ORIGINAL PAPER John W. Lawson á Jason A. Hare á Elizabeth Noseworthy James K. Friel Assimilation ef®ciency of captive ringed seals (Phoca hispida) fed different diets Received: 30 September 1996 / Accepted: 28 December 1996 Abstract Food type or quality can in¯uence assimila- tion eciency (AE). AE (digestibility of dry matter) of two captive adult ringed seals (Phoca hispida; one male, one female) was estimated for ®ve prey types. In trials, each of >8 days duration, the seals were fed red®sh (Sebastes spp.), capelin (Mallotus villosus), Arctic cod (Boreogadus saida), Atlantic herring (Clupea harengus) and a mixture of herring and shrimp (Pandalus boreal- is). Prey were marked so that faecal samples could be matched to individual seals, and AE was estimated by the relative concentration of Mn 2+ in food and faeces. AE was high but varied among the prey species (red®sh 83%; capelin 87%; Arctic cod 88%; herring 94%; herring/Pandalus mixture 92%). There was a weak, positive relationship between AE and prey lipid content or energy density, but a negative relationship with in- organic content. AE was lower than expected for cape- lin with high fat content. AE was not correlated with meal mass, number of ®sh in a meal, or seal mass. AE did not dier between the two seals. Introduction Food quality, meal size, time between meals, digestive tract morphology, and nutritional state can all aect the eciency with which animals digest their food (e.g. Golley et al. 1965; Heath and Randall 1985; Jackson 1986; Swart et al. 1993; Brekke and Gabrielsen 1994). Estimating prey requirements of a predator population using an energy model necessitates that digestive e- ciency be quanti®ed (Croxall et al. 1990; Olesiuk 1993; Balmelli and Wickens 1994; Stenson et al. 1995). In particular, estimates of prey requirements are needed for predator-prey systems in which predators are abundant and widespread; such a predator is the ringed seal (Phoca hispida). Ringed seals are the most numerous pinnipeds in the northern hemisphere, and are found throughout the Arctic (Finley 1979; Finley et al. 1983; Lydersen and Gjertz 1987; Harwood and Stirling 1992), and into Newfoundland waters in the northwest Atlantic (Sten- son 1994). The diet of this species varies seasonally and regionally, and consists mainly of small ®sh such as Arctic cod, capelin and crustaceans (Lowry et al. 1980; Smith 1987). Given their large population and diverse feeding habits, ringed seals are likely a signi®cant con- stituent of Arctic marine ecosystems. Assimilation e- ciency (AE) is an important measure required to produce estimates of ringed seal prey consumption using bioenergetic models. This study was undertaken to quantify ringed seal AE (digestibility of dry matter) on several prey species. One way to estimate AE is by determination of the ratio of inert markers in ingesta and faeces (Kleiber 1975). Naturally occurring manganese (Mn 2+ ) satis®es the requirements for an inert dietary marker (Friedman et al. 1987), so changes in AE are re¯ected in Mn 2+ concentration in the faeces. Mn 2+ has been used to estimate AE in a number of pinniped species (Fadely et al. 1990; MaÊ rtensson et al. 1994a; Lawson et al. 1997), including dead specimens (MaÊrtensson et al. 1994b). Ecologically important in¯uences on AE include food quality (de®ned in this paper as lipid content or energy density) and meal size. AE is low when food quality is low (Maiorino et al. 1986; Brekke and Gab- rielsen 1994). For example, harp seals (Phoca groen- Polar Biol (1997) 18: 107 ± 111 Ó Springer-Verlag 1997 J.W. Lawson (&) á J.A. Hare á E. Noseworthy Ocean Sciences Centre, Memorial University, St. John's, Newfoundland, Canada A1C 5S7 Fax: (709) 737 3220; e-mail: johnwl@morgan.ucs.mun.ca J.W. Lawson Psychology Department, Memorial University, St. John's, Newfoundland, Canada A1C 3X9 J.A. Hare Biology Department, Memorial University, St. John's, Newfoundland, Canada A1C 3X9 J.K. Friel Biochemistry Department, Memorial University, St. John's, Newfoundland, Canada A1C 3X9