Factors that affect Vaquita Mortality

Parasites
Parasites have been found on the flukes of some Vaquita found entangled in nets. Three types of trematodes have been found in the intestines of dead Vaquitas. It is not known to what degree parasites affects Vaquita´s health.

Predation

Various fishermen from El Golfo de Santa Clara who caught Vaquitas in their nets notified researchers that between February and May of 1990 and 1991 they found parts of Vaquita in the stomachs of several species of large sharks, including some species of sharks: the white shark (Carcharodon carcharias), the shortfin mako (Isurus oxyrinchus), the lemon shark (Negaprion brevirostris), the blacktip shark (Carcharhinus limbatus), two species of thresher shark (Alopias superciliosus and Alopias vulpinus), and the broadnose sevengill shark (Notorynchus cepedianus). Some vaquita tangled in nets showed scars on their flukes from teeth that could be shark or killer whale, and there have been sightings of killer whales and also of other species of sharks, like the tiger shark (Galeocerdo cuvier), scalloped hammerhead sharks (Sphyrna spp.) and bull shark (Charcharhinus leucas), among others. However there have been no direct reports of attacks on vaquita by these species of sharks or by killer whales.

Incidental mortality in fisheries activities

The vaquita is particularly vulnerable to incidental mortality in gill nets. Since they need to surface to breath air, when they get entangled in fishing nets they cannot surface to breath and they drown. This type of capture, when the vaquita is not the target species of the fishery yet they die in nets set for different species of fish, is known as incidental capture.

Gill nets
EIn the coastal fisheries of the upper Gulf, an area of strong currents, gill nets are the most common, most preferred and widely distributed fishing method. Different types and sizes of gill nets are used in the different fisheries. It is likely that vaquita have been captured in these nets since the middle of the 1920. During the 1940s with the significant expansion of the fishing industry to exploit the totoaba, the vaquita population was reduced. In 1992 when gill nets with mesh size greater than 10 inches were prohibited to protec the totoaba and the vaquita fishermen began using other types of nets. Nevertheless, vaquita still get entangled in 8 inches nets set for shark and manta rays, it has been reported that vaquita are also still trapped duringo fishing activities that use gill nets with mesh sizes between 2.5 and 8 inches, used for shrimp, sierra, chano, corvine and other species.

Trawling nets:
Incidental mortality of vaquitas in shrimp boat trawl nets has been mentioned by fishermen in the communities of El Golfo de Santa Clara and San Felipe. Eight dead vaquitas were reported between 1985 and 1990. Considering the number of shrimp boats operating in the upper Gulf of California, this is another fishery that threatens the existence of the vaquita population, especially in trapping the calves, which are slow moving. Recent regulations aimed at protecting the sea floor within the Reserve will reduce the number of trawling boats operating in the upper Gulf and at the same time will eliminate this risk for the vaquita.

In 1988, the Intercultural Center for the Study of Deserts and Oceans (CEDO), by conducting interviews with Puerto Peñasco fishermen on vaquita capture and the distribution of fishing activities and fishing effort, estimated that an average of 32 vaquitas annually were trapped in gill nets in the upper Gulf. Later, between 1993 and 1995, in a study by the Instituto Tecnológico y de Estudios Superiores de Monterrey en Guaymas (ITESM), direct mortality of vaquitas was observed at El Golfo de Santa Clara. As a result of this work and through an analysis of all the dead animals reported from gill nets, it was estimated that a minimum of 39 vaquitas die incidentally in nets each year in this port alone.

Even though t is known that vaquitas die in all types of nets, it has been difficult to estimate the exact mortality in each type of net. A study by CEDO of upper Gulf of California, fisheries determined that the fishing effort is different in each community in the region, and that the fisheries are dynamic and respond to changes of tides, seasons and other environmental factors, as well as economic factors.