I had the unique opportunity to study fish use of shallow inshore waters of the western Delta in 1978-79 and again in 2004-05. One of the biggest differences I noticed after 25 years was the increase in Largemouth Bass production. Mitigation areas where levees were breached allowing tides to enter-and-leave tidal ponds without flow-through were virtual Largemouth breeding factories. Areas where channel entrances had filled in and circulation reduced also were prone to aquatic plant proliferation and an abundance of non-native lake/pond fish including Largemouth, sunfish, and shiner minnows. Flow-through areas and tidal channels with two ends had lower Largemouth production (and more native fishes). Limited tidal circulation also caused prolific amounts of aquatic vegetation including water hyacinth, Egeria, milfoil, Parrots Feather, and Potamogeton. Dense beds of aquatic vegetation also occurred in bays, dead-end sloughs, breached islands, and protected shorelines.
A recent study1 relates higher Largemouth production to increases in aquatic plants, specifically relating the abundance of young Largemouth to Egeria. They also found young Largemouth more abundant in warmer waters, another feature of backwater areas. Aquatic plants slow currents, capture sediment, and absorb sunlight, which all contribute to warming of shallow waters.
One of the paper’s conclusions related to future habitat restoration:
“While these efforts will expand the largely missing shallow-water habitat in the Delta, a major concern is that increased shallow water area will expand the habitat for Brazilian waterweed and consequently increase the abundance of Largemouth Bass, creating a predation sink for target native fishes (Brown 2003).”
I have some points of disagreement with these conclusions. First, I do not believe the Delta lacks shallow water habitat. The problem, rather, is that too much of existing shallow water habitat is bad habitat more conducive to non-native warm water fish. Second, good shallow habitat along the edges of the bays and rivers has been and continues being lost to riprapping, ship-channel dredging, remnant soft-levee erosion, and filling with sediment.
I concur with the paper that much planned restoration will create more bad habitat. Instead we should be protecting good habitat and converting more of the bad habitat to good habitat.
For more on the subject of Delta habitat restoration see: http://calsport.org/news/cspas-assessment-of-historical-habitat-restoration-in-the-delta/ .