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And Atka Mackerel (Pleurogrammus Monopterygius) Can Be Used to Estimate the Length of Prey Consumed

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And Atka Mackerel (Pleurogrammus Monopterygius) Can Be Used to Estimate the Length of Prey Consumed A method to improve size estimates of walleye pollock (Theragra chalcogramma) Atka mackerel (Pleurogrammus monopterygius) consumed by pinnipeds: digestion correction factors applied to bones and otoliths recovered in scats Item Type article Authors Tollit, Dominic J.; Heaslip, Susan G.; Zeppelin, Tonya K.; Joy, Ruth; Call, Katherine A.; Trites, Andrew W. Download date 01/10/2021 12:43:29 Link to Item http://hdl.handle.net/1834/30930 ART & EQUATIONS ARE LINKED 498 Abstract—The lengths of otoliths and other skeletal structures recovered A method to improve size estimates of from the scats of pinnipeds, such as walleye pollock (Theragra chalcogramma) and Steller sea lions (Eumetopias juba- tus), correlate with body size and Atka mackerel (Pleurogrammus monopterygius) can be used to estimate the length of prey consumed. Unfortunately, consumed by pinnipeds: digestion correction otoliths are often found in too few factors applied to bones and otoliths scats or are too digested to usefully estimate prey size. Alternative diag- recovered in scats nostic bones are frequently recovered, but few bone-size to prey-size cor- Dominic J. Tollit1 relations exist and bones are also reduced in size by various degrees Susan G. Heaslip1 owing to digestion. To prevent under- Tonya K. Zeppelin2 estimates in prey sizes consumed techniques are required to account for Ruth Joy1 the degree of digestion of alternative Katherine A. Call2 bones prior to estimating prey size. 1 We developed a method (using defined Andrew W. Trites criteria and photo-reference material) 1 Marine Mammal Research Unit, Fisheries Centre to assign the degree of digestion for University of British Columbia, Room 18, Hut B-3 key cranial structures of two prey 6248 Biological Sciences Road species: walleye pollock (Theragra Vancouver, British Columbia, Canada, V6T 1Z4 chalcogramma) and Atka mackerel (Pleurogrammus monopterygius). The E-mail address (for D. J. Tollit): [email protected] method grades each structure into one 2 National Marine Mammal Laboratory of three condition categories; good, Alaska Fisheries Science Center fair or poor. We also conducted feeding National Marine Fisheries Service, NOAA trials with captive Steller sea lions, 7600 Sand Point Way NE feeding both fish species to determine Seattle, Washington 98115 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 Prey skeletal remnants from stom- There are at least three potential than smaller ones. Mean size reduc- ach samples and more recently from ways to deal with the effect of diges- tion varied between different types fecal (scat) samples are widely used tion on estimates of prey size. One is of structures (3.3−26.3%), but was to determine what pinnipeds eat to measure only relatively uneroded not influenced by the size of the prey consumed. Results from the observa- (Pitcher, 1981; Olesiuk et al., 1990; otoliths and assume that eroded oto- tions and experiments were combined Tollit and Thompson, 1996; Browne liths are from the same size fish as to be able to reconstruct the size of et al., 2002). Prey can usually be uneroded otoliths (Frost and Lowry, prey consumed by sea lions and other identified from taxon-specific hard 1986; Bowen and Harrison, 1994). pinnipeds. The proposed method has remains, the sizes of which often cor- Another is to apply a single species- four steps: 1) measure the recovered relate with the length and mass of specific digestion coefficient or correc- structures and grade the extent of the prey (Härkönen, 1986; Desse and tion factor (DCF), derived from feed- digestion by using defined criteria and photo-reference collection; 2) Desse-Berset, 1996). In the past, sag- ing experiments with captive seals exclude structures graded in poor con- ittal otoliths were commonly used to fed fish of known sizes and using dition; 3) multiply measurements of estimate prey size (Frost and Lowry, measurements of all the eroded oto- structures in good and fair condition 1981) but were recognized to erode or liths recovered in the scats produced by their appropriate digestion correc- become completely digested (Prime (Prime and Hammond, 1987; Harvey, tion factors to derive their original and Hammond, 1987; Harvey, 1989). 