Strontium:calcium ratios in juvenile Atlantic herring Clupea harengus L. otoliths as a function of water temperature

J. Exp. Mar. BioL EcoL, 160 (1992) 131-140 © 1992 Elsevier Science Publishers BV. All rights reserved 0022-0981/92/$05.00

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Strontium:calcium ratios in juvenile Atlantic herring Clupea harengus L. otoliths as a function of water temperature David W. Townsend a, Richard L. Radtke b, Susannah Corwin a and David A. Libby c aBigelow Laboratoryfor Ocean Science., I~: Boothbay Harbor, Maine. USA: b School of Ocean and Earth Science and Technology, Hawaii Institute of Geophysics, Universityof Hawaii, Honolulu, Hawaii. USA: cMaine Department of Marine Resources, West Boothbay Harbor, Maine, USA (Received 9 September 1991; revision received 1 April 1992; accepted 20 April 1992) Abstract: The effect of environmental temperature on the atomic concentration ratios of strontium to calcium in fish otoliths was investigated for young-of-the-yearjuvenile Atlantic herring Clupea harengus L. which were captured from the wild and held in the laboratory for I yr. The fish were maintained in large volume, flowing seawater tanks and were subject to the seasonal temperature cycle of Maine coastal waters (~-.218 °C). Otolith Sr/Ca ratios were measured for 136 fish and related to temperature by a hyperbolic Michaelis-Menten function. The greatest temperature effect occurred at lower temperatures, where Sr/Ca ratios increased with decreasing temperature. There was less of an effect at warmer temperatures where the Sr/Ca ratios approached an asymptote. The results are interpreted in terms of the effect of environmental temperature on the physiological processes that result in a discrimination against the passage of strontium from seawater into the saccular endolymph which baths the otoliths. We suggest that at lower temperatures, where those physiological processes become slowed or impaired, strontium passes more readily into the endolymph and becomes incorporated into the otolith aragonite, thus supporting the use of otolith Sr/Ca ratios in studies of life histories of fishes at environmental extremes.

Key words: Clupea harengus; Herring; Otolith; Sr; Temperature

INTRODUCTION

The use of concentration ratios of strontium to calcium in fish otoliths is a recent methodological development that promises to provide researchers with a powerful tool capable of revealing details of water temperatures experienced by an individual fish throughout its life history. Published accounts of the utility of the Sr/Ca otolith technique are few, however, and much remains to be learned as to just how universal the temperature dependence of Sr/Ca ratios may be. We present here results from experiments with juvenile Atlantic herring Clupea harengus L. that we believe will help to resolve some of the uncertainties encountered to date. Most fish otoliths are composed of the aragonite form of calcium carbonate which Correspondence address: D.W. Townsend, Bigelow Laboratory for Ocean Sciences, W. Bc,othbay Harbor, ME 04575, USA. Bigelow Laboratory for Ocean Sciences Contribution 92002.

