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With 4 figures in the text)Althoug frequency of occurrence (either as percentage or relative frequency) is the most common method of expressing the content of otter Lutra lutra faeces (spraints), the accuracy of the method, and the effects of varying sampling procedures (e.g. inter-collection interval) and sample sizes, have not been quantified. The validity of the technique was assessed in the present study by feeding trials involving captive, tame otters and computer simulation of various spraint sub-sampling regimes. Four animals were fed known quantities (numbers and biomass) of a total of nine fish species, two bird species and one mammal over a 28-day period. Most prey remains were passed in spraints within 24 h, although perch Percafluviatilis scales appeared up to 10d after consumption. Remains from single meals of perch were recorded in 60 subsequent spraints from two otters, and remains of individual eels Anguillu anguillu were recorded in up to 11 spraints. Some single spraints contained the remains of up to seven individual salmonids, Salmo spp. Minnows Phoxinus phoxinus placed within the body cavities of larger rainbow trout Onrorhynchus nzykiss were easily identified in spraints, as were the remains of Dytiscus spp. beetles which were not included in trial meals. The latter confirms that otters actively consumed large free-swimming insects. Spraint analysis accurately determined the rank order of prey groups for individual otters and for all four combined. However, few of the true proportions consumed fell within the 95% confidence limits calculated from spraints. Over the month-long trial, the overall picture of otter diet was altered little by increased inter-sampling period for spraints. But as samples were reduced, coefficients of variation for the mean estimates of each prey group increased and were often too large for estimates to be meaningful. It is not possible to quantify otter diet accurately by frequency of occurrence methods, and the results of previous studies attempting to quantify the amount of a specific prey item consumed by otters using this method must be treated with caution. Diet could be estimated more accurately from spraint analysis by concentrating on the main prey species and using keybones, which are resistant to digestion, to determine relative size-frequency distributions.

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The interactive effects of cooking and starch type on the dry matter, gross energy and protein digestibility of diets fed to fingerling silver perch were examined. Nine experimental diets were made: a reference diet either uncooked or cooked, six other diets comprising uncooked reference diet supplemented with 30% uncooked or cooked wheat, maize or potato starch and one diet comprising uncooked reference diet supplemented with 15% pregelatinized maize starch. Cooking involved autoclaving at 121 8C for 15 min. The indirect method was used to calculate apparent digestibility coefficients, and chromic oxide (1%) was used as the inert indicator. Both ingredient type and cooking influenced dry matter and gross energy digestibility of diets. All diets had significantly different digestibility coefficients in the following order (from most to least digestible): reference > wheat > maize > potato, for both dry matter and gross energy. Cooking significantly increased the apparent digestibility of diets for both dry matter and gross energy. There was no significant effect of ingredient or cooking or their interaction on protein digestibility. Dry matter and energy digestibility of the diet containing pregelatinized maize starch were significantly higher than for diets containing cooked or uncooked starch products. Ingredient digestibility followed the same trend as diet digestibility. The results from this study indicate that the successful use of starch in practical diets for silver perch is dependent on the origin and processing of the starch. The significant benefits of cooking in the reference diet suggest that pelleted diets should be cooked (e.g. steam conditioned or extruded).

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