Using Trap Catches of Salmonid Smolts Migrating Downstream to Index Population Abundance: Does It Work?

Quantifying the number of salmonid smolts migrating to the ocean can offer critical insights into population dynamics, stock performance in freshwater and potentially, effects of stream and watershed restoration. Downstream migrant traps and mark/recapture techniques are typically used to determine smolt population abundances on a weekly and seasonal basis. Given enough consecutive study years, population data is tested for trends over time to determine whether the smolt populations are increasing, decreasing, or remaining stable. However, a major obstacle can occur when mark/recapture experiments fail, and when the resulting point estimates (of abundance) are considered unreliable. I analyzed techniques that used total catch data instead of population data for trend analysis purposes, using eleven consecutive years of smolt data collected in a free flowing coastal stream supporting wild Chinook salmon and steelhead trout. Results showed that catch data closely followed population data on a weekly and seasonal basis for both 0+ Chinook salmon smolts and 2+ steelhead trout smolts. The same conclusions drawn from testing trends in population data occurred when using catch data. For 0+ Chinook salmon, both catch data and population data showed a non-significant trend (p > 0.05), and for 2+ steelhead trout, catch data and population data showed a significant, negative trend over years (p < 0.05). In those cases where mark/recapture assumptions cannot be met, or when sparse data lead to unreliable point estimates of abundance, trap catch data may prove to be useful to draw inferences about population trends.