5.2    Natural factors

The marine environment provides the bottlenose dolphins with the conditions under which they are able to live and flourish. A change to certain environmental parameters such as tidal currents, sediment regimes and wave exposure could possibly degrade or even improve the conditions required by the bottlenose dolphins.

a.   Current Status

Descriptions of the current status of environmental conditions within Cardigan Bay are given in section 2.2.

b.    Mechanism for effect on the feature

i.   Exposure

Wave exposure influences the degree of water mixing, water turbidity, dynamism of sea bed substrates and the level of physical turbulence that the shallow coastal waters and their marine life receives.

Differences in behaviour and possibly size exist between sheltered water and open coast bottlenose dolphins and the level of exposure may influence this to some degree. It has also been suggested that small populations of highly resident dolphins tend to be found in geographically closed habitats, while larger populations of both resident and transient animals are often found in open habitats [104].
It is not know what effect, if any, changes to the overall exposure within Cardigan Bay may have on its bottlenose dolphins.

ii.  Currents /Tidal Regimes

Bottlenose dolphin distribution and behaviour patterns are highly flexible and may vary from site to site. Short-term movements of the dolphins are often affected by tidal state, which also affects bathymetry and access to certain prey. In some areas around the world, activities such as feeding and resting are strongly related to current strength and tide state, whereas in other locations no significant relationship has been found between tidal regime and dolphin distribution or behaviour. However, coastal bottlenose dolphins tend to move with concentrations of food, both with and against the tide.

There is little evidence to date suggesting that bottlenose dolphin distribution and activity patterns in Cardigan Bay are influenced by tidal regimes or current strength. Although feeding behaviour has been observed near headlands and estuaries, where the generally weak inshore currents tend to be stronger, the dolphins have also been recorded feeding in most other coastal and offshore areas from the north to south of Cardigan Bay. Recent studies within the SAC addressing the possible relationship between dolphin behaviour, site use and tidal flow could provide further insight into any relationships between Cardigan Bay dolphins and local tidal regimes.

Tidal influences also affect the movement of marine sediments. Contaminant inputs from within and south of the Bay are generally circulated in a northerly direction, and may settle out into sediments or remain in the water column during the approximate 13 day flushing time [105] (time taken for natural processes to replace the waters of the bay and to ‘flush through’ any water borne pollutants). Contaminant inputs from the southern Irish Sea and the coasts of England and south Wales may also be washed into Cardigan Bay on the northerly residual and surface current flows. The levels of contaminants carried by currents may affect the dolphins as top predators in the Cardigan Bay marine ecosystem.

iii.  Habitat

The nature of sea bed habitats are a determining factor in the type, diversity and abundance of species living in and on the sea floor (see Figure 8). Sea bed communities are often highly specific to particular habitat types: small alterations in silt fraction of sediment, current strength and wave exposure can make an enormous difference. Alteration to any of the fundamental environmental parameters such as current strength, wave exposure, depth, temperature, substrate, geology, light and siltation which influence distribution of benthic species may result in an alteration in the distribution and abundance of some of the dolphins prey.

Sediments and sediment mobility in conjunction with tidal currents and weather conditions also affect the turbidity of coastal waters, which may influence the diversity and distribution of dolphin prey species. (See Turbidity below).

Sediments also act as a sink for trace metals and other pollutants that may be released when sands and gravels are disturbed by dredging, bottom trawling, or similar activities that disrupt the seabed. Changes to sediment movement and deposition may alter the pollutant burden that is biologically available and may be passed on to the dolphins.

A number of dolphin characteristics have been attributed to habitat variation, including density, group size and residence patterns of bottlenose dolphins in different populations [106] . Bottlenose dolphin behaviour (particularly feeding) and population characteristics vary in different habitats.

Dolphins living in large, open areas composed of numerous habitat types have been found to spend more time hunting, feeding and travelling between dispersed patches of food than those in restricted passes or channels that tend to concentrate fish and invertebrates migrating through on a daily/tidal and seasonal pattern [107] .

Differences in density, group size and residence patterns of bottlenose dolphin populations have also been attributed to variations in habitat. Research on other bottlenose dolphin populations has shown that in general, group size tends to increase with increased water depth or openness of the habitat. It has also been suggested that small populations of highly resident dolphins tend to be found in geographically closed habitats, while larger populations of both resident and transient animals are often found in open habitats [108] .

