away from the Harbour, and heavy industry along the foreshore has been removed, reducing the inflow of toxic wastes. But there is no room for complacency. The signature of past industrial pollution remains in the sediments in some parts of the upper Harbour such as Homebush Bay and Parramatta River and is found in the biota through the process of bioaccumulation, which is why fishing is still banned in these areas. Also, significant amounts of untreated urban runoff, loaded with sediment, pollutants and rubbish, continue to enter the Harbour, and the regular shipping traffic poses a constant threat of more invasive species arriving.
How do we ensure the Harbour survives and continues to flourish as a significant part of our natural heritage?
Despite the impressive results from the Museum’s database, no comprehensive marine survey of the Harbour has ever been undertaken, and many parts remain unexplored, including its darkest depths.
We need a sound knowledge of what species live in the Harbour, where they live and how they interact with each other. In this way, we can understand how the Harbour ecosystem works, and what affects the fauna, positively or negatively.
We also need to document how the fauna is changing. For example, are those tropical fishes and crustaceans coming into the Harbour in summer more likely to survive the winter if sea temperatures continue to rise, and what effect will this have?
We need to understand how our use or misuse of the Harbour affects the marine life. We also need to be far more careful about what is still finding its way into the Harbour, such as stormwater pollution and waste, and the potentially invasive species brought here by shipping.
Finally, we need to prevent the loss of the remaining natural shorelines and develop artificial walls and marinas that are more fauna friendly. To best manage the asset that the Harbour is, we need to know more about it. Just as the Museum database has provided these insights into what we know, it also highlights the gaps that we believe should be the target of further investigation. Imagine what we might find in a major, planned survey of the Harbour …
Dr Shane Ahyong, Senior Research Scientist; Dr Pat Hutchings, Senior Principal Research Scientist; Dr Mick Ashcroft, Spatial Analyst; Mark McGrouther, Collection Manager (Fishes); Dr Amanda Reid, Collection Manager (Molluscs).
First published in Explore 35(3), summer 2013.
Minor edits made 4 December 2013.
Download a pdf of this story here.
PA Hutchings, ST Ahyong, MB Ashcroft, MA McGrouther and AL Reid, 2013. Sydney Harbour: its diverse biodiversity. Australian Zoologist 36(3): 255–320, dx.doi.org/10.7882/AZ.2012.031