Growth of Crassostrea gigas spat and juveniles under differing environmental conditions at two sites in Wales

Croissance du naissain et de jeunes huîtres, Crassostrea gigas, sous diverses conditions environnementales de deux sites du Pays de Galles

¹ Centre for Applied Marine Sciences, Marine Science Laboratories, School of Ocean Sciences, University of Wales Bangor, Menai Bridge, Anglesey, LL59 5AB, UK
² Department of Zoology, Ecology & Plant Science, Aquaculture and Fisheries Development Centre, University College Cork, Lee Maltings, Prospect Row, Cork, Ireland

Received: 24 October 2005
Accepted: 25 March 2006

Abstract

Two sites in North Wales, UK, were compared for growth, survival, immunocompetence and gonad development of Crassostrea gigas. Juvenile C. gigas spat were purchased as a cohort and placed in bags on trestles at the Menai Strait (MS, two heights on the shore) and the Inland Sea (IS) in the summer of 2003. Performance was examined as a function of several environmental parameters; temperature, coloured dissolved organic material absorption coefficient (cDOM), organic and inorganic nutrient levels, chlorophyll a and phytoplankton community composition. Both sites produced variations in growth, as assessed by dry weight indices and condition index, which were found to be lower at IS, where the animals were permanently immersed, than at MS, where they were emersed for at least 1 h/tide (at the low shore height) and 4 h/tide (at the high shore height) during spring tides. Condition index was also significantly higher at MS high shore than at MS low shore. Mortality was low at all sites, though an increase at IS at the end of the trial led to a significantly greater final percentage mortality of 15.6%. Gonad development was limited, particularly at IS where most animals remained undifferentiated. Gonad development was significantly higher at MS high shore than at MS low shore. cDOM, organic and inorganic nutrients were similar at the two areas. At IS the temperature and chlorophyll a levels were higher than MS. The phytoplankton communities were significantly different, with a bloom of Prorocentrum micans dominating at the Inland Sea for much of the experimental period. Differences in the hydrodynamics, as well as Phytoplankton community appeared to be the only parameters that might cause a lower growth rate in the Inland Sea. The possibilities of P. micans having a sub-lethal effect on C. gigas are discussed.

Résumé

Deux sites du Nord du Pays de Galles (G.B.) sont comparés au niveau de la croissance, de la survie, de l'immuno-compétence et du développement gonadique de Crassostrea gigas. Du naissain de C. gigas a été acheté, considéré comme cohorte, et durant l'été 2003, placé en sacs sur des tables (tréteaux) à Menai Strait (MS, à deux différents niveaux de la zone intertidale) et dans la mer intérieure « Inland Sea » (IS). Les résultats sont examinés en fonction de plusieurs paramètres environnementaux, température, matière organique dissoute colorée (cDOM), niveaux de substances nutritives organiques et minérales, chlorophylle a et composition phytoplanctonique. Des variations sont observées dans les deux sites, au niveau de la croissance, établies par des indices de poids secs et de condition qui sont plus faibles à IS, où les huîtres sont immergées de façon permanente, alors que celles de MS restent émergées au moins 1 h par marée (en bas de la zone intertidale) et 4 h par marée (en haut) durant les grandes marées. L'indice de condition est significativement plus fort en haut qu'en bas de la zone intertidale à MS. La mortalité est faible quel que soit le site, bien qu'elle augmente à IS en fin d'expérimentation, ce qui conduit à 15,6 % de mortalité à IS. Le développement gonadique est limité, et particulièrement à IS où la plupart des huîtres restent indifférenciées. Le développement des gonades est plus important en haut de la zone intertidale qu'en bas. Les substances organiques et minérales, et DOM sont similaires dans les deux sites. La température et les niveaux de