Diet and Relative Abundance of the Neotropical river otter (Lontra longicaudis) in the Biological Reserve The Tirimbina.   

Karla Rojas Jiménez & Suní Marozzi Mata. Biological School of Sciences. Latina University of Costa Rica.  San José, Costa Rica.

karoji10@gmail.com, sunimarozzi@gmail.com. 

Resume:

We analyse the diet and relative abundance of the Neotropical river otter Lontra longicaudis (Carnivorous, Mustélidae) through depositions collected in the course of the Sarapiquí river and the One river, Heredia- Costa Rica, between september and november, 2005.

It was determined that in the diet of the otter a significant preference of prey category exists, where the greater incident was shrimps with 48,78%, followed by fish 37,8%, crabs 9,75% and insects 3,65%.  The previous thing due to that because the shrimps are more easy capture and they provide great performance of biomass by diet time unit; similar happens with the fish, since the greater frequency were the Cichlidos, considered as slow in their swimming capacity. 

By indirect methods we obtained that the population of the sampling area; a total of 3 lineal km al edge of the rivers is of 8 individuals, where one of these is represented for a youthful one. 

Keywords: diet, carnivorous, Mustélidae, depositions, Costa Rica 

The Neotropical river otter is a carnivore, considered as the most important predator of the sweet water environments of the tropic (Gómez 1999), although his role in the dynamics of this ecosystem is not know.  With respect to his ecology is little what has been investigated, in spite of being the only otter that habits in Central America and possesses an extensive geographical distribution that covers from Mexico to the central zone of Argentina (Manson 1990).   

This specie is generally observed alone, steel some times they are found in couples or in family groups; are assets during the day and at night (Wainwright 2002). They are used to establishing in specific zones, although from time to time they change territory in search of food and refuge. Its preferred habitat is situated to the edge of the rivers surrounded by forests, thick vegetation or walls of stone generally in crystalline water with stony funds; what gives us an indication on the good state of the river cause (Paschal 2000).   

The depositions are generally placed closer to the water, in high places of the edges of the river or in stones that excel of the river bed, mainly inside their action area (Gorman & Trowbridge 1989), which normally are gummy, without specific form and are covers by a thick layer of mucus.  When these are found dry they measure 1 cm of diameter, if they are fresh they have a black or brown greenish brilliant color and when they are drying, they return to gray and occasionally white color. Usually the depositions have a persistent and characteristic smell to grease of oil fish (Kruuk 1992).   

The aims of the project were; to describe the diet of the Neotropical river otter; analyse the preference of prey category in the diet of the Neotropical river otter; determine the Relative Abundance of the Neotropical river otter through the depositions in the Biological Reserve The Tirimbina, located in The Virgen of Sarapiquí- Heredia, Costa Rica Of the rainy season, in the period of the 17 of september to 13 of november of the 2005.   

Material and Methods  

Study Area

The Biological Reserve The Tirimbina, declared in december 2001 Wildlife National Park, is composed for 344 continuous hectares of forest, besides an island (2 km) formed by the Sarapiquí river.  It is found in Heredia Province with a 10° 24' 0 N latitude and 84° 7' 60W length; to an 182 m. s. n. m of altitude approximately (1).   

According to Holdridge, the reserve covers two life zones: very humid forest (bmh-P) and the tropical very humid forest (bmh-T).  Where the area categorised like forest, is a combination of areas that historically have been in diverse uses and know are found in regeneration process since over 40 years ago. There can be found secondary forest patches, the Cacao plantation and Tacotales in abandonment. Still nevertheless the great majority (a 85%) corresponds to forest(1).  

The Reserve has an average of 25,3°C annual temperature and its haste is of 3777 mm annual.  Inside the property passes a great part of the Sarapiquí river, besides the One river and Tirimbina river (1).   

Sampling Method 

A total of 20 days of sampling were carried, where 1,3 km of the Sarapiquí river where measuring (including two foothills of this), 2 km of an island formed by the Sarapiquí river (flanked in his totality) and 330 mts of the One river (tributary). A total of 3.6 km where measuring, including 5 mts from the edge of the river toward inside of the forest.   

The depositions collected were analysed during september 17 to november 13, 2005. 

