Tuesday 28 October 2014

Mackerel: easy to catch, good to eat - and beautifully designed

On the rare occasions when I visit the coast from land-locked Hertfordshire, I like to go out mackerel fishing, in company with other holidaymakers. Boat owners run these trips alongside those for sightseeing, and the necessary tackle is provided. Local knowledge of where shoals of fish are likely to be found is invaluable and, after chugging out to a suitable location, the boat’s engine is turned off and instruction given on how to lower the weighted line to the sea bed, raise it a little, let it drop, raise it again, etc. No bait is used, but hooks (at least three) are held out on traces that have brightly coloured feathers attached. In no time, the tug of fish is felt and the line is reeled in, sometimes with a mackerel on each hook, and everyone seems to have success. Occasionally, other fish are caught, but it is mackerel that make up almost all the catch and, on good days, they begin to pile up in trays, or buckets, on deck.

For many people on the trip, catching mackerel is pleasure enough but, for others, the freshly-caught fish make a splendid supper. Whereas meat, and especially game, improves with hanging, fish are best eaten as fresh as possible. There are a number of ways in which mackerel can be enjoyed and they are both delicious and good for one’s health. The simplest approach is to barbecue, or they might be eaten as a ceviche of raw fish, and they can also be smoked to allow preservation for several days, or even weeks. [1] My own favourite recipe comes from childhood when we were given freshly-caught mackerel by Mr Revell, who lived along our road and always seemed to have an excess when he went fishing from the end of Paignton Pier. We had them soused in vinegar, [2] with bay leaves placed into slashes in the flesh, and I can remember the taste well.

Everyone catching Atlantic mackerel (Scomber scombrus), whether to eat or not, is impressed by the appearance and muscularity of the fish. The muscles used for swimming are arranged in blocks and run along the body, from just behind the head to the tail, and they drive the caudal fins (tail). If we looked closely, we would see that three types of muscle are present. Running along the sides of the fish are red muscles, while ca. 90% of the bulk of the flesh consists of white muscles, or intermediate pink muscles. [3] Red muscles are very different in structure and function to the other types. They are well supplied with oxygen and do not tire easily and this contrasts with the white muscles, which show the opposite qualities, with the pink muscles (lying within the white muscle mass) having a slightly longer endurance. Red muscles are thus used in cruising, while the white and pink muscles are used for bursts of acceleration, such as are needed when avoiding prey or in predation.

In addition to powering the caudal fins, the muscles also provide the outline of the fish. Mackerel have an “idealised streamlined shape”; one where the widest part of the body is about one third from the front and where the length is about four times the width. This shape is shown in the diagram below and one needs to visualise it in 3-D. The mackerel makes a good fit and, to understand why this is advantageous, we need to imagine that water is arranged into sheets and that turbulence results when these sheets are disrupted. At some point along the body, the sheets of water passing over the mackerel peel away from close contact with the surface and this is referred to as the separation point (SP, see arrows in the diagram below). Beyond the SP, the sheets of water stop flowing smoothly and this results in drag, but the streamlined shape means that the water layers hold to the body for longer, resulting in a smaller turbulent wake behind the fish. Its shape thus results in less drag, so less power is then needed by the fish in both slow, and fast, swimming

Scales that are sunk into the skin cause micro-turbulence in the water flowing just over the surface of the fish and this provides a “lubricant” over which the fast-moving sheets just a little further away from the body can slide, further diminishing the risk of separation. Of course, this is only one function of the scales, as they also serve for protection. More obvious features of the surface are the fins and these are rigid and can be held out into the smooth flow of water passing over the fish when they are needed most during rapid swimming. They function like the flights of arrows, or darts, in counteracting pitch, roll and yaw and, like the body profile and the musculature, are elegant designs.

However, mackerel did not have a designer and all the structures that we see evolved over time. So, too, did other features of these fish, such as their good eyesight (that enabled them to spot the feathers on my fishing line); the swim bladder (an extension of the gut) that allows them to float without expending energy; their efficient means of acquiring oxygen through the gills; and many other modifications, including their extraordinary musculature (which is so good to eat). All evolved and we can only speculate how: were there dramatic mutations, or more gradual changes in anatomy, morphology and physiology? Sometimes, belief in a Creator seems like an easier way to get answers.

[3] J.J.Videler (1993) Fish Swimming. London, Chapman & Hall

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