'Fish are born, they grow, they reproduce and they die – whether from natural causes or from fishing. That's it. Modelers just use complicated (or not so complicated) math to iron out the details.' Andrew B. Cooper.
Whilst there are many technicalities along the way, the overall purpose of the International Bottom Trawl Survey is fairly straightforward: to work out the relative abundance* and distribution of the commercial heavyweights here: saithe, cod, haddock, whiting, Norway pout, herring, mackerel and sprat. Most of these live near the bottom of the sea (that's 'demersal' in marine biology) whilst others – herring, mackerel and sprat – shoal in the open water (called 'pelagic'). That naturally poses challenges for data collection, and there will be a blog covering the differences another day.
Once sorted, these fish listed above have various attributes recorded (more on that process later) before resulting data is channelled and ends up as information, maps and graphs that tell us where the various species are spread and the relative number of each per hour trawled. For the latter, this is categorised both by species and then by what's known as year class. So, although it's not written here, the graph underneath is for year class 1 of haddock: or haddock that is somewhere between its first and second birthday.
The vague dotted line cutting horizontally just short of 500 on the y-axis is the mean.
This if for haddock across the whole survey region; before that, an abundance index is produced for haddock per ICES statistical rectangle. There would also be separate graphs for haddock at various other ages as well as for other species.
All this is of course contingent on scientists knowing how old or young a given fish is, something also discovered in the wet laboratories of the Johan Hjort.
Species distribution is also an objective. Here's a map of the cod pulled from the water on the part of this year's survey that took place before I boarded the boat.
Alongside the finned creatures, distribution and relative abundance will also be monitored for selected crustaceans – such as the Norway lobster and spider crab. Also, biological parameters and hydrographical and environmental information will be observed at the different sampling points.
This is a summary so small it can barely be called a nutshell (more like a fish egg membrane), but it's the gist. Hopefully more of a detailed picture will develop as we go.
*As opposed to absolute abundance, the actual number of individuals in a population, relative abundance is the number of one fish species in a certain area (ICES rectangles) as a percentage of the total fish caught in that same area. This means that the same species can be compared in two different areas.