Deposition Analysed

Each deposition was collocated in a plastic bag with the date and place of harvesting (Paschal 2000); then we weighed them to quantify the rough residues and subsequently they were placed in a container with water and detergent laundry during 24 hr.  Continued we washed each sample with water on a sieve of 1 mm of netting; they were placed in role envelopes to dried up them in 60°C of temperature, where after two or three days of dried they were collected to proceed the identification of residues found in each depositions (Spinola 1995 & Paschal 2000).   

To identify the remainders fish consumed by the Neotropical river otter, we utilised the humid collection of the Natural Resources and Wildlife Laboratory of the Nacional University of Costa Rica (Heredia, Costa Rica) to carry out before the depositions analysis a key based on scales of the sweet water species of fish presents in the sampling area; where we used the species collected in Chilamate and The Virgen of Sarapiquí properly identifying of the LARNAVISI collection.  

For the key execution they were extracted scales of the sideline from above, the sideline underneath and in the high abundant stem of the each fish samplel; follow they were stained with red color, in order to observed differents characteristics of each specie or family and thus to facilitate their identification, by last they were mounted to be views by a microscope and subsequently to proceed to made the identification key. Besides parts of vertebrae, bones of the opercular apparatus, jawbones of fish, hair and feathers were utilised to identify the preys consumed by the otter as well as also help of ichthyologists and identification keys (Spinola 1995 & Utreras 1997).   

The identification level of the remainders found varied from families to species, depending on the available information. Once identifying the present species in each sample, we proceeded to divide them in four prey categories where we find: crustaceans (shrimps and crabs), fish and insects.   

In each prey categories there was expressed like occurrence (Or) the number of depositions in which a prey category occurs along with his corresponding relative frequency (Fr). Any evidence of a prey category in a deposition, still when could be more than one individual of that same category was considered like a single occurrence (Spinola 1995). Supposing that there not exist preference in the prey category is expecting that the value be fair in the four categories; where the observed value was utilised like the occurrence of prey in each one of the 52 samples of depositions.   

Relative Abundance

They where counted all the depositions that was considered recent  founding in a stripe that included the edge of the river and 5 mts of edge from the water line, one of the two margins of the Sarapiquí river, the island formed by the Sarapiquí river and the two margins in the case of the One river.  Where the counts were performed in both rivers, where we suppose that both rivers will have normal levels of beds, as well we the disorder rate of the depositions was similar in both places, since the frequency and intensity of the rains, temperature and humidity was considered similar.   

Since the rivers flow in certain point, is considered that the depositions found, can belong to individuals that make use of both rivers. And due to that we carried out the same number of measuring through both rivers, we controlled possible fluctuations in the counts because of the seasonal variation in the number of depositions placed by the otters.                                   

We applied the formula Relative Abundance Obtaining from fecal remainders (Fórmula1) that estimated the size of a population written down in a specific area. 

Fórmula 1:

Through the Tests for  (Gómez 2000), we obtained the youthful proportion of individuals that composed the population; supposing than the average weight of the depositions of an adult otter is equal or greater to seven grams, and of a youthful is smaller to seven grams (personal observations). The hypothesis test process was carried out: with a value Z calculated, where from the sample a critical value is compared of Z based on the value selected.  (Formula 2). To verify the sights reported by people of the zone and owns where there have been observed youthful of this species. Giving as Ho: that the 15% of the population is youthful and Ha: the 15% of the population is not youthful.   

Through the Tests for  (Gómez 2000), we obtained the youthful proportion of individuals that composed the population; supposing than the average weight of the depositions of an adult otter is equal or greater to seven grams, and of a youthful is smaller to seven grams (personal observations). The hypothesis test process was carried out: with a value Z calculated, where from the sample a critical value is compared of Z based on the value selected.  (Formula 2). To verify the sights reported by people of the zone and owns where there have been observed youthful of this species. Giving as Ho: that the 15% of the population is youthful and Ha: the 15% of the population is not youthful.   

Fórmula 2:

 Results 

Diet Analysis 

The size of the sample collected was of 52 depositions; where we registered 82 prey category occurrences in the total of the depositions analysed and classified in four prey categories (Chart 1).  The greater prey category occurrence was shrimps with 48.78% followed by fish with 37,8%.  Besides registration of crabs with 9,75% and insects with 3,65%.   

