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By Wayne Gustaveson
Western Staff Writer

March 20, 2009

Spawning Stripers

We all love to sight-fish for bass on the beds. It’s great to see a fish and then catch them. It just adds another enjoyable dimension to the act of fishing.

Spawning stripers don’t often offer the sight-fishing perspective, at least where spawning is concerned. Stripers spawn on the surface but it happens at night making it hard to see. You can hear them splashing and thrashing if you’re lucky enough to be in a spawning cove with the amorous school. They can certainly be caught as well. It seems stripers never miss an easy meal and they’re always ready to eat.

Stripers spawn a bit later than bass. The preferred spawning temperature is 64 to 70 F on Lake Powell. That usually happens during May but in some years it is delayed as late as early June. Bass spawn a month earlier.

Like most species, male stripers are ready to spawn at the waggle of a fin. Males ripen with the first warming of the water in April. Females need much more than that. When day length is right, spawning hormones are triggered only by a temperature spike or an increase in water flow for those fish staged near a tributary. Those that spawn in the flat water of the lake get their cue from a quick rise in temperature. In the desert southwest, a hot day in May can raise the surface temperature five to seven degrees. That rapid warming is the key for female stripers and should tip off knowledgeable anglers as well. 

Many not familiar with Lake Powell and other reservoirs in the Colorado River drainage will read the above paragraph and scoff at my reference to spawning in flat water. It is very well known that anadromous striped bass spawn in tributary rivers. Eggs must be suspended by current to keep them from settling to the bottom and being suffocated by lack of oxygen. That standard is right and true in almost all striped bass waters - with the exception of those in the Colorado River.

Shortly after striped bass were stocked in the Colorado River, reports of successful spawning were received from areas where no spawning had been expected. Results were the same in Lakes Mead, Havasu and Powell. Four years after stocking reproducing populations were discovered in each lake.

While fishing one night during May 1980, I was lucky enough to be able to find a spawning school in Wahweap Bay (Lake Powell). I saw them chasing and thrashing and suspected spawning. (Fishing was incredible, by the way.)  Sampling the next day proved striped bass eggs were in the water some 186 miles away from the flowing current thought to be necessary for successful reproduction. Reproduction was proven a month later as young stripers were caught near that spawning cove.

In-lake reproduction has been verified on Lake Powell each year since. The mechanism that allows that to happen in Colorado River reservoirs and not in most other waters is the nature of the water. Deep, clear canyon reservoirs with limited nutrients do not produce high amounts of algae and plankton and higher life forms that pass on through normal life cycles. For example, a huge algal bloom lives, thrives and then dies. Dead algae sink to the bottom, decompose and oxidize, returning the base nutrients, like nitrogen and phosphorus, to the water. This process uses a huge amount of oxygen creating oxygen-poor zones near the reservoir bottom where the organic mater settles. Striped bass eggs that settle to the bottom in one of these nutrient-rich systems do not hatch due to lack of oxygen. 

The nutrient-poor water in desert lakes does not produce a large amount of excess biota needing decomposition. Decomposition still occurs at Lake Powell but not at a high level. Therefore the oxygen depletion is small and adequate oxygen exists for eggs to hatch in the flat waters of the Colorado River.

That explains the process. The result is a huge population of striped bass that eat a tremendous amount of food. The preferred diet is threadfin shad. In most years there are more stripers than shad. Fishing is easy for hungry predators.  Much less frequently, shad have a good year. Food is plentiful making stripers grow larger but fishing success is less. 

The key to understanding fish life history in every body of water is to understand the habitat and how fish fit into the picture.