Atlantic horse mackerel

Atlantic horse mackerel occupy shelf waters as a schooling pelagic species, and their depth use changes on a daily cycle rather than a seasonal one.

THEY ARE SHELF PELAGICS, found over the continental shelf and shelf edge from the surface down to a couple of hundred metres, in dense schools.

DIEL VERTICAL MIGRATION IS THE DEFINING BEHAVIOUR. Schools spend the day near the bottom or in deeper water and rise towards the surface at night to feed on plankton and small fish. That daily movement, rather than any seasonal one, determines where the species is at a given hour and is why night fishing from piers and harbours produces them.

JUVENILES ARE STRONGLY INSHORE and are a familiar summer fish around harbours, piers and shallow coasts in numbers, frequently mixed with young mackerel.

JUVENILES ALSO SHELTER UNDER JELLYFISH, a well-documented association in which small horse mackerel take refuge among the tentacles — an unusual habitat use worth recording.

THEY TOLERATE A WIDE TEMPERATURE RANGE and occur from Norway to west Africa and into the Mediterranean, which is a broader envelope than most shelf pelagics.

AND THEY ASSOCIATE WITH STRUCTURE ONLY WHERE IT HOLDS PREY. Like other pelagic shoalers, horse mackerel are found around banks, wrecks and current features because those concentrate plankton and small fish, not because the fish want structure.

Horse mackerel carry a hazard that almost nothing else in British waters does, and the thing that makes it dangerous is that it only works in one direction.

WHAT A SCUTE IS. The lateral line of a carangid is armoured with a row of enlarged, hardened, keeled scales — scutes — running along the rear flank to the tail. On a horse mackerel they are prominent, they are sharp along the trailing edge, and they are angled backwards.

WHICH MEANS THE HAZARD IS ENTIRELY ABOUT DIRECTION. A hand run from head to tail passes over the scutes with the grain and feels nothing much: rough, ridged, harmless. A hand run from tail to head meets every one of them edge-on, and they cut. The injury is a series of parallel slices across the palm or the fingers, delivered in a single movement, by a fish that felt perfectly safe a second earlier.

The practical rule is therefore one sentence: HANDLE FROM HEAD TO TAIL, NEVER THE REVERSE. That covers the grip, the way the fish is slid out of a net, and the way it is held while a hook is removed. It is worth knowing before the first one is caught, because the mistake is made once and remembered permanently.

AND THE OTHER HALF OF THIS SPECIES' HANDLING IS ABOUT WHAT HAPPENS AFTERWARDS. Horse mackerel are frequently caught in numbers, often as bycatch on feathers, and are frequently destined for the bait bucket. A fish that is going to be used as bait is still a fish, and dispatching it properly rather than leaving it to suffocate in a bucket is both the humane course and — since flesh quality collapses in a fish that dies slowly and warm — the one that produces better bait. If it is going to be eaten, the flesh is oily, dark and quick to deteriorate, and bleeding and chilling it immediately is what decides whether it is good.

Horse mackerel seasonality is a shoal-movement story rather than a feeding-activity one, and the movement is driven by temperature and by prey.

THE FISH ARE OFFSHORE AND DEEP IN THE COLD MONTHS AND INSHORE IN THE WARM ONES. Horse mackerel occupy shelf waters and move shoreward and shallower as water warms through summer and into autumn, then withdraw to deeper offshore water in winter. Along northern European coasts they are consequently a late-summer and autumn fish inshore, and effectively absent from shore ground for much of the year.

THE AUTUMN CONCENTRATION IS THE PART WORTH UNDERSTANDING. As the season progresses, shoals become larger and more coherent and press closer to harbours, piers and headlands, following the small fish and planktonic prey that have themselves been concentrated by the season's production. That is why autumn produces the reliable inshore fishing and why it can be so sudden — a shoal arriving in a harbour is a mass event, not a gradual improvement.

DIEL MOVEMENT SITS ON TOP OF THE SEASONAL ONE. Horse mackerel rise in the water column at night and drop by day, following the vertical migration of the plankton their prey depends on. So the season decides whether they are there at all, and the time of day decides what depth they are at. Both are needed to make sense of when they are caught.

SPAWNING IS A SPRING AND SUMMER EVENT IN OFFSHORE AND SHELF WATERS, staged progressively along the coast so that the season begins earlier in the south and later in the north — a wave of spawning moving up the shelf rather than a single event. Fish are consequently in different reproductive condition at the same date in different places.

THE NORTHERN LIMIT MOVES BETWEEN YEARS AND BETWEEN DECADES. This is a warm-temperate species and its inshore abundance at the northern end of the range tracks sea temperature closely, so seasons vary considerably in length and intensity, and the species has become more prominent in northern waters as they have warmed.

AND THE SEASON DOES NOT DESCRIBE ANY RULE. Whether the species may be taken, in what quantity, and under what restriction is a matter for the local competent authority, and it varies. FishingHQ does not state what any current rule is.

Horse mackerel reproduction is a broadcast strategy staged across an enormous area and a long season, and the staging is the part that matters.

SPAWNING MOVES PROGRESSIVELY NORTHWARD ALONG THE SHELF. Rather than a single event, horse mackerel spawning begins earlier in the south and later in the north, producing a wave that travels up the continental shelf through spring and summer. Fish at the same latitude are therefore in the same condition at the same time, and fish a few hundred miles apart are not — which is why any single date for the species is wrong nearly everywhere.

THE SPAWNING IS BATCHED RATHER THAN SINGLE. Females release eggs in repeated batches across an extended period, which spreads reproductive risk across a whole season's worth of plankton conditions instead of staking it on one week. That is the same hedging strategy the red snapper and seatrout records describe, in a very different fish.

EGGS AND LARVAE ARE PELAGIC AND DRIFT, and where they end up is decided by the shelf circulation rather than by the adults. Year-class strength is therefore substantially a transport-and-survival question, and the relationship between spawning stock size and recruitment is correspondingly loose.

WHICH IS WHY THE STOCK STRUCTURE IS CONTESTED AND MANAGED AS SEPARATE UNITS. Horse mackerel in the north-east Atlantic are assessed as more than one stock with different spawning areas and different dynamics, and the boundaries between them are a live scientific question rather than a settled fact. FishingHQ records that as unresolved.

MATURITY IS REACHED AT A FEW YEARS AND THE FISH IS MODERATELY LONG-LIVED, so a stock accumulates several year classes and is not rebuilt annually — the opposite of the squid case.

RECRUITMENT HAS BEEN DOMINATED BY OCCASIONAL VERY STRONG YEAR CLASSES in the historical record, which have carried the fishery for long periods and whose causes are not fully explained.

AND WHAT MAY BE TAKEN IS AN ASSESSED AND NEGOTIATED MATTER revised annually by sea area; nothing above indicates any current recreational or commercial rule.