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is precipitated within a matrix of proteinaceous material (Carlstrom, 1963; Degens et al., 1969). Strontium is one of a number of elements known to contaminate the carbonate crystal in trace amounts; because it has a similar ionic radius and the same 2 + valence as calcium, strontium can substitute for calcium during the precipitation process. In studies of the kinetics of calcification, Kinsman (1969) and Kinsman & Holland (1969) showed that the degree of incorporation of strontium into inorganically precipitated calcium carbonate is inversely related to temperature over the range 1696 ° C. The process is much less straightforward for biologically precipitated carbonate, however. Smith et al. (1979) showed that although the Sr/Ca ratio in coral aragonite was approximately the same a'~ he Sr/Ca ratio of the seaw,~ ~r from which it was precipitated, there was some evidence of a biological control at increasing temperatures, beyond the purely physical chemical effect of temperature, that effected an additional fractionation to discriminate against the incorporation of strontium into the carbonate crystal of the coral. Based on studies of molluscs, Lowenstam (1963) suggested that more highly evolved organisms, with more complex physiological systems, may exhibit an even greater degree of discrimination against the incorporation of strontium into skeletal aragonitic materials, and that those organisms may have Sr/Ca ratios much lower than those seen in either coral or inorganically precipitated aragonite. It has been suggested that calcifying organisms in general are able to modify the ionic composition of the fluid used in calcification, as compared with that in seawater, and that this modification may reflect changes in either growth rate of the organism or environmental temperature, or both, and thus be of great utility as a paleo-ecological tool (Pilkey & Goodell, 1963; Milliman, 1974; Lorens & Bender, 1980). Several studies, however, have not supported the supposition that this biological fractionation, which discriminates against the incorporation of Sr into the calcifying material, is a function of either growth rate or temperature (Buchardt & Fritz, 1977; Lorens & Bender, 1981) as had been previously suggested (Pilkey & Goodell, 1963). Building on these earlier, albeit equivocal, results relaung Sr/Ca ratios in invertebrate carbonates to environmental parameters, initial work with fish otoliths has been promising and is suggestive of a discrimination against Sr incorporation that is temperaturedependent (Radtke, 1984). Radtke showed that otoliths from cod that had been reared at constant temperatures had Sr/Ca ratios between 2.8 and 3.8 x 103 and that these ratios decreased linearly with increasing temperature. Fish reared at lower temperatures had higher otolith Sr/Ca ratios (i.e., more Sr was incorporated), which is what one would expect if low temperatures physiologicallyprevented, or impaired, a fish's ability to discriminated a~ainst strontium from being incorporated into the otolith. Assuming a temperature dependence of otolith Sr/Ca ratios, Radtke & Target (1984) were able to validate the annual growth rings of an antarctic nototheneid by matching the rings with the annual Sr/Ca pattern, which appeared to reflect the annual temperature cycle of the region. Our earlier work on Sr/Ca ratios in otoliths of larval and juvenile Atlantic herring

Sr : Ca RATIOS IN CLUPEA HARENGUS OTOLITHES

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(Townsend et al., 1989; Radtke et al., 1990), though not without problems, has demonstrated the potential application of the Sr/Ca method in addressing ecological and oceanographic questions. We relied on validation experiments based partly on larval herring reared in the laboratory over a range of constant temperatures, as well as on field-caught larvae and early juveniles sampled seasonally, with which we related the Sr/Ca ratios of the outer-most regions of the otoliths to the ambient temperatures from where they were collected. Our larval rearing experiments at low temperatures (110 °C) gave unexpected results that should not be used to correlate Sr/Ca ratios with water temperatures (Townsend et al., 1989). Sr/Ca ratios were found that ranged between 3 and 4.5 x 103, similar to that found by Radtke (1984) for cod, but the temperature relationship for these rearing experiments was apparently much too steep, giving the relation: T(°C) = 32.4 - 7.3 x (Sr/Ca) x 10a. Larvae reared at the higher temperatures (5-10 °C) had relatively high St/Ca ratios ( 3 - 4 × 103) which were little different from Sr/Ca ratios of larvae reared at 1 and 2 °C (3-5 × 103). Applying this relationship to St/Ca ratios measured in otoliths of larvae from the wild resulted in exaggerated estimates of maximum temperatures experienced by the larvae, with some estimates as high as 25 °C. We assumed that the anomalous Sr/Ca-temperature relationship was the result of artificially introduced stress during the rearing experiments which interfered with the larvae's ability to discriminate against strontium incorporation at the higher temperatures where they would normally be capable of doing so. Therefore, we used a Sr/Ca vs. temperature expression based on 46 measurements of the outermost rim of the otoliths from field caught individuals, which gave the relationship T(°C) = 12.6 - 2.81 x (Sr/Ca) x 103 (To~vnsend et al., 1989). Though this relationship provided more realistic estimates of temperatures when applied to field samples, we nonetheless were suspicious of the small sample number and warned that our reconstructed temperature histories should be interpreted only as qualitative and indicative of general trends. The results of a second study where herring larvae were reared at warmer temperatures (Radtke et al., 1990; 7-13 °C) gave a St/Ca vs. temperature expression that compared more favorably with the one based on field-caught animals (T(°C) = 19.17- 2.95 x (Sr/Ca)x 103), but we again suspected that the reared larvae may have been stressed, as revealed by the lack of daily growth increments in the otoliths, and we warned once more that the expression was likely in error by some unknown amount. This early work on herring otoliths was more qualitative than quantitative in its application, primarily because of our inability to determine with any great deal of certainty the exact nature of the temperature relationship of otolith Sr/Ca ratios. Recent work by Kalish (1989) has cast doubt on the temperature effects on St/Ca ratios in fish otoliths, and one must assume that the issue of whether Sr/Ca ratios in fish otoliths can universally be related to temperature remains clouded. Our purpose here is to repert on the results of an experiment where juvenile herring were captured from the wild and held for one year in a flow-through sea water system that was subject to

