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Article

Eurasian Otters’ Urban Pond Use Patterns in Southern Spain: A Case Study

by
Jesús Duarte
1,*,
Diego Rodríguez
2 and
Miguel Ángel Farfán
3
1
Ofitecma Marbella SL, Avda. Ramón y Cajal 17, 29601 Marbella, Spain
2
Rewilding Spain, 19431 Corduente, Spain
3
Department of Animal Biology, Faculty of Sciences, University of Málaga, 29071 Málaga, Spain
*
Author to whom correspondence should be addressed.
Wild 2025, 2(4), 46; https://doi.org/10.3390/wild2040046
Submission received: 11 May 2025 / Revised: 19 October 2025 / Accepted: 13 November 2025 / Published: 18 November 2025
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Simple Summary

The presence of wild species in urban areas is becoming increasingly frequent. This trend may be due to animals favoring areas where resources like food are usually available and easy to exploit, or to the relative safety of urban areas during nighttime hours. However, there is little information on the patterns of resource use by these animals, and especially on whether these patterns differ between urban and natural settings. Using the Eurasian otter as a model, we have analyzed the differences in its pattern of visitation and prey capture between natural and artificial pools in urban environments. Otters use urban pools more compulsively than those in natural areas, with a prey capture pattern that is not random and seems to reduce prey numbers quickly. The otters’ use of these areas also creates a conflict with human interests that could lead to conservation problems.

Abstract

As human activities such as urbanization encroach on natural areas, some wildlife species adapt to these changes and learn ways to utilize newly available resources. We monitored the use patterns of the Eurasian otter in three urban ponds in southern Spain (Málaga province). We compared weekly otter visits and relative spraint abundance between the urban ponds and two control ponds located in natural areas, testing for differences in use patterns between them using Generalized Linear Mixed Models and the Wald–Wolfowitz run test. We also estimated prey survival rates through Kaplan–Meier estimator curves. We also assessed problems of coexistence with human interests. Relative spraint abundance was not affected by pond type (urban or natural). However, the number of otter visits was lower for urban ponds, and the pattern was not random but concentrated over a short time until prey depletion, suggesting consistent use of the urban feeding patches. Available food resources in urban settings can become a viable option for otters, which appear to explore urban habitats when it suits them. However, in the monitored urban ponds, otters competed with human interests and generated a conservation problem that almost led to lethal measures.

1. Introduction

Wildlife species are increasingly found in urban environments [1,2,3,4,5]. As human activities such as urbanization encroach on natural areas, some wildlife species adapt to these changes, primarily by finding efficient ways to utilize the newly available resources while continuing to live in wild habitats [6].
This new presence of wild species in urban environments can present potential conflicts with human interests. The use of habitats where human presence is common, as well as the use of urban resources by these species, often leads to a human–wildlife conflict and the need to mitigate damage or prevent the presence of certain wild species. The cases of ungulates in urban environments represent clear examples of these conflicts [7,8,9]. Cases of urban mesopredators are less understood, but also exist [10,11,12,13,14] and require new conservation measures adapted to new scenarios [15,16].
Wild animal species’ adaptation to urban environments can change their physiology, reproductive strategies, and overall fitness. In most cases, a species will adapt its behavior to the urban environment [17,18]. A common consequence of a species entering a novel environment is an increase in boldness [19]; some animals may display more exploratory behaviors and be less fearful of humans than others [15,20]. This allows animals to access the more predictable environment typical of urban settings, where they may be able to find more abundant resources [21,22].
The probability that an urban area is visited by a wild species will depend on the overall layout and design of its environment. For example, a matrix of natural-habitat patches within built-up developments is more likely to be invaded [23]. The type of vegetation and the presence of water and food are also important features that can attract animals. Urban parks usually meet these criteria by providing sources of anthropogenic food [24].
The Eurasian otter (Lutra lutra) is a semi-aquatic carnivore species linked to aquatic ecosystems and can be found from coastal waters to high-elevation streams [25]. Although marine environments are not favorable for the species due to waves and the effects of salinity [26], it will take advantage of shallow marine environments due to their high productivity when freshwater sources are also available. However, the otter’s typical habitat is the middle reaches and mouths of rivers, lakes, lagoons, marshes, and reservoirs [25,27], although it also occurs in artificial watercourses [28]. In southern Spain, otter habitats include ponds, ditches, river mouths, dams, weirs, and artificial lakes in golf courses [28,29,30].
The availability of food, shelter, and water is a limiting factor for the species’ presence [31]. As a possible consequence of this, the species often uses a combination of various environments, frequently using rivers and reservoirs within a single territory [27] and visiting areas with greater amounts of available food [32], especially in the case of females with pups. In fact, although otter populations tend to occur at low densities, there is a positive correlation between the species’ abundance and the productivity of the ecosystem it inhabits [32].
The otter primarily consumes fish [33], although it has a broad and diversified diet, also capturing crayfish (Procambarus clarkii), amphibians, reptiles, and other prey [34,35]. This species can be a piscivorous specialist or a more generalized, opportunistic predator, depending on the prevailing environmental and climatic conditions [33]. When the habitat is unstable, as is the case with the river environment in the Mediterranean basin, the quantity of crabs consumed increases [36]. In contrast, the otter’s diet is richer in fish and more diverse in more stable habitats [37]. In Mediterranean environments, the otter primarily uses stretches of freshwater streams during spring and summer, where it captures crayfish [35], moving to river mouths and other more open water environments during the rest of the year, where it can consume more fish. Its consumption of exotic fish is notable in reservoirs [38].
Although we present evidence herein of the use of urban habitats by the otter during a study period from 2017 to 2019, the presence of otters in the urban environments of southern Spain is not a recent phenomenon. The species’ presence in urban ecosystems appears to have increased over the past two decades [39,40,41]. In the south of Spain, there are records of road-killed otters on urban roads, evidence of animals permanently inhabiting golf courses surrounded by urban areas [42] and using harbors in highly urbanized areas, and clear signs of pond use in urban parks where otters have been able to deplete fish [43].
The purpose of this study was to test whether otter predation patterns and use of urban ponds differed from those of natural ponds. Our main hypothesis was that otters behave similarly in both types of pond; although the use of urban pools may be more sporadic, we expected to find a similar usage pattern. We also discuss the role of urban environments in providing otters with resources, and whether the presence of otters in urban areas may conflict with human interests and thus require mitigation measures.

