Observations on some population parameters of exploited stocks of Senilia senilis (=Arca senilis) in Sierra Leone

Abstract

The West African cockle Senilia senilis (L.) (=Arca senilis L., 1758) from some estuaries and muddy shores of Sierra Leone was studied from September, 1973 to February, 1975. Quantitative samples were obtained along transects using quadrats and sieves; some random hand-picked collections were also made. The length, number of rings and the maximum width of the inter-ring bands along the outer convex surface of the right valve were recorded from samples of ≥25 specimens. All the cockle beds examined were exploited and had stocks with a mean density of ≤9 large (>10 mm) cockles/m2, and at River No. 2 estuary at peak spatfall, of 130 seed cockles/m2. Spat settlement began in November-December after the rainy season, reached a peak in January-February and continued to the early part of the following rainy season (May-June). Seed cockles were absent from August to October. In the length-frequency distributions, modes were clear and associated with certain shell-ring numbers. Rings on shells were formed once a year, during July to September, the period of maximum rainfall, and they were successfully used in ageing the cockles. The first ring was formed at a mean age of 7 months and the subsequent ones annually. The growth period of the inter-ring bands, considered to extend from August to the following July (s. senilis growth year), may actually be of only 10 months duration. Sublittoral cockles from No. 2 estuary showed a higher growth rate than those from the levels exposed at low tide and subjected to greater exploitation. Cockles from the Sierra Leone River Estuary showed even greater growth. s. senilis grows slowly and lives long (up to 8 to 9 years), and its von Bertalanffy parameters are: River No. 2 estuary cockles →k=0.27 to 0.31 and L ∞=99 mm; Sierra Leone River Estuary cockles →k=0.22 and L ∞=145 mm. At No. 2 estuary, cockles enter the fishery at 22 to 25 mm (14 to 15 months old) and exploitation is heavy, resulting in the stocks being dominated by a few young year-classes.