Recently, I wrote about the fall Chinook salmon runs on the San Joaquin River and its three major tributaries over the past six years. Salmon counts in San Joaquin tributaries showed an increase in returning adults in 2012-2015 compared to the devastating returns in 2007-2009. This increase occurred despite the five-year (2012-2016) drought in the San Joaquin watershed. The number of spawners in 2012-2015 was still well below the returns in the eighties and nineties that corresponded to wet water year sequences. See Figure 1.
A close look at recruitment per spawner in the population over the past 40 years (Figure 2) provides clear evidence that recruitment suffers in years with dry winter-springs or dry falls. That relationship overwhelms the background relationship between spawners and recruits three years later.
- Recruitment is significantly depressed in drier years compared to wetter years. The major contributing factor is likely poor survival in winter-spring of juveniles in their first year.
- Recruitment is severely depressed for year classes rearing in critical years and returning as adults two years later in critical years (e.g., 88, 89).
- Recruitment can be depressed for year classes with good winter-spring juvenile rearing conditions but poor conditions when adults return (e.g., 05, 06).
- Recruitment can be enhanced for year classes with poor winter-spring young rearing conditions but very good fall conditions for adults returning (e.g., 81).
- Recruitment was enhanced in recent years likely as a consequence of increased flow requirements since 2009 (e.g., 09-13).
- There is an underlying positive spawner/recruit relationship, but it is overwhelmed by the effect on recruitment of flow-related habitat conditions.
- Poor ocean conditions in 2005-2006 likely contributed to poor recruitment.