2. Materials and Methods

2.1. Study Area

The study area consisted of three urban and two natural (control) locations (Figure 1, Figure 2, Figure 3 and Figure 4) in the province of Málaga, southern Spain. The urban ponds are located in Torre Leoneras park, Campanillas private estate, and Nueva Andalucía private estate (hereafter referred to as urban ponds A, B, and C, respectively). Urban pond A (actually comprising two ponds in close proximity) is located in an urban park in the town of Benahavís; the park has a surface area of 8400 m2 and contains two ponds. The nearest inhabited areas are 128 m away from the ponds and 35 m from the park perimeter. One pond spans 300 m2 with no perimeter fence, and the other spans 140 m2 and is enclosed by a metal fence 1.5 m high (Hercules rigid fence panels, 200 × 50 mm mesh size). The depth of the ponds does not exceed 0.5 m. In 2017, the local council introduced the common carp (Cyprinus carpio) into both ponds. The carp is a widespread domesticated freshwater fish that is common in eutrophic and human-altered environments worldwide, and is often an invasive species [44].
On 1 May 2017, 30 small common carp (16.2 ± 0.5 cm long from head to tail, mean ± SE) were introduced into the larger pond. In late April 2018, 35 Japanese koi (Cyprinus carpio koi, also called “brocaded carp”), a more colorful and larger variety of the common carp, were released into the smaller pond. These fish measured 72.6 ± 0.8 cm from head to tail.
The park is hedged around its perimeter and also fenced with Hercules rigid panels. Vegetation inside the park is mainly composed of native plant species, such as carob (Ceratonia siliqua), willow (Salix pedicellata), maritime pine (Pinus pinaster), wild olive tree (Olea europaea var. sylvestris), and black poplar (Populus nigra). Oleander (Nerium oleander), as well as the perennial and decorative papyrus sedge (Cyperus papyrus), is typically found around each pond. A weeping willow (Salix babylonica) also grows near the smaller pond.
The park is located near the Guadalmina riverbank. This river was declared a Special Area of Conservation of the Natura 2000 network (ZSC code: ES6170021 Río Guadalmina) to protect otters as well as other endangered species. Vegetation consisting of willows and other species found inside the park borders the river. An irrigation ditch in the riverbank farthest from the park is linked to a small dam 2 km upstream and a cultivated area near the river mouth (7 km from the park). The river has a reduced water channel during summer, with dry stretches interspersed with small ponds where fish tend to concentrate. The irrigation ditch always contains water during summer, although not enough for fish to survive. Common fish species in the Guadalmina river include Andalusian barbel (Luciobarbus sclateri), Malaga chub (Squalius malacitanus), and southern straight-mouth nase (Pseudochondrostoma willkommii) [45,46]. Mediterranean turtles (Mauremys leprosa), frogs—predominantly Iberian water frogs (Pelophylax perezi)—and crayfish are also found in the river and ditch.
Urban pond B is located in a private estate in an exurban area near Benahavís that is surrounded by a forest matrix in which homes, gardens, and green zones are interspersed. The estate includes a large garden with a 91-m2 artificial pond less than 1 m deep. The garden is surrounded by Mediterranean scrubland, including carob trees, cork oaks (Quercus suber), and maritime pines. Two small streams line the property. To the east is La Leche stream, which crosses the Los Arqueros golf course, where otters have been reported. To the west is the Puerco stream, which crosses another golf course and empties into the La Leche reservoir, like the eastern stream. Otters are present in this reservoir (Duarte and Rodríguez, pers. obs.), which is 2.2 km from the nearest dwelling. Both streams are less than 200 m from the pond. Although the garden pond does not contain fish, it is home to the Iberian water frog, Mediterranean tree frog (Hyla meridionalis), and crayfish.
Urban pond C is located in a residential complex of chalets located in a peri-urban area of the city of Marbella. This area is surrounded by several golf courses in which dwellings are interspersed with green zones and sporting facilities. One of the properties contains a Japanese garden with a 60-m2 artificial pond less than 0.3 m deep, into which the owner would release goldfish (Carassius auratus) less than 15 cm long from head to tail. Iberian water frogs and Mediterranean tree frogs are also present in the garden. This pond is located near the Benavolá stream, a narrow urban watercourse that crosses one golf course and connects two small reservoirs: El Ángel and Lagomar. Otters are present in both reservoirs (Duarte and Rodríguez, pers. obs.) and are frequently seen in the Benavolá stream. The stream is bordered by willows (Salix spp.), salt cedars (Tamarix spp.), and other ornamental garden trees. The pond contains water lilies (Nymphaea alba).
We established two control locations representing natural habitats for the species and consisting of small reservoirs in which otters were already present. These were the Guadalmina reservoir and the El Ángel reservoir (hereafter referred to as natural ponds Co1 and Co2, respectively). Natural pond Co1 is 1300 m from urban pond A and 2200 m from urban pond B, while natural pond Co2 is 1500 m from urban pond C.

