Many of the natural influences addressed within section 5.2 and activities addressed within section 5.3 have the potential to impact on the dolphins in similar ways. To avoid repetition, the most common mechanisms are described below and are extensively cross referenced, where appropriate, from within the assessment of each factor.

5.1.1.1    Disturbance

It is recognised that a range of human activities have the potential to disturb cetaceans, although it is difficult to assess the effects of disturbance. Under current UK [77] and European [78] wildlife legislation, it is illegal to deliberately disturb bottlenose dolphins. However, defining ‘disturbance’ and the need to prove any disturbance as deliberate weaken this legislation significantly. This weakness in the legislation has been brought to the attention of the relevant government departments by CCW, the other UK country agencies and Wildlife & Countryside Link (WCL).

However, leaving aside a precise legal definition, it is generally accepted that ‘disturbance’ in the current context is used to refer to any action which results in a change in the behaviour or physiology of an animal or group of animals, and it is to this definition of disturbance that this document works. Two main forms of disturbance occur; harassment, where there is repetitive close interaction with the dolphins; and noise which may constitute part of harassment or cause disturbance in its own right.

a.   Noise

Sound travels far more efficiently through water than air. Low frequency sounds can travel thousands of miles through water.

Dolphins use sound as their primary sense for communication, navigation and hunting. They rely, like humans, on a specific range of sound frequencies. Anthropogenic noise in the marine environment is a potential source of disturbance to the dolphins. Increases in the intensity or changes in frequency of underwater noise levels are likely to cause dolphins to move away from the source, at least initially, and in some cases permanently. Such noise may also increase physiological stress, change behaviour patterns including foraging, communication and navigation, or injure the animals.

Adverse effects on the dolphins’ energy budget in response to disruptive noise may affect their health and thus their reproductive success and survival.The shock waves created by powerful underwater explosions can contain sufficient energy to injure or kill dolphins. Seismic surveys using high pressure air guns produce sound waves that may be audible for tens of kilometers from the survey vessel [79]. Dolphins and their prey are known to avoid an area for several kilometres round a seismic vessel. Continuous or loud repetitious noise, especially of a limited frequency range, may not only disrupt normal behaviours but also damage the dolphins’ ability to hear and communicate, and could encourage them to abandon an area entirely.

Most anthropogenically created underwater noise in the shallow coastal waters of Cardigan Bay originates from boat activity. Most of the noise generated by boats comes from the engine and transmission, and through propeller cavitation. The operating condition of this equipment can significantly affect the intensity and frequency of noise developed. Other, potentially significant, generators of underwater sound include the oil and gas industry, underwater construction, dredging and dumping operations and military ordnance tests. Just like other animals it is likely that dolphins are startled by sudden unexpected noise.

They might be most likely to habituate to sounds which are predictable and unvarying [80], particularly if suitable alternative areas are not readily available.

b.   Harassment

Boating and other activity not only generates noises that can disturb cetaceans, but the behaviour and volume of operations in an area may also inadvertently or directly harass the animals. The mechanism for this type of disturbance is comprehensively defined within the New Zealand Marine Mammal Protection Regulations 1992, where the term ‘Harass’ includes “... to do any act that causes or is likely to cause injury or distress to any marine mammal, or disrupts significantly or is likely to disrupt significantly the normal behavioural patterns of any marine mammal.” Vessels in close proximity, travelling at high speed and/or behaving erratically in the vicinity of dolphins may elicit a range of subtle physiological responses as well as more observable or acoustically detectable changes in behaviour.

The effects of such disturbances are difficult to assess. However, cetacean responses to boats in the vicinity may include movement away from vessels, as well as changes in ventilation, vocalisation and social behaviour patterns [81].Dolphin response to vessels also varies with boat type and behaviour; quieter boats travelling at high speed appear to cause more disturbance than slower, larger boats that emit higher intensity noise [82].

Dolphins are more likely to become tolerant of predictable or regular boating activities, such as the slow movement of a large displacement vessel along a steady course. If disturbance persists, animals may move away for relatively short periods, or abandon an area altogether. However, emigration may not always be an option, if their social and biological requirements are not available in alternative locations. In such cases, there may be adverse effects on the dolphins in the long term, including reductions in health, reproductive success and survival.

5.1.1.2    Collisions

Boat activity in the vicinity of dolphins may result in inadvertent collisions with dolphins resulting in injury or death. The probability of collisions, which is typically low, increases with the density of dolphins and vessels, their proximity to each other and the dolphin’s physical or social condition. In Cardigan Bay, no dolphins are known to have been killed by boat collision, but there has been one dead harbour porpoise whose lesions were suspected to have been caused by a propeller [83]. Dolphins are also more capable of avoiding collision from a vessel if the vessels’ course is predictable and if there is an adequate room or depth for escape.

