Effects of environmental factors on the daily growth rate of Pecten maximus juveniles in the Bay of Brest (France)

The study of the effects of environmental factors on the daily growth of Pecten maximus juveniles (one- and two-year olds) in the Bay of Brest was prompted by the decline of the scallop fishery in the Bay. Scallops over 30 mm in shell length were collected monthly from October 1994 to November 1995, Daily shell,shell-growth rings were counted using image analysis. Multiple stepwise regression analyses were performed to identify environmental parameters significantly affecting mean daily shell growth rates of one- and two-year-old juveniles in 1994 and 1995. Normal growth of Pecten shells is mainly regulated by bottom-water temperature, salinity and, to a lesser extent, river flow, rather than food. In 1995, three major toxic blooms of Gymnodinium cf. nagasakiense were recorded, leading to major reductions in shell growth rates. The effect of the toxic blooms on the natural Pecten populations was all the more important as no wild scallop spat settled on artificial collectors during the 1995 summer toxic events, and no pre-recruits originating from the 1995 spawnings were sampled on the bottom, despite good-quality spawnings. Two growth retardations were observed for one-year-old juveniles during the summer of 1995: the first one appears to be related to the sedimentation of a Rhizosolenia delicatula-Chaetoceros sociale bloom. It is suggested that the large aggregates of these diatoms led to clogging of the scallop gills. The second growth decrease was explained by the first bloom of G. cf. nagasakiense. The second and third dinoflagellate blooms were not associated with daily growth rate decreases in one-year-old scallops. The toxic effect of the dinoflagellate blooms was greater for two-year-old than for one-year-old juveniles; higher filtration rates and/or a differently oriented metabolism, compared with younger individuals, would explain these variations. (C) 1998 Elsevier Science B.V.