2.2. Study Period and Data Collection

The study period ranged from early May 2017 to early October 2019—i.e., spring–summer, when drought causes Mediterranean stream levels to fall—with different ponds studied at different periods (see Table 1 for details of the study periods). We systematically monitored the presence of otters in the urban and natural ponds, as well as their effect on fish populations, following the UICN guidelines [47]. Our daily monitoring of all the study ponds was conducted by two teams (the municipal gardening staff and our team) by counting the number of fish present and looking for spraints and prey remains.
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Figure 4. Photos of some of the studied ponds, including urban ponds A and B (in Torre Leoneras and Campanillas, respectively), and natural pond Co1 (the Guadalmina reservoir plunge pool).
Figure 4. Photos of some of the studied ponds, including urban ponds A and B (in Torre Leoneras and Campanillas, respectively), and natural pond Co1 (the Guadalmina reservoir plunge pool).
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2.3. Urban Pond “A” Monitoring

In urban pond A, we monitored the ponds following the release of fish. We also installed three camera traps (Browning Strike Force HD cameras, Browning®, Birmingham, AL, USA) around the perimeter of the smaller pond to identify predators and estimate how frequently they fished in the pond. We considered predation events to be indicated by the presence of prey remains (e.g., a dead fish, or its scales or partially eaten remains) or a decline in the number of carp in the ponds. Direct predator sightings in the ponds were also recorded, and spraints were sampled and removed daily. Monitoring was halted when predation episodes ended.

2.4. Urban Pond “B” and “C” Monitoring

We monitored the shores of urban pond B daily and searched for otter spraints and amphibian remains. We counted and removed spraints and amphibian remains daily until they stopped appearing. At this pond, neither camera trapping was carried out nor frog population size estimated.
During the first episode of monitoring in urban pond C, the gardening team detected the presence of an otter following a release of goldfish. Fish were depleted, but no monitoring was carried out in this first year. The owner decided to release goldfish again the following year. This second year, daily monitoring was carried out as described in Section 2.3: we recorded any potential disappearance of fish as well as collecting data on the presence of spraints, scales, and fish remains on the pond shore. The number of goldfish in the pond was counted daily, and spraints were counted and removed. Camera trapping was not conducted at this pond.

2.5. Natural Pond Monitoring

In the control locations, we sampled only the plunge pools located at the toe drain outlets of the two reservoirs. These pools accumulated water that drained from the dam downstream. The pools were normally shallow (similarly to the urban ponds, at less than 1 m deep) and usually accumulated fish from the dam. Otters use these types of habitats for marking and fishing [31]. In both locations, we counted and removed otter spraints from the pool shores weekly. Additionally, in natural pond Co2, we conducted camera trapping by installing Browning Patriot cameras at two different downstream locations near the plunge pool. In Table 1, we summarize the characteristics of all sampling locations and the time periods in which they were studied.

