White sturgeon diet response to invasive species-mediated changes in a benthic prey assemblage

Annie Brodsky1, Steven Zeug

1

, Nina Kogut

2

, A. Robin Stewart

3

, Zachary Jackson

4

, and 

Joe Merz5

1Cramer Fish Sciences, Auburn, CA ; 2California Department of Fish and Wildlife, Monterey, CA; 3United States Geological Survey, Menlo Park, CA; 4U.S. Fish and Wildlife Service, Lodi, CA; 5Cramer Fish Sciences, West Sacramento, CA

Study Background and Purpose

. 

Results

Discussion

The San Francisco Estuary (SFE) is the most invaded estuary on the west coast of the United States and 

possibly the world. Since the arrival of the overbite clam (Potamocorbula amurensis), significant changes 

have been observed in the native benthic macroinvertebrate and plankton assemblages. 

Concomitant with the establishment of the overbite clam has been an overall decline in native SFE White 

Sturgeon (Acipenser transmontanus); however, this decline remains unexplained. Understanding White 

Sturgeon feeding ecology and how it may be impacted by invasive species like the overbite clam is essential 

to inform effective sturgeon management actions.

Pre-Invasion

• Dominant prey items by volume: several 

native and invasive clam species and Pacific 

Herring eggs (Clupea pallasii) (Figure 2).

• Clams displayed the highest frequency of 

occurrence (58% of all stomachs) (Figure 2).

Post-Invasion

• Dominant prey items by volume: clams 

increased in volume by 310% (2001-

2003) and 270% (2012-2013) relative to 

the pre-invasion period (Figure 2).

Isotope Results:

• Mixing model indicated a 95% probability that the overbite clam contributed 70.3% -

83% to sturgeon biomass (Figure 4).

• Starry Flounder (Platichthys stellatus) provided the next highest contribution by 

percentage (Figure 4).

• Contributions by all other taxa used in the model were minimal (Table 2).

• Niche breadth decreased from 4.8 in 

the pre-invasion period to 1.2 during 

2001-2003 and 1.5 during 2012-2013.

Methods

Sturgeon stomach collection

All sturgeon stomachs were collected from anglers’ harvested 

fish. Gut contents were analyzed in the lab (esophagus and 

stomach sections).

Time collected: 1965-1967, year-round

2001-2003, year-round

2012-2013 February (October 2012 only) 

Catch location: The majority of fish analyzed were caught in 

the northern San Francisco Bay, from the lower Sacramento 

River and the freshwater delta (Figure 1). 

Size: 117-168 cm total length

Isotope sample collection

Data for the isotope mixing model analysis : Stewart et al. (2004). 

White Sturgeon tissue and potential prey items were collected for analysis of carbon and nitrogen stable 

isotope ratios during fall 1999 and early winter of 2000 and 2001 (Stewart et al. 2004). 

All stable isotope values are reported as deviations relative to standards (delta notation):

δ

13C or δ15N =

Rsample

Rstandard

− 1 × 1000

where R = 13C/12C or 15N/14N. The standard for carbon was Pee Dee Belemnite limestone and atmospheric 

nitrogen was the nitrogen standard.

Fig. 1 Map depicting the location of the San Francisco Estuary (SFE) in California and the 

study region within the SFE. The box indicates the general location where most sturgeon 

were captured. 

• Fish were one of the few prey categories that increased in frequency during the 

post−invasion period. The SFE White Sturgeon population may have shifted to greater 

piscivory in response to changes in the abundance of other benthic prey following 

overbite clam invasion. 

• The mixing model yielded small estimates for crabs and shrimp despite being found in 

similar proportions to fish in White Sturgeon diets. Gut content analysis could be biased 

toward hard-bodied prey items.

Table 1. List of taxa identified from each prey category used in the comparison of 

white sturgeon diets during the pre- and post-invasion periods. Categories are 

listed in order of rank in total volumetric proportions.

Conclusion:

Although there was significant alteration of prey community structure after the overbite clam invasion, the trophic adaptability of 

White Sturgeon likely allowed them to exploit this new prey resource. However, this bivalve readily accumulates the trace element 

selenium, that has been shown to result in reproductive toxicity in surrogate species. 

Study Limitations:

• Stomach content analysis provides only a snap shot of White Sturgeon diets.

• Pacific Herring eggs: only available to sturgeon for a brief period during the year reflected in the low frequency of occurrence .