Research suggests that habitat alteration could potentially disrupt the social behaviour, food supply and health of bottlenose dolphins.

iv.  Turbidity

Studies in other areas have found that seasonal and short term prey distribution and abundance are related to variations in turbidity [109] , as well as water temperature, tidal flow speed and direction. Studies have also found that the distribution of many species of bottlenose dolphin prey is directly related to turbidity. There are indications that an area with muddy substrates that averages <1m visibility may be generally unsuitable for bottlenose dolphin prey species, and that dolphins may also avoid areas of high turbidity in warmer waters where there might be potential shark predators [110] . Research along the South African coast also suggests that seasonal, and perhaps shorter term environmental fluctuations of water temperature and turbidity govern the abundance and distribution of bottlenose dolphin prey in inshore areas [111] . Cardigan Bay bottlenose dolphins have been observed in both clear and turbid waters, but their use of areas during varying states of water clarity has not yet been studied.

v.  Depth

In general, bottlenose dolphin group size tends to increase with increased water depth or openness of the habitat. (See Figure 7 ). Preliminary indications suggest that Cardigan Bay dolphins may follow similar patterns. Mean dolphin group size is larger (12.35 individuals) in the deeper, more exposed waters of the Bay generally found greater than 3km (approx 2 miles) from shore than it is in the shallower waters (8.52 individuals) closer inshore [112] .

The reasons for these variations of group size may be related to foraging techniques and protection from predation. Shallow, inshore waters often provide relatively predictable, evenly distributed food resources associated with reefs or flat areas of seabed. When feeding on benthic prey, the bottlenose dolphin has to invest more energy in diving for deeper prey items than shallow ones. Shallow, enclosed areas may also provide protection from predation, though predation of bottlenose dolphins in Cardigan Bay is not currently known to be an issue.

In more open waters, schooling fish are the predominant prey available to the dolphins. Larger groups of dolphins combining their sensory capabilities increase the probability of locating these patchy but rich food sources and provide the number of individuals necessary to cooperatively locate and herd prey. Larger groups in open waters may also provide better protection from predators.

vi.  Climate & Seasonality

Climate encompasses regional conditions of temperature, humidity, rainfall etc. and is therefore covered in more detail within the relevant following sections. However, in general terms changes to the Cardigan Bay climate may result in improved or worsened conditions for its bottlenose dolphins. Climatic changes may not only affect the bottlenose dolphins directly, but possibly more importantly affect lower levels of the food chain on which they rely.

In general, some bottlenose dolphin behaviour has been related to seasonal changes in weather conditions. Seasonal variations in bottlenose dolphin group size have been noted in some locations around the world, including both increases and decreases in group size during the winter. However, some populations of a similar latitude differ in the degree of seasonality they show, showing some or none [113] .

There appears to be a seasonal pattern in group size of bottlenose dolphins using Cardigan Bay, with a peak in late September and October. The number of individuals using the area also appears to peak in late autumn. The reasons for such seasonal patterns in Cardigan Bay dolphins have not yet been identified. It is possible that seasonal peaks in feeding frequency seen in Texas and Florida dolphin populations may relate to dolphins building up fat stores in preparation for winter. In some tropical locations, bottlenose dolphin calves are born year-round, while in other populations calving appears to be seasonal. In the temperate waters of Cardigan Bay, newborn calves have been observed between May and September which suggests that breeding takes place during a similar period [114]. [115]

Some coastal bottlenose dolphins in higher latitudes show a clear tendency towards seasonal migration, while those in warmer waters show localised seasonal movements that probably have much to do with movements of food and need for safety in reproduction. In at least one case, off western Florida, movements of bottlenose dolphins correlated with movements of mullet. The fact that this species preys on many different food items may explain why their movement patterns are not as predictable as those of some other cetaceans.

vii. Sea Temperature

Short and long term abundance and distribution of prey species are related to a variety of natural environmental conditions, including sea temperature. An autumn feeding peak noted in a Texas population of bottlenose dolphins was most pronounced where water temperatures dropped significantly [116]. Recent research has also found that sea temperature is correlated with the prevalence and severity of skin lesions on bottlenose dolphins in Cardigan Bay and a number of other populations worldwide [117]. Sea temperature is thought to be a key limiting factor to the worldwide distribution of this species. Whilst they are seen commonly in the Moray Firth in Scotland and occasional sighting have been made further north, bottlenose dolphins are not thought to extend into arctic waters. The Cardigan Bay dolphins appear therefore to be toward the northern range of the species. (For temperature ranges in Cardigan Bay see Figure 10. and Figure 11.).

viii. Salinity

Short and long term fluctuations in salinity may have a direct effect on the dolphins’ susceptibility to infections and to their general health. For example, skin lesions in a variety of worldwide bottlenose dolphin populations, including those in Cardigan Bay, have been found to be correlated with salinity. Dolphins living in waters with the lowest salinity had the highest prevalence and severity of epidermal lesions [118]. The diversity and distribution of prey species is also strongly influenced by salinity, and dramatic changes in the natural characteristics of Cardigan Bay waters could result in changes in prey availability, with impacts on the dolphins’ energy budget, distribution, individual health and reproduction.