2.6. Data Analysis

We estimated the otters’ use of the ponds through spraint marking and otter visit frequencies. In the case of spraints, we estimated a relative abundance index per week by dividing the number of spraints collected by the length of shoreline sampled (in m). We also estimated the number of otter visits per week, but only for ponds with camera traps installed. To ensure independence of data, we considered only one visit per day to have occurred even if more than one photograph was captured on the same date. Table 2 summarizes the sampling effort.
We estimated survival rates in urban ponds A and C using the Kaplan–Meier product limit estimator [48] and compared the survival curves between ponds using the Kolmogorov–Smirnov test [49]. We used the Kruskal–Wallis test for non-normal data to test for differences in predation rates [50]. It was not possible to estimate predation rates or to produce a Kaplan–Meier survival curve for urban pond B.
We tested the effect of the factor location type (urban or natural) on the weekly relative abundance of spraints and number of otter visits (the dependent variables) using generalized linear mixed models (GLMMs) [51]. The first GLMM used a normal error distribution and an identity link function, and the second used a Poisson error distribution and log link function. The years and sites sampled were included as random effects in both models. We estimated model effect size and fit through McFadden’s pseudo R-squared measure.
We also tested whether the temporary pattern of otter visits fit the pattern expected if visits were random using the Wald–Wolfowitz runs test [52].
We conducted statistical analyses using the SPSS 24.0 software package (IBM, Armonk, NY, USA). Means are presented with their standard errors. All experiments complied with current laws on wildlife care, protection, and conservation in Spain and the Community of Andalucía.

3. Results

3.1. Urban Pond “A”

The pond containing common carp (i.e., the large pond) was monitored for 148 days. Only two carp (6.6% of the released fish) survived to the end of this period. Predators observed in the area included kingfishers (Alcedo atthis), gray herons (Ardea cinerea), and otters, although we were unable to attribute the numbers of fish caught to any individual predator species.
The pond containing koi (i.e., the small pond) was monitored for 157 days. Only 10 koi (28.5%) survived to the end of this period. Otters were the only predators observed here, and were responsible for all catches.
Predation events began in mid-July (80 days after the fish release) in both ponds and always occurred after the river water level had decreased. For 75–80 days, otters depredated the carp extensively (Figure 5), killing up to three fish per night (instead of the usual one per night). They also depredated the koi during the same incursion into the park, which always occurred at night. Kaplan–Meier survival functions were not significantly different between ponds (Kolmogorov–Smirnov test: Z = 0.649; p = 0.794), and mortality rates did not differ between the years assessed (Kruskal–Wallis test: χ2 = 0.258; df = 1; p = 0.612). Therefore, predation events followed the same pattern during the two years of sampling and in the two ponds.
We found no fish carcasses by the large pond, perhaps because of the carp’s small size, but we did detect fish scales and spraints near the pond border. However, the otters abandoned up to 10 koi carcasses in the grass (10 m around the small pond) and at least two carcasses in the nearby riverbank outside the park. Two of the carcasses were almost completely consumed (Figure 6); the remaining eight were partially eaten (Figure 7), either with their necks bitten or bodies skinned.
In the smaller pond, the relative spraint abundance per week (spraints/m of shoreline) was 0.01 ± 0.001 (n = 24). The number of weekly otter visits could not be estimated. In the larger pond, the relative spraint abundance was 0.03 ± 0.003 (n = 24), and the number of weekly otter visits was 0.77 ± 0.22 (n = 26).
Managers of urban pond A tried different mitigation measures in response to the rapid decline in koi numbers. Hercules fences were installed, and access to the pond was made more difficult for otters. However, as the otters continued to deplete the koi, these mitigation attempts escalated to the use of a cage trap and a snare.

3.2. Urban Ponds “B” and “C”

During the first survey at urban pond B (4 May to 8 May), we found 31 frog skins on the pond shore. Otters preyed on 10.3 ± 0.19 frogs per day during these five days. During the second survey (10 May to 15 May), 43 frog skins were found on the pond shore. During these six days, otters preyed on 12.3 ± 0.37 frogs per day. We also found otter spraints during the survey periods, but in low numbers. Sampling ceased when frog remains were no longer found on the pond shore.
In urban pond C, the survey began after the release of 20 goldfish (15.7 ± 0.3 cm head to tail length). An otter was first detected on 2 August, when nearby water lilies appeared to have been damaged. Between 2 August and 17 August, otters depredated all 20 goldfish and subsequently departed. Otter spraints, as well as fish scales and remains, were observed on the pond shore in low numbers. In both ponds, neither the relative abundance of weekly spraints nor the number of otter visits could be estimated from the data obtained.
The Kaplan–Meier survival function in urban pond C (Figure 8) did not differ from that of the smaller pond (Kolmogorov–Smirnov test: Z = 1.130; p = 0.156) or the larger pond (Kolmogorov–Smirnov test: Z = 0.719; p = 0.680) at urban pond A. Considering all the ponds together, the three survival curves were also not significantly different between years (Kolmogorov–Smirnov test: Z = 0.494; p = 0.968). We did not detect a significant difference in mortality rates among the three ponds (Kruskal–Wallis test: χ2 = 3.588; df = 2; p = 0.166) nor between years (Kruskal–Wallis test: χ2 = 1.533; df = 1; p = 0.216). Therefore, in urban pond C, otters followed the same pattern as in the two ponds that comprised urban pond A.
The owners of urban pond B were not concerned about predation and did nothing to counteract it. In contrast, the owners of urban pond C were unhappy about the depletion of their ornamental fish and stopped releasing more fish after verifying that the otters had eaten them all.

