Response on scallop culture to enhanced nutrient supply by experimental fertilisation of a landlocked bay

Abstract

The effect of an enhanced nutrient supply to coastal waters of a landlocked bay, Hopavagen in Central Norway, on the phytoplankton production and biomass, and on growth of scallops (Pecten maximus) was studied in 1997–1999. Nitrogen, silicon and phosphorous (N:Si:P = 16:8:1, atomic) were added daily between May and October in 1998 at a level of 0.4 mg P m−3 day−1. The concentration of nutrient addition was doubled in 1999 during the same period. High addition of nutrients (1999) resulted in a significantly higher phytoplankton biomass in the summer period, expressed as chlorophyll a content, than without nutrient (1997) and low nutrient (1998). The respective mean chlorophyll a levels were 2.4 in 1999, 1.6 in 1998 and 1.2 μg l−1 in 1997. The mean primary production during the summers generally increased with the addition of nutrients from an average level of 320 mg carbon m−2 day−1 in 1997 to 1200 mg carbon m−2 day−1 in 1999. Scallops placed at 10 m depth in Hopavagen showed an increase in growth rate of the outer scallop shell in the period July–September from 0.16% day−1 in 1997 to 0.53% day−1 in 1998. Scallops grown in an unfertilised control station in the fjord outside Hopavagen had a significantly lower growth rate than those grown in the fertilised water of Hopavagen. The results showed decreased growth rate with increasing shell sizes. However, for all size groups studied a higher growth rate of the scallops was observed when nutrients were added to the bay. The tissue dry weight content of scallops grown in Hopavagen was 2–4 times higher than in the control scallops.