Delta Smelt (Hypomesus transpacificus) are endemic to the San Francisco Bay-Delta in California. They were once common in the estuary, but are now endangered. Delta Smelt occurrence and abundance are strongly associated with higher turbidity in Interagency Ecological Program fish monitoring surveys conducted in the summer and fall. The number of Delta Smelt caught in the surveys is a function of their actual abundance (i.e., availability) at the site and the ability of the gear to catch them (i.e., catchability). Delta Smelt may select areas with higher turbidity because turbidity increases feeding success and reduces predation risk, increasing their availability at sites with higher turbidity. It has been suggested that Delta Smelt may be able to avoid the midwater trawl nets more efficiently in clearer water, presumably because they can see the nets from further away and thus have more time to react than they would in more turbid water. This reduced catchability in clearer water has been suggested as a factor contributing to lower Delta Smelt numbers in recent years, as the introduction of the invasive clams and macrophytes have reduced turbidity in the estuary. Using field data alone it is difficult to separate the positive effect of turbidity on availability from its negative effect on catchability. To assess the effect of decreased turbidity on the catchability of Delta Smelt, while holding availability constant, I developed a stochastic simulation model. This simulation uses published information on the swimming speed of Delta Smelt, predator avoidance behaviors, survey characteristics, and measurements of environmental variables to quantify the relationships in a conceptual model.
The simulation shows that the effect of turbidity on catchability is small, relative to other factors affecting the ability of Delta Smelt to avoid sampling gear. In particular, the combination of the specific mechanics of swimming for this species and the net speed protocols for the survey make escaping the net unlikely unless an individual was very close to the edge of the net’s path. This simulation supports the conclusion that higher catch of Delta Smelt in turbid waters can be attributed to increased availability or habitat preference.