Abstract
Many estuarine and freshwater ecosystems worldwide have undergone substantial changes due to multiple anthropogenic stressors. Over the past two decades, the Sacramento-San Joaquin Delta (Delta) in California, USA, saw a severe decline in pelagic fishes, a shift in zooplankton community composition, and a rapid expansion of invasive aquatic vegetation. To evaluate whether major changes have also occurred in the littoral fish community, we analyzed a beach seine survey dataset collected from 1995 to 2015 from 26 sites within the Delta. We examined changes in the Delta fish community at three different ecological scales (species, community, and biomass), using clustering analyses, trend tests, and change-point analyses. We found that the annual catch per effort for many introduced species and some native species have increased since 1995, while few experienced a decline. We also observed a steady pattern of change over time in annual fish community composition, driven primarily by a steady increase in non-native Centrarchid species. Lastly, we found that littoral fish biomass has essentially doubled over the 21-year study period, with Mississippi Silverside Menidia audens and fishes in the Centrarchidae family driving most of this increase. The changes in the catch per effort, fish community composition, and biomass per volume indicate that a shift has occurred in the Delta littoral fish community and that the same factors affecting the Delta’s pelagic food web may have been a key driver of change.
Highlights
Ecosystem shifts are often large-scale, abrupt, and can cause persistent ecological changes [1,2]
We examined the Delta Juvenile Fish Monitoring Program (DJFMP) beach seine survey dataset at three ecological scales: species, community, and biomass
Our results suggest that the environmental drivers that caused the Pelagic Organism Decline (POD) may have been largely beneficial to the littoral fish community; in particular, introduced fish species
Summary
Ecosystem shifts are often large-scale, abrupt, and can cause persistent ecological changes [1,2]. Most recorded ecosystem shifts have been driven by direct anthropogenic pressures [3,4], and have led to substantial changes in the provision of ecosystem services with significant effects on human well-being and resources [5]. Upon reaching a new stable state, these changes may be very difficult or almost impossible to reverse [2,7]. Even when such ecosystem shifts are reversible, the cost and time required to reverse the changes can be prohibitive without prompt and early intervention [8,9,10].
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