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the natural seasonal temperature cycle. We believe these results will help to clarify some of the uncertainties encountered earlier. MATERIALS AND METHODS

400 juvenile Atlantic herring were collected from Boothbay Harbor, Maine, in June 1989 using a Fyke net. The herring were ~ 9-month-old young-of-the-year fish, having been spawned the previous autumn. They were transferred immediately to 400-1 circular indoor tanks that were supplied with seawater pumped from the harbor. The flow rate was ~ 5 l.rnin -t which resulted in an exchange rate of 80 min. Four tanks were used, with each holding ~ 100 fish. Tank temperatures were recorded daily. The fish were fed a commercial fish food (BioDiet Starter, from Bioproducts, Warrenton, Oregon) four to six times a day. Subsamples of fish were sacrificed periodically, measured for total length and wet weights determined; a total of 277 fish of the original 400 were sacrificed. Otoliths from 136 individuals were extracted, mounted on glass disks, and polished and coated in preparation for analysis with a wavelength dispersive electron microprobe as described earlier in Towns, ~d et al. (1989) and Radtke et al. (1990). Only the outer edge (2-5 #m) ofthe otoliths were analysed for Sr and Ca, which we assumed represented the experimental water temperatures at the time of collection. RESULTS The average daily temperatures for the duration of the experiment are plotted in Fig. 1. Because the experimental tanks were indoors, there was some evidence o f a

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Fig. 2. (A) Plot of length (total length) of juvenile Atlantic herring sacrificed during course of holding experiment. Date is given as Day Number, with Day l = 21 July 1989. A third order polynomial has been fit to data: L = 52.7 + 0.94 (T) - 0.004 (T)'- - 0.000007 (T)3; T = temperature (° C), n = 277, r ~ = 0.938. (B) Same as Panel A, except for wet weight (g); W= - 1.1 + 0.16 (T) - 0.001 (T) 2 - 0.0000013 (T) 3, n = 277, r 2 = 0.968. (C) Plot of wet weight vs. total length for data in Panels A and B; W= 0,0000076(L) -'96.

136

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damping or lessening of the seasonal temperature extremes normally observed for coastal Gulf of Maine waters, which range from freezing to > 20 ° in some areas. Nonetheless, the tank temperatures mimicked quite well the general seasonal trend in coastal water temperature, with low temperatures in the range of 2 °C in winter and summer maxima that exceeded 18 ° C. Survival was quite high throughout the experiment, with the greatest mortality occurring during the initial part of the experiment. Over the 12-month period the total mortality was ,~30%. Periodic subsampling of the juveniles to be sacrificed was random; all fish sampled were exhibiting schooling behavior, swimming into the gently flowing current in the round tanks. The results of the length and weight measurements are given in Fig. 2. There is evidence of a cessation of growth in length and weight during the winter period of low temperatures, which is approximately the period between Days 125-150 and 260-280. This period coincides with the minimum water temperatures during the experiment, which were < 5 °C between Days 145 and 270. Despite this cessation of growth by the juveniles, there did not appear to be any departure from normal allometric growth as revealed by the weight-length relationship for the larvae during the mid-winter period (Fig. 2), suggesting no great departure in their morphometdc condition factors. The results of the measurements of Sr/Ca ratios of the outer edges of 136 individuals during the course of the experiment are plotted in Fig. 3 as Sr/Ca vs. temperature.

