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Middle Holocene Oyster Shells and the Shifting Role of History in Ecological Restoration

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At Western Australia’s Swan-Canning Estuary, extensive subfossil shellfish assemblages of Middle Holocene origin were largely destroyed through dredging for cement production in the first half of the twentieth century. Joseph Christensen discusses an extractive industry driving shellfish ecosystem decline builds on existing historical studies of commercial over-harvesting of oysters

Image courtesy of interviewee. December 11, 2023

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This video video discusses the ecological history and restoration efforts of shellfish ecosystems in the Swan Canning Estuary, located in Perth, Western Australia. The estuary serves as a central waterway for the city, which has a population of over 2.2 million. Since European colonization, the environment has undergone significant transformation, particularly affecting shellfish populations.

The speaker explains that their research aims to inform an ecological restoration initiative led by the Nature Conservancy Australia, focused on rehabilitating degraded shellfish ecosystems. Unlike other Australian regions where a history of overharvesting by settlers contrasts with sustainable Indigenous practices, the local Aboriginal groups, particularly the Noongar people, have not historically harvested shellfish from this estuary due to cultural reasons. As such, the ecological history here is distinct; the shellfish ecosystems observed by early settlers were remnants from an earlier geological time and not currently thriving.

The speaker emphasizes the challenges of restoration in a heavily altered environment, mentioning that typical restoration approaches may not apply due to changes in the ecosystem's character. Instead of simply reverting to historical conditions, efforts must creatively incorporate historical insights to establish functional ecosystems that can thrive in today's context. For example, the restoration program is focusing on establishing blue mussel reefs instead of the native oyster species.

The restoration initiative acknowledges the necessity of understanding the historical extent of shellfish ecological losses and learning from successful restoration examples, like those from Chesapeake Bay in the United States. The objective is to identify reference points for how past ecosystems functioned and to inform future ecological strategies, thereby ensuring the restoration efforts are relevant and effective in today's environment. Overall, the speaker highlights the importance of integrating cultural understanding and historical context into ecological restoration practices.

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X-TIMESTAMP-MAP:LOCAL=00:00:00.000,MPEGTS=900000

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This one canning street.

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It's the main waterway in the city of Perth,

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the capital city of Western Australia.

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And a, a waterway that's today surrounded

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by a very large city.

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So Perth has a population of in excess

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of 2.2 million people.

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Uh, the city is, is sort of, are built

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around the esry and the surrounding, coastal plain.

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Uh, the entrance to the estuary is, the Port

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of Fremantle, the main port for Western Australia.

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So, over here, it, it's a very familiar landmark,

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a very popular, destination for recreation,

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but also an environment that's been heavily transformed

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since European colonization, over the course

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of almost 200 years now.

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And that's really the context for the work that I've done,

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which is to, investigate the history

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of shellfish ecosystems in the estuary, essentially

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to inform a major ecological restoration initiative,

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which has been undertaken by the Nature Conservancy,

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which is of course, a global organization.

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This is, being undertaken by their Australian arm,

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and it's a part of a, a a a, a quite major national program

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of shellfish ecosystem restoration, where right

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around the country and often in,

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major capital cities.

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So Sydney, with Port Jackson, Melbourne,

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with Port Phillip Bay,

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and other parts of, well of, of Australia,

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the Nature Conservancy setting about

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to rehabilitate shellfish ecosystems

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that have been degraded,

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if not altogether destroyed over the past 200 years

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of European settlement.

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My case study is a little bit unusual in

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other parts of Australia.

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The history is fairly straightforward.

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You have a, period

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of often quite intense commercial harvesting of oysters

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because,

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the predominantly British settler population had an

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appetite for oysters.

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Uh, they're very, very popular,

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food in the 19th and 20th centuries.

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And, and, and, and wherever they were found, they were,

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they were over harvested for human consumption.

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And that drives quite significant declines

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of shellfish ecosystems, disease outbreaks.

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Um, and this is something which is also supported or, or,

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or, or, or fostered by commercial oyster farming is a

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secondary factor driving that decline that will contrast

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with evidence of quite long-term sustainable indigenous

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harvest in these same areas.

