The abundance and distribution of macrozooplankton and micronekton on Georges Bank

H. Brown, S. M. Bollens, L. P. Madin, E. F. Horgan, and G. S. Stoltz

The objective of this work is to quantify the abundance and distribution of macrozooplankton and micronekton in time and space on Georges Bank. These and other data will be used to estimate the predation rates on the target species Gadus morhua, Melanogrammus aeglifinus, Calanus finmarchicus and Pseudocalanus sp.

In total 873,527 specimens were collected in 1007 samples representing 224 taxa. The ten most abundant species in all the Broad Scale survey samples from 1995 - 1998 Neomysis americana, Salpa spp., Meganyctiphanes norvegica, Themisto gaudichaudii>, Pleurobrachia pileus, Nematoscelis spp., Euphausia krohnii, Sagitta spp., Spiratella sp. and Clione limacina.

Mean (+/- standard error) monthly abundance of predators from all Broad-Scale stations sampled between 1995 - 1998 (total water column weighted average; #/1000m3) were calculated. Striking seasonal and interannual variation in abundances can be seen for many species, particularly in Clupea harengus, Pleurobrachia pileus and our target species Gadus morhua. Relating this to biological (chlorophyll, mesozooplankton) and physical (temperature, ring activity) conditions is a major focus of our future and ongoing work.

Length frequency distributions for up to 50 specimens randomly selected from each net summed for any given Broad Scale survey cruise were determined. We have presented these ditributions for 1995 and 1996 for two species (Meganyctiphanes norvegica and Themisto gaudichaudii) to show how different species can have very different patterns of length frequency distributions. Meganyctiphanes norvegica shows interannual differences, especially later in year, particularly younger cohort production higher in 1995. Themisto gaudichaudii shows little interannual variation, with slow growth from February to May (no progression of modal length), but significant growth from May to June.

Bankwide distribution and abundance of Pleurobrachia pileus and Salpa spp. are presented for the months of May and June in 1995 and 1996. A large difference in the abundance of both species in the two years was shown, with Salpa spp. being almost completely absent in 1996. We attribute this difference to warm core ring activity on the southern flank in 1995 and the resulting intrusion of slope water and species onto the bank.