Thousands of kilometers across the sea: the great journey of the Norwegian Spring-spawning Herring
The Norwegian Spring-spawning herring (Clupea harengus) is a silvery forage fish belonging to the family of the Clupeidae.
It can live up to 25 years, reach a length of 40 cm and form very dense schools. Exploited for centuries, it constitutes one of the biggest fish stocks of the North-Atlantic. The herring is very tasty and is part of many traditional dishes of the Northern countries, often eaten salted, smoked or pickled. In addition to being a delicious dish, herring is one of the key species of the whole Barents and Norwegian Sea ecosystems, undergoing migrations of several thousands kilometers. Here is the story of its journey.
Everything starts with an egg. From January to March, male and female herring gather along the coast of Norway, off Møre, to spawn. While the female lay their eggs, they are fertilized in the water as males discharge their sperm. Each female can spawn around 40 000 eggs per year. Those are left in dense beds, on gravely sea floor, at depth down to 250m. After 2 to 3 weeks, the eggs hatch and a journey of 1 500 km starts for the newly born larvae.
The larvae start drifting with the Norwegian Coastal Current along the coast of Norway and up to the nursery grounds of the Barents Sea. On the way, they face several threats. First, they run the risk to be eaten by several predators such as adult herring, blue whiting and sometimes mackerel. Additionally, the amount food (Calanus eggs and nauplii) they can find while drifting will also determine how many will survive. Finally, whether they reach suitable nursery grounds or not strongly depends on the prevailing winds and currents, and this can seriously affect their survival probability. By late summer, the surviving larvae have metamorphosed in juveniles and settled in their nursery grounds where they will live the next 3–4 years. During this time, they feed mainly on zooplankton (euphausiids and Calanus finmarchicus) while being preyed on by sea birds, marine mammals and other fish.Once they reach 28-30 cm, their start maturing and leave the nursery grounds to join the adult stock, starting the most intense and demanding part of their life.
The first spawning takes place 1 to 2 years after leaving the nursery grounds, at around 3-5 years-old. The adult life-cycle of the herring is very particular. After, spawning along the Norwegian coast, the herring migrates towards the feeding grounds, in the Norwegian sea, where it feeds on zooplankton (Calanussp.) from April to August. Around September, it migrates towards the overwintering grounds. During winter, the herring does not feed and the energy used for gonads development and routine metabolism comes from the reserves it accumulated during the summer. In January, the herring migrates a third time towards the spawning grounds to release their eggs and sperm.
Figure 1: General migration pattern of adult herring during the last decade (from Kvamme et al. 2003).
The migration pattern of the herring changed quite a few times during the last century. In the late 60’s for example, it was going around the whole Norwegian sea, spawning on the Norwegian coast, overwintering near Iceland and feeding north of the Norwegian sea, swimming more than 3000 km in total. Nowadays, both spawning and overwintering grounds are located along the Norwegian coast, but it still migrates out in the Norwegian sea to feed, swimming a still impressive distance, between 1500 and 200 km (Figure 1).
During its extensive migrations, herring must still face many predators such as sea birds and marine mammal. On top of that, herring also supports an intense and economically important fishery, constituted of vessels from the surrounding countries (Norway, Russia, Iceland, Faroes Islands and Europe). The stock has been exploited for centuries, with both juveniles and adult harvested at any time of the year. However, the technological improvement of the fishery led to overfishing and the stock collapsed in the late 60’s. For 20 years, the stock numbers were at drastically low levels and fishing was banned. In the late 80’s, the stock recovered due to one exceptionally good year, that presented very favorable environmental conditions for larvae growth and survival. To avoid another dramatic event of the same kind, the fishery is now regulated with quotas and is restricted to adults above 25 cm long.
Because of its importance as a source of food and economy, the Norwegian herring has been extensively studied and an incredible amount of data on this stock is available. However, still a lot of mysteries remain. For example, the effects of the intense fishing on the herring stock are still to be understood. A lot is also to discover about the extraordinary adaptability of this fish to its environment, especially in the current context of climate change. Those issues are of great importance for the future management of this species and ensure that herring will always be available on our tables.
Kvamme, C., Nøttestad, L., Fernö, A. & Melle, W. (2003) Migration patterns in Norwegian spring-spawning herring: Why young fish swim away from the wintering area in late summer, Marine Ecology Progress Series,247:197-210
Health Benefits of herring, Organic Facts. URL: https://www.organicfacts.net/herring.html