Posts Tagged ‘midden analysis’

Identifying and Explaining Intensification in Prehistoric Fishing Practices XII: Specific Hypotheses to Explain Variation in Net-Use

November 28, 2009

The previous post in this series presented some high-level theory that might account for variation in fishing intensification and, thus, net use. This theory will now be tailored for my study area to generate some specific expectations. As noted elsewhere, I will not be dwelling on the details of the site and study area except as necessary to explain how I arrived at particular predictions.

The archeological assemblages that I have been analyzing derive from a single shell midden site. The analyzed deposit ranges from 15 to 55 centimeters below the ground surface. The site was excavated in five-centimeter arbitrary levels. I have treated each arbitrary level as a distinct analytic unit, which seems reasonable as spatial analysis of radiocarbon dates and other chronologically-diagnostic artifacts show very little vertical mixing or movement of artifacts. Poor environmental conditions appear to have occurred during the time period in which three levels–located between 35 and 50 centimeters below the ground surface–were deposited.

The period is characterized by widespread drought. These conditions disrupted settlement at many other sites. My site is one of the few sites in the region to have been continuously occupied during the period. The site lies near the mouth of a creek, an important source of fresh water.

Marine productivity may also have declined during the period of poor environmental conditions. Paleoenvironmental data regarding ocean conditions are complex and not entirely consistent. Proxy data derived directly from archaeological sites within the region, however, shows that sea surface temperatures during the period of drought were unusually high. These conditions may have affected the distribution and abundance of fish.

A variety of social and economic responses to the challenges of the period of poor environmental conditions have been documented. Economic specialization in artifact production emerged at my site and across the region. Local manufacturers produced trade goods. In exchange for these goods, these specialists presumably received food and other items that could not be produced locally as easily. Fishers at my site may have responded by changing their fishing strategies. The number of fish caught by the site’s inhabitants seems to peak during the interval of poor environmental conditions before declining.

This observation is consistent with other faunal analyses of the site’s deposits, but it could be attributable to a number of different factors. The peak in density of fish remains could be due to an increase in population at the site during the period of poor environmental conditions. The site may have served as a refuge for groups from elsewhere in this period, since fresh water was more readily available at the site. The increase in the density of fish remains could reflect a more widespread emphasis on fishing by the site’s inhabitants as other foods normally taken by them became less abundant. It could be attributable to increased economic specialization. Fish may have subsidized on-site specialized artifact production. Workers at the site may have specialized in both artifact production and fish procurement, as the local inhabitants had comparative advantages in these activities and exchanged their wares and fish for other goods. The greater density of fish could also be due to some quirk of cultural transmission, as fishers made choices about the appropriate gear to use and effort to undertake based on the work being done by their neighbors. A more detailed examination of the data will allow these possibilities to be distinguished.

© Scott Pletka and Mathematical Tools, Archaeological Problems, 2009.

Identifying and Explaining Intensification in Prehistoric Fishing Practices V: Quantifying the Relationship between Fish Size and Fish Bone Size

September 19, 2009

The previous post in this series established that a positive relationship exists between the live weight of fish and caudal vertebra height, providing support for the use of vertebra size as an index of overall fish size. I will attempt to quantify this relationship more precisely. Many different models could be chosen, but how should we select the most appropriate one?

The model needs to be appropriate for the structure of the data. The following graph shows a plot of the data with a linear model superimposed over the data points. The graph also depicts the deviation between data and the model with vertical lines. Notice that these deviations seem to get larger as the size of the fish gets larger. Linear models assume, among other things, that the variation remains constant. A linear model may not be appropriate.

Fish Live Weight and Vertebra Height Scatterplot with Linear Model

Fish Live Weight and Vertebra Height Scatterplot with Linear Model

A transformation of the data may help. Taking the log of both the live weight and the vertebra height produces more consistent variation. The next graph shows a linear model applied to the log of the data.