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So again, places like Port Phillip Bay, where the city

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of Melbourne is today or, or,

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or Port Jackson for Sydney, there will be the mid deposits,

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so discarded oysters that have been consumed

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by local indigenous groups.

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Um, and you can find in many, many different examples

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of these,

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and they can be analyzed, by archeologists

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and paleo ecologist,

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and it will essentially point to sustainable long-term

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Harvest. So oysters

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being fished on a relatively small scale

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by local indigenous groups,

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and this not having any significant impact

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on the population itself.

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That's a story in other parts of Australia,

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and a pattern that you'll also see repeated in other parts

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of the world over on, the western Australian coast.

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That, that at the s swung, canning esry, it's

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actually very different.

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Firstly, there is no archeological

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or eo ecological evidence of indigenous harvest,

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and that's essentially for cultural reasons.

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Local aboriginal groups that are,

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are the noar people in particular did not

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consume shellfish from the estuary.

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And that's essentially for cultural reasons, something

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that I don't know that much about.

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We didn't look into this in great deal,

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in great detail in our research,

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but that's, that's certainly the case.

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And then what really sets the case study apart is

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that the shellfish ecosystems that were encountered

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by the British when they settled, are,

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are from the late 1820s on, were not actually living

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shellfish ecosystems.

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So they had formed during the, the early

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to middle Holocene, at a time when the,

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esry environment was actually quite different.

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So sea levels were slightly higher.

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Uh, the rainfall in the southwest part

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of Western Australia was slightly lower,

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and the estuary compared

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to historical times had a more marine character.

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So in, in, in, in this, so, so more like an inlet

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or bay of the ocean in this environment,

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quite extensive anazi oyster, ecosystems flourished.

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Um, and then gradually died out as the sea levels drop

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and the winter rainfall increased,

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and the esry took on a more ester character.

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So as a historical case study, in a historical case study

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that's designed to inform A-A-A-A-A, a significant

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ecological restoration initiative, it's, it, it,

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it's really quite different.

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Um, there are overlapping timescales here.

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Um, so we've got a, a, you know, a short term history

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of European settlement and colonization.

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Um, and, and during this period,

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these historic shellfish deposits are actually destroyed

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quite systematically, but,

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but obviously not for human consumption

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as they would be elsewhere in other parts of Australia,

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but for the manufacturer of cement oyster

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to show us being a, a, a, a, a rich source of lime.

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Um, but then there, there is this, I guess,

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there's, there's no point of comparison

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as there is in other parts of the country between the, the,

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the, the, the, the, the over harvesting

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of European colonizers

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and the sustainable harvest of indigenous groups.

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So it's, it, it, it's really quite different

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and we've tried to explore that, unpack it

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as it were, and then apply that

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to inform the ecological restoration program.

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When you look at other parts of the country,

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and this is, it's perhaps a little bit simplistic,

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but I think it's broadly true, when you've got,

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on the one hand, a long term history

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of sustainable indigenous harvesting,

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and then a very short term

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and destructive history of European

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or colonial overconsumption, you can, you can s you can seek

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and set out to firstly document, what has been lost

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through the, the records of, of, of, of the commercial,

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over harvest and then set about to restore the lost

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or degraded ecosystems.

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So, you know, we began at point A over a period of decades,

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we have breached point B of a degraded

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or destroyed ecosystem, and we want to get back to point A,

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and that would be a very straightforward

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and more traditional application of historical knowledge,

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in a biological ecological restoration initiative.

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Of course, we don't have that in the swan canning.

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Actually, what we have is actually something

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that's more comparable to, the work.

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And it's, it, it, it's really quite extensive work

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that's been done at Chesapeake Bay on the east coast

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of America where,

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and this is really one of the, one of the original sites

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of oyster reef restoration programs.

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And, and, and the work there has been going on

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for a very long period of time,

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but in a context where the environment itself is so heavily,

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heavily modified, so heavily transformed

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that simply resetting the clock is not really an option.

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You're not going to get back to the abundance

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that was observed by early European explorers,

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in the 16th and 17th centuries.

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That's simply not an option.