3.3. Natural Ponds, Model Effects, and Random Patterns

The weekly relative spraint abundance was 0.03 ± 0.004 spraints per m of shore (n = 26) in natural pond Co1 and 0.03 ± 0.003 spraints per m of shore (n = 26) in natural pond Co2. In the latter pond, otters visited the plunge pool 1.25 ± 0.27 times per week (n = 32).
The relative abundance of spraints was not affected by the type of site (urban or natural); (normal GLMM: Wald χ2 = 1.624; p = 0.206; McFadden’s pseudo R2 = 0.98). The site type also did not affect the number of weekly otter visits to the ponds (Poisson GLMM: Wald χ2 = 1.778; p = 0.188; McFadden’s pseudo R2 = 0.83).
The temporary pattern of visits to the urban ponds differed from that expected given random visitation patterns (Wald–Wolfowitz test: Z = −2.602; p = 0.009), while visits to the plunge pools of the reservoirs did not differ from the expected random values (Wald–Wolfowitz test: Z = −0.457; p = 0.648).

4. Discussion

Our results suggest some pertinent findings. Firstly, otters hunted in urban settings, in a compulsive and non-random pattern, whereas their predation in natural ponds was random and did not seem to follow a depletion pattern; this may be because natural ponds are normally not closed systems with finite populations. Secondly, the otters created a human–wildlife conflict in some of the monitored urban ponds, which prompted managers to seek mitigation measures—almost lethal solutions—thus presenting a conservation problem.

4.1. Predation Patterns in Urban Pond “A”

Extensive predation by otters of reptiles [53] and amphibians [54,55,56], as well as of farmed fish in stocked streams [57], have been reported in natural ecosystems. To the best of our knowledge, our observations may be the first in the scientific literature of otter raids in urban areas in the Iberian region. Nevertheless, the frequency of otters entering urban areas may be higher than what is reported by the press [58,59,60,61]. Otters may, therefore, be following a similar trend to other wildlife by visiting urban landscapes more often [15,62].
Although our study’s results are a case study also suggest that there are no differences in the pattern of pond use between the urban environment and the natural environment, except that the use of urban ponds is not random, instead seeming to follow a more regular pattern. It appears that the otters found a foraging patch that they exploited quickly until it was depleted or was no longer profitable to continue visiting. These results are consistent with the otter as a food-limited species [63], adaptable to almost any habitat with abundant prey, and with otters selecting predictable habitats for foraging when they are available [57].
Therefore, the behavior of otters seems to depend more on prey availability than on prey type, raising the question of whether otters hunt until resources are depleted [29,64] or until resources reach a low density that does not favor continued hunting [32]. Both situations have been observed previously in Mediterranean otters, as well as in our case studies. In the urban park, small carp were caught until they were almost fully depleted. Moreover, the koi from the small pond of urban pond A would likely have been totally depleted if park managers had not relocated the surviving fish. We cannot be certain that frogs were completely depleted in urban pond B, as some could have survived; however, we confirmed that goldfish were totally depleted in urban pond C.
Otter predation decisions may be driven by easy access to available prey rather than by prey numbers or biomass [65]. Otters have been documented using scattered ponds for short periods [64]; the latter feeding patches were small areas close to rivers or shores inhabited by otters. The urban ponds we studied resemble such feeding patches, with the advantage that they are also shallow and contain abundant prey. Therefore, fishing must be very easy for the otters at these locations. Our results suggest that otters take advantage of these urban feeding patches similarly in different locations and years—probably shortly after detecting them—by visiting them on a few occasions over a concentrated period.
In the large pond of urban pond A, predation on carp could not be entirely attributed to otters. Although the sightings have not been verified, kingfishers and herons have been observed around the pond, suggesting that these birds may also have depredated fish. However, the park is very popular and was frequented by visitors during the day, so the predation pressure from daytime predators must have been very low. Furthermore, if kingfishers were fishing in the pond, they would have been limited to small fish. These species can develop a feeding association with otters, which may have benefited from their presence [66]. Although the small size of the carp made it difficult to find signs of predation, fish scales found near the pond and spraints containing scales confirmed that otters actively depredated the fish.
However, only otters raided the pond containing koi. Camera trapping and direct sightings, as well as the spraints and carcasses that we found, indicated that otters killed the fish. The otters did not fully consume most of the fish; instead, bites to the neck and skinning of the body suggest that they targeted the fattiest tissues. Several of the carcasses were found away from the pond and abandoned on the park grass. Some fish were found trapped in the pond fence, which possibly occurred as an otter attempted to remove them from the site. This pattern, along with the fact that up to three fish were killed on the same night, may suggest the presence of an otter family group, as they are usually very efficient at exploiting small, rich food patches within their territory [67].
Although our study period coincided with the otters’ breeding season [37,68], our data did not indicate the presence of cubs. Instead, our results could simply suggest that otters were eating only the most energy-rich parts of the fish and leaving the remaining parts [69] in a setting where prey was easily available. Otters rarely leave behind any part of their prey [63]. Nevertheless, when otters catch a large prey item, such as a koi, they eat a moderate amount and abandon the rest, including the head, tail, and vertebrae [70], which is the pattern we observed. Otters entered the park from the nearby river; in line with this, we located fish carcasses close to the park perimeter and on the riverbank outside the park.
During the time otters were present in the park, they had access to alternative prey (e.g., small fish, crayfish, amphibians, and terrapins), both in some of the river ponds and in the irrigation ditch. Nevertheless, some otters—or perhaps always the same individual—focused on the park ponds. It was likely more profitable to catch energy-rich fish in a small and shallow artificial pond, where the fish could not escape [71,72], than to search for other prey in the river. As such, for otters, the park was like a well-stocked pantry.