5.1.1.3    Pollution

Pollution in the marine environment can be highly complex in nature, and may have either a direct effect on the bottlenose dolphins, or an indirect effect, by reducing the availability of food to that species or changing the habitat significantly [84] . With the current level of regulation of discharges to air and water, the most significant UK sourced pollutants can be chemicals that were only discharged in substantial amounts a long time ago. Highly persistent toxic substances, such as organochlorines (that degrade very slowly) and heavy metals (that never degrade), can remain a problem for decades or centuries after they were released into the environment.

This is probably the case in Cardigan Bay, where bottlenose dolphins may be accumulating, or have accumulated, significant amounts of polychlorinated biphenyls (PCBs) and perhaps some heavy metals through bioaccumulation [85] and biomagnification [86] via the food chain on which they depend. Regulated sources of these contaminants are tiny compared with the amounts already in the marine environment. Toxic burdens of persistent chemicals (particularly those stored in blubber) such as organochlorine compounds may remain within a population for some time, a proportion being passed from mother to calf during gestation and weaning.As a top predator, the bottlenose dolphin is very reliant on the success of the trophic levels beneath it.

The dolphins and their prey may range widely and thus become exposed to pollutants over a wide geographical area. In this situation the contamination status of Cardigan Bay may not be the only, or even the most important, factor in the influence that pollution has on the dolphins encountered there. Relatively high levels of organochlorines, or heavy metals such as mercury, could be leading to chronic effects on the health of the dolphins (acute effects are far less likely) and may reduce their ability to resist disease or reproduce.

The routes by which these contaminants accumulate in the Cardigan Bay dolphins are, however, very uncertain, and a considerable amount of research will be required to understand them and their significance for the continued well-being of the Cardigan Bay dolphin population.

5.1.1.4    Entanglement & Ingestion

Bottlenose dolphins may become entangled in underwater debris and equipment particularly in fishing gear such as rope, line and netting [87]. Marine debris including fishing hooks and line may also be ingested directly, or inadvertently within or attached to prey items, causing internal injury and death [88]. Primary sources of debris are litter and active, lost and discarded fishing and angling gear. The harder the debris is to detect the greater the likelihood of entanglement. Likelihood of entanglement will be density (of bottlenose dolphins, fishing gear, debris and litter) dependent.

Significant problems have been identified elsewhere in the world with mono and multi-filament nylon line and netting.As dolphins breathe air, entanglement underwater may result in drowning. Animals may also die due to choking, restriction of blood vessels or during removal from gear. Struggling animals typically injure themselves in an attempt to get free or during removal. For example, a young bottlenose dolphin calf died in Cardigan Bay in 1994 as a result of severe abdominal injuries; the injuries it sustained indicated that it was probably entangled in a net and fatally injured during removal [89]. Linear cuts and indentations around the head and fins are typical signs of a once entangled dolphin.

Ingested items are known to cause internal injury and death.It is often difficult to document bycatch of marine mammals in small-scale coastal gill net fisheries due to difficulties in placing observers in these fisheries. In such cases, identification of bycatch may rely on documentation of physical evidence of entanglement in stranded animals. For example, harbour porpoises are known to be entangled in small-scale gill net fisheries that operate seasonally along the coasts of North Carolina [90], Virginia, and Maryland, but these by-catches are not reported to state or federal agenciesAnalysis of dead stranded dolphins is one way in which we are able to look for evidence of entanglement problems within the bay, and a strandings programme is currently in place.

The largest single cause of death for cetaceans found along English and Welsh coasts between 1990-1995 was fisheries bycatch [91]. Accidental capture in fishing gear was also the single most important cause of death for harbour porpoises in Great Britain from 1988-1995 [92] . Over 20% of examined harbour porpoises and other cetaceans stranded on the Welsh coast from 1989-1991 were found to have been caught in fishing gear [93]. However, not all bycaught cetaceans show clear net marks or other injuries, and the ability of even an experienced examiner to detect evidence of entanglement is often determined by the condition of the carcass.

It is likely that among the harbour porpoise and other species of English and Welsh cetaceans in which physical trauma of unknown cause was diagnosed, there were further bycatch cases. It is therefore possible that the true incidence of bycatch in UK stranded cetaceans has been underestimated [94]. Similarly, studies in North Carolina show that evidence of fisheries interaction such as net marks, removed appendages, slit abdomens, attached gear and propeller cuts diminish with deteriorating condition of the carcass [95]. This too suggests that evidence from strandings underestimates the impact of human interactions on cetaceans.