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holdingexperimentas a functionof the averagedailytemperature(°C and °K) of exFerimentaltanks. An equation of Michaelis-Mentenform: Sr/Ca (x 103)= a* T/(b + T ) has been fit to data using iterativeleast squares, where T is absolute temperature, and a and b are constants; a =0.137, sD=0.008, ¢v =6.16~o; b = - 265.7, st) = 0.702, cv = 0.260/o.

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Temperature (K) Fig. 4. Data from Townsend et al. (1989). Plot of Sr/Ca ratios of samples of outermost edges of otoliths of Atlantic herring larvae sampled from Sheepscot Estuary of Maine as a function of water temperature ( ° C and ° K). An equation of Michaelis-Menten form: Sr/Ca ( x 103) = a* T/(b + T ) has been fit to data using iterative least squares, where T is absolute temperature, and a and b are constants: a = 0.08, SD = 0.006, c v = 7.78 %; b = - 269, SD = 0.465, CV = 0.17 %.

The trend of the data is similar to our earlier results based on field-caught larvae (Fig. 4; Townsend et al., 1989). Although, because of small sample size we chose in our earlier study to fit a simple linear regression model to the data, we have fit to these data an expression of the Michaelis-Menten form, Sr/Ca ( x

10 3) =

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where T is absolute temperature, and a and b are constants. It is arguable which expression might be more appropriate in nature~ a linear or nonlinear form, but as we will discuss below, we felt that an expression that allowed for an initial slope based on low temperature stress, or a physiological inhibition of ability to discriminate against strontium, followed by a period that approached an asymptote at higher temperatures, was more in keeping with recent studies of Sr/Ca ratios on other species (Kalish, 1989).

DISCUSSION

The use of Sr/Ca ratios in fish otoliths as biological recorders of past water temperatures experienced by an individual fish can be of immense value to researchers. Our earlier study on the overwintering distributions of herring larvae in the Gulf of Maine (Townsend et al., 1989) was the first attempt to use this technique to address a particular research question, and served as an example of the kind of information that can

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D.W. TOWNSEND ET AL.

be gleaned from otoliths. Based on profiles of Sr/Ca in otoliths from larvae collected in inshore areas along the Maine coast in spring, we concluded that those larvae had spent the mid-winter months in waters further offshore, which remain somewhat warmer than inshore waters in winter. As pointed out earlier, those results suffered from the lack of a precise expression relating the St/Ca ratios to temperature, and only general trends in individual temperature histories were inferred. Kalish (1989) has justifiably expressed doubt about the underlying premise of a universal temperature dependence of Sr/Ca ratios in fish otoliths. In his temperaturecontrolled studies (ranging from 13 to 22 °C) of juvenile Australian salmon Arripis trutta, he found that there was only a slight correlation between temperature and otolith Sr/Ca ratios, and that there was no significant linear relationship. He suggested that the Sr concentration in the saccular endolymph, and hence in the otoliths, may vary seasonally and with age for a particular species, but not directly with temperature, and he urged caution in relating otolith chemistry to environmental variables. Our results might at first appear to be in direct contradiction to Kalish's conclusions. We found a nonlinear relationship between Sr/Ca and temperature, in which the otolith Sr/Ca ratios are elevated at lower temperatures, and become more asymptotic at higher temperatures (Fig. 3). The data in Fig. 3 are similar to results we reported for fieldcaught herring larvae in our earlier work (Fig. 4; Townsend et al., 1989) in that those earlier data, albeit fewer in number, also appear to reflect a nonlinear relationship. The hyperbolic Michaelis-Menten expression fit to the data in Figs. 3 and 4 includes a parameter for what we suspect may be an asymptotic portion of each data set at higher temperatures. At the lower end of the temperature range, the St/Ca ratios appear to increase rapidly with decreasing temperature. We argued in our earlier paper (Townsend et al., 1989) that at low temperatures the fish may become physiologicallyimpaired with respect to their abilities to discriminate against the incorporation of strontium. The data from Kalish (1989) provide strong evidence that increases in otolith Sr/Ca ratios are well correlated with the Sr/Ca ratios in the endolymph. We suggest, therefore, that at temperatures of < ~ 5 °C herring become increasingly unable to suppress the penetration of strontium into the endolymph, and hence into the otolith aragonite. The exact nature of '.his physiological dependence of Sr incorporation on temperature is not certain, although in a more recent paper, Kalish (1991) has explored this more fully. He suggests that Sr incorporation may be related to the total quantity of calcium-binding protein in the blood plasma, which in turn may vary with reproductive cycle and season, and thus be collinear with, but not necessarily a direct function of temperature. K alish's (1991) results with bearded rock cod Psevdophysis barbatus do, however, demonstrate a negative correlation between temperature and otolith Sr/Ca ratios, and apart from the direct physiological mechanism, his results provide evidence that Sr/Ca ratios can be used as a proxy for environmental temperatures. The lack of a well-defined Sr/Ca vs. temperature relationship in his earlier work (Kalish, 1989) may simply reflect his having selected experimental temperatures that did not adversely affect the fish's physiological mechanism, i.e., the titer of calcium-binding protein, to

