CAPTOR update – Plankton sampling – 5 August 2019

Sean Fennessy 

After a fairly lengthy break owing to logistic challenges, the second phase of CAPTOR sampling resumed, with a plankton sampling trip to Aliwal Shoal south of Durban, on the ACEP vessel Phakisa. In Phase 1, plankton sampling took place at five transects across the continental shelf, from iSimangaliso in the north to Aliwal Shoal in the south. These transects will be repeated in Phase 2 to assess if there are inter-annual differences in plankton composition, and to expand the library of genetic information on the variety of species off our coast.

Sampling always takes place after dark, as this is when plankton rise in the water column. We use three different net types to sample at different depths: a WP2 net is hauled vertically from close to the sea bed, a Manta trawl is towed to sample the surface, and a Ring net is towed about 5 metres below the surface. Plankton is collected from sites over four water depths: 200, 100, 50 and 20m – for comparative purposes, and mesh sizes are 0.2 mm and 0.5 mm. A CTD probe (to measure water conductivity, temperature and depth) is also deployed at each sampling site. The plankton is preserved on board and returned to the lab for processing.

We left Durban around 4:30 pm, timed to arrive at Aliwal Shoal around 6:30 pm once it was dark, preparing the nets on the way. The weather and sea conditions could not have been better – flat calm, and no wind. Some humpback whales were seen on the way south, and a large ocean sunfish jumped a few times as we passed by; gannets were on the hunt too, suggesting the possibility of sardine shoals in the vicinity. Numerous skiboats were out catching bait before heading to favourite reefs to catch geelbek and kob. The plankton sampling took about four hours, starting in a water depth of 200m and moving inshore. Catches were dominated by copepods, but a variety of other plankton was caught, including a large lobster larva, spiky pteropods, plenty of amphipods and some iridescent Noctiluca. At the 50m station, masses of fish eggs were caught in the ring net – time will tell if these are from sardines. Plenty of work is required to extract and sequence the DNA from these bulk samples – but, if the method can be refined successfully, it will be a lot easier than having to identify the plankton in the traditional way using a microscope.