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
No comments:
Post a Comment