Predation by marine-feeding otters (Lutra lutra) in south-west Portugal in relation to fluctuating food resources

Patterns of predation by otters ( Lutra lutra ) inhabiting the Portuguese south-west rocky coast, were inferred from the analysis of 757 spraints collected along a 5-km stretch of shore, from November 1990 to October 1993. Undigested remains were identified, counted and measured; the length and weight of ingested prey was estimated from regression equations. Monthly fluctuations in diet were compared to changes in the abundance of the primary prey species along the coast. The most commonly taken fish species had median lengths between 6.7cm and 15.0cm, and median weights between 3.5 g and 34.2g. Corkwing wrasse dominated the diet in terms of biomass (51.6%), and was the most important prey in terms of numbers (22.6%). Gobies (19.4%), Valenciennes' blennies (15.0%), shannies (1 1.4%), crayfish (7.2%), and shore rocklings (4.1%) were other numerically important prey. The contribution in biomass of gobies (14.5%), shore rocklings (6.7%) and conger eels (5.5%) was also significant. The dominant prey species and sizes in the diet were consistent over the years. From late winter to early spring the diet was numerically dominated by corkwing wrasse and gobies, whereas for the rest of the year shannies, Valenciennes' blennies and crayfish were also important. The contribution of these last species in biomass was small at all times of the year, and the corkwing wrasse was always the dominant prey. Prey was twice as heavy in winter as in spring. There was close correspondence between seasonal variation in diet and fluctuations in the abundance of the primary prey along the coast, but most dietary variation could be accounted for simply by fluctuations in the abundance of corkwing wrasse. In south-west Portugal, the most important food resource for otters seems to be the corkwing wrasse, probably because they are large and easy to catch. Corkwing wrasse are also exploited by local fishermen, and the possibility of competition between otters and humans should be investigated.     

Effects of freshwater availability on the summer distribution of otters Lutra lutra in the southwest coast of Portugal

The availability of freshwater has been suggested to strongly influence the distribution of coastal otters Lutra lutra The test this hypothesis, a study was undertaken on the relationships between otter distribution and freshwater availability in a coast where freshwater is a very scarce resource during the summer In this area otters occur mostly in a few small coastal streams but feed largely in the sea Twentynine streams were surveyed in the summer of 1990, and the presence/absence of otter signs was related to nine habitat variables Freshwater availability was found to be the most important factor influencing the occurrence of otters in summer It is suggested that otters favour large streams with good vegetation cover, for these are the most likely to maintain frestwater during the dry periods The distribution of otter signs m streams was also analysed, and it was found that signs tend to concentrate particularly near the mouth of the streams This habitat analysis has some conservation implications, indicating that the decrease of freshwater availability or quality of streams can make these unsuitable for otters, severely decreasing the areas suitable for the species in the southwest coast of Portugal     

Diet of otters (Lutra lutra) in closely associated freshwater, brackish and marine habitats in south-west Portugal

The diet of otters was studied in closely associated freshwater, brackish and marine habitats, from spraints collected on the Portuguese south-west coast over an 18-month period. In areas where marine prey was the only available resource, diet was dominated by blennies, wrasses and gobies, but other prey was taken in areas near coastal lagoons, marshes and estuaries. Eels and amphibians were considered the typical prey of freshwater habitats, and grey mullet the typical prey of brackish water habitats. If the inland habitats near the coast were large enough, otters preferred to forage there rather than in the sea. In one area where otters alternatively used marine and inland habitats, the former were used most extensively in autumn and winter, and the latter in spring and summer. It is suggested that otters prefer to forage inland rather than in the sea.     