About place of sampling the depositions, generally they were found in accessible places for otters, normally over conspicuous rocks, exposed to the sun and in the edge of the rivers, where the currents were from moderates to fast.  Only a sample was found in a path on the floor and another on a trunk exposed in the edge of the river.   

Chart 1.  Occurrence and Relative frequency of prey categories found in 52 depositions collected in the Biological Reserve The Tirimbina- Costa Rica, setiembre to November 2005 

It was shown that exists a prey category preference (shrimps) by part of the Neotropical river otter (Lontra longicaudis).  (X2 = 46, 5; gl = 3; p?  0,01).   

Likewise it was appreciated that the most representative family of fish corresponds to Cichlidae, followed by the Characidae and Poecilidae with the same number of species and finally Eleotridae.  Besides two kinds of shrimps were identified (Atya and Machrobiachium), crabs of the Pseudotelpusa kind and insects (Corydalus sp) (Chart 2).   

Chart 2.  Species reported in the diet of L. longicaudis in the Biological Reserve The Tirimbina- Costa Rica, september to november 2005.   

Relative Abundance

En el presente trabajo se estima que la población presente en el área de estudio (3 km lineales en  un área de 14 500 m²), es de ocho individuos; donde además se demostró por medio de la Prueba para , que el 15% de la población son juveniles y el 85 % son adultos. Es decir, la población cuenta con una cría en la época que se desarrollo el estudio.  

Discussion  

Diet Analysis

In the Biological Reserve the Tirimbina, the diet of the Neotropical river otter was based mainly by the shrimps of the kind Atya sp and Machrobiachium sp; fish distributed in eight species, dominating the family of the Cichlidae where the individuals pertinent to Pseudotelpusa sp kind is found with smaller occurrence that the rest of the families, with a frequency of  9, 75%.  The prey category of insects, independently if they indirect consume by the otters or direct prey of these obtained the smaller frequency of occurrence as prey category. Where the kind found (Corydalus sp) is characterised for being one of the insects with the must hard shell that lives in clean currents water, rivers with great size and certain muddy; what explains that this specie is was not very abundant in the diet of the otter (Peinador 2001).   

On the other hand there where not found remainders of amphibians, reptilian, birds or other mammals in the analysed depositions, preys that were reported by Rooster (1989) for the same species and by various authors for other species of the same kind:  L. lontra (Adrián & Delibes 1987; Ruíz- Olmo 1995) and L. longicaudis (Quadros 1999). This due to the reason that otters feeds of this species when by low seasonal conditions the river bed out of the rivers, generating remote bodies of water, where these prey remain retained and if it is accessible to the otter to arrive at the water bodies they will fed of these preys; as also is known that they are capable by the otter, if for some reason they have direct contact with the water (Kufmann 2000).   

The crustaceans were the most frequent prey and with high proportions of occurrence in the diet of the otter Neotropical in both rivers.  The most consumed were the shrimps of the kinds Atya and Machrobiachium. Which according to Barnes (1996) they are related to rivers with fast currents and rocky substrates, as also with observations carried out in the area of study by the residents, was confirmed they are easily accessible for the otters, as well as also they observed that otters they turn the stones for thus find these individuals.   

By the other hand, fishes were an important component in the diet of the Neotropical river otter, due to that the more representative family consumed Cichlidae is characterised for being considered as a slow fish. Besides families such as Poecilidade and Characidae are considered like fast swimming families, witch explains its smaller presence with regard to the previous is justified.  Ryder (1955) in a study about the predation of fish by the American otter (L. canadensis), concluded that the fish are consumed in direct proportion to their abundance and inverse to their swimming ability.   

As for the Eleotridae family they presents benthonic habits and lethargic customary and that’s why they are of easy capture for the otter (Bussing 1998), as well as also there habit generally is rivers estuaries, what differs with our area sampled, that corresponds to intermediate zones of high to moderate current.  The fish known as bearded pertinent (Ramdia sp) are a common species in these rivers and one of their characteristic is that are fishes without scales, motive by which is not included in the list of species that corresponds to the diet of the otter; although be to affirmed that they form part of the diet of L. longicaudis since fishermen of the zone have reported that the Neotropical river otter capture this species of fish (pers .com).   