Log Transform of Live Fish Weight and Vertebra Height

Log Transform of Live Fish Weight and Vertebra Height

The deviations from the model are much more consistent. This model now seems reasonably appropriate to the structure of the transformed data in the sense that it doesn’t appear to violate the model assumptions. Those assumptions include normally-distributed variation and constant variation. Ideally, however, I’d like to fit a model that has an easier interpretation. Is there any theoretical basis for applying a particular type of model to fit to this data? As it turns out, the answer is “yes”, and I will discuss this model in the next post in the series.

© Scott Pletka and Mathematical Tools, Archaeological Problems, 2009.

Identifying and Explaining Intensification in Prehistoric Fishing Practices IV: The Relationship between Fish Size and Fish Bone Size

September 18, 2009

The previous post argued that size-frequency distributions of fish reflect the gear used to capture those fish. Middle-level theory, however, must also verify that the size of fish bone reflects live fish size, since archeological deposits will only contain the bones and not whole fish. In general, the size of particular animal parts reflects overall body size.

Previous studies of a number of species have demonstrated a strong, positive relationship between vertebra size and fish size, allowing inferences to be made regarding the original size of a fish based on vertebra size. The exact nature of this relationship varies by species and by the specific vertebra measured.

For my midden assemblages, vertebrae seem to be a useful bone type on which to focus. Vertebrae are generally the most well-represented bone type among all of the species likely to occur in these midden assemblages. Vertebrae will therefore be most likely to provide adequate sample sizes for further analysis.

Ideally, we should obtain multiple specimens for each species represented in the middens and develop functions relating bone size to fish body size. Such a project requires large collections, spanning dozens of species and including a range of sizes for each species. Most faunal analysts probably do not have access to collections of this scope.

A preliminary analysis using a variety of common species from my study area demonstrates that a positive relationship between vertebra size and fish size exists among many species of bony fish (teleosts). The individuals used in the analysis derive from an unsystematic sample of specimens for which live weight had been recorded. The table provides the data, which are illustrated in the graph.

Fish Live Weight and Vertebrae Measurements

Fish Live Weight and Vertebrae Measurements

Fish Live Weight and Caudal Vertebra Height Scatterplot

Fish Live Weight and Caudal Vertebra Height Scatterplot

The graph shows that a positive relationship obtains between vertebra size and live weight for the specimens in this sample. An inspection of the graph might be sufficient to establish that vertebra size reflects fish size. The exact nature of this relationship will be important for some of the analyses that I will do later. For this reason, I would like to model this relationship more explicitly. In the next post in the series, I will begin this task.

© Scott Pletka and Mathematical Tools, Archaeological Problems, 2009.

Identifying and Explaining Intensification in Prehistoric Fishing Practices III: Identification of the Fishing Gear Used from the Size-Frequency Distribution of Fish

September 16, 2009

The previous post concluded that intensification of fishing could be identified from the kind of gear used to capture those fish. Having decided that fish bone assemblages should be subdivided based on the gear used to capture the fish, the issue then becomes: how can we identify that gear? Answering this question requires middle-level theory that can link physical characteristics of the fish assemblage to gear type.

Gear types differ in the sizes of fish that can be captured by them. Nets should capture a larger range of fish sizes than other gear such as hook and line or spear. Hook and line or spears can not effectively capture smaller species. Assemblages formed primarily from net-caught fish should have a larger proportion of small fish than those assemblages that formed from fish primarily caught by hook and line or spear. To verify this intuition, additional sources of data from which middle-level theory could be derived would be very helpful.

Baseline data on the size-frequency distribution of fish from nearshore ocean habitats, drawn from modern sources, could be compared to the size- frequency distribution of fish bone from archeological assemblages. Prehistoric fishers presumably selected a portion of the natural range of variation in fish size through their use of particular fishing gear. Thus, the comparison would facilitate the identification of such selection. Published modern data of this sort are surprisingly rare. Beach seine netting around an estuary in Alaska produced fish assemblages whose size-frequency distributions were largely unimodal with a long tail to the right. The size-frequency distribution of individual species varied from unimodal to multimodal, depending on the number of age-classes present. The applicability of these data as an analogy to fish from my study area can obviously be questioned. I don’t have any reason to believe that the form taken by the Alaskan size-frequency distributions is exceptional, however, and a consideration of the factors that produced these distributions may be useful.

