A comparison of growth rates of wild rainbow trout (Oncorhynchus mykiss) in the upper Sacramento River before and after the Cantara Spill of 1991

In July of 1991, a train car filled with metam-sodium pesticide fell into the upper Sacramento River, California, killing more than 300,000 wild rainbow trout and all aquatic life in the 38-mile stretch of river below the spill. I compared growth of wild rainbow trout prior to and following the spill based on analysis of scale samples collected by California Department of Fish and Game in 1978, 1980 and 1986 (pre-spill) and 1994, 1996 and 1997 (post-spill). Scale analyses were supplemented by diet analysis of wild trout collected in 1998-2001. Back-calculated lengths at age were determined using a natural log-transformed Fraser-Lee equation, and specific growth rates before and after the spill were compared using analysis of covariance (ANCOVA). Back-calculated lengths and specific growth rate comparisons between pre-spill and post-spill periods provided strong evidence that growth of trout decreased following the spill, but the effect apparently was not immediate. Back-calculated trout lengths and specific growth rates from 1994 scale samples were not significantly different from pre-spill scale samples, but analysis of 1996 and 1997 scale samples showed significant decreases in both length at age and specific growth rates for age 3 and 4 trout. Specific growth rates of rainbow trout were lowest in 1995, when a severe winter occurred and high winter and spring flows were recorded. Diet analysis revealed that wild rainbow trout were consuming a diverse selection of macroinvertebrates. Large trout were getting most of their consumed energy from Dicosmoecus sp., a large caddisfly larva, but showed no evidence of sculpin consumption. The upper Sacramento River trout population may have been more susceptible to the effects of natural disturbances in the process of recovery following the spill, as trout growth rates appear to partially reflect winter severity.