Armstrong et al.: Food habits of Leptocottus armatus 



467 



Cancer magister 

 S% 



Nereis brandti 



25% 



Intertidal Eelgrass 



n=55 



Other Nereis brandti 



5% 5% 



Neotrypaea californiensis 



7% 



Upogebia pugettensis 

 6% 



Cancer magister 



77% 



Intertidal Shell 



«=44 



Figure 9 



Staghorn sculpin, Leptocottus armatus, diets, expressed as % IRI 

 from intertidal eelgrass and intertidal shell habitats, Grays Har 

 bor, Washington. All data combined, April to August 1989. 



in sculpin sampled from the shell habitat (higher 

 %NC and %FO in the IRI calculation). 



Size at which 0+ crab were no longer vulnerable to 

 sculpin predation was hypothesized to be about 25 

 mm CW (about instar J5) (Reilly, 1983) based on 

 mouth gape width of the most prevalent size sculpin 

 (gape limited predation [Zaret, 1980]). Theoretically, 

 small crab newly settled to the estuary would be 

 available and vulnerable to sculpin predation for 

 much of their first full summer (as J1-J6), whereas 

 1+ crab resident in the estuary during summer would 

 be too large (50-100 mm CW, Fig. 6; Stevens and 

 Armstrong, 1985; Gunderson et al., 1990) for stag- 

 horn sculpin to consume. These assumptions were 



confirmed because only J1-J4 (below 25 mm 

 max. CW) were consumed (Fig. 8). Generally 

 there were relatively few sculpin with an es- 

 timated gape width of 25 mm (Fig. 2), thus 

 sculpin were restricted to Jl— J4 crab. Cara- 

 pace width is the larger body dimension of 

 this species of crab, but if attacked from the 

 side (i.e. laterally at the walking legs rather 

 than face-on towards the chelae), then body 

 length (from the posterior of the carapace to 

 the orbit of eyes) could be the limiting dimen- 

 sion with respect to sculpin mouth gape. 

 Based on data of Weymouth and MacKay 

 (1936), the carapace length (CL) of J3, J4, 

 and J5 instars is about 73%, 71%, and 70% 

 of width, respectively. From this perspective, 

 the average size sculpin of this study could 

 possibly consume instars up to J5 with re- 

 spect to length (approximately 18.8 mm CL). 

 Very few sculpin longer than 175 mm TL 

 were caught; even their estimated gape width 

 was only about 24.6 mm, less than the aver- 

 age carapace width of most 0+ crab by Sep- 

 tember (Wainwright and Armstrong, 1993). 

 The six other potential fish predators of 0+ 

 crab found in greatest abundance through- 

 out the summer included juvenile English 

 sole, Pleuronectes vetulus, shiner perch, 

 Cymatogaster aggregata, snake prickleback, 

 Lumpenus sagitta, saddleback gunnels, Pholis 

 ornata, sand sole, Psettichthys melanostictus, 

 and starry flounder, Platichthys stellatus. 2 

 The first five species have relatively small 

 mouths, are most prevalent as juveniles 

 within the Grays Harbor estuary (English 

 sole) or are not documented as being impor- 

 tant crab predators 1 (Williams, 1994). Starry 

 flounder may prey on 0+ Dungeness crab both 

 in Grays Harbor 1 and San Francisco Bay 

 (Reilly, 1983) but are uncommon as adults in 

 the former estuary (Rogers et al., 1989). 

 The results of this study expand current knowl- 

 edge of staghorn sculpin's diet composition and feed- 

 ing behavior, establish the importance of 0+ Dunge- 

 ness crab as part of the estuarine summer diet of 

 sculpin, and provide a perspective of the potential 

 impact that sculpin predation has on juvenile Dunge- 

 ness crab survival after settlement. Posey (1986) 

 showed that staghorn sculpin predation limits the 

 distribution of ghost shrimp N. californiensis on in- 

 tertidal sandflats and labeled this fish a keystone 

 predator controlling the depth distribution of newly 

 settled ghost shrimp and the expansion of established 

 beds of adult shrimp. Our data demonstrate that 

 staghorn sculpin are a major predator of small 



