Diet, Prey Selection, and Body Condition of Age-0 Delta Smelt, Hypomesus transpacificus, in the Upper San Francisco Estuary

Abstract

The Delta Smelt, an endangered fish, has suffered a long-term decline in abundance, believed to result from, in part, to changes in the pelagic food web of the upper San Francisco Estuary. To investigate the current role of food as a factor in Delta Smelt wellbeing, we developed reference criteria for gut fullness and body condition based on allometric growth. We then examined monthly diet, prey selectivity, and gut fullness of larvae and juvenile Delta Smelt collected April through September in 2005 and 2006 for evidence of feeding difficulties leading to reduced body condition. Calanoid copepods Eurytemora affinis and Pseudodiaptomus forbesi remained major food items during spring and from early summer through fall, respectively. Other much larger copepods and macroinvertebrates contributed in lesser numbers to the diet of older juvenile fish from midsummer through fall. In fall, juvenile Delta Smelt periodically relied heavily on very small prey and prey potentially associated with demersal habitat, suggesting typical pelagic food items were in short supply. We found a strong positive selection for E. affinis and P. forbesi, neutral to negative selection for evasive calanoid Sinocalanus doerrii, and neutral to negative selection for the small cyclopoid copepod Limnoithona tetraspina and copepod nauplii, which were consumed only when extremely numerous in the environment. Feeding incidence was significantly higher in 2006, but among successfully feeding fish we found no between year difference in gut fullness. However, we did detect differences in fullness across months in both years. We found no difference in body condition of Delta Smelt between years yet our sample sizes were low in September when Delta Smelt reverted to feeding on very small organisms and fullness declined, so the longer-term effect remains unknown. Our findings suggest that: Delta Smelt had difficulty obtaining prey in spring 2005 or obtaining propersized prey in fall of both years. We detected these difficulties in some regional feeding incidence and fullness indices, but not in body condition indices.

A Covered Cod-End and Tow-Path Evaluation of Midwater Trawl Gear Efficiency for Catching Delta Smelt (Hypomesus transpacificus)

“…This could be because of a strong surface orientation in visually feeding sub-adult Delta Smelt (Slater and Baxter 2014). Feyrer et al (2013) observed Delta Smelt throughout the water column on flood tides and only in the lower portion of the water column on ebb tides.…”

Effects of freshwater flow and phytoplankton biomass on growth, reproduction, and spatial subsidies of the estuarine copepod Pseudodiaptomus forbesi

“…However, the decline in delta smelt is due at least in part to food limitation at various life stages (Bennett, 2005;Slater & Baxter, 2014;Hammock et al, 2015). Primary productivity and abundance of the copepod prey of this fish have been low since * 1987 mainly because of grazing by the clam Potamocorbula amurensis, introduced in 1986 (Cloern & Jassby, 2012).…”

“…After its introduction, P. forbesi was most abundant in waters ranging in salinity from 0 to 16 (Orsi & Walter, 1991), although it is now limited to salinity below * 5 (Kayfetz & Kimmerer, 2017). P. forbesi is an important food source for larvae and juveniles of pelagic fishes (Meng & Orsi, 1991;Moyle & Leidy, 1992;Bryant & Arnold, 2007) and makes up over half of the summer-autumn diet of delta smelt (Nobriga, 2002;Slater & Baxter, 2014). P. forbesi is a suspension-feeder, consuming phytoplankton, and microzooplankton (Bouley & Kimmerer, 2006;York et al, 2014;Bowen et al, 2015).…”

A Conceptual Model of the Aquatic Food Web of the Upper San Francisco Estuary

“…As most fish grow, they progress to larger foods, such as mysids, amphipods and other epibenthic invertebrates. Filter feeders such as American (Alosa sapidissima) and threadfin (Dorosoma petenense) shad, and planktivores such as delta and longfin (Spirinchus thaleichthys), smelt remain largely tied to the pelagic food web, although they use epibenthic organisms such as mysids at least periodically (Moyle et al 1992;Feyrer et al 2003;Feyrer et al 2007;Slater and Baxter 2014).…”

“…There are few opportunities for grounded carbon to return to the pelagic web, except through microbial production that supplies the cyclopoid copepod L. tetraspina (Linkage 4.25) and some calanoid copepods (Linkage 4.22) with supplemental support. Although L. tetraspina is the numerical and biomass dominant copepod in the LSZ, it is not an important prey item for fishes (Slater and Baxter 2014), which may account for its high abundance (Bouley and Kimmerer 2006). Thus, L. tetraspina is probably grounded back into the detrital web, or advected out of the system.…”