Any nearshore habitat will likely contain a range of species, each represented by specimens from one or more age classes. Different species will vary in mean size within a particular age class. The aggregate of the individual size-frequency distributions is therefore likely to produce a highly variable unimodal distribution, particularly when individuals from many different species are represented. Assemblages formed from a mix of fish caught by net and fish caught by hook and line or spears should have a bimodal size-frequency distribution. The proportion of fish in each mode should reflect the emphasis placed on netting and other fishing gear. Variation in the size-frequency distribution among archeological assemblages should provide some indication of variation in the techniques used to take fish.

© Scott Pletka and Mathematical Tools, Archaeological Problems, 2009.

Identifying and Explaining Intensification in Prehistoric Fishing Practices II: Determining What to Measure and Figuring Out What It Means.

September 15, 2009

In the previous post in this series, I suggested that we needed a way to take observations on archaeological data like fish bone and to determine what those observations implied about how hard people worked. The intellectual machinery used for this transformation is called middle-level theory by archaeologists. Well-justified middle-level theory is essential for archeological interpretation. It directs attention to the kinds of observations that archeologists need to make, and it makes those observations meaningful. For these reasons, I will dwell extensively on the development of some appropriate middle-level theory for the identification of intensified fishing.

Middle-level theory can come from a variety of places. Sometimes archaeologists can conduct experiments to understand what kind of material will be left as a result of particular activities. Historic or modern observations on people engaged in daily activities similar to those activities expected to have occurred in the past are often another useful source. Such observations will be important for the interpretation of fish bones. Middle-level theory can also come from established physical principles. I will also call upon a few physical principles to interpret the fish bones examined in this study.

An ideal source of middle-level theory would be data on the return, in calories, for finding, capturing, and processing different fish species. Fish bone in an archeological collection could then be sorted by their return rate and the effort expended to capture such fish could be quantified. Unfortunately, no comprehensive database of such return rates exists. Ethnographic and archeological data from my study area indicate that fishers took a wide variety of fish from a range of habitats using several different types of gear. The ethnographic data does not provide any basis for quantifying return rates. Experimental evidence on return rates would also be very difficult to acquire, given the large number of fish, habitats, and gear involved. Ethnographic evidence from elsewhere indicates, however, that gear type varies in terms of the amount of work required to manufacture that gear. Hook and line or spears are much easier to make than nets, for example. Sorting and quantifying the amount of fish represented in an archeological collection by the type of gear used to catch them would provide an indication of the effort expended to catch fish. This is the approach that I chose. In the next post, I will describe the methods that I used to determine the type of gear employed by fishers to take the fish in my archeological assemblages.

© Scott Pletka and Mathematical Tools, Archaeological Problems, 2009.

Identification and explanation of intensified prehistoric fishing practices I

September 13, 2009

In this series of posts, I will be exploring ways to identify and explain intensified prehistoric fishing practices. Intensification refers to the input of greater amounts of labor per unit capita to procure resources. As this formal definition implies, people are working harder at subsistence activities when they intensify their way of making a living. How do we determine when people are working harder from archaeological evidence? And what factors would induce people to intensify their efforts?

A lot of theories exist to address the latter question, but the former question is the more immediate problem. We need middle-level theory (sometimes labeled middle-range theory) appropriate to the nature of the archaeological evidence. Middle-level theory links archaeological data to phenomena of interest. It allows archaeologists to say with some confidence what happened in the past, based on that evidence.

My evidence comprises collections of fish bones and other artifacts from an organic-rich trash dump at a single archaeological site. Such trash dumps are often called middens. The occupation of the site spans several hundred years. The trash dump, however, has been sufficiently undisturbed since it was deposited that it could be excavated to recover evidence representative of much shorter spans of time. The mathematical tools that I found useful as I developed appropriate middle-level theory for this evidence included regression analysis, mixture models, and maximum likelihood models. In the next post, I will begin to develop the middle-level theory in detail, talking about the blind alleys down which I went, mistakes that I made, and solutions at which I arrived. Check back soon.

© Scott Pletka and Mathematical Tools, Archaeological Problems, 2009.