Using Otolith Microstructure to Determine Natal Origin of Lake Ontario Chinook Salmon NATHAN G. SMITH* 1 AND PATRICK J. SULLIVAN Department of Natural Resources, Cornell University, Ithaca, New York 14853, USA LARS G. RUDSTAM Cornell Biological Field Station, 900 Shackelton Point Road, Bridgeport, New York 13030, USA Abstract.—Stocking of hatchery-reared Chinook salmon Oncorhynchus tshawytscha in Lake Ontario has led to the development of a sport fishery that provides high economic returns to local communities. However, increased natural or ‘‘wild’’ production resulting from the naturalization of hatchery Chinook salmon to the system could result in changing salmonine dynamics that would require alteration of management practices in Lake Ontario. Using young-of-the-year (age-0) Chinook salmon of known origin—from hatchery and wild sources—we established a baseline for separating these two groups using otolith microstructure. Hatchery- reared Chinook salmon hatch earlier than wild Chinook salmon, and back-calculated hatch dates from otoliths correctly classified 97% of fish of known origin. A second protocol developed for determining the origin of Chinook salmon used the daily growth characteristics in the vicinity of 300 lm from the center of the otolith. Measuring the width of 20-d growth from 300 lm inward toward the origin correctly classified 100% of known hatchery fish and 89% of known wild fish. These measurements were used to determine the origin of Chinook salmon smolts caught in the nearshore of Lake Ontario adjacent to the Salmon River, New York, in 2000 and 2001. In both years, the nearshore population was dominated by naturally produced fish (85% to 89%). These results indicate that natural reproduction of Chinook salmon may play a larger role in Lake Ontario than previously thought. The Chinook salmon Oncorhynchus tshawytscha is an important component of the Lake Ontario sport fishery. Connelly et al. (1997) estimated that Lake Ontario anglers account for annual expenditures of over US$70 million, of which a large proportion is derived from the Pacific salmon fishery. In the early 1990s managers became concerned that the large number of salmonids being stocked in Lake Ontario could adversely impact the alewife Alosa pseudoha- rengus population that sustains the salmonid fishery (Jones et al. 1993; Kocik and Jones 1999). Stocking rates of salmonids were decreased briefly but have since been increased again because of pressure from stakeholders. Compounding the problem of the effect of high hatchery stocking levels was the lack of information on the contribution of natural reproduction to the system. Natural reproduction of Chinook salmon was first identified in the Salmon River, New York, in the late 1970s (Johnson and Ringler 1981). In general, natural reproduction is thought to contribute minimally to the Lake Ontario population (Johnson and Ringler 1981; Rand et al. 1992; Crawford 2001) but, recently, high abundances of wild young-of-year (age-0) Chinook salmon have been observed in and around the Salmon River (Dean 2002; Smith 2005). Researchers in Lakes Michigan (Carl 1982; Crawford 2001) and Superior (Peck et al. 1999) have reported substantially higher contributions of wild Chinook salmon than have been reported for Lake Ontario. Fish otoliths provide an excellent tool for stock identification. Because otoliths grow throughout the life of a fish and are metabolically inert, the material and growth patterns laid down at young ages do not change over time (Campana and Neilson 1985; Campana 1999). Thus, differences in growth and environment are recorded in otoliths and can be used to distinguish groups or stocks. Both otolith micro- structure and microchemistry have been widely used for stock identification (Campana 1999; Campana and Casselman 1993; Begg et al. 2001). Otolith microstructure has been used successfully to investigate many aspects of the early life history of Chinook salmon. Daily increment formation has been validated in juvenile Chinook salmon otoliths (Neilson and Geen 1982; Neilson et al. 1985). Neilson and Geen (1986) used otolith microstructure to evaluate the first- year growth rate of Chinook salmon and its relation to * Corresponding author: nsmith@odu.edu 1 Present address: Center for Quantitative Fisheries Ecol- ogy, 800 West 46th Street, Norfolk, Virginia 23508, USA. Received March 9, 2005; accepted February 13, 2006 Published online June 29, 2006 908 Transactions of the American Fisheries Society 135:908–914, 2006 Ó Copyright by the American Fisheries Society 2006 DOI: 10.1577/T05-079.1 [Article]