Irregular Variations

Although usually conforming to the described monthly pattern, the plankton is characterised by short-period fluctuations which are non-seasonal.

In several cases plankton samples were neither quantitatively nor qualitatively representative of the month in which they were taken. The following volumes were recorded for successive samples from Station One in June, 1961 and July, 1961:

June 1	270cc
June 13	120cc
June 15	190cc
June 22	120cc
July 5	20cc
July 12	180cc
July 20	270cc
July 26	200cc

The sample taken on 5 July produced only a small fraction of the volume normal for these months, and Obelia geniculata medusae and Pleurobrachia pileus which usually dominate the June and July plankton were very scarce. The larvae of several brachyuran species present in the majority of the above samples were absent on 5 July.

A similar example occurred on 25 January 1962, when a volume of 10cc was recorded compared with 230cc, 180cc and 230cc recorded on 3, 10 and 15 January respectively. Neither the volume nor the faunal composition of the sample taken on 5 January compared with other January samples. Brachyuran larvae representing 15 species dominated this sample, while Copepoda, porcellanid larvae, and Jaxea sp. were rare, and Salpidae, Octophialucium funerarium and Phialella quadrata were absent.

The larvae of species occurring seasonally were occasionally identified in samples taken outside the expected seasonal boundaries. A single stage one larva of Petrocheles spinosus was recorded on 2 May 1962. This species was absent from all samples between December 1961 and 2 May 1962, and did not reappear in the plankton until July 1962. Several stage one larvae of Jaxea sp. were recorded on 24 May 1962. Only late stage larvae were present in February 1962, and stage one larvae of this species were not subsequently recorded until September 1962. In such out of season occurrences as these larvae did not appear in sufficient numbers to suggest a secondary period of liberation.

During the spring and summer months occasional plankton samples were dominated by many hundreds of early stage larvae of any one of several crustacean species. These monospecific swarms were probably produced by large numbers of adults liberating their larvae simultaneously, and the larvae were not dispersed by tidal action. This is supported by the observation that species occurring sparsely in the plankton for several weeks may be suddenly represented by an abundance page 26 of first stage larvae in a single plankton sample, with subsequent samples yielding very few.

Identified decapod Crustacea forming such larval swarms are as follows: Hemigrapsus edwardsi, Petrolisthes novaezelandiae, Callianassa filholi, Jaxea sp. Of these, Callianassa filholi and Jaxea sp. swarm quite frequently in the months of their greatest abundance.