Some remarks on seasonal dating of fish remains by means of growth ring analysis

D.C. Brinkhuizen

1. Introduction

Since many commercial fishes are available for measurement only on a few occasions in their life cycle, fishery biologists have sought methods to reconstruct their rates and patterns of growth. Most useful - at least for fishes of the temperate zones - is the study of the year marks on hard tissues, such as scales, otoliths, fin spines, opercular bones and vertebrae. Yearly zones of growth on these parts are caused by a slowing down of temperature-dependent processes during the winter and a resumption of a more rapid metabolism in the spring. It is generally supposed that a narrow zone is formed during winter, when limited growth occurs. Such a zone is called an annulus, arrest line, or winter ring. In transmitted light it is translucent and in reflected light relatively dark. The wide zone or summer ring, which is opaque in transmitted light and white in reflected light, is formed during rapid growth. Together one annulus and one summer ring represent, in many cases, the annual addition of hard materials. Thus, by counting the number of annuli visible in a particular bone, otolith, fin ray or scale, one may determine the age of the individual fish.

Ageing of fish by means of counting annular configurations in hard parts of the body has become one of the standard methods in fishery research. Elements most frequently considered in this work are otoliths and scales, but other elements such as opercular bones, cleithra and fin rays have been used for age determination of particular fish species. There is an extensive literature on ageing of various species of fish, mostly commercially attractive ones (e.g. Bagenal 1974). With respect to the accuracy of the age determinations of modern fish we should bear in mind that a lot of data on the material is known, e.g. the species involved, mostly the sex of the individuals, the location and type of water where the individuals were caught and the date of capture. For excavated material such data are lacking.

2. Archaeological application of the method

In the Western World (i.e. outside the former USSR), it was Richard W. Casteel (1972) who first drew attention to the possibility of seasonal dating of excavated fish remains by means of growth ring readings. According to this author the season of death can be established by determining the kind of growth ring which forms the outer edge of an excavated fish bone. Moreover, the increment width of this ring gives information for estimating the time of death of the individual more precisely within the established growth period. The method and the validity of its application was demonstrated by Casteel on the basis of results from his study of excavated fish vertebrae. In another publication the author summarized the results of age readings and seasonal datings carried out by other workers on excavated fish scales (Casteel 1974). In 1976, Casteel brought these and other studies of his together into a book entitled Fish remains in archaeology and paleo-environmental studies. This work contains a substantial body of literature on incremental growth structures in otoliths, scales and vertebrae, with archaeological applications.

Thus, for deriving seasonal information from incremental growth structures in fish, a number of archaeologists and archaeozoologists have applied the method as described by Casteel. Some of these investigators first studied growth rings in recent material and relevant literature on the species involved before studying growth rings in excavated material (e.g. Hüster 1983; Noe Nygaard 1983). Others, however, had the impression, after reading Casteel's publications, that the method is a priori applicable to vertebrae of any species (e.g. Torke 1981; Le Gall 1981; 1984; IJzereef 1981; Desse 1983). Except for IJzereef (1981: 122), who states: "As it was possible to study the three stages of growth in the remains of Bovenkarspel quite well, use of the method was justified", none of the authors have commented on the validity of its application to the species studied. It will now be argued that the results of seasonal dating which Casteel derived in 1972 from vertebrae, and used to justify application of the method, are less reliable than the author demonstrated.

Some observations on Casteel's 1972 statements

In the first section of his 1972 publication, Casteel gives information on fish growth and the method of age estimation of fish. In addition to this, the author (p. 406) states:

"....., and of special importance to the archaeologist, is the corollary that by observing the kind of ring occurring on the margin of one of these scales or bones it may be possible to establish during which period of the year the fish died or was captured."

Casteel demonstrates the validity of this by using the results of his study of four concentrations of fish remains which were associated with two superimposed burials from a burial complex in California. The archaeological question asked was: were these burials contemporary or not? The answer would be given by the seasonal dating of the fish remains. Therefore Casteel assigned the excavated vertebrae, on the basis of the growth stage of their outermost ring, to one of the following three categories:

  1. vertebrae with the annulus present on the margin (March-June)
  2. vertebrae with a small amount of growth beyond the annulus (July-October)
  3. vertebrae with much growth beyond the annulus (November-February)

By studying the readable vertebrae from three concentrations which were associated with one burial, Casteel found that these belonged to individuals caught between July and October. By using statistical tests Casteel demonstrated that the three concentrations belonged to the same or identical populations. The concentration of vertebrae found in association with the other burial, however, represented a distinctly different season of burial, namely November-February. Thus, on the assumption that the vertebrae belonged to fish which were freshly caught at the time of interment, Casteel concludes that the bodies were not buried contemporaneously. With regard to the analyzed vertebrae Casteel says (p. 409):

"..... in spite of individual sample sizes ranging from 445 down to 21 there is a consistent value of from 12% to 14% of the readings which do not occur with the majority."

