A million miles away

Morning, readers! Did any of you won the IEO-logoed lottery yesterday? If so, we hope the winner will invite us to have some tapas when we arrive home. With or without tapas we continue our trip west and our current location is 43º04.9430 – 031º49.3500.

Last night we crossed the Mid-Atlantic Ridge. It is very impressive to think of all those mountains over 3000 m high on the bottom of the ocean. New readers can see the profile of the sea bottom during the 2012 crossing in the old blog. Or even better, type Mid-Atlantic Ridge in Google and read about it. The Ridge can only be enjoyed in dry conditions in Iceland (dry conditions being actually a manner of speech and depending of the very temperamental weather). The whole of Iceland is emerged Mid-Atlantic Ridge.

We had to wait until the evening, but we finally saw the sun, when it was lower than the clouds. The wind decreased from 30 to 8 knots. The clouds prevent us from seeing the stars, but we will be happy if we have calm weather over the next days.

We explained yesterday the relevance of our work in the Grand Bank, and today we will start describing our sampling in detail.

Once the catch is on board (fishing itself will have its own entry next week), the officers drop it down a slip so it reaches the working deck, situated below. There we have two large doors we can open slowly to let the fish flow at the right pace to sort it by species. We put it into boxes, all of which are weighted. At this time physical activity is very intense. On average we catch 1-1.3 tons of fish per tow, and each individual can weight anything from a few grams to 10-15 kg. The boxes have to be carried to the weight. Depending of each species (flatfish packs better), a box can weight 30-35 kg. 

After sorting the catch, teams organise themselves to work in pairs. One measures and/or cuts and the other writes down the data. For all species except for grenadiers and shrimp we record total length (from the snout to the end of the tail) to the inferior cm. This means that if the fish measures 25.5 cm we record 25 cm. For grenadiers we use pre-anal length (from snout to anal fin) because they have a long and thin tail that gets easily damaged, and the length unit is 0.5 cm. Of shrimp we will talk another day, since it is hardly present. To measure fish we use ichtiometers, which are boards engraved with a measuring tape and shaped with a right angle at the 0 cm to hold the fish at the right position. A good rule of thumb to know how much fish to measure is to multiply by four the difference between the sizes of the largest and smallest fish. For example, being the smallest fish 15 cm and the biggest 40 cm, we need to measure 40-15= 25; 25 x 4= 100 fish. Sometimes, as it is the case with Greenland halibut, we do not catch enough fish and all of it must be measured.

Measuring fish takes little time and gives quite a lot of information. Besides knowing the proportion of fish at each size we can find out if there are more than one generation (cohorts, we call them) of fish in our sample. They can be identified by the number of peaks (maximums) in the resulting curve.

Length distribution for American plaice in 2009. The untrained eye can distinguish easily four cohorts.

For species investigated in international stock assessment meetings (cod, American plaice, Greenland halibut, yellowtail flounder, redfish, roughhead grenadier, thorny skate and witch flounder) and others of special interest we record as well individual weight, sex and maturity stage, to monitor overall fish condition, sex ratio and proportion of the population inmature and at different stages of their reproductive cycle (maturing, spawning, spent). Sexing fish can be relatively easy, but identifying the maturity stage is another issue altogether. Besides having different scales for different species, as our colleague Lola Garabana from IEO Coruña told us during the Maturity workshop celebrated at IEO Vigo last Friday, the problem arises from trying to put into categories stages of a biological process that occurs in a continuum. In theory and with the superb material they shared with us during the workshop it sounded easy, but it is not. The general aspect of the gonad has to be noted, as well as colour, texture, presence of ovocytes (early stages of eggs) in female gonads, ovocyte size... despite all this, the maturity stage of over 11000 fish was recorded in Platuxa 2013. We usually collect as well gonads from Greenland halibut, American plaice and cod, which are stored in formalin and analysed at the Coruña lab later in the year. To this purpose, extremely thin cuts of the gonads are obtained, and coloured for easier recognition of the different tissues. The microscopic analysis of these samples allows monitoring the quality of the macroscopic samples (those identified visually on board) during the survey. Last year a total of 583 gonads were collected. With the samples analysed to date it has been concluded that this sampling can hereafter be carried out biennally, which is very good news considering we are short of two people this year.

Our work with fish does not finish here. We also extract otoliths, but we will tell you about this tomorrow. Be there or be square!