Growth of juvenile Chilkat Lake sockeye salmon in response to density-dependent and environmental factors

Abstract

Chilkat Lake, in northern Southeast Alaska, is home to a Sockeye Salmon Oncorhynchus nerka population that is an important component in local commercial, sport, and subsistence fisheries, and has been monitored since the late 1960s. The population began declining in the late 1980s, prompting fishery managers to evaluate the production potential of Chilkat Lake to determine if it could be a candidate for enhancement efforts such as fry stocking or lake fertilization. Sockeye Salmon fry were stocked into Chilkat Lake intermittently from 1989 to 2004 in both small- (<50,000) and large-scale (2.6-5.3 million) events. The purpose of this study was to determine whether stocking of fry resulted in decreased freshwater growth due to density-dependent processes. Fish scales from the Alaska Department of Fish and Game's archived collection of adult Chilkat Lake Sockeye Salmon were measured and used as a proxy for fish growth. The objectives of this thesis were to 1) examine changes in juvenile Chilkat Lake Sockeye Salmon freshwater growth over time (1978-2012); 2) determine whether increased density of juvenile fry coupled with simultaneous climate events negatively affected freshwater growth of Chilkat Lake Sockeye Salmon; and 3) determine whether increased density of juvenile fry affected age at smoltification of Chilkat Lake Sockeye Salmon. We hypothesized that high fry density would slow growth and delay smoltification; however, these analyses produced variable results. We did not detect an effect of increased fry density on growth in the first year of fresh water, but found evidence for a subtle, negative relationship between fry density and second year freshwater growth of those fish that delayed migration. We also found that age at smoltification decreased with increasing fry density. Overall, the model results indicated that no factor or combination of factors related to stocking activity or climate consistently affected juvenile Sockeye Salmon scale growth, suggesting either unidentified, equally influential, or confounding mechanisms (e.g., high adult escapement and anomalous weather patterns).