The spatial organization of otters (Lutra lutra) in Shetland

The dispersion of otters Lutra lutra was studied along the coasts of an area of Shetland. The population was relatively stable, with about one adult otter per kilometre of coast. They fished diurnally in a strip of water usually within 100 m of the shore. Females lived in exclusive group ranges, occupied by up to four females, with well-defined boundaries which were respected by successive occupants. For two groups, where relatively complete information was available, range size was 4.7 and 6.4 km, occupied by two and four females, respectively; a third range was about 14 km, with possibly 4–5 females. Within the group ranges individual females moved on their own or with their cubs. Each had a core area, where they were observed > 50% of the time; these core areas were separate from each other, although each individual used the entire group range. Several or all females in each range bred in any one summer. Male ranges were larger than those of females, and overlapped with two or more female group ranges; there were several resident males along each section of coast, but the information on males was incomplete. The habitats of the sexes were different; males spent more time on exposed coasts than did females. There were frequent visits to all coasts from non-resident otters of both sexes.
Behavioural interactions related to territoriality are described; otters showed little overt aggression, except between adult males, but there was some individual avoidance. It is argued that the survival value of this spatial organization can be understood in terms of resource dispersion, but several types of resources have to be considered (including prey and fresh water), and detailed predictions of spatial organization from resource dispersion are unwarranted.     

Scent marking by otters (Lutra lutra): signaling the use of resources

Otters (Lutra lutra) deposit feces as scent marks ("spraints")throughout their range, and observations on this behavior inShetland were used to test the hypothesis that carnivores ingroup ranges use scent marking to signal priority of use ofresources to other group members. Sprainting was seasonal (highrates coinciding with low prey availability), and there wasno significant, overall difference in sprainting rates betweenotters of different sex or status. There were no concentrationsof spraints near group territorial boundaries. Sprainting wasassociated with the beginning of feeding bouts, as well as withthe utilization of other resources, such as fresh water anddens ("holts"). More than 30% of spraints were deposited inplaces that flooded within hours, and the spraints were functionalonly for a short time. It is argued that the temporal patternof use and subsequent replenishment of resources makes it advantageousfor otters to signal to other group members when they are exploitinga "patch" and for other members to avoid resources already partlydepleted by a prior arrival. With such a signaling system thereis no need for actual aggressive encounters to reinforce themessage of scent marking. Sprainting could be the mechanismfor the observed spaced-out use of resources among the inhabitantsof a group territory.     

Seasonal and spatial differences in food selection by otters (Lutra lutra) in Shetland

Otters Lutra lutra in Shetland feed almost exclusively in the sea. Their diet was assessed from direct observations of 13, 313 dives, of which 27% were successful, and 2028 prey could be identified. The results are compared with published data on the availability offish for different seasons, time of day, state of tide, and different types of coast. The most frequently taken prey was eelpout Zoarces viviparus (34%), but they were relatively small, and rocklings Ciliata mustela dominated the diet in terms of biomass (18%, against eelpout 15%). Small eel-shaped, bottom-living species dominated in the diet at all times, but of those, otters took the larger fishes in the populations (median weight of prey 28 g). The size and species composition of prey of males was the same as that of females with cubs, but females without cubs took smaller fish.
In spring, prey was smaller than at other times, and otters were least successful when diving. Diving success and mean prey weight was highest in winter, although prey availability was highest in summer.
Eelpout were caught mostly along sheltered coasts, rocklings in more exposed areas. It is argued that for optimal exploitation of the seasonal availability of different species of prey, otters need to use different types of coast, and the size of their ranges would be related to the spacing of coastal types.
Most fishes were caught in daytime, and around high tide otters fed less than at other times; hunting therefore took place during periods of inactivity of prey. For the main prey species, Ciliata mustela , it was estimated that otters took a substantial part of the total population.     

Factors influencing use of freshwater pools by otters, Lutra lutra, in a marine environment

Eurasian otters ( Lutra lutra ) inhabiting a marine environment need to wash in freshwater in order to maintain the insulating properties of their fur. Use of freshwater pools by otters on the coast of Skye was studied in order to determine factors influencing pool choice. A number of characteristics were recorded for all pools present. Otter spraints were recorded as an indication of pool use. Only 34% of available pools were utilized by otters. Stepwise logistic regression revealed that choice of pools is influenced by pool depth, the percentage of a pool's substratum composed of flat rock, and the surrounding percentage cover of short grass. Possible reasons for these associations are discussed. The information may be of use when selecting and managing coastal areas for otter conservation.     

