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The former occurrence of sturgeon in the North Sea: the contribution of archaeozoology and ancient DNA

Book - Dissertation

Traditionally, it was assumed that only one sturgeon species, the European sturgeon, Acipenser sturio L. 1758, used to occur in the North Sea. This species is currently almost extinct, with only one remaining population in the Gironde River in France. Since the early 2000s, different studies have indicated the historical presence of another species, the American Atlantic sturgeon, Acipenser oxyrinchus Mitchill 1815, in France, the Iberian Peninsula, the Baltic Sea, and, based on a limited number of samples, also in the North Sea. In this thesis, the former occurrence of sea sturgeons (A. sturio and A. oxyrinchus) in the southern North Sea is examined more into detail through a genetic and morphological analysis of museum specimens from different European collections and of archaeological sturgeon remains from the Netherlands, Belgium, the UK and France. The studied material represents the bulk of the sturgeon remains found in the Netherlands, the UK and Belgium; France is represented by a limited number of remains. Most of the archaeological sturgeon remains consist of dermal bones, mostly found on the head and pectoral girdle of the animal, and in one dorsal and paired ventral and lateral scute rows alongside the body. The exposed part of those dermal bone bears a typical surface pattern, consisting of pits and ridges. For A. sturio, this surface pattern is described as tubercular, while the bones of A. oxyrinchus would be characterised by an alveolar ornamentation pattern. This difference in surface pattern is the only characteristic suitable for the morphological identification of the dermal bones of both species. Since some earlier studies showed that this characteristic is not always accurate for species identification, the validity of the ornamentation pattern as a diagnostic criterion is evaluated through the genetic and morphological study of different sized museum specimens and of 34 archaeological dermal bones. A total of 65 museum specimens were examined, 37 of which the morphological species identification was validated through genetic analysis. The study indicates that, although dermal bones with a tubercular surface pattern indeed occur in A. sturio, they also occur in different A. oxyrinchus specimens smaller than 1 m. The alveolar surface is observed mostly in A. oxyrinchus specimens, both larger and smaller than 1 m. However, A. oxyrinchus also displays variations in surface patterns of different bones of the same individual, and sometimes also within the same bone. Lateral scutes towards the end of the row often have a more tubercular structure and in some dermal bones, the alveolar structure becomes more tubercular-like towards the edge of the bone. This shows that morphological species identification of dermal bones based upon their surface pattern is not that straightforward. In case of bones with a tubercular ornamentation pattern, a correct identification requires size reconstruction, and in some cases, also the positioning of the bone within the individual. Following guidelines for the species identification of archaeological dermal bones are described: bones with an alveolar surface structure can be assigned to A. oxyrinchus, disregarding reconstructed lengths. Dermal bones with a tubercular structure can be identified as A. sturio, but only when the reconstructed length is larger than 1 m. Bone fragments which consist of the edges of dermal bones and which have a tubercular surface ornamentation can not be assigned to species, and neither can lateral scutes with a tubercular surface structure from the caudal end of the row. Hybrid individuals can not be identified based on the surface pattern of dermal bones. To back-calculate the length of fish from archaeological sturgeon remains, different methods for size reconstruction based on dermal bones of skull and pectoral girdle and the 5 scute rows are explored. For the bones of the head and pectoral girdle, measurements on museum specimens indicate that the length can be back-calculated accurately based on the regression of the total length of the animal on measurements on individual bones. Consequently, regression equations are constructed, based on 56 A. oxyrinchus and A. sturio specimens. These regression equations enable the back-calculation of size based on measurements on bones of the head and pectoral girdle. As an example, these equations were applied to archaeological remains of Vlaardingen, a Dutch Neolithic site. The 318 reconstructed lengths all fall within the known size ranges of A. sturio and A. oxyrinchus, and indicate that the animals were probably caught during their spawning run. For the size reconstruction based on the scutes from the 5 rows, both traditional and geometric morphometric techniques are explored. It seems that the relationship between measurements on the scute and the total length of the fish is not that straightforward. Size reconstruction based on the linear regression of total length of 58 museum specimens on different measurements on their scutes only gave limited accuracies. In an attempt to improve those models, the possibility to estimate the original place of an individual scute within its row, based on the scute dimensions or shape, is evaluated. For lateral and dorsal scutes, a difference in dimension and shape between scutes situated in the beginning and the end of the row is observed. However, this difference is to diffuse to allow an accurate estimation of place within the row. Next to differences within the same individual, differences in scute shape and dimensions are also observed between large and small animals. However, these differences are, again, not large enough to allow an accurate estimation of a size class. Although the scutes do not allow an accurate size reconstruction, it is possible to determine whether a sturgeon was larger or smaller than 1 m TL. When the linear regression models are used for length reconstruction and the reconstructed length is larger than 1 m, this is almost always correct. If those scutes display a tubercular ornamentation pattern, they can be assigned to A. sturio. Findings of the previous chapters are bundled in a practical guide for the identification of the most commonly found archaeological sturgeon remains. For the correct identification of archaeological skeletal remains, a reference collection, preferably consisting of different sized (modern) specimens, is necessary. However, for some species, especially those that are threatened or (locally) extinct, such as sturgeon (A. sturio/A. oxyrinchus), it is difficult to acquire specimens. This guide is an aid for the identification of archaeological sturgeon remains when a reference collection is not available. The identification guide is used to identify 7382 archaeological remains from in total 76 Dutch, Belgian, British and French sites, dated to the Mesolithic up to Late Modern times. Thirty-four of those remains are also genetically identified for the study of the surface pattern. Traditionally, all these archaeological sturgeon remains have been identified as A. sturio, since this was the only sturgeon species assumed to occur in the North Sea. Given the discovery of the historical occurrence of A. oxyrinchus in Europe, these identifications need to be reconsidered. As a first step, the remains are identified to skeletal element and, when possible, size is reconstructed based on bone measurements. Next, the ornamentation pattern is evaluated. The reconstructed lengths of in total 637 remains also give some indications on possible place and season of capture. The lengths suggest that the sturgeons in the examined Neolithic sites were captured during their upstream spawning run, while those from the examined Roman excavations were fished primarily in the estuary. Furthermore, the results of this research indicate a continuous, sympatric occurrence of A. sturio and A. oxyrinchus in the southern North Sea from Neolithic times onwards, with remains identified as A. oxyrinchus always outnumbering those of A. sturio. Moreover, genetic analysis of the 34 archaeological sturgeon remains gave evidence for historical hybridisation between both species. Further genetic analysis of archaeological sturgeon remains from the North Sea and Europe is needed to elucidate the extent of this hybridisation. Moreover, this would also give more indications on the timing of the colonisation of Europe by A. oxyrinchus.
Number of pages: 209
Publication year:2015
Accessibility:Closed