ix.  Fronts

Fronts are regions of strong horizontal density gradients in the ocean. The strong horizontal pressure gradients generated at the front often balanced by the Coriolis force [119] , leading to strong along-front currents. When these two forces are not perfectly balanced, vertical circulations can be generated at the front. These vertical circulations have the potential of bringing deep, nutrient-rich waters into the well-lit surface waters, and stimulating phytoplankton growth. Fronts are typically the site of subsurface patches of phytoplankton, however, we do not have a good understanding of the physical and biological processes that maintain these patches.

An increase in productivity and diversity is usually associated with marine fronts. However, the scale and nature of the Cardigan Bay front is unlikely to reflect the increases in productivity and diversity associated with the larger oceanic fronts. Its importance to the bottlenose dolphins and the impact which may result from its loss or alteration are unknown. Likely areas of frontal formation are shown in Figure 12.).

c.    Management Response

i.   Rationale

Changes to any of the above natural variables has the ability to affect the bottlenose dolphins, either directly or more likely indirectly through changes to prey distribution and abundance. Natural changes to these processes are outside any management control and negative impacts on the dolphins as a result of this are considered acceptable. However, many of these natural processes can be influenced by humans to a lesser or greater degree and management controls may exist to provide a means of managing such activities. Consideration of the potential for human activities to alter natural processes and thereby effect the bottlenose dolphins is given in the following section.

Bottlenose dolphin communities are not known to be resident further north in the UK than the Moray Firth, and this is believed to represent the northern limit of their normal natural range. Sea temperature is the key influencing factor on the dolphins range, and any reduction to sea temperatures in Cardigan Bay would bring this northern limit closer. Global trends are currently showing an increase in sea temperature and variation in the flow and direction of the Atlantic drift current may also result in increases or decreases in temperature.

Individuals existing close to the edge of their natural range are typically, though not always, less able to tolerate increases in ‘stress’ than individuals living towards the centre of their natural range.There is limited knowledge on the sediment movement into and within Cardigan Bay and the influences of this on the cSAC habitat. There is a need to review the likely impact on the SAC of pollutants in the sediments.

ii.  Type of Response

F6 : There is evidence to suggest that the factors are having a significant effect on the feature, but they are outside management control (i.e. natural process, or there is no current mechanism for management).

iii.  Actions, including links to other policies/plans/measures

• Maintain a watching brief on natural processes in Cardigan Bay and the Irish Sea, noting alterations or trends that may be anthropogenically influenced.
• Ensure that consideration of plans and projects (see page) take full account of the potential for alterations to be made to natural processes and the potential effects these may have on the bottlenose dolphins and their habitat.
• There is limited knowledge on the sediment movement into Cardigan Bay and influences on the cSAC. There is a need to review the likely impact of pollutants in the sediments.
• Promote research which aims to gain a better understanding of the key habitats of the bottlenose dolphins and the key natural factors that determine their importance.

[104] Ballance, L.T. 1990. Residence patterns, group organization, and surfacing associations of bottlenose dolphins in Kino Bay, Gulf of California, Mexico. In: Leatherwood, S. and Reeves, R.R (Eds.) The bottlenose dolphin. Academic Press, San Diego, pp. 267-283.

[105] Nichols et al (1992)

[106] Shane, S.H., Wells, R.S. & Wursig, B. 1986. Ecology, behaviour and social organization of the bottlenose dolphin: a review. Marine Mammal Science, 2(1), 34-63.

[107] Shane et al (1986)

[108] Ballance (1990).

[109] Cyrus & Blaber 1987 as cited in Cockcroft, V.G. 1992. Incidental capture of bottlenose dolphins (Tursiops truncatus) in shark net: an assessment of some possible causes. Journal of Zoology, London (A), 226: 123-134

[110] Cockcroft, V.G., Cliff, G & Ross, G.J.B. 1989. Shark predation on Indian Ocean bottlenose dolphin Tursiops truncatus off Natal, South Africa. South African Journal of Zoology 24: 305-310.

[111] Cockcroft, V.G. 1992. Incidental capture of bottlenose dolphins (Tursiops truncatus) in shark net: an assessment of some possible causes. Journal of Zoology, London (A), 226: 123-134.

[112] Grellier et al (1995)

[113] Shane et al (1986)

[114] Dolphin gestation is about 12 months

[115] Grellier et al (1995)

[116] Shane et al (1986)

[117] Wilson et al (1999)

[118] Wilson et al (1999)

[119] Force generated by the turning of the earth.