Schoener (1971) showed that otters presents diet strategies to obtain an optimum diet, selecting food of great performance of biomass by diet time unit and thus same of easy capture.  Explaining thus the shrimps consumption tendency (Atya and Machrobiachium) which can have significance in their number of individuals regard besides the species present in the area of study and on the other hand, they can be consumed in short periods of time with the smaller expense of energy.  (Spinola 1995) 

The majority of publications that treat on the diet of the otter present data as frequency of presence (Adrián & Delibes 1987; Carss et al 1990; Ruíz- Olmo 1995) giving an importance equal to each species appeared in a deposition, independently of the quantity of remainders of each type and of the weight of the excrement, what carries to an over- estimation of rare species with the consequent that one of the most frequent species.  (Wise 1980; Carss & Parkinson 1996; Jacobsen & Hansen 1996).  Thus same, studies carried out by Erlinge (1968) of the Eurasian otter (L. lontra) showed that the analysis of the diet through occurrences by prey categories in the depositions tend to over- estimate the preys with large proportions of hard parts of fish in the samples and to sub- estimate larges fishes or without scales; even so, provides with reasonable accuracy, the relative importance of The same.   

Another study carried on the white-collar otter (A. capensis), Rowe- Rowe (1977) reported that the occurrences analysis method of the depositions was the more exactly and that satisfactorily reflected the proportions of the different prey consumed.  Therefore this methodology is adequate to analyse the diet of the otters.  Because other methodologies as stomach content analysis and direct observation turn out to be difficult at the moment of identification of prey, since this is a difficult species to observe in its natural environment and would present problems of conservation.  (Sielfeld & Castile 1999; Spinola 1995)  

Also this studies shown the importance of the existence of the Neotropical river otter in the zone, since this regular one the population of fish, shrimps and crabs; because they fed of those that are sick or very old, leaving thus the most apt individuals to be reproduced maintaining thus a healthy population in the ecosystem of the area studied (Spinola 1995).   

Relative abundance

The direct observation of the otter is difficult in their own environments, being a constraint for the population size estimations execution (Sielfeld 1999).  By said reason, the estimation of relative abundance was carried out with indirect methods utilising the depositions remainders.  Said methodology has been utilised satisfactorily in studies carried out by Jacobsen & Hansen 1996.  The otters are individuals that present territorial bearing behavior, using depositions, urine and anal glands secretions; which generally are found fence or in the burrows and resting places (Kufmann 2000), which coincides that in many occasions the depositions were placed in recurrent and specific places. 

It is important to emphasise that there not exist a global population estimation of this species, although studies carried out by Bertoratti & Parera (1994) shows that density of the Neotropical river otter varies from 0,81 to 2,76 otters by lineal km, which coincides with our population with an individual by lineal kilometer.   

Thus same, it fits to stand out that the study was carried out in the rainy season, reason by which, the number of depositions found could have diminished in some samplings or the variation in the number of individuals that place the depositions as olfactory marks (Kruuk 1992).  Since as they are found in exposed rocks and near the margin of rivers, they remain in direct contact with the water (especially the rain) that washes the depositions placed in the rocks.   

One of the causes that carry the Neotropical river otter to reduce its population, was the indiscriminate hunt by its skin, reason that nowadays has been restricted thanks to the creation of laws.  Nevertheless at the present the decrease of this species has followed itself observing due to diverse factors such as the loss of habitat, contamination and the decrease in the availability of food.   

Conclusions 

The population of the study area it was determined that the zone counts with eight individuals, of which one represents a youthful one.  The Neotropical river otter is a species that prefer to eat fish, but the shrimps are the species of greater occurrence in this season of year.   

Recommendations 

It is recommended to carry out other studies on the availability of food in dry season inside the same study area, as well as also to expand the study area in the lower and high part of the rivers.  Also it is important to carrying out studies in other areas to determine in which conditions is found the Neotropical river otter in the country.   

Acknowledgements  

Ours thanks goes to our unconditional family that supports us along our investigation. 

Msc.  Lilliana Piedra guardian of our work of investigation.

PhD.  Marine Marozzi by his support in the development of the work.

Bernal Rodriguez, Mariamalia Quirós, Jose and all the personnel of the Biological Reserve the Tirimbina by its guide and aid in the harvesting of data. 

To the personal of Natural Resources and of Wildlife Laboratory, National University of Costa Rica (LARNAVISI).   PhD.  Mauricio Protti, the students of Degree:  Mauricio Herrera and Meyer Guevara.

To all our friends and companions that supported us along the investigation.   

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