Using Archaeological Freshwater Drum Otoliths to Detect Long-Term Changes in Age and Growth

1. Metzig Garden 1 otolith 8500-6500 BC N Doty Island 448 Otoliths 1680-1712 Sauer Resort 9 otoliths 1390-1530 (calibrated 1317-1419)* Kargus Site 50 otoliths 1330-1390 Bell Site 2 otoliths 1222-1275 58 otoliths 1680-1730 Menominee Park I 12 otoliths 1150-1350 Fahrney Point 38 otoliths 1728-1730 Error bars represent +/- one standard error Fig. 3 Using Archaeological Freshwater Drum Otoliths to Detect Long-Term Changes in Age and Growth Shannon Davis-Foust and Ronald Bruch Introduction Results (cont.) Methods Results Sagittal otoliths for this study were recovered from archaeological sites surrounding the Lake Winnebago system (Fig. 3). The lengths of 690 otoliths were measured in mm on the longest axis, weighed to the nearest tenth of a mg, embedded in epoxy, sectioned with a low-speed diamond bladed saw, and examined through a dissecting scope for age determination. Age determinations were made by two experienced readers by counting the opaque zones as the boundary for annual growth increments (Casselman 1987). Precision was evaluated by calculating the coefficient of variation (Campana 2001). The best fit equation for the relationship between fish total length and otolith length for drum otoliths <15.5 mm long was Log10(TL) = 1.0387 * Log10(OL) + 1.3290) r2 = 0. 9806, and the equation for otoliths ≥15.5 mm was Log10(TL) = 1.3683 * Log10(OL) + 0.9591, r2 = 0.8560 (Fig. 4). Freshwater drum (Aplodinotus grunniens) have been a dominant part of the aquatic community of the shallow Lake Winnebago System for centuries. Since European settlement in central Wisconsin began in the early 1800’s, the Lake Winnebago system has faced an onslaught of anthropogenic changes including loss of thousands of acres of emergent wetlands, eutrophication, and invasions of non-native species such as carp and zebra mussels. In addition to these changes, millions of pounds of the native freshwater drum (Acipenser fulvescens), a species considered a rough fish,were removed annually throughout most of the later half of the 20th century in a combined effort by commercial fishermen and state-funded programs (Fig. 1). The rough fish removal program was terminated in 1990 in part because the desired impact on drum (fewer drum) was never detected. Despite their modern reputation as a rough fish, drum were an important part of the diet of Native Americans inhabiting the shore of the Winnebago lakes, evidenced by the 1000’s of drum otoliths that have been found in middens in excavated Native American campsites and villages in the area. These archaeological otoliths provided an opportunity to compare age and growth of Winnebago System drum from pre-historic times to that of drum from modern times, and perhaps provide some insight as to the impact of 60 years of intensive drum removal in the 20th Century. Fig. 7 Using mean total length at age for archaeological drum ages 2-64 and modern drum ages 2-41, archaeological drum grew slower yet attained similar lengths to modern drum (likelihood ratio test, F(3,96)=3.8886, p=0.011) (Fig. 8). Similar differences between von Bertalanffy parameters were obtained between lotic modern and lotic archaeological drum. Fig. 4 The TL’s of the lotic drum (M=502.3, SD=96.3)were greater than lentic drum (M=362.7, SD=103.2), F(1,1214)=405.619, p<0.001 (Fig. 5). However, the length distributions between the two time periods were not significantly different, F(1,1214)=3.667, p=0.056, and there was significant interaction between habitat type and time period, F(1,1214)=66.11, p<0.001. Fig. 8 Conclusions • Archaeological drum from lotic habitats had greater longevity and slower overall growth rates than modern drum. • Archaeological drum less than age 10 grew more quickly than modern drum, suggesting that the food base for small drum may have been better and the food base for large drum may have been poorer (in relation to population densities) prior to European settlement. • The inflection points observed between TL and OL, and OL and age in both archaeological and modern drum corresponds to the length at which there is a diet shift in modern drum (unpublished