Researcher, PhD

Anders F. Opdal

Spawning migration and the value of an egg

For many species, the ideal nursery areas for their offspring are often less profitable or even completely unsuitable as habitats for the parents. A vast number of species, both on land and in the sea, are capable of performing astonishing migrations to overcome such conflicting interests. Finding a spawning location which maximises fitness is therefore a complex decision circling around the trade-off between parental cost and offspring benefit.

In 1878, fisheries scientist G.O. Sars unveiled a long kept secret on where the cod spawning along the Norwegian coast in early spring came from, namely the Barents Sea. Then, as now, fisheries scientists were particularly concerned about the fluctuating cod fishery in Lofoten, also known as the NA cod, or Lofoten-cod, given that the majority of spawning cod was predominantly located there. The fact that the NA cod actually spawned along the entire Norwegian coast is poorly reflected in scientific literature, despite the fact that there were years when the majority of NA cod actually spawned on the southwest coast of Norway, and not in Lofoten.

Fisheries scientists had made important discoveries on the particular conditions, such as at what temperatures and depths fish eggs and larvae need to grow and proliferate. These findings eventually culminated in the classical work by Johan Hjort, who linked early life survival, year class strength and stock recruitment in the NSS herring. The concept that fishes in general, and the NA cod and NSS herring in particular, utilize specific life history strategies to maximize their reproductive success, must therefore have been relatively well understood. However, in the case of the NA cod, the cost of spawning migration differed widely between individuals spawning off the Finnmark coast and the ones that spawned more than 2000 km further south, outside Vest-Agder. These differences in spawning locations, known through the commercial fisheries statistics , were never considered to be variations in spawning strategies, but thought to be a somewhat “natural distribution”, without any particular link to life history theory.

Today we believe that a size-selective trawl fishery in the Barents Sea has altered the spawning ground distribution of the NA cod. From theoretical state dependent optimisation models we hypothesise that a “feeder fishery” has promoted earlier age at maturation compared to a historic “spawner fishery”, and that due to physiological changes to the spawning stock, optimal spawning grounds have shifted north. We also propose that latitudinal effects on survival outweigh inter-annual climatic influence, suggesting that location of spawning grounds could be more important than climatic variability in determining ambient larval conditions. Thus, it is plausible that a reduced distribution of spawning grounds would strengthen the link between climate and recruitment. Through empirical analysis of commercial fisheries catch data from 1866 to 1969, it is suggested that the influence of climatic periods on spawning ground distribution becomes negligible when factoring in the onset, and rapid increase, of the trawl fishery in the Barents Sea (feeder fishery).

References

	A.F. Opdal, F.B. Vikebø, and Ø. Fiksen. 2011. Parental migration, climate and thermal exposure of larvae: spawning in southern regions gives Northeast Arctic cod a warm start Marine Ecology Progress Series 439: 255-262. [ pdf ] [open access]
	A.F. Opdal. 2010. Fisheries change spawning ground distribution of northeast Arctic cod Biology Letters 6: 261-264. [ pdf ]
	C. Jørgensen, E.S. Dunlop, A.F. Opdal, and Ø. Fiksen. 2008. The evolution of spawning migrations: state dependence and fishing-induced changes Ecology 89: 3436-3448. [ pdf ]
	A.F. Opdal, F. Vikebø, and Ø. Fiksen. 2008. Relationships between spawning ground identity, latitude and early life thermal exposure in Northeast Arctic cod Journal of Northwest Atlantic Fishery Science 41: 13-22. [ pdf ]
	A.F. Opdal, O.R. Godø, O.A. Bergstad, and Ø. Fiksen. 2008. Distribution, identity, and possible processes sustaining meso- and bathypelagic scattering layers on the northern Mid-Atlantic Ridge Deep Sea Research II 55: 45-58. [ pdf ]