5.1.1.5    Pathogens

Bottlenose dolphins are susceptible to a range of macroscopic and microscopic parasites and, like many wild animals, often support naturally high parasite burdens. Whilst most parasites typically have a preferred host in which they are most successful, certain pathogens are able to exist in multiple host species, although the type and degree of effect on their host often varies.Certain human activities, particularly those that put dolphins in contact with other mammals or their waste products, increases the potential for transmission of any such organisms to the bottlenose dolphins.

Humans touching dolphins, the introduction of relocated dolphins, human and cattle wastes are all methods by which this could potentially occur [96]. Whilst increasing the burden of existing parasites will be detrimental to the dolphins, it is the introduction of a virulent novel parasite or parasite strain, one to which they are not already exposed, which may have the most rapid and serious effect.

It should be noted however that immunosuppression resulting from other impacts on the bottlenose dolphins is probably of greater threat in facilitating the rapid spread and increased host mortality of a parasite. The spread and impact of Morbillovirus in striped dolphin populations of the Mediterranean [97] and seals of the Dutch Wadden Sea [98] possibly as a result of PCB induced immunosuppression, for example, illustrates this well.

5.1.1.6    Prey depletion

Bottlenose dolphins, in general, are known to feed on a variety of fish and invertebrates such as molluscs [99] and crustaceans. As opportunistic feeders, different dolphin populations in different habitats have different diet preferences. Unfortunately there is very little information on the diet of the Cardigan Bay dolphins. Existing information is based upon a few dolphin autopsies and observation of surface feeding behaviour. From visual observations it is believed that pelagic and seasonal fish such as mackerel, herring, sprat, bass, sewin (sea trout), salmon and mullet form a proportion of their diet within the bay [100].

The turbidity of local waters and the general nature (behaviour) of the bay’s dolphins has made observations of underwater feeding behaviour difficult. Analysis of the acoustic behaviour of dolphins as well as their distribution and that of their prey may in the future improve our understanding of dolphin diet. Research projects are currently trying to identify potential dolphin prey on or near the seabed.Certain human activities have the potential to reduce the prey available to the bottlenose dolphin both in variety and abundance.

This can be through direct removal of the prey species by fishing activities or by impact on the prey or their habitat through noise, pollution, fisheries and marine development.A reduction in the diversity, abundance or size of prey, or changes in their distribution will detrimentally impact the dolphins energy balance and may result in a change in dolphin distribution, reduced growth rate, reproductive success and survival. In addition, toxic contaminants like PCBs that are stored in fatty tissues will be mobilised when starving animals draw on their blubber reserves for nourishment. The degree of effect is likely to be largely determined by the level of reliance of the dolphins on the species of prey affected.

5.1.1.7    Habitat Impacts

Many activities have the potential to alter or detrimentally affect the sea bed and its marine communities. As a top predator, bottlenose dolphins are heavily reliant on the condition of the trophic levels beneath them in their food chain. Changes to the dolphins habitat may ultimately result in changes to the abundance or distribution of their prey.The presence of a pollution burden within sea bed communities may result in some of this being passed up the food chain to the dolphins. Bioaccumulation and biomagnification may result in high pollutant burdens in the dolphins.

[77] Wildlife and Countryside Act (1981) as amended.

[78] The Conservation (Natural Habitats, &c.) Regulations (1994) - (Taken as the Habitats & Species Directive transposed into UK law)

[79] Goold, J.C. 1996. Acoustic assessment of populations of common dolphin Delphinus delphis in conjunction with seismic surveying. J. Mar.Biol.Ass. UK

76:811-820.Engas, A., Lokkeborg, S. , Ona, E. & Soldal, A.V. 1993. Effects of seismic shooting on catch availability of cod and haddock. Institute of Marine Research, Norway Fisken og Havet, No. 9.Lokkeborg, S. and Soldal, A.V. 1993. The influence of seismic exploration with airguns on cod (Gadus morhua) behaviour and catch rates. ICES Mar.Sci.Symp., 196:62-67.

[80] Gordon, J.C.D., Gillespie, D., Potter, J., Frantzis, A., Simmonds, M., Swift, R. 1998. The effects of seismic surveys on marine mammals. In: Centre for Marine & Petroleum Technology,  Seismic & Marine Mammals Workshop Documentation. 23-25 June 1998, LSE, London.

[81] Arnold, H. 1997. The Dolphin Space Programme: The development and assessment of an accreditation scheme for dolphin watching boats in the Moray Firth. A Report for Scottish Wildlife Trust, Scottish Natural Heritage and the EU LIFE Programme. Inverness.