1989). The third is to estimate and size; and 4) calculate the size of prey from allometric regressions relating Thus, otolith measurements likely correct for the degree of digestion corrected structure measurements to underestimated sizes and numbers (based on defined losses of morpho- body lengths. This technique can be of fish ingested (Jobling and Breiby, logical features) of each recovered readily applied to piscivore dietary 1986), thereby preventing a reliable otolith by using estimates from ref- studies that use hard remains of assessment of overlap of prey con- erence material (Sinclair et al., 1994; fish. sumed with catch taken by commercial Antonelis et al., 1997) or by applying fisheries (Beverton, 1985). Accurate condition-specific DCFs derived from Manuscript submitted 28 April 2003 estimates of size of prey consumed by fish fed in captive seal feeding studies to Scientific Editor’s Office. pinnipeds are also important in order (Tollit et al., 1997). Manuscript approved for publication to understand foraging behavior and Of the three approaches to correctly 25 March 2004 by the Scientific Editor. to explain spatial and temporal vari- estimate prey size from skeletal re- Fish. Bull. 102:498–508 (2004). ability in diet composition. mains, there is the assumption with ART & EQUATIONS ARE LINKED Tollit 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 2 [SSL2] [ID no. F97SI], mean mass 150 kg) between degree of digestion is independent of otolith size, result- October 2000 and April 2002 at the Vancouver Aquarium ing in a potentially biased fraction. For certain species Marine Science Centre. Over the experimental period, it can also result in a notable reduction in sample size the sea lions were fed pollock for 52 days in 16 separate because relatively few otoliths pass through the gut feeding experiments, and Atka mackerel for 31 days in good condition. The second approach of applying in 5 separate feeding experiments, at between ~4−8% mean species-specific DCFs is an improvement to not of body mass per day. Fork length (FL) and weight of accounting for size reduction (Laake et al., 2002); how- all fish were measured to ±0.1 cm and ±1 g. Sea lions ever, there is the assumption with this approach that were fed meals of pollock of three size categories (small, all structures are reduced in size by the same amount. 28.5−32.5 cm FL; medium, 33.5−38.7 cm FL; large, Consequently, mean fish mass may be overestimated if 40−45 cm FL) and meals of Atka mackerel of one size such correction factors are applied to relatively undi- category (30−36 cm FL). Fish of one particular size cat- gested otoliths, or they may be underestimated if ap- egory were fed either as a single meal or as a seven-day plied to very digested otoliths (Hammond et al., 1994; block of meals. Full details of a typical experimental Tollit et al., 1997). The third method accounts for the protocol can be found in Tollit et al. (2003). Size ranges intraspecific variation in size reduction caused by di- for any category of fish fed within separate experiments gestion, reduces systematic error (see Hammond and were usually ≤3 cm. Fecal material was collected until no Rothery, 1996), yields estimates of mass that compare other remains of experimental meals were found (7 days favorably to those fed to captive animals (Tollit et al., after feeding), and was washed through a 0.5-mm sieve 1997), and hence may well be the most promising ap- to remove hard parts. Each animal was maintained on proach to reconstructing prey size. whole Pacific herring (Clupea pallasi) between experi- The dramatic decline of the western population of ments at ~6% body mass per day. Steller sea lions (Eumetopias jubatus) in the 1980s The strong relationship between fish size and otolith (Loughlin et al., 1992; Trites and Larkin, 1996) has size also exists for other skeletal structures (Desse and prompted a number of studies to determine what they Desse-Berset, 1996). Thus, we quantified the types and eat and the extent of dietary overlap (prey consumed) numbers of the prey structures recovered in the scats with catch taken by commercial fisheries. Stomach con- of free-ranging Steller sea lions (from the collections tents analysis was used to determine diet until the late of Trites et al.1 and Sinclair and Zeppelin, 2000) and 1980s when scat analysis became the preferred method selected seven of the most commonly occurring struc- (e.g., Pitcher, 1981; Frost and Lowry, 1986; Sinclair and tures for pollock and Atka mackerel. These were the Zeppelin, 2002). However, unlike in stomachs, there is sagittal otolith (OTO), as well as the interhyal (INTE), an overall sparsity of otoliths in Steller sea lion scats hypobranchial 3 (HYPO), pharyngobranchial 2 (PHAR), (Sinclair and Zeppelin, 2002) and, therefore there is a angular (ANGU), quadrate (QUAD), and the dentary need to also use other skeletal structures to describe (DENT). The structures selected also had particular the size of prey consumed. morphological features that seemed to be relatively The following outlines a method (using defined crite- resistant to digestion and could effectively be used to ria and photo-reference material) to assign the degree estimate fish size (Figs. 1 and 2, Table 1). of digestion for otoliths and alternative key skeletal Concurrent with our feeding study, we measured structures of walleye pollock (Theragra chalcogramma) selected structures (Figs.
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