According to Casteel there are two explanations for this: either the author himself may have made errors in reading these vertebrae, or they may represent random elements which were in the deposit already before the interments took place.

There is, however, another possible explanation, for neither in his 1972 nor in the 1976 publication does Casteel establish the species of the excavated fish vertebrae. This omission is striking, for Casteel himself states in the first section of his 1972 publication (p. 405):

"It appears that each species will have to be dealt with individually, both in contrast to other species and with regard to populations of the same species in various regions."

The question arises why Casteel fails to mention the species name. Possibly all the vertebrae studied were from the same species, and he simply forgot to mention it. However, there may be another reason: the growth periods Casteel had chosen are March-June, July-October and November-February. The explanation for this choice may be found in the following sentence (Casteel 1972: 406):

"From what little data are available, it appears that among the majority of the native fishes of California spawning stress is the most likely cause of annulus formation and this occurs between March and June depending upon species and location."

Because of this, it is most likely that the author assumed that identification of the vertebrae to species level was not necessary at all. Formation of the winter ring takes place with all Californian species at a certain time during the period March-June.

If this is the real reason for not mentioning the name of the species, then we can say that the vertebrae have been studied as if they belong to the same species. But the consequence of regarding them as one species is that Casteel overlooked the possibility that the material studied could have contained vertebrae belonging to species which spawn at the beginning of the period March-June as well as those belonging to species which spawn at the end of this period. What, then, can happen to the results of seasonal dating when such a mixed sample of vertebrae is analyzed?

The climate in California is subtropical and the differences in temperature are far less extreme than those in the temperate regions. From a theoretical point of view, fish growth can continue during the whole year; in practice, however, it is interrupted by "spawning stress". Species which spawn at the beginning of the spawning period (March) will start to grow soon after this event. This means that they will have an advantage in growth compared with species which spawn at the end of the spawning period. Consequently the vertebrae of "early spawning" species show an amount of bone growth beyond the annulus by June. This is not the case, however, with the vertebrae of the "late spawning" species. Thus, if individuals of an "early spawning" species and individuals of a "late spawning" species are caught simultaneously in, say, September, and we read their vertebrae, which are not identified to species level, then the results of these readings will indicate that the individuals were caught in different periods.

It is concluded that if Casteel had studied vertebrae which he did not identify to species level, but which belonged to "early spawning" species and "late spawning" species simultaneously caught in September, then he will have placed the vertebrae of the first mentioned species, on the basis of much bone growth, in the period November-February and those of the 'late spawning species', on the basis of less bone growth, in the period July-October. This could be a third explanation for the "error" readings. However, further criticisms can be made.

Before reading the vertebrae, Casteel also should have taken into consideration the possibility that the vertebrae over-represent a particular season. He himself says, concerning this point (Casteel 1972: 408):

"The vertebrae to be analyzed were limited to those which had complete centra with margins intact in order to avoid duplication which could result in over-representation of a given season."

Casteel is correct in making this point, but he does not go far enough. It is necessary to test for the possibility that a number of vertebrae may come from one and the same individual. In other words: every vertebra used must represent a separate individual! For example: an assemblage of fish remains contains 32 vertebrae. Thirty of these come from an individual caught in the period July-October and two come from an individual caught in the period November-February. If we are not aware of the fact that we are dealing with vertebrae from only two individuals, and we read these 32 vertebrae, then the period July-October will be represented fifteen times more strongly than the period November-February. In reality, however, both periods are represented equally.

When seasonal dating is applied to an assemblage of vertebrae which belongs to several fish species, one also has to keep in mind that one or more of these species can over-represent a particular season. The first reason for this is that the number of vertebrae of the vertebral column differs from species to species. Thus the chance of finding vertebrae belonging to species with many vertebrae is greater than that of finding vertebrae of species with few vertebrae. Secondly, some species have skeletal elements which are less resistant than those of other species. Thus it is likely that any assemblage contains more vertebrae from species with resistant bones than from species with fragile bones.

On the basis of the results from his 1972 study, Casteel states in his 1976 publication (p. 81):

"Because of the occurrence of seasonal annuli on vertebral centra, it is possible to assess not only the age of the fish, but also the approximate season in which it died. Thus, it is possible to estimate the time of the year during which the fish, constituting the fossil or sub-fossil assemblage died."

From the foregoing arguments it will be clear that "it is possible" is not justified, because the reliability of the results of seasonal dating from vertebrae is, as yet, doubtful.


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