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And it's not really an option at the Swan River either,

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because, as I've said, I mean the, the, the, the, the,

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the, the, the, the ecosystems that were first encountered,

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the shellfish deposits that were first encountered

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by early European explorers had actually formed

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during an early geological era.

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Um, so in that sense, we, we, we have

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to think a little bit more creatively about

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how we actually apply historical insight and information,

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and we can look not to other Australian case studies,

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but to examples like Chesapeake Bay for those insights.

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So it's essentially around, identifying reference points

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for how past ecosystems may have functioned, using that

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to better understand how re re restored

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or rehabilitated ecosystems,

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albeit in a highly altered environment might

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function in the future.

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And using that to clarify the objectives,

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and the strategy in ecological restoration.

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And of course, that's something that,

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that the scientists will then set about implementing,

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but it's, it, it, it's really about, as I said,

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moving away from a, from a more simple process

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of resetting the clock, restoring what is being lost,

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and using historical insight to better formulate strategies

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for, restoring functional ecosystems

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and ma maximizing ecological

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benefits from healthy ecosystems in the context

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of the 21st century, the work

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of the Nature Conservancy is ongoing.

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Um, and they're obviously working, as I've said,

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in a major urban estuary.

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So they have to deal with all the constraints

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that would apply, in such a location,

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a major urban waterway, a competing demand

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for transportation, for recreation,

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various sanctuary zones across EST history as well.

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So they have to work within all those parameters.

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Um, they've, they've, they've, they've set about

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to restore functional shellfish ecosystems.

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They're restoring, or,

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or their program is based on establishing reefs

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of a blue mussel species as opposed to the

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anazi oyster species that we would see, as the basis

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of the, the preexisting oyster,

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deposits in the estuary.

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So, and, and, and, and, and this reflects

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what I was saying in terms of using the historical insight

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and the historical knowledge to inform, the strategy

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and the targets in ecological restoration.

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Um, in terms of the historical insight,

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I think we've been able to, to, to, to, to,

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to highlight a few key points.

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First is that not withstanding the differences

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of our case study, there are still very enduring cultural

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ties, between the,

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the local environment and the community.

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Um, so we've been able to draw those out in our case study,

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and, and,

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and in particular by looking at the way

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that understanding knowledge about the local environment

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and what actually caused these, these,

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these pre-existing oyster deposits to die out, how, how, how

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that understanding amongst the colonial population was

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informed by knowledge transfer from

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the indigenous population.

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Um, so this, this would, this would point

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to those very enduring cultural ties

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that I've been speaking about.

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Uh, so that's one, one benefit of the historical

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research that's been done.

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A second, was to go to the records

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of the company that actually dredged up all these historic

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oyster deposits, to manufacture cement.

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And they were actually, very careful

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and it's, it's a reflection of the,

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I guess the policies they were operating under

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where they had to pay, they had

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to pay duty on the volume of shell

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that they dredged up from the,

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from the estuary to the government.

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So the records are actually quite exact,

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and the maps, that they produced to

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identify the different deposits in the Esry

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and where they would like to dredge next are quite detailed.

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And we were able to use these to essentially using, using,

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GIS technology to recreate the extent

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of these historical deposits,

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and that then provides a point of comparison for

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what the Nature Conservancy is doing.

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So, we can, we can show just

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how extensive these historic assemblages

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were bearing in mind.

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As I said, they weren't living ecosystems,

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so the oysters in them weren't filtering water, which

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of course is one of the main ecological benefits

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of shellfish ecosystems,

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but they were providing all the other environmental

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benefits, particularly habitat,

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for fish and crustaceans.

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Um, so in that sense, they were, they were a, a, a,

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a cornerstone of the Eine environment,

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and they were really quite extensive.

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And it, it, it, it, when we understand the full extent of

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what is being lost, we begin to appreciate the full extent

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of ecological degradation in the estuary across

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the course of the 20th century.

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And that, of course, becomes a very powerful argument for

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what the nature uncertainty is doing today

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to begin the process of restoring these loss

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and degraded ecosystems.

