Dolphin mortality results from both natural and anthropogenic causes.

Natural causes include old age, failure to thrive as a calf, intra-specific agonistic interactions, predation, stingray wounds, disease, storms and biotoxins from harmful algal blooms such as red tides. Threats of human origin include entanglement in nets, entanglement in or ingestion of fishing gear, pollution, boat collisions, noise damage and fishery operations. In some cases the distinction between natural and anthropogenic causes of death can be hard to determine, for example exposure to toxic contaminants which increases a dolphin’s susceptibility to natural pathogens or biotoxins.

For most marine mammals, the risk of injury is continual, whether from natural sources or human activities as named above. Nowadays more animals die in accidents. Interaction with fisheries is a leading cause of death and injury: pelagic odontocetes die in drift nets, coastal cetaceans in gill net and trawl fisheries and some river dolphins from fishing methods that use electricity and explosives. Discarded net fragments, ropes, packing bands, monofilament line, and other debris also put animals at risk. For the individual victim, the problem is always serious. An animal that does not drown immediately may escape with fractures and internal injuries or may carry net fragments, ropes, or bands that increase drag and impede swimming ability.

Pollution

• chemical: heavy metals (lead, mercury), PCB (synthetical organic chemicals), DDT (synthetical pesticydes, genotoxic). Cancirogenic substances cause central nervous system malfunctions and disable reproduction. Fat-soluble substances, such as the persistent DDT, PCBs, and related organochlorines, reside in fatty tissues like blubber, liver, and brain; heavy metals are found in liver but also distribute in muscle, kidney, and other organs. Low levels of testosterone were associated with high levels of PCBs and DDE.
• mechanical: plastic bags, packaging, fishing gear and other synthetical materials. All these materials can be eaten by mistake or ingested as mycroplastics throughtout their lifetimes;
• contaminants are adversely affecting population growth
• the impact of pollution on marine mammals can occur throughout the entire chain from exposure, uptake, metabolism to excretion. Concentration in prey is a determining factor.
• generally, coastal species are exposed to higher environmental levels than pelagic species, and species occurring in industrialized (including intensive agricultural) areas usually have higher pollutant levels compared to animals in less developed regions. Among marine mammals dolphins usually carry the highest tissue residue levels.
• Species that reach sexual maturity at a younger age are in an advantage as compared to those that start reproducing at an older age. Early reproduction is positive for the offspring as the amount of pollutants descendants receive is lower if mothers initiate reproduction activities early, because they have not yet built up high tissue concentrations.
• a factor likely to lead to differential vulnerability between species is body size. Small species have generally higher levels of pollutants relative to their bodyweight, than those of larger body mass.
• oil spills: the impact of spilled oil depends on its composition, environmental conditions, and the species involved. During the first few hours or days after a spill, low molecular weight fractions are the most acutely toxic. They irritate and harm tissues, especially the sensitive membranes of the eyes and mouth; they can be ingested during feeding or when a fouled animal is grooming; or their vapors can be inhaled and damage the lungs. Light fractions are absorbed into the blood where they can attack the liver, nervous system, and blood-forming tissues. 
• ingesting debris: some marine mammals are likely to ingest various types of discarded items and trash that enter the oceans—mostly from land sources—at a rate of over 6 million metric tons each year. Some items may block or perforate the gastrointestinal tract, leading to slow starvation or sudden death.

Noise

• noise pollution is a particular source of concern because many marine mammals rely on sound emission and detection for finding their prey, communication, and navigation;
• is a concequence of human activities in marine environment. Activities producing noise-related disturbance include increasing vessel traffic, turism, building of the infrastructure in the oceans, military maneuvers, seismic testing, oil and gas drilling, building or functioning of energy generating machines, etc. It all distracts marine life from their normal activities.

