To Hit et al.: A method to improve size estimates of Theragra chalcogramma and Pleurogrammus monopterygius 



499 



the use of only uneroded otoliths that recovery and the 

 degree of digestion is independent of otolith size, result- 

 ing in a potentially biased fraction. For certain species 

 it can also result in a notable reduction in sample size 

 because relatively few otoliths pass through the gut 

 in good condition. The second approach of applying 

 mean species-specific DCFs is an improvement to not 

 accounting for size reduction (Laake et al., 2002); how- 

 ever, there is the assumption with this approach that 

 all structures are reduced in size by the same amount. 

 Consequently, mean fish mass may be overestimated if 

 such correction factors are applied to relatively undi- 

 gested otoliths, or they may be underestimated if ap- 

 plied to very digested otoliths (Hammond et al., 1994; 

 Tollit et al., 1997). The third method accounts for the 

 intraspecific variation in size reduction caused by di- 

 gestion, reduces systematic error (see Hammond and 

 Rothery, 19961. yields estimates of mass that compare 

 favorably to those fed to captive animals (Tollit et al., 

 1997). and hence may well be the most promising ap- 

 proach to reconstructing prey size. 



The dramatic decline of the western population of 

 Steller sea lions (Eumetopias jubatus) in the 1980s 

 (Loughlin et al., 1992; Trites and Larkin, 1996) has 

 prompted a number of studies to determine what they 

 eat and the extent of dietary overlap (prey consumed) 

 with catch taken by commercial fisheries. Stomach con- 

 tents analysis was used to determine diet until the late 

 1980s when scat analysis became the preferred method 

 (e.g., Pitcher, 1981; Frost and Lowry, 1986; Sinclair and 

 Zeppelin, 2002). However, unlike in stomachs, there is 

 an overall sparsity of otoliths in Steller sea lion scats 

 (Sinclair and Zeppelin, 2002) and, therefore there is a 

 need to also use other skeletal structures to describe 

 the size of prey consumed. 



The following outlines a method (using defined crite- 

 ria and photo-reference material) to assign the degree 

 of digestion for otoliths and alternative key skeletal 

 structures of walleye pollock (Theragra chalcogramma) 

 and Atka mackerel (Pleurogrammus monopterygius) 

 recovered from scats. We also present the results of 

 a feeding study with captive Steller sea lions used to 

 determine the extent of erosion and to derive condition- 

 specific digestion correction factors to reconstruct the 

 original sizes of the pollock and Atka mackerel struc- 

 tures consumed. Finally, we combine these DCFs with 

 newly developed regression formulae that estimate fish 

 length to derive a more accurate method of estimating 

 size of pollock and Atka mackerel consumed by Steller 

 sea lions and other pinnipeds (see Zeppelin et al., 2004, 

 this issue; Tollit et al., 2004, this issue). 



Materials and methods 



Experimentally derived digestion correction factors 



Feeding experiments were conducted with two 3-year- 

 old female Steller sea lions: Steller sea lion 1 (SSL11 

 [ID no. F97HA], mean mass 129 kg; steller sea lion 



2 [SSL2] [ID no. F97SI], mean mass 150 kg) between 

 October 2000 and April 2002 at the Vancouver Aquarium 

 Marine Science Centre. Over the experimental period, 

 the sea lions were fed pollock for 52 days in 16 separate 

 feeding experiments, and Atka mackerel for 31 days 

 in 5 separate feeding experiments, at between -4-8% 

 of body mass per day. Fork length (FL) and weight of 

 all fish were measured to ±0.1 cm and ±1 g. Sea lions 

 were fed meals of pollock of three size categories (small, 

 28.5-32.5 cm FL; medium. 33.5-38.7 cm FL; large, 

 40-45 cm FLi and meals of Atka mackerel of one size 

 category (30-36 cm FL). Fish of one particular size cat- 

 egory were fed either as a single meal or as a seven-day 

 block of meals. Full details of a typical experimental 

 protocol can be found in Tollit et al. (2003). Size ranges 

 for any category offish fed within separate experiments 

 were usually <3 cm. Fecal material was collected until no 

 other remains of experimental meals were found (7 days 

 after feeding), and was washed through a 0.5-mm sieve 

 to remove hard parts. Each animal was maintained on 

 whole Pacific herring (Clupea pallasi) between experi- 

 ments at ~6'7( body mass per day. 



The strong relationship between fish size and otolith 

 size also exists for other skeletal structures (Desse and 

 Desse-Berset, 1996). Thus, we quantified the types and 

 numbers of the prey structures recovered in the scats 

 of free-ranging Steller sea lions (from the collections 

 of Trites et al. 1 and Sinclair and Zeppelin, 2000) and 

 selected seven of the most commonly occurring struc- 

 tures for pollock and Atka mackerel. These were the 

 sagittal otolith (OTO), as well as the interhyal (INTEl, 

 hypobranchial 3 (HYPO), pharyngobranchial 2 (PHAR), 

 angular (ANGU), quadrate (QUAD), and the dentary 

 (DENT). The structures selected also had particular 

 morphological features that seemed to be relatively 

 resistant to digestion and could effectively be used to 

 estimate fish size (Figs. 1 and 2, Table 1). 



Concurrent with our feeding study, we measured 

 selected structures (Figs. 1 and 2) from randomly 

 subsampled fresh fish and combined these data with 

 unpublished NMFS data to generate allometric regres- 

 sion formulae relating structural measurements to fish 

 length (see Zeppelin et al., this issue). Fork lengths 

 (±0.1 cm) and weights (±1 gl of an extended subsample 

 of pollock (8.3-47.7 cm FL) were measured to generate 

 an appropriate regression formula for estimating fish 

 mass from fork length estimates. All selected structures 

 are located in the cranium as illustrated in Zeppelin et 

 al. (2004, this issue). Naming offish structures follows 

 Rojo (1991). 



Initial inspection of selected structures found in scats 

 from the wild revealed high intraspecific variation in 

 the degree of digestion, ranging from no apparent size 

 reduction to about a 60% size reduction (heavily digest- 

 ed material). Consequently, we extended the condition- 



Trites, A. W., D. G. Calkins, and A. J. Winship. 2003. Un- 

 publ. data. Marine Mammal Research Unit, Fisheries 

 Centre, University of British Columbia, Hut B-3, 6248 Bio- 

 logical Sciences Road, Vancouver, B.C., Canada, V6T 1Z4. 



