Population dynamics and secondary production of the cockle Cerastoderma edule: A comparison between Merja Zerga (Moroccan Atlantic Coast) and Arcachon Bay (French Atlantic Coast) |
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| Authors: | Mériame Gam Xavier de Montaudouin Hocein Bazairi |
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| Institution: | 1. Dpto. Ecoloxía e Bioloxía Animal, Facultade de Ciencias do Mar and Estación de Ciencias Mariñas de Toralla, Universidade de Vigo, 36200 Vigo, Spain;2. Instituto Português do Mar e da Atmosfera, I.P./IPMA, Avenida 5 de Outubro, s/n, P-8700-305 Olhão, Portugal;3. Department of Biological Sciences, University of South Carolina, Columbia, SC 29208, USA;1. Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” (MACN-CONICET), Angel Gallardo 470, Buenos Aires 1405DJR, Argentina;2. School of Ocean Sciences, College of Natural Sciences, Menai Bridge, Anglesey LL59 5AB, UK;3. Universidad Nacional de Mar del Plata, Facultad de Ciencias Exactas y Naturales, CONICET, Funes 3250, Mar del Plata B7602AYL, Argentina;4. Grupo de Investigación y Educación en Temas Ambientales (GrIETA), Mar del Plata 7600, Argentina;5. Instituto de Biología Marina y Pesquera “Almirante Storni”, Universidad Nacional del Comahue, CONICET, Güemes 1030, San Antonio Oeste R8520CXV, Río Negro, Argentina;1. Ifremer, Laboratoire des Sciences de l''Environnement Marin (LEMAR), 29840 Argenton-en-Landunvez, France;2. Fisheries and Oceans Canada, Moncton, NB E1C 9B6, Canada;3. Laboratoire des Sciences de l''Environnement Marin (LEMAR), UBO, CNRS, IRD, Ifremer, Plouzané, France |
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| Abstract: | Cockle (Cerastoderma edule) population dynamics were studied at the southern limit of the distribution of this marine bivalve in Merja Zerga, Morocco. Parameters such as growth, mortality, and production were compared with those of a population at Arcachon Bay (France) a site in the center of the cockle's range. At each sampling period between two and three cohorts were simultaneously observed at each site and the average total abundance was usually higher at Merja Zerga. Recruitment occurred at both sites in spring when temperature rose above 19 °C, independently of the month. In Merja Zerga, winter recruitment was also observed at one occasion, following high sediment disturbance. The first year (2005–06) at Merja Zerga, the mortality rate was close to nil for juveniles and was Z = 1.5 yr? 1 for adults, providing a high production (64 g dry weight m? 2 yr? 1). At Arcachon during the same period, the juvenile mortality rate was Z = 10.9 yr? 1, the adult mortality rate was 3.4 yr? 1 and production was 26 gDW m?2 yr? 1. The second year (2006–07), mortality after recruitment was much higher (Z = 8.6 yr? 1, for juveniles) and similar to what was observed at Arcachon (Z = 8.4 yr? 1). Mortality rate of adults was higher at Merja Zerga (Z = 3.0 yr? 1) than at Arcachon (Z = 1.5 yr? 1). Production was lower at Arcachon than at Merja Zerga although growth performances were higher at Arcachon. The higher growth performance at Arcachon (Φ′ = 3.3) was mainly due to high asymptotic length (L∞ = 38 mm) and was related to low intraspecific competition compared to Merja Zerga where cockle abundance was higher (Φ′ = 3.1, L∞ = 31 mm). P/B was low in both sites and slightly higher at Arcachon (1.1–1.5 against 1.0–1.1 yr? 1). At Arcachon, recruitment was correlated with temperature, a peak occurring when temperature rose above 19 °C (June–July). At Merja Zerga, recruitment was already 2–3 months earlier but was not significantly correlated to temperature.This study showed that population dynamics of cockles at the southern limit of this distribution fell in the range of what was observed elsewhere in the North-Eastern Atlantic coast. Most factors that were involved in population regulation (intraspecific competition, predation and sediment dynamics) were not strictly dependent on latitude. The direct role of temperature (latitude dependent factor) was not obvious. Variation in temperature could explain the recruitment delay between Arcachon and Merja Zerga and the low maximum shell length at Merja Zerga. |
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