Excessive fishing

• is one of the prime reasons for destroying of habitats, which leads to depletion of marine resources. Depletion of food stocks, whether from overgrazing, overfishing, climatic or oceanographic fluctuation, can affect entire populations;
• dolphins themselves used to be hunted for meat, skin, bones and oil;
• interactions due to bycatches in fisheries constitute one of the major threats to marine mammals. Bycatch in gillnet (trammel and other bottom set gillnets, driftnets, etc.) fisheries throughout their range is of great concern.
• in many areas of the world, coastal fishermen consider dolphins as competitors. Common dolphins (Delphinus delphis) in the Mediterranean have been perceived as a threat to the  fisheries operating there, and as a result have been deliberately caught. In the mid-1900s, tens of thousands of dolphins and porpoises were killed every year as a result of fishing industry claims of competition.
• Prey Depletion: depletion of prey may often be caused by overfishing by commercial fisheries. Depending on the extent of the depletion, marine mammals may respond to the reduced supply of prey either by switching to other species, or by temporarily moving to another area. The resilience of animals in such populations is negatively affected, potentially increasing the impact of pollutants present in their environment.

Human interactions

• in European countries marine mammals are protected by law. Even attempts of swimming with dolphins is treated as harrassement or annoyance and is punishable. It can also be dangerous as these animal are about 3 m long and weigh about 200 kg. They are wild, unpredictable animals and must be treated accordingly;
• traffic: speeding and carelessness in more closed areas with heavy traffic with small depths can lead to collisions which always leads to severe damage or deaths of dolphins;
• there is a possibility of zoonosys or deseaseases transferring between mammal species (including human-dolphin, both ways);
• marine mammals have adapted over millions of years to the often harsh conditions of the marine environment. In the past few decades, environmental change has proceeded at a rate far exceeding the slow pace of evolution. How can marine mammals cope with urban and industrial wastes, coastal dredging, undersea construction, vessel traffic, noise, and intense competition from humans for food resources?
• Because of increasing pressure from expanding human populations (especially in coastal zones), the major threat to all populations is degradation and destruction of coastal habitats. This degradation is mainly being caused by coastal zone development, overfishing of prey, pollution and vessel traffic;
• hunting for delphinariums: dolphins are legally protected in most countries of the world, so hunting for such reasons is rare, but is continued in some parts of the world.

Parasites

Almost all marine mammals are infected by parasites. Most of the parasites have evolved with their hosts and, under normal circumstances, cause little damage to otherwise healthy animals. Among these are the amphipods and copepods that eat bits of whale skin, seal lice that normally occur in small numbers and consume insignificant amounts of blood, and gastrointestinal helminths (“stomach worms”). Other are harmful enough to affect the well-being of individuals and even segments of a population. Consequences can be serious when individuals are exposed to parasites for which they have evolved no tolerance. For example marine mammal infections with the protozoan Toxoplasma gondi, a parasite of cats, may be one example of “pathogen pollution” as oocysts shed in cat feces enter coastal waters in runoff and sewage. Any parasite can become destructive when the mechanisms that maintain the host–parasite balance break down, as they do when an animal is ill or starving.

Microorganisms

Microorganisms of all kinds—bacteria, fungi, and viruses among them—abound in the sea. Some are found on land and in land dwellers, others, including certain Vibrio bacteria, thrive only in aquatic habitats. Like terrestrial mammals, marine mammals harbor many organisms that are considered normal. Most of these are potentially pathogenic, meaning they do not always cause infectious disease, but some are more threatening than others. The fine line between infection and infectious disease depends on both the aggressiveness of the organism and the susceptibility of the host, that is determined by the condition of its immune system.

1. Bacteria

The nature and severity of bacterial infections can be influenced by the animal’s behavior and age and environmental conditions. Habitat also plays a determining role. A few bacteria are inherently pathogenic. Leptospirosis, caused by the spirochete Leptospira spp., occurs in domestic and wild animals worldwide. Bacterial pneumonia, often associated with lungworms, can be serious enough to cause death or stranding.

