Mortalities of sea-cage salmon ( Oncorhynchus tshawytscha) due to a bloom of Pseudochattonella verruculosa (Dictyochophyceae) in Queen Charlotte Sound, New Zealand

Abstract

Over a 6 day period in mid winter (June) 2010 about 200 tonnes of Chinook salmon ( Oncorhynchus tshawytscha) on a sea-cage farm at Ruakaka Bay, Queen Charlotte Sound died as a result of exposure to a bloom of the dictyochophyte Pseudochattonella verruculosa. The alga was isolated and its identity confirmed by morphology and large subunit ribosomal DNA sequencing. Cells densities of P. verruculosa over several tens of thousands of cells per litre were sufficient to cause fish mortalities. The bloom originated in the inland reaches of the sound in the Grove Arm and the farm was located near its seaward extension. Fish were affected and recovered as the bloom front moved on and off the farm site under the influence of tidal currents. Before the bloom reached its peak (at about 1.0 × 10 5 cells L −1) the farm was relocated to a site outside the affected area where surviving fish quickly recovered. The bloom took place when day length and seawater temperatures were near their annual minimum (9 h and 12 °C respectively), and after its peak it was rapidly displaced by the in situ development of a diatom bloom dominated by Skeletonema costatum. The P. verruculosa bloom arose after several weeks of persistent rainfall. This may have assisted in the development of a circulation pattern that led to the movement of increasingly buoyant, relatively warm, nutrient-enriched bottom water into the Grove Arm, shoaling to shallow depths at the head of the inlet. The high productivity of waters in this area at the time is believed to have been largely driven by this mechanism. Larger size (∼3.5 kg) fish were mainly killed by the bloom and examination of affected salmon tissues showed pathologies in the liver and kidneys. There are ambitious plans for the expansion of sea cage fish farming in New Zealand and this event is a timely reminder of the hazard these events pose to sea cage fish farming operations.