Factors Affecting Cockle Population
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Cockles are small, bivalve molluscs from the family Cardiidae that are found in sheltered beaches around the world. There are over 200 species of cockles found in various places, but the most common species around the UK is the Cerastoderma edule, also known as the Common Cockle. The Common Cockle is found on beaches on the northern European coastline from the Bearing Sea as far south as the north western coast of Africa. The Common Cockle is the most abundant species of cockle found in Britain and is found at a wide range of tidal flats and estuaries where it burrows just below the surface of the sediment using a strong muscular foot. This allows it to feed on plankton easily using its siphons but still providing it with relative safety from predators and the tide (figure 1).
Traeth Melynog, also known as Traeth Abermenai, is a large mudflat of approximately 3.5km2. It is located at the southern point of Anglesey with the mouth of the Menai Strait to the east, and the Irish Sea to the west. It is buffeted from the Irish Sea by a stretch of land called Abermenai Point at its South-western border, which protect it from the full force of the waves. This beach is marked with 10 transects of 680m length which were 120m apart, covering an area of 0.75km2 from the high shore to the low shore. At the high shore level, ten areas at the end of the transect lines were sampled for any organisms present in the top layer of sand. This was repeated 220m lower on the transect line at mid shore level, and a further 220m at the low shore level. This allows researchers to get a good idea of the population structure and density of species on each level of the beach.
The population of cockles in any area is affected by many different factors, both biotic and abiotic. Examples of biotic factors include population density and predation, while abiotic factors include nutrient availability and exposure of the environment. If the population density in an area is large, the growth of the cockle will be affected due to the lack of room to grow sideways. This results in the cockle growing "upwards" in the sediment to gain more space. This affects the shape of the cockle, which at normal conditions will grow in length and width at the same rate. Having to grow upwards due to lack of space will show cockles that are positively allometric, or longer than they are wide. The amount of predators in an area also affects the cockle population. Cockles in the sublittoral zone will be preyed upon by marine organisms such as the common shore crab (Carcinus maenas), while those at the high water zone of the beach will be preyed on by Oystercatchers (Haematopus ostralegus) (Sanchez-Salazar et al. 1987). This is because the common shore crab will hunt for smaller, younger cockles found in the low shore zone which are easier for them to feed upon, while the Oystercatcher will select older, larger cockles that have managed to grow and move up the beach.
The objective of this study is to see whether these factors influence the cockle population at Traeth Melynog, the mortality and longevity of the cockles, and whether the population has a higher density on the low shore, mid shore, or high shore.
The results of the samples taken from transect 8 shows us that the majority of cockles were found in the middle shore. In transect 8, 147 cockles were found, with 6 on the high shore, 114 on the middle shore and 27 on the lower shore. Of the collected sample, 4.1% of the cockles were found on the high shore level, 77.6% of the cockles were found on the middle shore level, and 18.4% of the cockles were found on the low shore level. This would suggest that the cockles were more able to grow and feed on the mid shore level.
If we look at the results of table A, we can see that the vast majority of cockles sampled were found in the middle shore level of the beach. Transect 8 on the above map shows that of the 165 cockles collected during the sampling, 4.2% of the cockles were found on the high shore level, 77.0% of the cockles were found on the middle shore level, and 18.8% of the cockles were found on the low shore level. This is very close to the sample taken on the 27th October, with only a 0.1% variance in most results. This would imply that the cockle population along transect 8 were to an undetermined degree, constant.
Of the 147 cockles sampled, 47 were aged using growth bands on the shell that are secreted during growth. During the winter months, there is little growth in the cockle, which leads to marked bands on the shell. These bands can then be used to age the cockle. The results of the aging show that on the high shore, the cockles were small, not getting past 2 years of age, while at the middle shore the ages are spread between 2 and 7 years. This would seem to show that cockles in the middle shore are able to mature and grow more than the higher and lower shores, were it not for the results of the lower shore aging. On the lower shore, the cockles were aged from younger than 1 year to over 7 years of age. This could be explained due to the lack of nutrients on the upper shore, due to only being covered by tide for short periods of the day.
From the results, we can see there is a big difference in Cerastoderma edule size, density, and mortality across the three shore heights. This can be explained in many different ways. The size difference at the three different shore heights are influenced by the availability of nutrients, the presence of predators, and the population size of the cockle itself.
At high shore, the sediment is exposed to air throughout most of the day due to the action of the tides. This affects the cockles' ability to gain nutrients in the form of plankton, and therefore its ability to use those nutrients to grow, as shown by the small amount of cockles found in the samples. At low shore, the cockles are submerged for a majority of the day in water, but they do not grow beyond the mean length of the cockles found at the middle shore levels. Even though the low shore cockles have long periods of access to the nutrients, they are not growing very much in length.
This can be explained by the presence of predators in the high shore and low shore areas. In the low shore zones, the common shore crab Carcinus maenas prey on young, small cockles as the shell is thin or weak enough to be broken by the crabs' claws. On the high shore zones, the Oystercatcher Haematopus ostralegus prey on older, larger cockles. This allows the smaller cockles to grow until they become large enough to be considered a worthy meal by the oystercatchers. This can be witnessed by the biodiversity of Traeth Melynog; Oystercatchers were seen around the high shore zone of the beach, while the Common Shore Crab was found in larger numbers at the low shore area than the middle or upper shore areas. This suggests that although predation is common in the upper and lower shore areas of the beach, that predation by birds and crustaceans are limited in the middle shore, allowing for the large number and longer life spans of the cockle at this level. The middle shore also receives equal time of air exposure and water submergence, giving the cockles a good chance to gain nutrients, while staying relatively safe from predation.
The high population of cockles at the middle shore area of the beach was also shown to be affecting the growth pattern of the cockles. In the higher and lower shore areas, the cockles' length grew at the same rate as their width and weight, but in the middle shore, the length of the cockle was sometimes shorter than the width and the weight. This is because due to the high population density in the middle shore, some cockles close together have had to grow upwards rather than growing sideways. This force surrounding the cockle restricts the growth of the shell itself, but not the growth of the flesh of the cockle. This gives the cockle a higher weight than would be normal for a cockle of the same length.
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