2. Mycotic Infections

Fungal organisms rank low on the list of primary pathogens of marine mammals. They tend to infect animals that are weakened, perhaps by other chronic disease. After a long illness, an animal can be riddled with systemic fungi. The usual source is soil, dust, or water. The wide variety of fungi found in marine mammals includes Candida, Aspergillus, Coccidioides, Blastomyces, Histoplasma, Fusarium, Nocardia, and Lacazia.

3. Viruses

To spread rapidly, a virus requires a naive host population of a minimum density, which can arise either through population growth or changes in social behavior. Once infected, a migrating or wandering animal may carry the virus to new habitats. Poxviruses, e.g., commonly cause skin lesions in cetaceans; pox disease can appear and disappear in conjunction with other illnesses or stress. Herpesviruses are also common in cetaceans and, usually nut serious, they have been associated with fatal pneumonia and hepatitis. The number of viruses and other pathogens continues to grow. Some may represent new or emerging diseases in marine mammal populations.

Tumors

Marine mammals develop all kinds of tumors. As studies on marine mammals have increased, so have the numbers and variety of reported tumors. In other mammals, tumors have been associated with a variety of factors, including hormones, viruses, congenital and hereditary defects, and physical and chemical agents.

Predation

Killer whales and sharks are responsible for most  attacks on whales, dolphins, and porpoises. In addition to these aquatic predators, one terrestrial predator, the polar bear, hunts beluga whales (Delphinapterus leucas) and narwhals (Monodon monoceros) in Arctic areas. River dolphins appear to be the only cetacean group free from natural predation, although it has been suggested that freshwater caiman in South America may occasionally take young dolphins.

Actions being taken

Endangered marine mammal species are currently protected by a variety of domestic and international laws and collaborative efforts by nations, individuals, and organizations to reduce takes and prevent extinctions. Such as the 1991 “Agreement on the Conservation of Small Cetaceans of the Baltic and North Seas (ASCOBANS),” “Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea, and contiguous Atlantic area (ACCOBAMS),” and the International Whaling Commission’s global moratorium on commercial whaling in 1986. Other efforts, such as those by the International Union for the Conservation of Nature and Natural Resources (IUCN), the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) have focused on recovering marine mammal species or populations threatened with extinction on a global scale.

Most conservation efforts have been directed at reversing the impacts of commercial exploitation. Unfortunately, as huntung has declined or even ceased, other anthropogenic threats have arisen,
including those having direct (e.g., fishery by-catches, vessel strikes) or indirect (e.g., loss of prey to commercial fisheries) impacts. These threats have kept some marine mammal populations at low levels and slowed the recovery of others.

New challenges are emerging, which will have to be addressed further. These include anthropogenic activities related to habitat destruction, degradation of water quality, disturbance of animals (anthropogenic noise) and bioaccumulation of toxins. These impacts are less obvious than direct mortalities and are not easily studied or measured, but may be equally as deadly.

Recovery of a species requires that threats be removed or, at the least, mitigated. If the threat is as simple as over-harvesting, then the required management actions are obvious. However, the causes for population declines or lack of recovery are, in many cases, obscure or difficult to address.

If properly managed, marine mammal-watching activities can benefit the animals conservation through promoting increased public awareness of their biology and threats. However, dolphin watching is also recognized as a potential threat to the dolphins. Careful management, official dolphin-watching codes and enforcement are needed in order for the industry to be sustainable. Some environmental conditions are more difficult or impossible to control. Harmful algal blooms have increased in frequency and distribution in the past few decades. Blooms in some areas are linked to human activities. There is also growing evidence that changing environmental conditions can influence the prevalence or virulence of pathogens. Adaptation by some marine mammal species or populations may become evident with changes in general distribution, migratory patterns, and feeding areas or prey. Species already pressed to the edge of their habitat may not be able to bear much more change. Barring a catastrophic event, many—if not most—species will, as they have throughout their evolution, muster their pre-adaptive genes to secure a niche that will assure their survival and their health.

Reference:
Encyclopedia of marine mammals

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