Estimativa de abundância e impactos antrópicos sobre o peixeboi marinho (trichechus manatus) no estuário dos rios Timonha e Ubatuba, divisa dos estados do CE e PI

Abstract

The West Indian manatee is one of the most threatened aquatic mammals of Brazil, and now has a patchy distribution from the State of Alagoas to the State of Amapá. The difficulty of observing the species, especially in estuarine waters, is a challenge for conservation. The goal of this study was to test active acoustic method of detection to estimate manatees density and abundance and to identify the areas and the frequency of occurrence of the species in the estuaries of Timonha and Ubatuba rivers, between the states of Ceará and Piauí (NE Brazil). Moreover, we tried to identify human activities that threatens the species in these estuaries. Data collection was conducted from a wooden boat with outboard motor using a side-scan sonar (SSS). Sonar images were collected along line transects in three regions within the study area. Between 2012 and 2014, a total of 21 field trips and 94 days of effort were conducted. For the estimation of density and abundance, the distance sampling method was used through Distance 6.0 software. The methodology used allowed to obtain accurate data with a low margin of error, showing that the SSS is a viable method in the detection of manatees. We recorded 77 sonar detections of manatee groups, totaling 139 individuals. The average speed of the boat during the detections was 3.8 km / h and the depth of detection varied from 2 to 10 m, with 87.1% of the depth detections of 2 to 6 meters. The manatee was distributed throughout the study area, with greater occurrence in Rio Carpina. The relative frequency of occurrence of manatees in the region was 60%, with an average of 1.85 manatees and 1.03 detections per survey. The relative abundance (maximum number of manatees detected on the same day) was 44 individuals (19 detections). The probability of occurrence of manatees per hour of effort and of recording a detection were 0.73 person / hour and 0.41 detection / hour, respectively. Of the 77 detections, in eight of them there were calves. The use of the estuary by the manatees seems to have a seasonality, having more use in the first semester of the year, with greater number of detections in the month of May. The social structure (group size) varied from solitary individuals (44.16%) to groups of up to seven animals (1.30%). There was no difference in frequency of detection related to the phases of the moon and for tides. For the estimation of density and abundance, we considered 1396 transection lines that totaled 863.6 km traveled in the study area. The observed general meeting rate was 0.089 groups detected per kilometer traveled. The probability of detection of groups of manatees during the study was estimated to be 33.7%. The best detection model resulted in an estimated density for the entire study area of 9.19 manatees per km2. The estimated density resulted in a total population of 37 manatees (CV% = 30.2, 95% CI = 21- 66). The best detection model was the Hazard-rate with no adjustments terms, resulting in a detection probability of 33.7%, and a estimated density for the entire study area of 9.19 animals per km2 , and a total population of 37 manatees (CV% = 30.2, 95% CI = 21 to 66). Among the threats we recorded vessels and fishing activities, shrimp farms and salt marshes, generating impacts such as solid waste pollution, accidental catches in catch nets, trawling and fishing corral, and destruction of feeding areas by trawl nets. It is recommended that the zoning and management of EPA Delta do Parnaíba take into account the intense use of the Carpina river by manatees, with specific restrictions to boat traffic to avoid strikes and harassment of this endangered marine animal. This study contributes with data on the distribution of manatees in the region, which are key to conservation, such as population status, and are expected to be used in the development of species conservation strategies.