498 



Abstract— The lengths of otoliths and 

 other skeletal structures recovered 

 from the scats of pinnipeds, such as 

 Steller sea lions iEumetopias juba- 

 tus), correlate with body size and 

 can be used to estimate the length 

 of prey consumed. Unfortunately, 

 otoliths are often found in too few 

 scats or are too digested to usefully 

 estimate prey size. Alternative diag- 

 nostic bones are frequently recovered, 

 but few bone-size to prey-size cor- 

 relations exist and bones are also 

 reduced in size by various degrees 

 owing to digestion. To prevent under- 

 estimates in prey sizes consumed 

 techniques are required to account for 

 the degree of digestion of alternative 

 bones prior to estimating prey size. 

 We developed a method (using defined 

 criteria and photo-reference material) 

 to assign the degree of digestion for 

 key cranial structures of two prey 

 species: walleye pollock (Theragra 

 chalcogramma) and Atka mackerel 

 (Pleurogrammus monopterygius). The 

 method grades each structure into one 

 of three condition categories; good, 

 fair or poor. We also conducted feeding 

 trials with captive Steller sea lions, 

 feeding both fish species to determine 

 the extent of erosion of each structure 

 and to derive condition-specific diges- 

 tion correction factors to reconstruct 

 the original sizes of the structures 

 consumed. In general, larger struc- 

 tures were relatively more digested 

 than smaller ones. Mean size reduc- 

 tion varied between different types 

 of structures (3.3-26.3%), but was 

 not influenced by the size of the prey 

 consumed. Results from the observa- 

 tions and experiments were combined 

 to be able to reconstruct the size of 

 prey consumed by sea lions and other 

 pinnipeds. The proposed method has 

 four steps: 1) measure the recovered 

 structures and grade the extent of 

 digestion by using defined criteria 

 and photo-reference collection; 2) 

 exclude structures graded in poor con- 

 dition; 3) multiply measurements of 

 structures in good and fair condition 

 by their appropriate digestion correc- 

 tion factors to derive their original 

 size; and 4) calculate the size of prey 

 from allometric regressions relating 

 corrected structure measurements to 

 body lengths. This technique can be 

 readily applied to piscivore dietary 

 studies that use hard remains of 

 fish. 



Manuscript submitted 28 April 2003 

 I" Scientific Editor's Office. 



Manuscript approved for publication 

 25 March 2004 by the Scientific Editor. 



Fish. Bull. 102:498-508(2004). 



A method to improve size estimates of 

 walleye pollock (Theragra chalcogramma) and 

 Atka mackerel (Pleurogrammus monopterygius) 

 consumed by pinnipeds: digestion correction 

 factors applied to bones and otoliths 

 recovered in scats 



Dominic J. Tollit ' 

 Susan G. Heaslip 1 

 Tonya K. Zeppelin 2 

 Ruth Joy' 

 Katherine A. Call 2 

 Andrew W. Trites 1 



1 Marine Mammal Research Unit, Fisheries Centre 

 University of British Columbia, Room 18, Hut B-3 

 6248 Biological Sciences Road 



Vancouver, British Columbia, Canada, V6T 1Z4 

 E-mail address (for D. J Tollit): tollit 5zoology.ubc.ca 



2 National Marine Mammal Laboratory 

 Alaska Fisheries Science Center 

 National Marine Fisheries Service, NOAA 

 7600 Sand Point Way NE 



Seattle, Washington 98115 



Prey skeletal remnants from stom- 

 ach samples and more recently from 

 fecal (scat) samples are widely used 

 to determine what pinnipeds eat 

 (Pitcher, 1981; Olesiuk et al., 1990; 

 Tollit and Thompson, 1996; Browne 

 et al., 2002). Prey can usually be 

 identified from taxon-specific hard 

 remains, the sizes of which often cor- 

 relate with the length and mass of 

 the prey (Harkonen, 1986; Desse and 

 Desse-Berset, 1996). In the past, sag- 

 ittal otoliths were commonly used to 

 estimate prey size (Frost and Lowry, 

 1981) but were recognized to erode or 

 become completely digested (Prime 

 and Hammond, 1987; Harvey, 1989). 

 Thus, otolith measurements likely 

 underestimated sizes and numbers 

 of fish ingested (Jobling and Breiby, 

 1986), thereby preventing a reliable 

 assessment of overlap of prey con- 

 sumed with catch taken by commercial 

 fisheries (Beverton, 1985). Accurate 

 estimates of size of prey consumed by 

 pinnipeds are also important in order 

 to understand foraging behavior and 

 to explain spatial and temporal vari- 

 ability in diet composition. 



There are at least three potential 

 ways to deal with the effect of diges- 

 tion on estimates of prey size. One is 

 to measure only relatively uneroded 

 otoliths and assume that eroded oto- 

 liths are from the same size fish as 

 uneroded otoliths (Frost and Lowry, 

 1986; Bowen and Harrison, 1994). 

 Another is to apply a single species- 

 specific digestion coefficient or correc- 

 tion factor (DCF), derived from feed- 

 ing experiments with captive seals 

 fed fish of known sizes and using 

 measurements of all the eroded oto- 

 liths recovered in the scats produced 

 (Prime and Hammond, 1987; Harvey, 

 1989). The third is to estimate and 

 correct for the degree of digestion 

 (based on defined losses of morpho- 

 logical features) of each recovered 

 otolith by using estimates from ref- 

 erence material (Sinclair et al.. 1994; 

 Antonelis et al., 1997) or by applying 

 condition-specific DCFs derived from 

 fish fed in captive seal feeding studies 

 (Tollit et al„ 1997). 



Of the three approaches to correctly 

 estimate prey size from skeletal re- 

 mains, there is the assumption with 



