LaCosta Creek is a tributary of Alameda Creek and is blocked from San Francisco Bay by Turner Dam at San Antonio Reservoir. Trout in San Antonio Reservoir spawn in LaCosta Creek during the winter months.
Very little rainfall fell during December and January, but the rains of February and March have produced a run of large rainbow trout from the reservoir. Whether these trout are related to the steelhead trout of Alameda Creek is a mystery to me, but I’m sure there is significant genetic data somewhere, as biologists have been investigating these fish for years.
Here are some photos taken last week.
It appears that eggs are present at the rear of the female fish (the one on the right). Note that this male is quite large (thick) and his back is out of the water most of the time.
The male would occasionally nudge the female in an effort to instigate spawning behavior. When the female began to release eggs should would turn on her side and flop wildly. Unfortunately this lively action didn’t result in any useful photos.
This male was very large for LaCosta Creek probably about four pounds – maybe more.
Once again the male nudges the female. He is probably double her size.
Some of the fry manage to move up into the deep canyons where apparently they manage to survive through the hot summer days and also avoid predators. Occasionally the large trout get trapped in the creek as creek levels subside, but if that happens, their doomed as none of the creek can support large fish through the summer months.