Data from: Dietary complexity and hidden costs of prey switching in a generalist top predator

Summary 1. Variation in predator diet is a critical aspect of food web stability, health, and population dynamics of predator / prey communities. Quantifying diet, particularly among cryptic species, is extremely challenging however, and differentiation between demographic subsets of populations is often overlooked. 2. We used prey remains and data taken post mortem from otter Lutra lutra to determine the extent to which dietary variation in a top predator was associated with biotic, spatial and temporal factors. 3. Biotic data (e.g. sex, weight, length) and stomach contents were taken from 610 otters found dead across England and Wales between 1994-2010. Prey remains were identified to species where possible, using published keys and reference materials. Multi-model inference followed by model prediction were applied to test for and visualise the nature of associations. 4. Evidence for widespread decline in the consumption of eels (Anguilla anguilla) reflected known eel population declines. An association between eel consumption and otter body condition suggested negative consequences for otter nutrition. Consumption of Cottus gobio and stickleback spp. increased, but was unlikely to compensate (there was no association with body condition). More otters with empty stomachs were found over time. Otter sex, body length, and age class were important biotic predictors of the prey species found, and season, region, and distance from the coast were important abiotic predictors. 5. Our study is unique in its multivariate nature, broad spatial scale, and long-term dataset. Inclusion of biotic data allowed us to reveal important differences in costs and benefits of different prey types, and differences between demographic subsets of the population, overlaid on spatial and temporal variation. Such complexities in otter diet are likely to be paralleled in other predators, and detailed characterisation of diet should not be overlooked in efforts to conserve wild populations.Individual X and Y coordinates were used in earlier data preparation steps to identify Region and Distance to coast (data provided). X and Y coordinates themselves are not not directly used in our models, and are not provided, because of sensitivities associated with otter conservation status. Funding provided by: Natural Environment Research CouncilCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000270Award Number: NE/ K00719X/1Funding provided by: Natural Resources WalesCrossref Funder Registry ID: http://dx.doi.org/10.13039/100009261Award Number: Funding provided by: European CommissionCrossref Funder Registry ID: http://dx.doi.org/10.13039/501100000780Award Number: Leonardo da Vinci programmeOtters found dead were collected across England and Wales between 1994 and 2010, as part of a national scheme coordinated by Cardiff University. The entire stomach and intestine were taken during post mortem examination of 610 otters, and frozen at -20°C. The year, month and location where each otter was found were recorded. Distance to the coast via the nearest river (river distance) was measured using ArcMap GIS (V.9.2, ESRI 2006). Where otters were found more than 1000 m from a river, river distance was omitted. Each otter was assigned to one of eight regions, based on aggregations of adjacent river catchments. Each otter was examined in detail at post mortem; sex, age class (adult, sub-adult, juvenile), reproductive status (females only), weight (kg, to the nearest 10 grams), length (mm, to the nearest 5 mm, measured nose to anus and anus to tail tip) and cause of death were determined. Morphometric and reproductive data were used to assign age class, such that males below 3 kg and females below 2.1 kg are recorded as juveniles. Above these weights, otters are considered adult if reproductively mature (males with baculum length ≥60 mm, females with signs of reproductive activity such as placental scarring, prominent teats), and sub-adult if reproductive maturity is not apparent. Based on examination of the uterus and teats, female reproductive status was categorised as never reproduced, quiescent, pregnant, or lactating. Length and weight were used to calculate a condition index (K = W/aLn, where W = total body weight in kg, L – total length in m; for males a = 5.87 and n = 2.39; for females a = 5.02 and n = 2.33). Stomach and intestine samples were defrosted and rinsed through a fine sieve. Recognisable dietary items (e.g. feathers, fur, etc) were recorded. Remaining solid material was transferred to a liquid detergent solution (water:detergent, 10:1) to aid removal of soft tissues. Samples were filtered repeatedly, and detergent solution replaced until only hard prey items remained. After air drying, prey items were examined using a binocular microscope (Leica 2000, x7-x30) and identified with the aid of published keys and by comparison with reference material. Prey items were identified to species level whenever possible.