Nicholas C. Halliday

Spawning season and temperature relationships for sardine (Sardina pilchardus) in the eastern North Atlantic

Spawning temperature preferences for sardine (Sardina pilchardus) in the eastern North Atlantic were determined from egg survey data. These were compared with climatological temperature cycles (1986 2002) derived from satellite observations, by geographical region, to predict spawning seasons. Optimum spawning temperatures were determined as 14.0-15.0°C from the English Channel to Portugal and 16.0-18.0°C for all north-west African regions. Spawning seasons were closely related to the general latitudinal trend of the annual temperature cycle, with modification by upwelling in the western Iberian and north-west African regions. Some differences between temperature-based spawning season predictions and field observations were related to variations in seasonal plankton production. Correlations in the annual time-series of favourable spawning temperatures suggested relatively strong linkages between the southern areas from Portugal to Senegal. There was no consistent relationship between annual variations in extent of temperature-predicted spawning seasons and observed field abundance of eggs.

A comparison of LHPR and OPC data from vertical distribution sampling of zooplankton in a Norwegian fjord

Abstract Sampling was carried out using a double Longhurst-Hardy Plankton Recorder (LHPR) system (200 µm and 53 µm mesh nets) in June 1993 in Storfjorden on the west coast of Norway to describe the vertical distribution of zooplankton. The results are compared with concurrent Optical Plankton Counter (OPC) data. Zooplankton was most abundant in a superficial (0–30 m depth) layer ofwarrn (8 to 12 °C), low salinity (23 to 34) water. Included in this shallow distribution were cladocerans, which comprised 62.7 % of all zooplankton identitied in the 200 µm LHPR samples. Most of the small zooplankton taken in the 53 µm LHPR samples was distributed in the upper 50 m of the water column|abundant amongst these small organisms were adult Microsetella norvegica together with nauplii and early copepodite stages of other copepods. Both the number and biovolume of particles recorded by the OPC were consistently about 4 times higher throughout the water column than for an equivalent size range of zooplankton identified in the 200 µm LHPR samples. A comparison of particulate size frequency distributions from the OPC and the 200 µm LHPR samples suggests that this difference could be due partly to the detection of flocculants and detritus by the OPC. However, the 53 µm LHPR results indicate that there was also significant under-representation of zooplankton < 0.35 mm Equivalent Spherical Diameter (ESD) in the 200 µm LHPR samples due to extrusion through the liltering mesh.

Distribution and abundance of sardine (Sardina pilchardus) eggs in the English Channel from Continuous Plankton Recorder sampling, 1958–1980

Continuous Plankton Recorder (CPR) samples from the English Channel and adjacent Celtic shelf, taken over the period 1958–1980, were analysed for sardine (Sardina pilchardus) eggs. Results showed the progression of sardine spawning along the English Channel from west to east from March to August and a return from east to west from September to November. This corresponds with the two seasonal peaks of sardine egg abundance in the western Channel: the main summer peak being in May/June, with a smaller autumn peak in October/November. Long-term changes in sardine egg abundance in CPR samples showed a decline in summer spawning from the late 1960s, but no clear trend in autumn-spawned egg abundance. Similar patterns were observed in the numbers of sardine eggs sampled by conventional plankton net tows at the time-series Station L5 off Plymouth. This supports the use of the longer time-series of sardine egg data at L5 as being representative of a wider area and emphasizes the importance in continuation of the L5 time-series.

A comparison of morphometric and geometric methods for the estimation of individual zooplankton volumes

Abstract Morphometric and geometric methods were compared for the determination of volumes of individual zooplankton sampled in a Norwegian fjord, the Storfjorden, using a double Longhurst-Hardy Plankton Recorder (LHPR). Both methods of volume estimation, after correction for shrinkage during preservation, gave satisfactory results and have applications in the determination of the biomass of individual taxa and their developmental stages.

Occurrence of the physonect siphonophore Apolemia uvaria off Plymouth and in south-west England

In September 2007, observations were made of a siphonophore in surface waters and near to the seabed by sea users off south Devon and south-east Cornwall. The same siphonophore was also recorded from regular samples collected offshore of Plymouth. The species is identified as Apolemia uvaria, which had not previously been recorded off Plymouth. It was sampled until March 2008 and re-appeared, in smaller numbers, in autumn 2008 until February 2009 but was not reliably reported in autumn 2009 (to end of October). The occurrence is unlikely to be due to sea warming, but more likely some variation in oceanic currents, possibly influxes of Atlantic water