Sr : Ca RATIOS IN CLUPEA HARENGUS OTOLITHES

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exclude strontium from the endolymph. Rather, his data may represent the asymptotic portion of the hyperbolic function described here. Kalish (1989) underscored the apparent differences in Sr/Ca ratios in both the endolymph and otoliths of fish of different ages and among different species. Based on a data set for 10 marine fish species, all collected within approximately the same temperature range, he reported that Sr/Ca ratios in otoliths increased linearly with the Sr/Ca ratios of the surrounding endolymph fluid. Further, the wide range in otolith Sr/Ca ratios among the 10 species ( ~ 2-6 x 103) clearly demonstrated the existence of differences among species. Interspecific differences have been discussed in the literature with reference to invertebrates, as we discussed earlier. Radtke (1989) has reported St/Ca ratios in Fundulus heteroclitus larvae that ranged from ~ 9.5 to 12.5 x 103, which are greater than those we found for herring, and greater than those reported in Kalish (1989) for all the species he studied. The two data sets we present here in Figs. 3 and 4, for herring juveniles and larvae, are similar to one another in shape, but the Sr/Ca ratios in the juveniles are slightly greater than in the larvae, perhaps suggesting that older herring have greater St/Ca ratios, in keeping with Kalish's (1989) conclusion that Sr/Ca ratios increase with the age of a fish. The asymptotic portions of the two curves in Figs. 3 and 4, defined by the parameter a in the Michaelis-Menten equation, are statistically different from one another, being greater for the juveniles in Fig. 3. The temperature-dependence, or the sensitivity of St/Ca to low temperatures, on the other hand, which is expressed by b, a constant that represents the Sr/Ca ratio half way between the maximum and the asymptote, though also statistically different between the two, does not differ as much. The increase in Sr/Ca ratios in herring otoliths at low temperatures would appear to become most significant when the temperature falls below 5 or 6 ° C, where the ratios increase at a greater rate as temperature falls. Evidence in support of the hypothesis of an interference with the physiological processes that may be responsible for discriminating against the passage of Sr into the endolymph, and hence into the otolith, comes from the growth information presented in Fig. 2. Growth of the herring, both in length and weight, is clearly retarded during the mid-winter period when the temperatures ranged from ~ 2 to 5 ° C, suggesting a significant slowing oftheir metabolism. This same phenomenon of retarded growth in cold waters has been observed in the field and does not appear to be an artifact of the experiment (Townsend & Graham, 1981). In conclusion, the Sr/Ca ratios of young herring can be described by the hyperbc,lic Michaelis-Menten equation, where the greatest temperature effect begins at some low-temperature threshold, such that physiological processes become significantly slowed, and the ability of the fish to discriminate against the incorporation of strontium becomes increasingly diminished. At higher temperatures, the St/Ca ratios begin to reach an asymptote with respect to temperature, and measurable changes in Sr/Ca as a function of temperature become more difficult to detect. Thus, we suggest that the use of Sr/Ca ratios in fish otoliths can be a useful tool in studying life histories of fishes exposed to environmental extremes.

