# W hite sturgeon diet response to invasive species
- mediated changes in a benthic prey assemblage Annie Brodsky 1 

# , Steven Zeug 1 

# , Nina Kogut 2 

# , A. Robin Stewart 3 

# , Zachary Jackson 4 

# , and Joe Merz 5

1 Cramer Fish Sciences, Auburn, CA ; 2 California Department of Fish and Wildlife, Monterey, CA; 3 United States Geological Survey, Menlo Park, CA; 4 U.S . Fish and Wildlife Service, Lodi, CA; 5 Cramer 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 S turgeon ( Acipenser transmontanus ); however, this decline remains unexplained. Understanding W hite S turgeon 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 S turgeon 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 13 C or δ 15 N = R sample

R standard − 1 × 1000 where R = 13 C/ 12 C or 15 N/ 14 N. 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 S turgeon 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 S turgeon 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 S turgeon 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 S turgeon 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 i nfo rmation on the sturgeon fishery. We are indebted to many charter captains and anglers who allowed us to collect samples from th e 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 Wi ldlife Service grant number F11AC01162. For this study: We compare W hite S turgeon 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 S turgeon 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 S turgeon and all potential prey items used in the mixing model. The prey category each species belongs to is listed in parentheses. 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.

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.