4.2. Predation Patterns in Urban Ponds “B” and “C”

In urban pond B, we determined that the pattern of predation fit that of an otter [34], and this was supported by the presence of its spraints. Although we did not count the number of frogs in the pond, the otter seemed to deplete most of the existing amphibians by systematically visiting the habitat over a few days, probably until it was no longer profitable to catch the remaining individuals. This behavior was repeated at the same time of year for two consecutive years and, according to the estate staff, had also occurred in previous years. This implies that otters either systematically patrol their entire territory or remember places with a potentially high abundance of resources at any given time.
This pond was located near two streams, but these had low flows and were of little importance. However, these streams were connected to the artificial lakes in the golf course and a reservoir, and we confirmed that otters were present in all those water bodies. In addition, the predation episodes in this pond always occurred in May, before the summer drought began. Therefore, the otters’ behavior may not have been due to prey scarcity but to prey concentration.
The reproductive period of the Iberian water frog is delayed compared to that of other frog species, and is concentrated in late spring in southern Spain [73,74]. This period aligns with predation episodes observed in a pond that met all the requirements for the frogs’ optimal breeding habitat [75,76] and in which numerous individuals congregated for amplexus and spawning. Furthermore, when otters encounter high seasonal densities of amphibians that make them easy to capture, they tend to deplete those amphibians [77], especially in lentic ecosystems.
Regrettably, we were unable to obtain data from more than one season in urban pond C. However, the pattern observed was similar to that of the other ponds. As otters were often in the stream close to the pond, they may have noticed the fish in the pond and depleted them over a short time period. A similar occurrence in the previous season (i.e., during the first release of fish) likely occurred, as the property owner reported seeing an otter eating a fish in the garden.

4.3. Implications of Urban Prey Depletion by Otters

Our data suggest that once otters discover new resource-rich habitat patches in urban environments, they exploit and can deplete them. There are some considerations to keep in mind regarding this finding.
Firstly, the density of prey at which otters stop predating seems to be low. In urban pond C, otters completely depleted the goldfish. In urban pond A, the otters stopped when there were two common carp left in the large pond and ten koi had been translocated (making no more koi available in the small pond). It is likely that the otters would have completely depleted their prey had the koi not been translocated. Unfortunately, the frog population in urban pond B could not be estimated, but all our evidence indicates that otters try to deplete their prey as much as possible in these types of urban ponds.
Secondly, urban environments may have expanded so much that they have reached the otters’ natural habitats. In this case, opportunistic mammals such as otters would be expected to exploit the resources readily available to them. Our data confirm that otters can comfortably hunt in urban and peri-urban environments, and are congruent with the pattern of population recovery and increase that otters typically display in Spain [78]. The latter authors related this population growth to the appearance of new favorable and anthropogenic habitats, among other factors. Further, in the same study area, the effect of artificial ponds located on golf courses [79] on otter presence and distribution has been described.
Therefore, the results of this study are also important for the conservation of the species, since urban feeding patches can contribute to maintaining populations at specific times of the year, or provide important food resources during the reproductive period of the species. The potential behavioral plasticity of otters suggests that the species will use natural or urban ponds according to their availability and need, and thus contribute to population dynamics on a larger rather than local scale.

4.4. Limitations of the Study

Although the results of this case study are revealing, there are certain limitations that must be considered, indicating that it is necessary to delve deeper into the ecology of this species in urban areas. Firstly, the small number of ponds studied is a clear limiting factor. Low site number and uneven sampling effort (some ponds sampled and others not because it was not possible) can limit inference and the generalization of findings. However, as a case study, our results still have a representative value that encourages focusing greater effort on this type of research. The fact that the sampling was carried out in different years, although this factor was considered in the analysis and was not found to be significant, is a second factor. The sampling period was spring–summer, i.e., during the Mediterranean stress period; we do not know if a similar pattern might occur in autumn–winter.
On the other hand, we do not have data on possible intra- to interspecific predation events between the prey in the ponds. Although this kind of predation may occur, we consider it unlikely; fish in each of the different ponds were of the same species and similar size, reducing this probability. However, it is likely that some fish in urban pond C were preyed upon by kingfishers or herons, as occurred in the large pond of urban pond A, i.e., a small part of the mortality presented in the Kaplan–Meier curve of the large pool A could not be attributed to the otters. However, the pattern does not significantly differ from large pond B, where we are certain of the otter’s responsibility. This suggests that predation carried out by non-otter predators was minimal. In any case, this did not prevent otters from preying on most of the fish in each pond, as suggested by the fact that mortality patterns in urban pond C were not significantly different from those in urban pond A.
Finally, we cannot determine whether predation was carried out by a single otter or by several individuals in each pond; more precise sampling methods are therefore needed for future research. We also cannot know if there has been an underestimation of the predation rate due to sampling errors, which would have led to faster and more intense mortality in any case. Nevertheless, it must be remembered that the sampling was carried out by two teams simultaneously, which reduces the likelihood of sampling errors.