data). • Sixty years of intensive drum removal operations may have sufficiently decreased densities to result in a younger age composition and increased growth rates in the adult Lake Winnebago drum population. *Calibrated by quickcal2007 ver.1.5 Sagittal otoliths were obtained from modern drum (n=148 in 1986, n=177 in 2003, n=126 in 2004, n=107 in 2005, n=184 in 2006, n=351 in 2007, n=240 in 2008, and n=106 in 2009) captured by fall assessment trawling and angling at tournaments. A random otolith length (OL) (left or right) was used from 1016 drum to determine the model the best described the relationship between OL and total body length (TL). Modern otoliths were embedded and sectioned following the same methods as the archaeological otoliths. TL’s of archaeological drum were calculated using the equation derived from the relationship between OL to TL for modern drum. TL distributions of both archaeological and randomly sampled modern drum were examined by grouping them into length categories proposed by Gabelhouse (1984). Mean length at age was compared using paired t-tests between modern and archaeological drum. Von Bertalanffy models were used to compare growth rates by calculating mean length (OL’s and TL’s) at age. Significant differences between growth parameters were evaluated drum using likelihood ratio tests. To further evaluate differences between archaeological and modern drum, subclasses were created by categorizing drum as lentic or lotic (Rypel et al. 2006). Modern drum captured during tournaments (n=287) were classified as lotic because the majority of competitors in these tournaments target large drum in riverine habitat. Drum captured by trawling (n=1152) were classified as lentic. Archaeological otoliths were assumed to be lentic if they were recovered from sites along the lakes (n=110) and considered lotic (n=508) if they were from Doty Island, which was the only site located at the opening of a river. Differences among length and age distributions of archaeological/lentic, archaeological/lotic, and randomly captured modern/lentic and modern/lotic drum were examined using two-way ANOVA. Fig. 5 The CV for age determinations between the two readers for archaeological otoliths was 0.37%. Ages of lotic drum (M=31.66, SD=15.23) were older than lentic drum (M=16.0, SD=10.65), F(1,1251)=178.01, p<0.001(Fig. 6). Ages of archaeological drum (M=34.5, SD=15.82) were older than modern drum (M=17.6, SD=9.98), F(1,1251)=234.98, p<0.001. There was no significant interaction F(1,1251)=3.674, p=0.056 indicating that the ages of lotic drum were greater than lentic drum during both time periods. Fig. 1 Fig. 2 Objectives Sagittal otoliths from drum provide accurate estimates of age (Davis-Foust et al. 2009) (Fig. 2), and total lengths of freshwater drum have been estimated from their otoliths with remarkable accuracy (Witt 1960, Priegel 1963). Age has been estimated from a small set of archaeological otoliths (Bergquist 1996). Witt (1960) Priegel (1963) both concluded that archaeological drum grew larger than modern drum collected from archaeological sites near the Mississippi River and Lake Winnebago; however, no studies have used otoliths to evaluate changes in age composition or growth rates. We evaluated changes in age and growth of freshwater drum from prior to European settlement (circa 1850) to modern times within the Lake Winnebago system. We compared (1) length distribution, (2) age distribution, and (3) growth rates between archaeological and modern drum within lotic and lentic subclasses. References Fig. 6 Acknowledgements Many thanks goes to Richard Mason and Dr. Jeffrey Behm, Department of Anthropology, UW-Oshkosh for providing archaeological otoliths, and to the many WDNR fisheries technicians that have assisted with capturing drum and otolith processing. An inflection point was detected between modern and archaeological mean OL’s at age (Fig. 7). Mean OL’s at age for archaeological drum were greater than modern drum from age 2-9 (one tail paired Student’s t test: t=2.29, p=0.028), and mean OL’s for modern drum were greater than archaeological drum from ages 10-49 (one tail paired Student’s t test: t=-6.56, p<0.001).