140

D.W. TOWNSEND ET AL. ACKNOWLEDGEMENTS

Funding for this research was provided by NSF Grants OCE-88-16662 to D.W. Townsend and OCE-88-00686 to R.L. Radtke.

REFERENCES Buchardt, B. & P. Fritz, 1977. Strontium uptake in shell aragonite from the freshwater gastropod Limnaea stagnalis. Science, Vol. 199, pp. 291-292. Carlstrom, D., 1963. Crystallographic study of vertebrate otoliths. Biol. Bull., Vol. 125, pp. 441-463. Degens, E.T., W.G. Deuser & R.L. Haedrich, 1969. Molecular structure and composition of fish otoliths. Mar. BioL, Vol. 2, p. 105-I 13. Kalish, J.M., 1989. Otolith microchemistry: validation of the effects of physiology, age and e~vironment on otolith composition. J. Exp. Mar. Biol. Ecol., Vol. 132, pp. 151-178. Kalish, J.M., 1991. Determinants of otolith chemistry: seasonal variation in the composition of blood plasma, endolymph and otolith of bearded rock cod Pseudophysis barbatus. Mar. Ecol. Prog. Ser., Vol. 74, pp. 137-159. Kinsman, D.J.J., 1969. Interpretation of Sr +2 concentrations in carbonate minerals and rocks. J. Sed. Petrol., Vol. 39, pp. 486-508. Kinsman, D.J.J. & H.D. Holland, 1969. The co-precipitation of cations with CaCO3 - IV. The coprecipitation of Sr ~ 2 with aragonite between 16 ° and 96 ° C. Geochim. Cosmochim. Acta, Vol. 33, pp. 1-17. Lorens, R.B. & M.L. Bender, 1980. The impact of solution chemistry on Mytilus edulis calcite and aragonite. Geochim. Cosmochim. Acta, Vol. 44, pp. 1265-1278. Lowenstam, H.A., 1963. Sr/Ca ratio of skeletal aragonites from recent marine biota at Palau and from fossil gastropods. In, Isotopic and cosmic chemistry, edited by H. Craig et al., North-Holland, Amsterdam, pp. 114-132. Milliman, J.D., 1974. Marine carbonates, Springer-Verlag, New York, 375 pp. Pilkey, O.H. & H.G. Goodell, 1963. Trace elements in recent mollusc shells. Limnol. Oceanogr., Vol. 8, pp. 137-1.18. Radtke, R.L., 1984. Cod tish otoliths: information storage structures. FlodevigenRapp., Vol. 1, pp. 273-298. Radtke, R,L., 1989. Strontium-calcium concentration ratios in fish otoliths as environmental indicators. Comp, Biochem. PhysioL, Vol, 92A, pp. 189-193. Radtke, R.L. & T, E. Targett, 1984. Rhythmic structural and chemical patterns in otoliths of the Antarctic fish Notothenia larseni: their application to age determination. Polar BioL, Vol. 3, pp. 203-210. R'~dtke, R.L., D.W. Townsend, S.D. Folsom & M.A. Morrison, 1990. Strontium:calcium concentration ratios in otoliths of herring larvae as indicators of environmental histories. Environ. Biol. Fish., Vol. 27, pp. 51-61. Smith, S.V., R. W. Buddemeier, R. C. Redalje & J. E. Houck, 1979. Strontium-calcium thermometry in coral skeletons. Science, Vol. 204, pp. 404-407. Townsend, D.W. & J.J. Graham, 1981. Growth and age structure of larval Atlantic herring, Clupeaharengus harengus, in the Sheepscot River estuary, as determined by daily growth increments in otoliths. Fish. Bull. U.S., Vol. 79, pp. 123-130. Townsend, D.W., R.L. Radtke, S.D. Folsom & M.A. Morrison, 1989. Recruitment implications of larval herring overwintering distributions in the Gulf of Maine, inferred using a new otolith technique. Mar. Ecol. Prog. Ser., Vol. 55, pp. 1-13.