A primary focus of steelhead monitoring in the Central Valley is capturing migrating smolts moving downstream and adult fish moving upstream to spawn. Using data collected on anadromous fish in the Stanislaus River (one of the most comprehensively monitored rivers in the San Joaquin Basin), the authors show that data from this type of monitoring alone does not provide a full picture of the river’s O. mykiss population. Data from a fish-counting weir show that few adult steelhead migrate into the Stanislaus River to spawn in any given year. While a fair number of O. mykiss are captured in a rotary screw trap on the river used to monitor out-migrating juvenile fish, not all of them exhibit smolt characteristics and some are even resident adults. These data provide some indication of timing of peak movement, but they do not reveal how the young and mature resident rainbow trout in the population are doing. Luckily, water users have also funded annual snorkel surveys on the Stanislaus, which are used as a non-invasive method to estimate trout abundance. This additional monitoring component revealed that the estimated O. mykiss abundance was stable until a decline during consecutive low-water years from 2013–2016, and showed signs of recovering until 2020. 

 

The complexity of O. mykiss life history requires a new approach to monitoring, one that covers all key life stages. The authors reviewed 12 sampling techniques that have been used in past O. mykiss monitoring and discussed the best applications of each. Weirs and rotary screw traps are effective at sampling the migratory component of a population, but as was seen on the Stanislaus, when anadromous smolts or adult steelhead are relatively rare, a low number of encounters can limit the ability to assess the population as a whole. On the other hand, snorkel, hook-and-line, and electrofishing surveys are well suited for monitoring juvenile and adult trout, and large numbers can easily be sampled where abundances are high. In addition to enumerating the resident and anadromous components of Central Valley O. mykiss, genetic monitoring of both forms is needed given the growing understanding of the genetic basis for life-history expression.

 

Management agencies tasked with the recovery of steelhead can benefit from monitoring that looks beyond migratory individuals and leverages the largely untapped resource of information represented by resident and juvenile O. mykiss. Recovery actions can be implemented to increase the number of anadromous individuals, but managers should be mindful that creating conditions that encourage anadromy may come at the expense of freshwater residents. The only way to prevent unintended consequences is to monitor both population components concurrently. If the goal of recovery is for Central Valley O. mykiss to be self-sustaining, then understanding how juvenile and adult residents contribute to O. mykiss population resiliency should be a high priority.