Australia: The Land Where Time Began

A biography of the Australian continent 

Aboriginal Stone Walled Intertidal Fishtraps – Morphology, Function and Chronology

Around the coastline of Australia there are stone-walled intertidal fish traps that are among the largest structures that were built by Australian Aboriginal people. Globally, fishtraps are considered to be important elements of the production of food, domestication, territoriality, and ceremonial landscapes, but there is highly variable viewable level of detail in their documentation and there is scarce scholarly knowledge of these structures. There is a lack of detail and reproducibility of recording, which hinders the comparative analysis of their morphology, function and chronology. Kreij et al. used a high-resolution, close-range aerial vehicle (UAV) [aka drone] photogrammetry and a suite of spatial information analytical techniques in this study to investigate the stone-walled fish traps of the Kaiadilt Aboriginal people, Sweers Island, southern Gulf of Carpentaria, Australia. Modelling of tidal inundation was undertaken to assess:

1)    Working range of fish traps,

2)    Individual and simultaneous function of fishtraps,

3)    Seasonal fluctuation, and

4)    Chronology that was based on the history of sea level.

There are 13 fishtraps that were identified in the study area, which ranged from 38 m long to 287 m long. It was shown by inundation modelling that all fishtraps operate most efficiently at sea level of the present mean sea level (PMSL), which indicates that they were built in the last 3,500 years. An opportunity to improve approaches to the recording of large-scale stone features and standardise documentation of stone-walled intertidal fish trap sites is provided by quantitative recording techniques, analytical procedures and terminology that were developed in this study.

In the Australian archaeological record stone-walled intertidal fishtraps are among the largest structures that have been documented. Fishtraps, which were constructed with rock and/or organic matter, are argued to be designed primarily to trap or control the movements of marine resources across tidal cycles in coastal or riverine contexts (Campbell, 1982; Dortch et al., 2006; Jeffery, 2013; Rowland & Ulm, 2011). Stone-walled intertidal fishtraps are defined for the purposes of this study as structures that can control the movements of marine animals.

As structures testifying to local subsistence, the organisation of labour, occupation, and social strategies, fishtraps have been cited as features of early domestication (Codding & Bird, 2015; Smith, 2014; Zeder, 2015), anthropogenic niche construction (Lepofsky & Caldwell, 2013; Lourandos, 1980; Smith, 2014, Zeder, 2015), and mid- to Late-Holocene economic social intensification (Lourandos, 1980, 1983; McNiven et al., 2012, 2015). A variety of approaches to the recording of sites have been led to by physical and conceptual challenges of characterising fishtraps in spite of interest in fields such as archaeology, evolutionary biology, and human behavioural ecology. Resulting from this researchers often adopt vague definitions of fishtraps and terminology (Bannerman & Jones, 1999; Jeffery, 2013; Ross, 2009; Rowland & Ulm, 2011, and fundamental questions concerning the construction and function of fishtraps remain to be addressed(Caldwell et al., 2012; Elder et al., 2014; Moss et al., 1998).

Access is often restricted and is dependant of movements of tides; as a result of the location of fishtraps in intertidal and riverine settings, and in some parts of the world the work of field workers can be made hazardous by the presence of marine predators. Tides also control the times of visibility and recording time, and are restricted further by wind which causes swell and sediment that obscures structures. As well as such environmental factors, there are impacts of recreational vessels, which cause intertidal stone features to be eroded partially or completely, which underlies the urgency of recording the fishtrap structures that remain (Elder et al., 2014; Memmott et al., 2008; Roberts et al., 2016; Rowland & Ulm, 2011; Rowland et al., 2014). In spite of the urgency of documentation, and a global interest in fishtrap construction (Greene et al, 2015; Jeffry, 2013), most recordings consist of basic sketch maps of limited detail, with a few quantitative data or photographic records (see Coutts et al., 1978; Greene et al., 2015; Koivisto et al., 2018; Langouët and Daire, 2009; McNiven et al., 2012; O’Sullivan, 2004). There has been a proliferation of terms that describe attributes of fishtraps as a result of varied approaches to the recording of sites, which leads to challenges for the management of sites, comparisons between sites, and the ability for fishtraps to be considered in meaningful debates. The focus of this study was intertidal stone-walled fishtraps, and proposes a standard high-resolution recording scheme for intertidal stone features on a large scale, to improve knowledge of the construction, function and age of fishtraps.