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It's really about moving away from a more simplistic view

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of the past, looking at environments as, as,

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just a more deletion as process of decline, usually,

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you know, brought about

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by human activity in the relatively recent path,

12:51.075 --> 12:52.495
the 19th and 20th century.

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Um, and that's, that's something which is particularly

12:55.935 --> 12:57.895
relevant in a settler society context

12:58.005 --> 12:59.135
like Australia, of course.

12:59.715 --> 13:01.415
Um, and, and, and, and,

13:01.415 --> 13:04.175
and that of course would be feeding into the notion

13:04.175 --> 13:08.255
that we can begin to restore,

13:08.915 --> 13:10.415
the what is being lost.

13:10.475 --> 13:11.895
We can reset the clock,

13:12.155 --> 13:15.175
but of course we can't, we can't really do that in

13:15.195 --> 13:17.135
so many different locations

13:17.205 --> 13:18.495
because the fully extent

13:18.495 --> 13:20.295
of human impacts on particularly marine, Marine

13:20.295 --> 13:22.295
and coastal environments does not allow for that.

13:22.355 --> 13:25.335
So, you know, habitat modification, the fact that, you know,

13:25.335 --> 13:29.375
with climate change, there are, other,

13:29.375 --> 13:32.375
impacts that won't allow, you know,

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the full scale water has being lost to be recovered.

13:35.235 --> 13:37.295
It might be sea level rises,

13:37.555 --> 13:39.375
whatever the case may be.

13:39.515 --> 13:42.695
So, so, so, so moving, moving away from

13:42.695 --> 13:44.815
that more simplistic notion of

13:44.895 --> 13:47.295
what a historical baseline looks like

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and using an understanding of the past to inform

13:53.375 --> 13:55.935
strategy in ecological restoration.

13:56.235 --> 13:57.935
So what does, what does a healthy

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ecosystem actually look like?

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What can we then hope to recreate in a context

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of often a highly altered, marine environment?

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Um, so, me

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using, using the historical understanding

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to move away from that, that more simple notion of,

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a baseline to providing reference points for,

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for past ecosystems to better understand

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how altered ecosystems might work in the future,

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and to guide decision making strategy

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and targets in ecological restoration.

00:00 - This one canning street. It's the main waterway in the city of Perth, the capital city of Western Australia. And a, a waterway that's today surrounded by a very large city. So Perth has a population of in excess

00:12 - of 2.2 million people. the city is, is sort of, are built around the esry and the surrounding, coastal plain. the entrance to the estuary is, the Port of Fremantle, the main port for Western Australia. So, over here, it, it's a very familiar landmark, a very popular, destination for recreation, but also an environment that's been heavily transformed since European colonization, over the course of almost 200 years now.

00:44 - And that's really the context for the work that I've done, which is to, investigate the history of shellfish ecosystems in the estuary, essentially to inform a major ecological restoration initiative, which has been undertaken by the Nature Conservancy, which is of course, a global organization. This is, being undertaken by their Australian arm, and it's a part of a, a a a, a quite major national program of shellfish ecosystem restoration, where right around the country and often in,

01:21 - major capital cities. So Sydney, with Port Jackson, Melbourne, with Port Phillip Bay, and other parts of, well of, of Australia, the Nature Conservancy setting about to rehabilitate shellfish ecosystems that have been degraded, if not altogether destroyed over the past 200 years of European settlement. My case study is a little bit unusual in

01:44 - other parts of Australia. The history is fairly straightforward. You have a, period of often quite intense commercial harvesting of oysters because, the predominantly British settler population had an appetite for oysters. they're very, very popular, food in the 19th and 20th centuries. And, and, and, and wherever they were found, they were,

02:09 - they were over harvested for human consumption. And that drives quite significant declines of shellfish ecosystems, disease outbreaks. and this is something which is also supported or, or, or, or, or fostered by commercial oyster farming is a secondary factor driving that decline that will contrast with evidence of quite long-term sustainable indigenous harvest in these same areas. So again, places like Port Phillip Bay, where the city of Melbourne is today or, or,