Some factors influencing the distribution of otters (Lutra lutra)

The number of sites at which evidence of otters was found and the total number of signs were recorded on fifty 5-km stretches of rivers in Wales and the West Midlands of England. Data were collected on fifteen parameters of habitat considered to be of importance to otters and subjected to a multivariate analysis.
Evidence of otters was found at thirty-four stretches. The distribution of sites and signs was correlated with the presence of potential holts (of which 107 were found) and with mature sycamore and ash trees. The measured variables accounted for 52 5% of the overall variation in otter sites and 60 2% of the variation in otter signs. The conservation of otters is discussed in light of the results of the analysis.     

Grooming and resting of otters Lutra lutra in a marine habitat

The weight gain in lactating harbour seal pups and sex-specific growth curves are described. The relationship between body length, body weight and age were derived by regression analysis based on length and age data from 365 seals, and weight values from 136 seals. The asymptotic values of the curves describing body length were 148.0 cm and 147.2 cm in females and 153.9 cm and 155.5 cm in males using Gompertz and von Bertalanffy, respectively. The corresponding body weight values were 72.8 kg and 76.7 kg in females and 90.7 kg and 88.4 kg in males.     

Seasonal and area-related variations in the diet of otters Lutra lutra on Mull

The diet of otters Lurra lutrn living in a sea loch on the Isle of Mull was described by means of spraint analysis. Seasonal and spatial differences in the spraint composition were explored in the context of fluctuations in prey availability, as revealed by trapping. Butterfish Pholis gunnellus and the cottids Myoxocephalus scorpius and Taurulus bubalis were the most commonly occurring prey in spraints. In the winter, the proportion of rocklings ( Ciliata spp. and Gaidropsarus spp.) and non-rockling gadoids (Gadidae) in the diet increased. Their relatively large size makes these latter species an important constituent of the winter diet. Seasonal and spatial variation in fish consumption largely reflected fluctuations in availability. The mean size of principal prey fishes in spraints was similar to that observed in the traps. These observations suggest that otters were not strongly selective with regard to fish species and size. Shore crabs, in contrast, were not taken in proportion to availability. The findings are discussed in the light of other studies of the diet of coastal otters. It is suggested that the lack of selectivity demonstrated in this study, compared with previous studies, may be explained by area-related variations in prey availability.     

Habitat factors and the presence or absence of otters Lutra lutra in Denmark

The purpose of the present study was to examine the relative influence of different habitat factors on ottersLutra lutra (Linnaeus, 1758) and to develop a predictive model to better understand the distribution of the otter in Denmark. During the National Otter Survey in 1991 data were collected on 19 variables which reflected aspects of habitat structure, composition, organic pollution and human disturbance. Multiple logistic regression analysis was used to estimate probabilities of the presence of otters as a function of one or more explanatory variables. Six variables (county, pH, water depth, presence of trees, bottom substrate and Saprobien-Index) were identified. In Denmark, otter habitat typically consists of water courses with depths > 1 m over a varied bottom, with pH > 7.0, Saprobien-Index on II–III to III (indicating slight organic pollution) with no trees on the banks. Some of these variables reflect highly productive waters. The use of the otter as an indicator of good water quality and/or aquatic habitat should be used with care.     

Seasonal exploitation of amphibians by otters (Lutra lutra) in north-east Scotland

The dietary importance of amphibians to otters was studied in north-east Scotland in 1987. The proportion of amphibians identified in otter droppings varies seasonally and according to the habitat. The consumption of amphibians increases in late winter and spring while they are either hibernating or spawning. Estimations of amphibian population density at spawning sites have been carried out. There is a significant correlation between the availability of amphibians and their consumption by otters. The main amphibian species involved in the diet is the common frog, Rana temporaria (60.8%), while the common toad, Bufo bufo , represents only 1.7% of the amphibian prey.     

Spacing and foraging of otters (Lutra lutra) in a marine habitat

The distribution of otter dens (holts) and otter droppings (spraints) which also function as scent markers, is described for an area of coast in north-west Scotland, and some observations are presented of the otters' movements and foraging. Otter holts were spaced out at an average distance of 11 km, and spraints were concentrated around the holts. The foraging areas were probably used communally, and the difference between this utilization and that observed elsewhere is discussed.