DOI: 10.5061/dryad.5dv41ns3k
title: Data from: Dietary complexity and hidden costs of prey switching in a generalist top predator

description: 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.
description: Otters 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/aL<sup>n</sup>, 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..
description: 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.