Discussion

It was demonstrated by the GIS-generate sea level scenarios that the stone-walled structures of Ngathald and Kabar Bays would not have had effective ranges earlier than about 3,500 BP, which, according to Kreij et al., correlates with the earliest dates of occupation of Sweers Island and Bentinck Island, which date to about 3,500 BP (Memmott et al., 2016). There was a continuous signal of occupation from about 2,000 cal. BP in the archipelago, and ethnographic records that outline the need for fishtrap maintenance that is on-going for optimal function (e.g. Tindale, 1960), it can be concluded that the fishtraps were constructed within the last 2,000 years. With the current placement and a consistent height of fishtraps below 1 m, the traps of Ngathald Bay and Kabar Bay would have functioned with tides of 0.5 m ± PMSL. With a fishtrap range that was relatively narrow, sea level shifts of ±0.5 would have an impact that was significant on the functioning of the trap. It will become possible to further refine construction chronologies as regional and local sea level curves become available. It was acknowledged that the height of a trap may be decreased over time; however it cannot be demonstrated that such erosion has occurred without monitoring of morphology in the long term, this study was based analysis on current properties of the structure.

Based on fishtrap construction findings, and interaction with past and current tidal regimes, high resolution documentation of fishtrap metrics (length, height and width) were recommended by Kreij et al., particularly elevation in relation to a national height datum. Photogrammetric UAV recording proved particularly useful for documenting features in the intertidal zone, while the method and majority of the analytical procedure can be applied to other archaeological site types. Earlier documentation of fishtraps has varied in detail, and there is limited cross-site comparison due to ambiguous descriptions. Quantitative analysis of geometric properties was enabled by GIS assessment of the physical features of fishtraps, and formed a basis for robust comparative studies. The risk of misidentification is minimised and the potential for comparison of sites and modelling is maximised, by describing the structures through geometric variables. It is recommended to move towards a standardised objective industry practice, involving simple, though inclusive terms, such as fishtraps, which represent structures, either natural or constructed, that enclosed bodies of water, described by metric properties. Geomorphological studies of the environmental impacts on local ecosystems within the structures will assist in gaining an understanding of local resource management, and impacts on the wider intertidal zone of potential erosion and sedimentation. Site-specific cultural heritage management plans for stone-walled features in intertidal environments of low energy can be contributed to by such recent and historical information, of structural function and impact on the intertidal environments. Indictors of concepts, such as domestication, cultural niche constructions, and potential aquaculture paradigms, will be furthered by evidence of local marine resource technologies, as well as their impacts.

Conclusion

In this study 13 Kaiadilt fishtraps were identified by the use of a dataset that was highly accurate that had been obtained by high-resolution UAV photogrammetric mapping of Ngathald Bay and Kabar Bay, Sweers Island, where it was determined that the local topography was determined to be the dominant consideration of the placement and construction of fishtraps. Operation of fishtraps throughout the year was effectively enabled by the intertidal locations, and the stone-walled fishtraps are placed for optimal use during PMSL. In times of increased sea levels the structures can be effectively used during low tide up to +1 m above PMSL, which indicates, in combination with archaeological records, a date for construction at some point in the last 2,000 years. In this study the basis for further refinement of the chronology of construction is provided by the UAV documentation technique and GIS analysis that is presented, when regional sea level histories of higher-resolution are available for the Late Holocene. Considering the working range of fishtraps at times of past sea levels is the most recent, viable option to date stone-walled fishtraps that are located in the intertidal zone of the present. Future impact of sea level rise on fishtraps can also be modelled for applications of cultural heritage management. Management plans for preservation of this type of site can benefit further from aligning recording methods and terminology to a standardised quantitative technique, and can contribute to a broader cross-disciplinary understanding of stone-walled intertidal fishtraps.

Sources & Further reading

1. Kreij, A., et al. (2018). "Aboriginal stone-walled intertidal fishtrap morphology, function and chronology investigated with high-resolution close-range Unmanned Aerial Vehicle photogrammetry." Journal of Archaeological Science 96: 148-161.