4.5. Interaction with Human Interests

One key finding of this research is the existence of a human–wildlife conflict caused by otters’ presence in urban areas. This conflict prompted measures to prevent otters’ presence in the park, and almost led to their control by lethal means. This highlights the need to better manage the presence of wild species in urban settings, as well as the lack of conservation policies for urban wildlife and specific measures for managing such events.
The managers of urban pond A attempted a series of mitigation measures against otters that progressively increased in aggressiveness. The Hercules fences in the park failed to stop otters from entering; we found evidence of digging under the fence and observed hair on the fence, suggesting that otters were attempting to climb it. Faced with the evidence that the fence was not an obstacle, the park managers tried to seal the base of the pond fence with concrete to avoid raids, but otters overcame the obstacles, found new gaps, and continued hunting. A tree near the fence was even pruned to prevent otters from climbing over it, without success.
Following several consecutive raids after the previous mitigation measures were unsuccessful, the park managers installed a cage trap to try to capture otters alive. The otters disregarded the cage trap, so this new attempt at damage control also failed. In a final attempt to solve the problem, the managers installed a snare in one of the gaps under the fence through which the otters were known to access the pond. At this point, pressure from local citizens forced managers to remove the snare and discontinue lethal measures, and they decided to relocate the surviving koi to a new pond more than 3 km from the park. After the relocation, otters stopped visiting the park.

5. Conclusions

As urbanization modifies trophic dynamics by increasing the availability of anthropogenic food, wildlife communities are also altered, and predator numbers may increase [80]. The urbanization of endangered or protected species will likely become more frequent as changes occur in natural and human-made environments. Therefore, it is imperative to ensure the survival of these species in the wild by protecting their natural habitats, and developing and implementing adequate management and damage-control strategies within urban settings. These substitution habitats may allow for similar or higher population growth rates than in the species’ original habitats [21], thereby becoming a source of conflict between humans and wildlife.
Our findings suggest that the incursion of a protected wildlife species into urban areas can become a problem when it threatens human interests, even those as inconsequential as the enjoyment of ornamental fish in a pond—this apparently slight conflict between wildlife and human interests even led to attempted lethal solutions. Our study highlights the need for new policies for wildlife management in a changing world [16] in which urban habitats will play an important role. Further research is needed to understand how otters can find prey in urban patches apparently outside their usual ranges. Most importantly, research should focus on strategies for effective mitigation of the damage caused by otters in urban areas before such damage becomes frequent and lethal solutions become the only options.

Author Contributions

Conceptualization, J.D. and D.R.; methodology, J.D.; software, J.D.; validation, M.Á.F. and J.D.; formal analysis, J.D.; data curation, J.D.; writing—original draft preparation, J.D.; writing—review and editing, M.Á.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Institutional Review Board Statement

All sampling was conducted with permission from the properties and pond managers. No special authorization was required, as no wildlife was captured or managed and these were not protected areas. The authors declare that they did not participate in or advise on the management measures implemented to mitigate damage caused by otters. All management decisions were made by the space or pond managers.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Acknowledgments

We would like to thank the Torre Leoneras park gardening staff for their help during the monitoring period. Diego Zumaquero, environmental technician at the Benahavis Town Hall, put us on notice about the fish releases at the park. Juan Vicente García, manager of the Campanillas estate, told us about frog remains in the garden of the estate, allowed us to visit the pond and facilitated sampling. Finally, the manager of the Nueva Andalucía dwelling allowed us to visit the place and monitor.

