The majority of species of direct interest to fisheries are higher trophic level predators. For ecosystem modelling purposes their prey are simply pooled into several forage functional groups. Estimation of the corresponding forage biomass was done using the information from acoustic backscattering collected by the scientific sonars on the Celtic Explorer and information on the species and size composition from the catches collected using the pelagic trawls.
The Celtic Explorer was equipped with two pelagic trawl systems. The ‘herring trawl’ had a single cod-end trawl and was considerably smaller than the ‘multipurpose trawl’. Both trawls are strictly pelagic, they do not make contact with the seafloor and are suspended in midwater between floats on the top edge, weights at the bottom and the pull of the trawl ‘doors’ which are attached to the ships’ warps.
The multipurpose trawl was equipped with a multisampler, a device that incorporates three separate cod-ends, and hence allowed three discrete samples to be collected in a single trawl set.
The Ecosystem survey completed 44 fishing stations and obtained 1 to 3 trawl samples from discrete depths for a total of 96 samples. Fishing samples were taken day and night however the dawn-dusk periods were avoided because of the vertical migration behaviour of many forage species. On the echogram time runs from left to right and the vertical axis represents the depth. This segment shows the upward migration of mesopelagic species from 400m and deeper as dusk falls. The fine red lines near the left edge are the echoes of the oceanographic instrument which was deployed and recovered just before dusk.
The vast majority of the trawl catches were made up small sized fish and invertebrates, the overall mean length was less than 60mm. The catches were highly diverse, with 75 families of fish species and 50 families and higher taxa of non-fish groups. In many cases, taxonomic identification was limited to the family level.
An electronic atlas of photographs of fish and invertebrates was assembled to assist in field taxonomy. Identification was time-consuming as only a small fraction of the species encountered were of interest to fisheries.
In the following sample pages the 1cm background grid provides a size reference: