The feeding dynamics of out-migrated age-0 chinook salmon (Oncorhynchus tshawytscha) in Lake Ontario

Abstract

The purpose of this study was to examine gastric evacuation, feeding chronology, daily ration, growth, diet, and prey selectivity of recently out-migrated young-of-the-year (YOY) chinook salmon (Oncorhynchus tshawytscha) in Lake Ontario. To determine gastric evacuation rates, chinook salmon ranging from 40-80 mm in total length, were fed to satiation and maintained at 10, 13, 16, or 19oC in the laboratory for 24 hours. Five fish were sampled randomly every four hours from each temperature treatment and the complete digestive tract (CDT) contents were removed. The CDT contents and the remainder of each fish were subsequently dried and weighed. Results show that evacuation rate (R) was dependent on temperature, with estimates of R ranging from 0.214?h-1 to 0.352?h-1, at 10oC and 19oC, respectively. Field sampling for daily ration estimates was conducted on five dates from May 21 ? June 25, 2001. Sub-yearling chinook salmon were captured using a seine near the mouth of the Salmon River, beginning 30-60 minutes after sunrise and continuing at approximately 4-hour intervals for 24 hours. Estimates of mean daily ration ( ) were derived using the Eggers (1977) model. Results showed that gut fullness varied significantly (p < 0.05) with both date and time of day, but there was no indication of synchronous diel variability in gut fullness between dates. Because did not vary as a function of date (p > 0.05), an overall (28.3 g dry wt?100 g dry wt-1?d-1) was calculated using the grand mean for all five sampling dates. Daily ring counts of sagittal otoliths revealed that in 2001, YOY chinook hatched between February 21 and April 2, and exhibited a mean growth rate of 0.65 mm?day-1. The early spring hatch dates indicated that the captured chinook salmon were naturally produced, as 2001 hatchery chinook hatched in November. For diet analyses, out-migrated YOY chinook salmon, along with potential prey items, were sampled biweekly, at dusk, at two near-shore sites (i.e., Sandy Creek and the Salmon River) in Lake Ontario from April ? July of 2000. In 2001, weekly diet and prey sampling was conducted from April ? July at the Salmon River site only. On five dates from mid-May to late-June 2001, prey and fish samples were collected throughout the day to assess diel shifts in prey availability and selection. Mid-water and surface prey sampling was conducted using a 1000 ?m neuston net pulled across the surface and parallel to the shoreline in water about one meter deep. Out-migrated, age-0 chinook salmon were captured using a seine concurrent with prey samples in both 2000 and 2001. Results showed that YOY chinook salmon in Lake Ontario were primarily diurnal feeders as indicated by both a decrease in gut content wet weight and the lack of identifiable prey in stomach samples examined after midnight. Sub-yearling chinook sampled at dusk at both sites in 2000 and 2001 fed heavily on aquatic taxa, with mature chironomids constituting the bulk of the diet. Similarly, chinook salmon sampled during the day at the Salmon River site in 2001 consumed at least 71% aquatic taxa. Moreover, there was little evidence of a diel diet shift. Although amphipods, homopterans, and developing chironomidae often dominated the prey samples, daytime diet data collected at the Salmon River site in 2001 revealed that mature chironomidae remained the most frequently consumed prey. Strauss?s (1979) index of prey selection (L) revealed that in general, YOY chinook actively selected (L = 0.3-0.5) for chironomidae, while negatively selecting for non-chironomids. This study shows that naturally produced chinook salmon are thriving in the near-shore areas of Lake Ontario and consequently have an excellent chance to recruit to the lake?s Pacific salmon fishery.