[82] Evans, P.G.H., Canwell, P.J. and Lewis, E.J. 1992. An experimental study of the effects of pleasure craft noise upon bottle-nosed dolphins in Cardigan Bay, West Wales. In: Evans, P.G.H. (ed.) European Research on Cetaceans - 6. European Cetacean Society, Oxford. pp 43-46.

[83] Kirkwood et al 1997

[84] Morris, R.J., Law, R.J., Allchin, C.R., Kelly, C.A., Fileman, C.F. 1989. Metals and organochlorines in dolphins and porpoises of Cardigan Bay, west Wales. Mar. Poll. Bull. 20(10): 512-523.Law, R.J. 1994. Collaborative UK Marine Mammal Project: Summary of data produced 1988-1992. MAFF Fisheries Technical Report No. 97, Directorate of Fisheries Research, Lowestoft

[85] Bioaccumulation is the net accumulation of a substance within an organism resulting from the rate off uptake exceeding the rate of loss through excretion or metabolisis.

[86] Biomagnification is the increase in amount of a substance per organism up the food chain as a result of the transfer of substances from prey to predator. i.e. a predator takes on the entire pollution burden of all its prey.

[87] Wells, R.S., Hofmann, S., Moors, T.L. 1998. Entanglement and mortality of bottlenose dolphins, Tursiops truncatus, in recreational fishing gear in Florida. Fishery Bulletin,  96(3):647-650. Mann, J., Smolker, R.A. and Smuts, B.B. 1995. Responses to calf entanglement in free-ranging bottlenose dolphins. Mar.Mamm.Sci. 11:100-106.

[88] Gorzelany, J.F. 1998. Unusual deaths of two free-ranging Atlantic bottlenose dolphins (Tursiops truncatus) related to ingestion of recreational fishing gear. Marine Mammal Science. 14(3):614-617.

[89] Kirkwood et al 1997

[90] A. Read, T. Cox, S. Barco, J. Evans, D. Gannon, H. Koopman, B. McLellan, K. Murray, A. Pabst, C. Potter, M. Swingle, V. Thayer, K. Touhey, & A. Westgate.1997. Documenting bycatch of marine mammals in coastal fisheries: observations of stranded harbour porpoises along the Mid-Atlantic coast. Fifth Annual Atlantic Coastal Dolphin Conference. University of North Carolina.

[91] Kirkwood, J.K., Bennet, P.M., Jepson, P.D., Kuiken, T., Simpson, V.R. and Baker, J.R. 1997. Entanglement in Fishing Gear and Other Causes of Death in Cetaceans Stranded on the Coast of England & Wales, The Veterinary Record, 141: 94-98.

[92] Law 1994; Kirkwood et al 1997

[93] Baker & Martin 1992

[94] Kirkwood et al 1997

[95] Touhey, K., Hohn, A., and V.Thayer.1997. Detecting Evidence of Human Interaction in Stranded Bottlenose Dolphins: It’s Conditional. Fifth Annual Atlantic Coastal Dolphin Conference. University of North Carolina.

[96] A summary of anthropogenically derived or transmitted pathogens which may affect bottlenose dolphins is given in Grellier et al (1995).

[97] Aguilar, A., & Raga, J.A. 1990. Mortality of dolphins in the Mediterranean Sea. Politica Cientifica, 25: 51-54 (in Kuiken, T., Bennett, P.M., Allchin, C.R., Kirkwood, J.K., Baker, J.R., Lockyer, C.H., Walton, M.J., Sheldrick, M.C. 1993. Is there a link between PCB levels and cause of death in harbour porpoises (Phocoena phocoena)? In: Evans, P.G.H. (Ed.) Research on Cetaceans - 7. Proceedings of the seventh annual conference of the European Cetacean Society, Inverness, U.K., February, 1993. 195-197)

[98] Brouwer, A., Reijnders, P.J.H., and Koeman, J.H. 1989 Polychlorinated biphenyl (PCB)-contaminated fish induces vitamin A and thyroid hormone deficiency in the common seal (Phoca vitulina). Aquat. Toxicol., 15: 99-106 in Kuiken et al (1993).

[99] Mostly Cephalopod molluscs such as squid and cuttlefish.

[100] Arnold, H. 1993. Distribution, abundance, and habitat use of bottle-nosed dolphins in Cardigan Bay, Wales, 1992. In: Evans, P.G.H. (Ed.) European Research on Cetaceans – 7. European Cetacean Society, Oxford. pp. 63-66