THE APPLICATION OF EQUATION MODELS TO DETERMINE THE AGE OF PINK RIVER DOLPHIN SKULLS - Biology, Evolution and Conservation of River Dolphins Within South America and Asia - page 84

I NTRODUCTION
For several decades now, society has known about the negative impact of fishery
activities on the world's Cetacean populations. The main cause of their death is fish nets, a
problem, which also affects other animals such as turtles and is happening in both marine and
in river ecosystems (Northridge & Hofman, 1999).
The case of the pink river dolphin ( Inia geoffrensis ) is no exception as there are regular
cases of pink river dolphins accidentally caught and found dead in monofilament nets. Several
studies have shown the negative impact of these nets on the populations of this river dolphin
(Trujillo, 2000; Trujillo et al., 2006). Additionally, the fishermen of large catfish species
believe that this dolphin is a competitor of their resources and consequently, sometimes they
resort to guns and harpoons to kill the dolphins. More recently, another threat developed that
is affecting the pink river dolphins. The dolphins are purposefully killed and left to rot to
attract some small catfishes, like the ―mota‖ or ―mapurito‖ ( Calophysus macropterus ) in the
Orinoco and in the Amazon (Colombia and Brazil, mainly). Some indigenous people have a
demand for dolphin parts which bolsters a dolphin market. For example, dolphin oil is used as
a form of medicine against pulmonary illness (for instance, this practice is extended into the
Mamoré River within Bolivia, Ruiz-García, unpublished observation), whereas teeth and
genitalia are utilized as love charms by Indian ―witches‖ (for instance, the ―pasaje Paquita‖ in
the Mercado de Belén at Iquitos, Peru; Ruiz-García, unpublished observation).
However, when a dolphin is accidentally killed by a net or fisherman, the real age of the
animal is unknown. Only the size of the animal has sometimes been recorded and thus we
only know if they are an adult, young or calf. For this reason, the main objective of this
chapter is to explain a method of how to determine the age of a dead pink river dolphin
through the analyses of craniometric measurements and different mathematical
methodologies. We will show the existence of a relationship between the age of dead pink
river dolphins estimated by means of teeth analysis and several craniometrical measures. This
relationship has not yet been demonstrated with this species, although several craniometrical
and morphometric studies have been carried out with Inia (Da Silva, 1994; Ruiz-García et al.,
2006). Therefore, if one specimen is found dead, some skull variables could be measured and
the age of this animal could be estimated. It could be extremely useful to determine which age
fraction of the pink river dolphin populations are caught in nets or killed by firearms or
harpoons or, if the dead animals represent a random sample of a population. Ages of dead
animals were determined through the analysis of teeth growth layer groups (GLGs).
The first works determining the age of mammals by means of GLGs were those carried
out on pinnipeda by Scheffer (1950) and Laws (1953) in Callorhinus ursinus and Mirounga
leonina, respectively. Subsequently, other research on this topic was accomplished by Laws
(1957, 1958), Mc Laren (1958), Nishiwaki & Yagi (1953) and Sergeant (1959) in pinnipidea
and odontocetes. During 1968, in Oslo, The International Whaling Commission (IWC)
accepted that the formation of one GLG was equal to one year of life in odontocetes. Later
works were in agreement with this finding (Nielsen 1972) with Phocoena phocoena , Hohn
(1980) and Hohn et al ., (1989) with Tursiops truncates , Collet (1981) with Delphinus delphis ,
Manzanilla (1989) with Lagenorhynchus obscurus and Lockyer (1993) with Globicephala
macrorynchus . Also the Platanistoid dolphins show an annual GLGs. This was demonstrated
by Kasuya (1972) for Platanista gangetica and by Anli & Zhou (1992) for Lipotes vexillifer .