Ecological Significance of the Ctenophore Pleurobrachia pileus off Southwestern Nova Scotia

Abstract

The population dynamics and quantitative feeding ecology of the ctenophore (Pleurobrachia pileus) were investigated in the spring of 1983 and 1984 on Browns Bank, southwestern Scotian Shelf. In both years, large crustacean Zooplankton (>1050 μm) were the dominant item in the diet of Pleurobrachia among all sizes captured (range = 5–21 mm diameter) averaging 70% by weight of the total prey consumed. The daily ration of Pleurobrachia was strongly size dependent increasing from 2.6 mg dry wt./d at a body size of 6 mm to 8.5 mg/d at 12 mm. At the population level, daily consumption of crustacean zooplankton during 1983 and 1984 averaged 8.8% (range = 0.2–28%) and 0.7% (range = 0.2–2%) of the prey standing stock, respectively. This difference was consistent with the observation that there was 28% less zooplankton, on average, during 1983 relative to 1984 at the same time and location. An abnormal spring peak in ctenophore abundance was evident in 1983, but not in 1984, that consisted of large, mature individuals at average densities in excess of 1/m3. Ctenophore abundance during spring of 1984 averaged 0.1/m3 Increased overwintering survival of ctenophores resulting from anomalously high sea surface temperatures (SST) appears to have been responsible for the exceptional spring abundance of Pleurobrachia on Browns Bank during 1983. Over 50% of the variation in a 10-yr time series of spring Pleurobrachia abundance for the southwest Nova Scotia region was explained by SST conditions prevailing during January to April. I suggest that the extremely low levels of haddock (Melanogrammus aeglefinus) larvae produced on Browns Bank during the spring of 1983 resulted from food shortages induced by severe depredation of zooplankton by Pleurobrachia.