02:40 - or Port Jackson for Sydney, there will be the mid deposits, so discarded oysters that have been consumed by local indigenous groups. and you can find in many, many different examples of these, and they can be analyzed, by archeologists and paleo ecologist, and it will essentially point to sustainable long-term Harvest. So oysters being fished on a relatively small scale

03:01 - by local indigenous groups, and this not having any significant impact on the population itself. That's a story in other parts of Australia, and a pattern that you'll also see repeated in other parts of the world over on, the western Australian coast. That, that at the s swung, canning esry, it's actually very different. Firstly, there is no archeological or eo ecological evidence of indigenous harvest,

03:24 - and that's essentially for cultural reasons. Local aboriginal groups that are, are the noar people in particular did not consume shellfish from the estuary. And that's essentially for cultural reasons, something that I don't know that much about. We didn't look into this in great deal, in great detail in our research, but that's, that's certainly the case. And then what really sets the case study apart is

03:45 - that the shellfish ecosystems that were encountered by the British when they settled, are, are from the late 1820s on, were not actually living shellfish ecosystems. So they had formed during the, the early to middle Holocene, at a time when the, esry environment was actually quite different. So sea levels were slightly higher. the rainfall in the southwest part of Western Australia was slightly lower,

04:13 - and the estuary compared to historical times had a more marine character. So in, in, in, in this, so, so more like an inlet or bay of the ocean in this environment, quite extensive anazi oyster, ecosystems flourished. and then gradually died out as the sea levels drop and the winter rainfall increased, and the esry took on a more ester character. So as a historical case study, in a historical case study that's designed to inform A-A-A-A-A, a significant

04:45 - ecological restoration initiative, it's, it, it, it's really quite different. there are overlapping timescales here. so we've got a, a, you know, a short term history of European settlement and colonization. and, and during this period, these historic shellfish deposits are actually destroyed quite systematically, but, but obviously not for human consumption as they would be elsewhere in other parts of Australia,

05:08 - but for the manufacturer of cement oyster to show us being a, a, a, a, a rich source of lime. but then there, there is this, I guess, there's, there's no point of comparison as there is in other parts of the country between the, the, the, the, the, the, the over harvesting of European colonizers and the sustainable harvest of indigenous groups. So it's, it, it, it's really quite different and we've tried to explore that, unpack it

05:39 - as it were, and then apply that to inform the ecological restoration program. When you look at other parts of the country, and this is, it's perhaps a little bit simplistic, but I think it's broadly true, when you've got, on the one hand, a long term history of sustainable indigenous harvesting, and then a very short term and destructive history of European or colonial overconsumption, you can, you can s you can seek

06:04 - and set out to firstly document, what has been lost through the, the records of, of, of, of the commercial, over harvest and then set about to restore the lost or degraded ecosystems. So, you know, we began at point A over a period of decades, we have breached point B of a degraded or destroyed ecosystem, and we want to get back to point A, and that would be a very straightforward and more traditional application of historical knowledge, in a biological ecological restoration initiative.

06:39 - Of course, we don't have that in the swan canning. Actually, what we have is actually something that's more comparable to, the work. And it's, it, it, it's really quite extensive work that's been done at Chesapeake Bay on the east coast of America where, and this is really one of the, one of the original sites of oyster reef restoration programs. And, and, and the work there has been going on for a very long period of time,

07:03 - but in a context where the environment itself is so heavily, heavily modified, so heavily transformed that simply resetting the clock is not really an option. You're not going to get back to the abundance that was observed by early European explorers, in the 16th and 17th centuries. That's simply not an option. And it's not really an option at the Swan River either, because, as I've said, I mean the, the, the, the, the, the, the, the, the ecosystems that were first encountered,

07:33 - the shellfish deposits that were first encountered by early European explorers had actually formed during an early geological era. so in that sense, we, we, we have to think a little bit more creatively about how we actually apply historical insight and information, and we can look not to other Australian case studies, but to examples like Chesapeake Bay for those insights. So it's essentially around, identifying reference points for how past ecosystems may have functioned, using that

08:08 - to better understand how re re restored or rehabilitated ecosystems, albeit in a highly altered environment might function in the future. And using that to clarify the objectives, and the strategy in ecological restoration. And of course, that's something that, that the scientists will then set about implementing, but it's, it, it, it's really about, as I said, moving away from a, from a more simple process

