Winter halibut spawning

Admittedly, it’s already the second half of February, but any time is a good time to take on challenges and make resolutions, especially regarding the environment, one’s health and household budget. Changes in diet are easily reconciled with concern for the climate and biodiversity, if only by choosing fish from certified sustainable fisheries. In winter and the pre-winter, halibut are particularly tasty and healthy, and relatively inexpensive. Why just now? Because they are in the process of spawning, so they are easy to catch, and richest in proteins, fats and other nutrients.

Disappearing giants, gnomes on our tables

One of the most economically important fish of the last century was the Atlantic halibut Hippoglossus hippoglossus, also known as the white halibut, the white bullet or simply the bullet fish. This true giant reached more than 3.5 meters in length and up to 300 kg in weight. It is by far the most impressive member of the flounder family Pleuronectidae. Today such huge specimens are no longer found. In general, it has become a conservation-requiring rarity. So what halibuts reign supreme on our tables, as their harvesting is still considered relatively nature-friendly by scientists and authorities? In Poland and Europe, it’s primarily black halibut Reinhardtius hypoglossoides, also known as blue, Greenland halibut, or black limpets [4, 5, 7, 9, 10].

They are much smaller and much more common than their white namesakes. They usually reach up to 1 meter in length and a dozen or so kilograms in weight. Hence their other fishing and commercial name: lesser halibut. As far as commercial fish substitutes go, they are exceedingly closely related to the original. After all, both species belong to the same family (flounder), prefer similar habitats (the depths of cool waters with temperatures below 4°C), even originate more or less from the same regions of the Omni-Cocean (the North Atlantic, around Greenland, with the white halibut reaching further south through the White and North Seas to the northern edge of the Bay of Biscay, occasionally even entering the Baltic Sea).

This is a striking difference from freshwater predatory fish, i.e. European catfish, burbot or perch, which are being replaced in markets and restaurants by representatives of other families and other continents, with not very similar biology and ecology such as: clarias (“African catfish”), kinglip (“New Zealand burbot”) and “Nile perch”(Nile lates) or “sea bass” (sea bass) [2, 6, 7, 9].

Time and gender

The reproductive biology of the fish determines not only their taste and dietary value, but also the risk of overfishing the stocks. Let’s discuss this using the example of the ongoing winter spawning of both culms. Atlantic (white) halibut – as befits a giant of the seas around the Arctic – grows, then matures sexually extremely slowly. “Gentlemen” become ready to spawn at seven or eight years of age, “ladies” at ten or eleven. We should add that individuals of both sexes usually live to be 25-30 years old, although true mature adults of half a century have been caught.

The number of egg grains (eggs) laid by females sometimes varies widely: from a few thousand to almost four million (on average about two million). White-bellied Kulbaki are unique in the flounder family and in the order of flatfish, as they spawn in winter, while most prefer spring or summer. Specifically, between December and April, just off the seabed, in the densest waters of 5-7°C. Fertilized egg grains measure 3.0-3.8 mm.

The larvae that hatch from them reach 6.5 mm in length, and their further fate strongly depends on intra- and interspecies competition. Food is key, which may be the reason for choosing such an absurd spawning season. In winter, competition for polar water resources diminishes, which is why emperor penguins also breed in the middle of winter, albeit on the opposite side of the globe [3, 8].

Big trouble with small eggs

The current king of Polish tables, the black halibut, lays eggs from January through March. Thanks to bathyscaphes and research buoys, and now also underwater drones and tagging of individuals, mankind has learned about its spectacular spawning flights. It then rises from the bottom through almost the entire column of water, to a depth of 350-200 meters from the surface. Females release only one and only one batch of eggs each year. Males, on the other hand, mate annually with multiple partners in succession. The fertilized eggs, and later the larvae of the lesser curlews, remain in the pelagial water column for longer.

Only having completed the transformation, at 60-85 mm in length, they will sink to the bottom to take up the lifestyle typical of adult specimens. Blue halibut differs from many other fish not only in the timing of spawning, but also in the form of development of gonads and oocytes, two batches of which are formed simultaneously. One of them is laid during the current spawning season, the other only a year later, in the next spawning. This is due to certain thermodynamic and biochemical constraints on the production of spare materials (yolk).

The lesser bulbul produces relatively large (for its rather micro spawner size) egg grains (2.5-3.0 mm in diameter with a maximum body length of 1.2 m, but usually 0.8-1.0 m). The production of the yolk and its subsequent compartmentalization in the egg cell takes a very long time at the low temperatures of the Arctic winter. If black halibut, following the example of typical flounders, developed only one batch of oocytes at a time, it could reproduce every two to three years [3, 5, 8, 9].

Riddle chases riddle

Thanks to tagging news on live specimens, scientists and fishermen have been astonished to learn that blue whimbrels – although flounder-like – not only trumpet, but quite often also hunt in the water column, quite high above the bottom. This aspect of their biology makes it easier to fish without trawling the bottom, which undoubtedly helps to take care of bottom biodiversity. Greenland halibut were initially thought to move vertically in the water column, but tag readings do not confirm this. Perhaps this has something to do with the species’ wider field of vision than the rest of the flatfish? After all, it has long been puzzled as to why exactly this kulbak has its left eye on top of its skull, rather than both on one side of the body, as befits a flatfish.

The substitution of species of large yet slowly reproducing fish for their smaller and more prolific cousins (or non-cousins), both in the wild and in our kitchens, is a common phenomenon. A similar story can be told about tuna and catfish. However, the real revolution in the ecosystem occurs when the entire landscape, along with the food web, changes after the annihilation of apex predators. The bays of our Baltic Sea are also threatened by something like this, since pike, cod and fish-eating herring have been declining for years (as we wrote about in the article: Do herring eat fish?), while sticklebacks have been arriving [1, 2, 4, 10].

pic. main: NOAA Fisheries

In the article, I used, among other things. z:

1. Boje, J., Neuenfeldt, S., Sparrevohn, C., Eigaard, O., Behrens, J. (2014). Seasonal migration, vertical activity, and winter temperature experience of Greenland halibut Reinhardtius hippoglossoides in West Greenland waters. Marine Ecology Progress Series. 508: 211-222.
2. Clover, C. (2004). The End of the Line: How is overfishing changing the world and what we eat? Ebury Press, London.
3. Kennedy, J., Gundersen, A., Høines, Å., Kjesbu, O. (2011). Greenland halibut (Reinhardtius hippoglossoides) spawn annually but successive cohorts of oocytes develop over two years, complicating correct assessment of maturity. Canadian Journal of Fisheries and Aquatic Sciences. 68 (2): 201-209.
4. Nelson, J. (2006). Fishes of the World. John Wiley & Sons, Hoboken.
5. Nikolski, G. (1970). Detailed Ichthyology. PWRiL, Warsaw.
6. Orlov, A., Binohlan, C. (2009). Length-weight relationships of deep-sea fishes from the western Bering Sea. Journal of Applied Ichthyology. 25 (2): 223-227.
7. Rutkowicz, S. (1982). Encyclopedia of marine fish. Maritime Publishing House, Gdansk.
8. Siwicke, K., Seitz, A., Rodgveller, C., Echave, K. (2022). Characterizing spawning behavior of Greenland halibut (Reinhardtius hippoglossoides) in the eastern Bering Sea and Aleutian Islands. Fishery Bulletin. 120 (1): 55-67.
9. Terofal, F., Militz, C. (1996). Marine fish. Świat Książki, Warsaw.
10. https://www.msc.org/pl/dla-mediow/informacje-prasowe/informacja-prasowa/niebanalny-przepis-na-halibuta [dostęp 11.01.2025]