Conflicts of Interest

Author Jesús Duarte was employed by the company Ofitecma Marbella SL. The remaining authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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Figure 1. General location of the study area within the municipalities of Benahavís and Marbella (Málaga province, southern Spain).
Figure 1. General location of the study area within the municipalities of Benahavís and Marbella (Málaga province, southern Spain).
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Figure 2. Location of the sampling points in the municipality of Benahavís, including urban ponds at Torre Leoneras park (A) and the Campanillas private estate (B), and the natural pond at the Guadalmina reservoir (Co1). Cartographic base: WMS service-Instituto de Estadística y Cartografía de Andalucía–IDEAndalucía. UTM coordinates are shown (ETRS89, zone 30).
Figure 2. Location of the sampling points in the municipality of Benahavís, including urban ponds at Torre Leoneras park (A) and the Campanillas private estate (B), and the natural pond at the Guadalmina reservoir (Co1). Cartographic base: WMS service-Instituto de Estadística y Cartografía de Andalucía–IDEAndalucía. UTM coordinates are shown (ETRS89, zone 30).
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Figure 3. Location of the sampling points in the municipality of Marbella, including the urban pond in the Nueva Andalucía private estate (C) and the natural pond at El Ángel reservoir (Co2). Cartographic base: WMS service-Instituto de Estadística y Cartografía de Andalucía–IDEAndalucía. UTM coordinates are shown (ETRS89, zone 30).
Figure 3. Location of the sampling points in the municipality of Marbella, including the urban pond in the Nueva Andalucía private estate (C) and the natural pond at El Ángel reservoir (Co2). Cartographic base: WMS service-Instituto de Estadística y Cartografía de Andalucía–IDEAndalucía. UTM coordinates are shown (ETRS89, zone 30).
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Figure 5. Kaplan–Meier survivor functions and number at risk of carp in urban pond A.
Figure 5. Kaplan–Meier survivor functions and number at risk of carp in urban pond A.
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Figure 6. Koi fish carcass almost completely consumed by an otter. Photo: J. Duarte.
Figure 6. Koi fish carcass almost completely consumed by an otter. Photo: J. Duarte.
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Figure 7. Koi fish carcasses are skinned or partially consumed by otters. Photo: J. Duarte.
Figure 7. Koi fish carcasses are skinned or partially consumed by otters. Photo: J. Duarte.
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Figure 8. Kaplan–Meier survivor function and number at risk of goldfish in urban pond C.
Figure 8. Kaplan–Meier survivor function and number at risk of goldfish in urban pond C.
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Table 1. Location, sampling periods, structural features, spraint relative abundance, and weekly otter visits for each of the study locations.
Table 1. Location, sampling periods, structural features, spraint relative abundance, and weekly otter visits for each of the study locations.
Urban Pond A (Large)Urban Pond A (Small)Urban Pond BUrban Pond CNatural Pond Co1Natural Pond Co2
Location31°31′06″ N, 5°02′36″ W31°31′06″ N, 5°02′37″ W36°31′13″ N, 5°01′25″ W36°30′26″ N, 4°57′52″ W36°31′53″ N, 5°02′43″ W36°31′08″ N, 4°58′35″ W
Sampling periodMay–Oct
2017		Apr–Oct
2018		Mar–May 2018 and 2019Jul–Aug
2018		May–Oct
2018		May–Oct
2019
Water sheet size (m2)3001409160460204
Shore length (m)794396394732
Depth (m)0.50.50.90.3<1<1
Distance to nearest river basin (m)48
(Guadalmina)		67
(Guadalmina)		1225
(Guadalmina)		1480
(Verde)		On the riverbed1485
(Guadaiza)
Distance to nearest water stream–natural habitat (m)55
(Guadalmina ditch)		75
(Guadalmina ditch)		190
(La Leche stream)		60
(Benavolá
Stream)		On the riverbedOn the riverbed
Spraint abundance
(spraints/meter of shore; mean ± SE)		0.01 ± 0.0010.03 ± 0.003Not estimated Not estimated0.03 ± 0.0040.03 ± 0.003
Weekly otter visits (camera trapping)Not estimated0.77 ± 0.22Not estimatedNot estimatedNot estimated1.25 ± 0.27
Table 2. Sampling effort in the urban and natural ponds in relation to camera trapping and spraint and prey-remains detection.
Table 2. Sampling effort in the urban and natural ponds in relation to camera trapping and spraint and prey-remains detection.
Number of CamerasCamera Sampling Effort (Trapping Days)Sampling Effort Spraint and Prey Remains (Searching Days)Main Prey Present
Urban pond A (large)--148Common carp
Urban pond A (small)3157157Brocaded carp (koi)
Urban pond B--11Iberian water frog
Urban pond C--33Goldfish
Natural pond Co1--160Andalusian barbel
Malaga chub
Southern straight-mouth
Crayfish
Natural pond Co23153160Andalusian barbel
Malaga chub
Southern straight-mouth
Crayfish
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Duarte, J.; Rodríguez, D.; Farfán, M.Á. Eurasian Otters’ Urban Pond Use Patterns in Southern Spain: A Case Study. Wild 2025, 2, 46. https://doi.org/10.3390/wild2040046

AMA Style

Duarte J, Rodríguez D, Farfán MÁ. Eurasian Otters’ Urban Pond Use Patterns in Southern Spain: A Case Study. Wild. 2025; 2(4):46. https://doi.org/10.3390/wild2040046

Chicago/Turabian Style

Duarte, Jesús, Diego Rodríguez, and Miguel Ángel Farfán. 2025. "Eurasian Otters’ Urban Pond Use Patterns in Southern Spain: A Case Study" Wild 2, no. 4: 46. https://doi.org/10.3390/wild2040046

APA Style

Duarte, J., Rodríguez, D., & Farfán, M. Á. (2025). Eurasian Otters’ Urban Pond Use Patterns in Southern Spain: A Case Study. Wild, 2(4), 46. https://doi.org/10.3390/wild2040046

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