08:34 - of resetting the clock, restoring what is being lost, and using historical insight to better formulate strategies for, restoring functional ecosystems and ma maximizing ecological benefits from healthy ecosystems in the context of the 21st century, the work of the Nature Conservancy is ongoing. and they're obviously working, as I've said, in a major urban estuary. So they have to deal with all the constraints

09:03 - that would apply, in such a location, a major urban waterway, a competing demand for transportation, for recreation, various sanctuary zones across EST history as well. So they have to work within all those parameters. they've, they've, they've, they've set about to restore functional shellfish ecosystems. They're restoring, or, or their program is based on establishing reefs of a blue mussel species as opposed to the

09:32 - anazi oyster species that we would see, as the basis of the, the preexisting oyster, deposits in the estuary. So, and, and, and, and, and this reflects what I was saying in terms of using the historical insight and the historical knowledge to inform, the strategy and the targets in ecological restoration. in terms of the historical insight, I think we've been able to, to, to, to, to, to highlight a few key points.

10:02 - First is that not withstanding the differences of our case study, there are still very enduring cultural ties, between the, the local environment and the community. so we've been able to draw those out in our case study, and, and, and in particular by looking at the way that understanding knowledge about the local environment and what actually caused these, these, these pre-existing oyster deposits to die out, how, how, how

10:31 - that understanding amongst the colonial population was informed by knowledge transfer from the indigenous population. so this, this would, this would point to those very enduring cultural ties that I've been speaking about. so that's one, one benefit of the historical research that's been done. A second, was to go to the records of the company that actually dredged up all these historic

10:56 - oyster deposits, to manufacture cement. And they were actually, very careful and it's, it's a reflection of the, I guess the policies they were operating under where they had to pay, they had to pay duty on the volume of shell that they dredged up from the, from the estuary to the government. So the records are actually quite exact, and the maps, that they produced to

11:22 - identify the different deposits in the Esry and where they would like to dredge next are quite detailed. And we were able to use these to essentially using, using, GIS technology to recreate the extent of these historical deposits, and that then provides a point of comparison for what the Nature Conservancy is doing. So, we can, we can show just how extensive these historic assemblages were bearing in mind.

11:48 - As I said, they weren't living ecosystems, so the oysters in them weren't filtering water, which of course is one of the main ecological benefits of shellfish ecosystems, but they were providing all the other environmental benefits, particularly habitat, for fish and crustaceans. so in that sense, they were, they were a, a, a, a cornerstone of the Eine environment, and they were really quite extensive.

12:10 - And it, it, it, it, when we understand the full extent of what is being lost, we begin to appreciate the full extent of ecological degradation in the estuary across the course of the 20th century. And that, of course, becomes a very powerful argument for what the nature uncertainty is doing today to begin the process of restoring these loss and degraded ecosystems. It's really about moving away from a more simplistic view of the past, looking at environments as, as,

12:41 - just a more deletion as process of decline, usually, you know, brought about by human activity in the relatively recent path, the 19th and 20th century. and that's, that's something which is particularly relevant in a settler society context like Australia, of course. and, and, and, and, and that of course would be feeding into the notion that we can begin to restore,

13:08 - the what is being lost. We can reset the clock, but of course we can't, we can't really do that in so many different locations because the fully extent of human impacts on particularly marine, Marine and coastal environments does not allow for that. So, you know, habitat modification, the fact that, you know, with climate change, there are, other, impacts that won't allow, you know,

13:32 - the full scale water has being lost to be recovered. It might be sea level rises, whatever the case may be. So, so, so, so moving, moving away from that more simplistic notion of what a historical baseline looks like and using an understanding of the past to inform strategy in ecological restoration. So what does, what does a healthy ecosystem actually look like?

13:59 - What can we then hope to recreate in a context of often a highly altered, marine environment? so, me using, using the historical understanding to move away from that, that more simple notion of, a baseline to providing reference points for, for past ecosystems to better understand how altered ecosystems might work in the future, and to guide decision making strategy and targets in ecological restoration.

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