Errors associated with otter Lutra lutra faecal analysis. I. Assessing general diet from spraints

Abstract

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.

Importance of small fishes and invasive crayfish in otter Lutra lutra diet in an English chalk stream

“…Notwithstanding, Kloskowski et al (2000) suggested that bones from larger fishes, such as C. carpio, are often under-represented in spraints and thus if larger fish were taken by otters within the study area then it might be that the methods used were not able to detect these. The lack of quantitative data on the fish assemblage also prevented the use of analytical methods such as relative frequency of occurrence and relative size frequency distributions (Carss and Parkinson, 1996). Thus, if managers require greater insights into otter predation in the river, then these potential confounding issues need to be addressed through further study.…”

Otter Surveys of the North West Scottish Islands

“…Percentage frequency of occurrence of prey remains is the most easily applied spraint analysis method, and it has been used in numerous studies Mason and Macdonald [35], Carss [21], Beja [36]. However, there seems to be little agreement among authors on precisely how to interpret the results.…”

“…Carss [21] also undertook feeding trials on four captive otters and attempted to put confidence estimates on the analysis of spraints using percentage frequency of occurrence. He found strong positive correlations between the values obtained for each of the ten prey groups and their true proportions in the diet, but found that this gave a poor estimate of the true proportions of prey by number or biomass for the ten prey groups.…”

“…He therefore concluded that for prey which does leave remains, percentage frequency of occurrence allowed dietary items to be ranked with a high degree of accuracy, but gave poor estimates of the true percentage proportions of the various prey groups in the diet. Carss [21] also found that prey was over-estimated if sequentially deposited spraints are collected regularly from frequently used sites.…”

“…No method of expressing the results of spraint analysis describes the diet of otters accurately [16][17][18][19][20][21]. However, the two most commonly used methods are described by Conroy et al [11].…”