We thank the Foundation Sportsman’s Club that allowed us to collect samples during their annual derby and provided valuable information on the sturgeon fishery. We are indebted to many charter captains and anglers who allowed us to collect samples from the sturgeon they harvested and provided information on catch location. Ian Anderson, Bill Beckett, Jillian Slingsby, and Jamie Sweeney identified sturgeon stomach contents. Staff from Cramer Fish 

Sciences and the United Stated Fish and Wildlife Service participated in sample collection during sturgeon derbies. Funding for this study was provided by the United States Fish and Wildlife Service grant number F11AC01162.

For this study:

We compare White Sturgeon gut contents from the pre− and 

post−invasion period to determine how prey composition and 

importance have responded to changes in benthic assemblage 

structure. We then combined stable isotope ratios of White Sturgeon 

and their prey from the post-invasion period in a mixing model to 

estimate the proportional contribution of overbite clams and other 

potential prey to sturgeon biomass to determine if dietary proportions 

observed in stomachs match assimilation into tissue. 

• The difference between gut content and biomass assimilation of clams indicated by isotope ratios is likely explained by the 

percent content of clam shells and other difficult-to-digest material included in volumetric assessment of prey contribution.

Figure 4. Bi-plot of mean stable isotope values with standard errors for White Sturgeon and all 

potential prey items used in the mixing model. The prey category each species belongs to is 

listed in parentheses.

Crangon 

spp./Metacarcinus 

magister (shrimp and 

crab)

Ampelisca spp. 

(Amphipod)

Corophium spp. 

(Amphipod)

Isopods

Potamocorbula 

amurensis (clam)

Acanthogobius 

flavimanus (fish)

Acipenser 

transmontanus

Platichthys stellatus 

(fish)

10.00

11.00

12.00

13.00

14.00

15.00

16.00

-29.00 -27.00 -25.00 -23.00 -21.00 -19.00 -17.00 -15.00

d 15N

d

13C

Figure 2. Volumetric proportion of prey items identified in sturgeon guts during the pre-invasion 

(1965-1967) and post-invasion periods (2001-2003, 2012-2013). Clam values appear on the 

secondary y-axis. The species composition of each category is listed in Table 1.

0

0.5

1

1.5

2

2.5

0

0.05

0.1

0.15

0.2

0.25

0.3 Proportion of clams Proportion of total prey volume

Prey category

2012-2013

2001-2003

1965-1967

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6 Frequency

Prey Category

2012-2013

1965-1967

Figure 3. Frequency of occurrence for prey categories identified in sturgeon guts during the pre-

(1965-1967) and post-invasion period (2012-2013). Meta-data for stomachs sampled during 

2001-2003 was incomplete and frequency could not be accurately calculated.

Table 2. Proportional contribution of each prey item to sturgeon biomass at five cumulative probability 

levels from the stable isotope mixing model. 

• All identifiable clams were introduced 

species (Table 1).

• The largest change in frequency was the 

22% increase in clams, followed by a 

17% increase in fish.

Prey Item 0.05 0.25 0.50 0.75 0.95

Crangon spp./Metacarinus magister (shrimp/crab) 0.001 0.006 0.013 0.026 0.055

Ampelisca spp. (Amphipod) 0.001 0.009 0.020 0.039 0.080

Corophium spp. (Amphipod) 0.001 0.006 0.014 0.027 0.071

Isopoda (Isopod) 0.001 0.007 0.016 0.033 0.076

Potamocorbula amurensis (clam) 0.703 0.746 0.770 0.796 0.830

Acanthogobius flavimanus (Yellowfin Goby) 0.001 0.003 0.008 0.016 0.037

Platicyhys stellatus (Starry Flounder) 0.037 0.097 0.127 0.152 0.190

Prey 

Category Taxa identified Pre-invasion period Post-invasion period

Clams

Macoma sp.

1 X

Tapes semidecussata 2 X

Potamocorbula amurensis 2 X

Corbicula fluminea 2 X

Fish/fish 

eggs Morone saxatilis 2 X

Platichthys stellatus 1 X

Clupea pallasi (eggs)1 X

Engraulis mordax 1 X

Porichthys notatus 1 X

Leptocottus armatus 1 X

Menidia beryllina 2 X

Gobidae2 X X

Cottidae1 X X

Barnacles Balanus spp.

2 X

Crabs Rhithropanopeus harrisii 2 X X

Metacarcinus magister 1 X

Shrimp Palaemon macrodactylus 2 X X

Crangon spp.

1 X X

Neomysis spp.

3 X

Crayfish Astacidae3 X

Isopods Synidotea sp. X

Gastropods Opisthobranchia X

Physidae3 X

Ploychaetes Nereis sp.

3 X

Mussels Mytilus sp.

3 X

Amphipods Gammaridae3 X

Nematodes Unidentified X

1

 native, 2 introduced, 3 native and introduced species within this taxonomic classification