MONITORING CAIMAN POPULATION TRENDS ON THE IBERA MARSH

 
 
 

Tomás Waller (email: twaller@fibertel.com.ar)
Universidad del Salvador - Instituto de Medio Ambiente y Ecología (IMAE)
& Fundación Reserva del Iberá, Argentina

Introduction

Two species of caiman occurs in Argentina, the broad-nosed caiman, Caiman latirostris, and the common caiman, Caiman yacare. Both species were exploited during the past century to utilize their skins for the leather industry. From 1940 until 1990, thousands of skins were ilegally obtained and exported from Argentina, proceeding mainly from caimans inhabiting the swampy habitats of the province of Corrientes.

In 1990 Argentine CITES Management Authorities together with the CITES Secretariat promoted a survey of argentinian caiman populations. During that year the illegal hunting and trade stopped completely due to internal and international control measures. The CITES survey concluded that Corrientes province is one of the main strongholds for the conservation of these species in Argentina. Both habitats and caiman populations were relativelly healthy in spite of the past intensive hunting. The survey recommended the implementation of a monitoring program to assess the recovery process and to detect any threat to the existing populations (Waller & Micucci, 1993).

As part of the current project we are monitoring the population trend of Caiman yacareat Ibera marshes. These wetlands comprises two thirds of the total wetland habitat available for this species in Corrientes province. By evaluating the population trend in this area we expect to have a good picture of what is going on with common caiman populations in all the region.
 

Methodology

Trends in yacare populations are evaluated by monitoring changes in relative abundance and population structure along a time-frame, in this case, by comparing our results with those obtained in a previous survey 10 years ago (Waller & Micucci, 1993). Increments in the relative abundance and/or an increase in the average age/size of the population are interpreted as a positive trend.

Our work is carried out at the same monitoring sites assessed 10 years ago by the CITES study: lakes Ibera, Luna and Ipucú, and different creeks and channels inside the marshes, Arroyos Miriñay, Disparo, and Carambola. Census are conducted by night navigating the waterbodies with a boat and using a powerful spotlight (400,000 candlepower) to detect caiman's eyeshines. Only non-hatchling caimans are counted  (>35 cm total length), hatchlings of the year are disregarded due to the ephemeral nature of this size class. Water and air temperatures, general weather conditions and water level (high, normal, low) are recorded for each census. We tryed to work during ideal weather conditions (moderate to high water/air  temperatures and low wind speeds). Census are initiated preferably no more than 2 hours after sunset and we always tryed to finish before midnight. Exact positions are taken with the aid of a GPS.  Results are expressed in animals per km of lake shoreline or km of creek. During specific night surveys caimans are sighted and their size estimated in order to obtain the proportion of juveniles, young adults and old ones in the population.

The information gathered is clustered and analyzed by habitat type (creek or lake), by areas inside the Ibera system (Galarza, Loreto, Concepcion and Ibera), and compared with previous studies.
 

Preliminary results

Until now, we monitored caiman densities at three sites on the Iberá marshes: Galarza (Lake Luna and Disparo Creek), Colonia Pellegrini (Lake Iberá and Miriñay Creek) and  Ea. San Juan Poriahu, Loreto (Lake at Ipucu Marsh). We surveyed 90.7 kms of lake shorelines and 10.6 kms of waterways (creeks, channels). Census results are summarized in Table 1.

Creeks/channels harbors more caimans per km than lake shorelines (46.2 c/km vs. 14.8 c/km). Lake populations are exposed to wind/waves with possibly more variable water temperature conditions. Creek habitat is characterized by the presence of dense submerged vegetation praires while open lakes only offer this condition in very localized places or pockets. The association of caimans with submerged vegetation and protected waters is also confirmed by the fact that the highest caiman densities in lakes are found only at highly spatially heterogeneous shores as those found on the south section of Luna lake (26.5 c/km). At this place we found twice and half the densities found in other Luna shorelines (26.5 c/km vs ca.10 c/km). This area is characterized by the presence of profound bays with protected waters that during summer are covered by dense praires of submerged and floating vegetation, paralleling conditions usually offered by linear waterways as creeks and channels.

Current caiman populations showed a 2.5 fold increase both for creeks and lakes as compared with 1991/92 census performed by the author (46.2 c/km vs 17.7 c/km for creeks and 14.8 c/km vs 6.4 c/km for lakes) (Waller y Micucci, 1993). This seems to be a direct result of a complete cessation of caiman hunting in Argentina since 1990 onwards. However, other parameters (as the water level rise detected since 1990) could eventually explain part of this significant augmentation of the Iberá caiman population.

Repetitions carried out at Disparo creek (Galarza) showed slight fluctuations in the number of caimans inhabiting the creek, in apparent response to water regimes (high/low water level cycles). These repetitions will continue during the following year with the expectation of having a better understanding of this density variation pattern.

Future research will involve the estimation of the size structure of these populations with the aim of comparing current size composition with that assessed in 1991/92.
 

Reference:

Waller, T. & P. A. Micucci. 1993. Relevamiento de la distribución, habitat y abundancia de los crocodilios de la provincia de Corrientes, Argentina. Pp. 341-385. In: IUCN, Zoocría de los Crocodylia, Memorias de la I Reunión Regional del CSG, Grupo de Especialistas en Cocodrilos de la UICN - Unión Mundial para la Conservación: I Taller sobre Zoocría de los Crocodylia. Santa Marta, Colombia, 11 al 14 de Noviembre de 1991.